The purpose of this design guide is to assist a design engineer to better understand contact
design in general. This guide will give specific design guidelines and formulas needed to
design the types of contacts commonly used in consumer products powered by alkaline
batteries.

There are many criteria that must be considered when designing an electrical contact. The
designer must consider all criteria to satisfy the electrical signal and the specifications of the
application. It would be beneficial if there were a step by step process or recipe that could be
applied; unfortunately, this is not
possible. Years of experience are needed to learn how to design contacts and to determine
what type of contact would be good for each application. The following discussions will
analyze each criteria that should be considered.

Usually, there is a minimum contact force needed to satisfy an electrical signal. The contact
force is dictated by several factors: the voltage, the quality of the contact material and the type
of plating. These factors are again interrelated; the lower power required, the lower the
contact force required.The more conductive the material, the lower the power required, the
lower the contact force. The more conductive the plating, the lower the contact force. The
best way to determine the exact contactforce for each application is empirically, however
there are some typical rules of thumb to follow. There are also specifications written by
many industries that set minimum requirements for their products. Underwriters
Laboratories (UL) set standards mainly for household products powered by AC voltage of
120 and 240 volts. There are also military specifications that set standards for electronics
and computer type interconnections. Currently there are no specifications for battery type interconnections due to low DC voltage. Low DC voltage is very safe and does not present
a safety hazard to people, for this reason, a standard has not been required.

Most contacts used in electronic applications require 50 to 100 grams of contact force due to
the fact that the voltage is usually very low DC. However the force would be higher if gold
plating was not used for these applications along with a dimple type contact area. Gold
plated contacts have high reliability as well as low electrical resistance, but the cost is
usually higher. These types of contacts are usually used in military or computer type applications.

Fig-1

F= K x D

Hooke’s Law Where F= force, K= spring rate, D= distance.

The spring rate is measured in force per unit distance, such as pounds per inch, or grams
per inch or grams per millimeter. The formula shown in (Fig-1) is known as Hooke’s law.
This law pertains to any spring type material used in contact design.

The spring rate is determined by a number of variables. Modulus of Elasticity or Young’s Modulus is the inherent stiffness of a spring material. It is the ratio of the applied stress over
the resultant strain of the material. The Modulus is measured in pounds per square inch
(PSI) or megapascals (MPa). The modulus for steel is 28,000,000 to 30,000,000 PSI. The modulus for most brass and copper alloys used in stamped contact design is 15,000,000
to 20,000,000 PSI. The other variables that affect spring rate are thickness of the material
used, the type of spring used and the physical dimensions and shape of the spring as well
as the loading and constraints applied to the spring. In following sections we will examine
four different springs in great detail.

For example, to demonstrate these general relationships, let’s look at the typical cantilever
beam type spring.

Fig-2 Typical Cantilever Beam

From the strength of materials, we know that the formula for the force produced by deflecting
a cantilever beam is:

Where “D” is the deflection inches, “E” is the Modulus of Elasticity in PSI , “I” is the moment
of inertia in inches to the 4th power, and “L” is the length in inches. The spring rate in pounds
per square inch for this type of beam is then:

Fig-4

For a rectangular section or flat metal spring the moment of inertia becomes:

Fig-5

Where “W” is the width and “T” is the thickness or the height of the material used. By
substituting this into the force equation we get:

Fig-6

The possibilities derived from this formula are numerous. The two most dramatic variables
in this formula are length of the beam and the thickness of the material. A small change in
these variables will have a much greater effect than the other variables. Any variable on the
top of the equation that is increased will increase the contact force; alternatively, an increase
in the length will decrease the contact force.

A designer must find the best combination of length, thickness, width, and deflection to
satisfy contact force needed. This is done by an iterative process and sometimes trial and
error to get the right solution. Computer aided Finite Element Analysis (FEA) can be used to determine the right combination of variables. FEA can also be used to determine high
stress areas so they may be reduced to safe operable levels.

Batteries and other contacts that interconnect with the contacts being designed determine
the deflection required. A big concern is the tolerance of the mating parts and the insulator material that constrains the contact, which is usually a thermal plastic. The contact itself
also has tolerances that must be taken into consideration. The manufacturing of the contact
will also dictate the tolerances. The sum, of the tolerances must be considered when
developing mean minimum and maximum deflections to yield adequate contact force.

Let's look at the simple cantilever beam again as an example to demonstrate how to predict deflection. If we solve for deflection for the above force equation we get.

Fig-7

D = deflection
P = force
L = beam length
W = beam width
T = beam thickness
E = modulus

From this equation we can see that a greater contact force and a greater beam length will
cause a greater deflection. Also we can see that by increasing the width or the thickness
of the beam or increasing the modulus by going to a stiffer material we will reduce the
deflection. Again it is noted that the length and the thickness of the beam have a more
dramatic effect on the deflation than the other variables because of the cubed factor. The
contact deflection required is determined by device contact requirements and how the
device uses batteries.

When designing a contact, the stress is as important as the contact force. Force and stress
are interrelated to one another. As the force increases, the stress increases. This
relationship presents even more design challenges. While trying to design an adequate
contact force, the designer must keep track of stress and keep it low enough not to damage
the contact. If the stress increases enough, it may exceed the elastic limit of the material. If
this occurs, the contact may take what’s known as a permanent set. Which means the
contact will not return to its original shape or size. This set will then cause a lower force on
the batteries, which may not be satisfactory. There are other cases when exercising a contact
by changing batteries frequently may cause what is known as fatigue, which will induce a set
or even fracture. The section on fatigue will cover in greater detail how to choose allowable stress levels based on fatigue.

As seen in later discussions, each type of contact has a unique formula for deriving force.
This is also true for stress. Let us examine the typical cantilever beam and discover how its stress is related to its contact force. Again, from strength of materials we find the maximum
outer fiber stress of the beam is:

Fig-8

Where “S” is the stress in PSI, “F” in pounds, is the load, “L” in inches, is the length of the
beam, “W” in inches, is the width of the beam and “T” in inches, is the thickness. The
formula for the average stress for the beam section is:

Fig-9

It is better to use the maximum outer fiber stress in most applications due to the fact that this yields a higher stress and will provide for a safer design. However, we will use the average stress for the following discussion. If we substitute the force equation for “F” into the above stress formula we get:

Fig-10

We can see from this formula that increasing the deflection, the thickness or the modulus
will increase the stress. Also by increasing the length we can reduce the stress. When designing a contact the above stress equation and the equation for contact force must be
viewed simultaneously. The safe stress of the material must not be exceeded. Computer programs can be written to make these calculations quickly; analyzing the stress and the
contact force, varying the thickness of the material as well as the length of the beam and
the deflection until the optimum conditions exist with a safe stress value.

Stress relaxation occurs in loaded springs due to the effect of time. There are several factors
that affect stress relaxation: the material of the spring and how it is manufactured, the environment, the initial loading, temperature, and the time that the contact is under load.
Stress relaxation can occur over several thousand hours. The behavior is linear in nature.
Stress will drop off at a constant rate and continue to drop off linearly with time. It is good to
keep the initial stress as low as possible, this will limit the affect of relaxation. Temperature,
due to contacts heating as current passes through them, also contributes to relaxation. Most manufactures of spring materials will have extensive data that can be used to predict the
effect of relaxation. The only true verification of this affect of relaxation is to perform extensive testing. Testing of a prototype that closely resembles the actual production contacts is highly recommended. It is a good practice in battery contact applications to instruct the user to
remove the batteries from the device while not in use. This reduces the stress relaxation in
the contacts as well as the risk of galvanic corrosion.

Fig-13

Plating for most battery connections are usually nickel to nickel. It is not advised to mate dissimilar platings due to galvanic corrosion. Since most alkaline batteries have nickel cans, nickel plating for the contacts would be preferable. Nickel, being harder than gold, will tend
to wipe and aid in breaking down the oxide that can form on the battery contact surfaces. It is preferable to put a dimple or similar detail that will produce a surface stress high enough to
aid in the wiping, but not high enough to scratch the nickel coating of the contacts or
batteries. Nickel connections usually require between 500 to 1000 grams of force to carry
the current required, however, most design conditions differ slightly. Testing should be conducted to determine the best design.

The conductivity of a material can affect the contact force and the reliability of the signal passing through the connection. This is most true in high quality electronic equipment where the reliability of the signal is paramount. In most battery applications, the conductivity level can be satisfied with a wide variety of materials. Plating can be used to enhance the base material to create a reliable interconnection. High conductivity materials such as Beryllium Copper are expensive and may not be needed in most battery applications from a conductivity standpoint. However, high conductive base materials can be used to eliminate the need to plate, which can also reduce cost.

Plating for most battery connections are usually nickel to nickel. It is not advised to mate dissimilar platings due to galvanic corrosion. Since most alkaline batteries have nickel cans, nickel plating for the contacts would be preferable. Nickel, being harder than gold, will tend to wipe and aid in breaking down the oxide that can form on the battery contact surfaces. It is preferable to put a dimple or similar detail that will produce a surface stress high enough to aid in the wiping, but not high enough to scratch the nickel coating of the contacts or batteries. Nickel connections usually require between 500 to 1000 grams of force to carry the current required, however, most design conditions differ slightly. Testing should be conducted to determine the best design.

There are several different designs of contacts that can be used in battery connections. The most common is a simple compression spring design. This design offers low cost and
large deflection capability, which helps reduce the need for tight tolerances on mating parts. They are great for designs where batteries may be stacked in series to produce higher
voltages such as flashlights and inexpensive toys. Compression springs can be wound on
a variety of winding equipment at relatively low cost. They are usually made from music wire
or stainless steel and must be plated to yield the conductivity required. They do however
require more room than other springs.

Cantilever type contacts are more expensive to make and plate than compression springs, however they offer higher reliability and tighter tolerances. They are used in higher quality products such as cameras, radios, CD players and other electronics. They are usually
made by stamping dies or four slides and can be made from higher conductivity materials
than compression springs. The conductivity of these materials can be high enough that
plating may not be required. Cantilever springs can be many shapes and can be pre-
loaded in nature. This pre-loading allows the batteries to be inserted more easily. They do
not lend themselves to stacking batteries in series due to variable battery sizes.

More about these contacts and their respective force deflection and stress formulas will be discussed in the following sections.

This section will discuss several different versions of contact designs that can be used specifically for battery applications. The pros and cons of each type will be discussed. The formulas for deflection for each type will be given. Example problems for each type will be worked out to solve for deflection and stress.

These springs are very common. Many books exist to educate a designer and aid a
designer in the design process. A very popular book is Spring Designers Handbook by
Harold Carlson Published by Marcel Decker. Also the Spring Manufacturers Institute
publishes the Handbook Of Spring Design. SMI also has a web site SMIHQ.org. This site
offers additional help including spring design software that simplifies much of the work
needed to calculate deflection and stress. Many of the tables and graphs in this section
come from the principle spring characteristics; spring rate, deflection, the force at a desired deflection, allowable torsion stress of the material, wire diameter, mean coil diameter,
modulus of rigidity of the material, number of active coils, spring length, pitch or number of
coils per inch and the type of ends used on the spring. There are four types of ends used;
plain not ground, plain with ends ground, squared not ground and squared ground. There
are also other variables such as round wire or square wire. So, as you can see, there are
many issues to be concerned with. The following table (Figure 14) will show the
relationships and formulas between most of these variables.

Fig-14

The type of spring end used dictates the number of the active coils in a spring. The following table shows the relationship between each spring type and the number of active coils.

Fig-15

Spring end types also effect other characteristics. The following table shows how spring
“end types” can effect pitch, solid height, total coils and free length.

Fig-16

Most design problems for coil spring contacts usually have some initial given conditions,
which are dictated by the application itself. The force needed to make the contact is
determined by the circuit requirements. The desired deflection needed to satisfy that force
is usually determined by the tolerances of the spring itself, its means of retention and the tolerances of the battery or batteries it may be holding. These tolerances will also help to determine the needed deflection of the spring. The solid height of the spring must also be
taken into consideration. There may also be size limitations dictated by the application that
will determine the free length of the spring and the diameter of the coil.

Once these conditions are set the designer must pick a material and type of wire and wire
size; this is usually done through experience. The type of wire will dictate the torsional
modulus and the allowable stress for that material. Wire manufacturers readily provide
these values. The initial wire size and the number of coils must be chosen as a good first
guess by the designer, which again is done through experience. Now we will use the
formula for P (force) taken from the table shown in (Fig-16.)

Fig-17

Where “G” is the torsion modulus, “d” is the wire diameter, “F” is the deflection, “N” is the
number of coils, and “D” is the diameter of the coil. If the force yielded from this equation is
too small, the diameter of the wire or the deflection can be increased or decreased. Simultaneously the designer must also calculate the stress for this initial condition as done
for the force, using the formula in the above table to calculate the stress.

Fig-18

If the stress (S) is too high, the diameter of the coil and the number of coils can be
increased as well as the deflection or the diameter of the wire size could be decreased. An iterative process must be completed to arrive at the force needed and the allowable stress
not exceeded. This is where computer aided software that calculates these formulas
quickly and can be used to reduce the time needed to arrive at a satisfactory design.

Conical compression springs as shown below are tapered from top to bottom in the shape
of a cone and are commonly used in applications where space is a concern. They can be designed in such a way that when compressed each coil can fit inside the next so the solid height of a conical spring is less than a traditional spring.

Fig-19

The easiest way to determine the force needed to deflect a conical spring is to use the
average of the large and small diameters and use the same procedure as spelled out in the section on traditional springs. This method, in most cases, is very accurate. Another way is
to treat each coil separately, calculating the force stress and deflection of each coil. This
process is very time consuming and usually not much more accurate than the preceding method. This type of spring is much more simply analyzed by using computer aided
software or FEA type analysis.

Charging of primary batteries should be avoided. Duracell has evaluated several devices
that offer both primary battery and AC power options. In these devices, mechanical switches
are preferred over electrical versions (i.e., diodes) due to the absence of leakage current.

The shape chosen to use in a design is usually dictated by the circumstances of each
design. To some extent the lengths and radii used can also be dictated by the space confinements of the design. The designer can vary the thickness of the stock material, the
type of material used, the width of the beam, the length of each section and the size of each radius. Again the process of the design is iterative in nature. Once the initial shape of the
contact is determined the contact force should be plugged into the deflection formula. Just
like compression springs the stress of the contact should then be calculated. If the stress
is too high, the shape of the contact, the stock material thickness or width of the beam
should be adjusted to reduce the stress.

This section will show diagrams of three different types of contacts that may be used. It will
also show the corresponding free body diagram, which is the equivalent representation commonly used in a "strength of material" analysis. Also the deflection formula for each
shape will be given.

Cantilever beam with curved and straight sections:

Fig-20

The above is an isometric view of a typical stamped contact that may be manufactured for a battery application. It has a curved spring arm that will apply a load to the battery when
deflected. It also has four tabs that will locate the contact in a space in the insulator of the
battery compartment.

Figure 21 shows two other views of this contact

Fig-21

This contact is a cantilever beam with a radius and a straight member. Even though the
straight member is at an angle when the battery deflects this arm, the arm will be very close
to vertical. Therefore a free body diagram as shown below can represent the above contact.

Fig-22

The formula for deflection for a cantilever beam loaded this way can be found through
several strength of materials integration methods.

Fig-23

Example: Let’s design the above contact using an Olin C151 copper alloy spring hard which
has a modulus “E” of 17.5 million PSI and a yield strength of 60 thousand PSI, using a
material thickness of .007 inches and a beam width of .120 inches. The load required by
the battery is 500 grams, which is equal to 1.1 pounds. When substituting these values into
the above equation we get ».200 inches of deflection. We then must check the stress for
these applications using the formula shown in Figure 24 for beam stress.

Fig-24

S = M Y / I

Where "S" is the stress in PSI, “M” is the maximum moment, “Y” is half the thickness of the beam, and “I” is the moment of inertial for the beam. Assuming the maximum moment for
this beam is the load 1.1 pounds times the length of the distance of the load to the fulcrum
point which is .150 inches. Solving for the stress we get 27,000 PSI which is less than the
yield strength of this material.

Fig-25

The above is an isometric view of a contact that has two radius members and two straight members. This particular contact will offer more deflection than the previous contact for the same force without increasing the stress. It can be used in applications where more
deflection is needed. Below are front and side views of the same contact.

Fig-26

The diagram (Fig-27) is the free body diagram of the above contact. We assume that the deflection of the contact is such that the members again are close to vertical.

The deflection for the above cantilever beam can again be found through strength of
material methods.

Fig-28

As before, once the contact shape is designed a material can be picked along with a
thickness for the width of the beam. The deflection must be calculated and the stress must
be checked. The moment arm for this beam is the load multiplied by the length plus the
radius.

Fig-29

M = P(L + R) and S = MY

If the stress is too high, the designer must change the material or the thickness or the
material or the beam width to reduce the stress.

These types of beams are commonly used in electronic connector applications where
there is great need for low mating force between male and female connectors without losing contact force. This is accomplished by pre-compressing the spring in the assembly such
that the beam has an initial deflection. The designer will create the beam longer than is
required and the insulator is used to deflect the beam during assembly. This will cause a
pre-load up against the insulator. With the prior cantilever beams the contact force begins at zero, as the deflection starts at zero, the force increases linearly with the deflection
according to Hooke’s Law. As the pre-loaded contact is deflected a small amount, the force
will increase very quickly until the beam is lifted off the insulator and then the force deflection curve resembles a traditional cantilever beam. This results in a very low mating force with
very low deflection but high contact force. So, if this design was incorporated into a battery design, the battery could be inserted into place with little effort. The only draw back to this
design is it limits the amount of deflection. Therefore, this design can only be used in applications where tight tolerances can be held. The following drawing illustrates an
example of a pre-loaded contact. This contact is a modified version of the contacts shown
in Fig-10 and Fig-11.

Fig-30

The drawing below shows the pre-loaded contact with the pre-load applied. Notice the
contact is deflected and the angles are closer to vertical. A wall designed into the insulator
at the pre-load point will cause this initial deflection without the battery in place.

Fig-31

When the battery is put in place, it will deflect the contact at the contact point and deflect the contact further so the bottom of the pre-load arm is lifted off the insulator and the total load is now on the battery. This is what causes the lower insertion force and the same contact force. The deflection equation and the stress equation for the pre-loaded contact is the same as
the one pre-loaded contact because when the battery is in place the contact point is the
same so the moment is the same and the load is the same. Once the contact is in place
the pre-loaded leg could be cut away because its job is done and no longer has any load.

The following will be a general discussion of materials used in both compression springs
as well as stamped cantilever type springs used in battery contact applications. The five
basic materials discussed will be Music Wire, Stainless Steel, Spring Brass, Phosphor
bronze, and Beryllium-copper. Each of these materials have many different alloys. They all
differ in composition and manufacturing process to a slight degree to yield different
performance results. Scientists and engineers have devoted considerable time
developing and studying these materials. It would be impossible to discuss them all in
detail.

The following pages will provide a brief overview. All five materials can be used to make
wound compression springs; however, music wire and other high carbon steels as well as stainless steels are seldom used to make stamped contacts because the cost of the
tooling would be too great.

Music wire, commonly called piano wire, is a high carbon cold drawn steel. It is looked
upon as the best quality steel used for springs. It has high tensile strength, high elastic
limit, and can withstand high stresses under repeated loads. As you might guess from the name, this type of wire was developed for musical string instruments. A thin coat of tin is
usually applied to help with corrosion. This tin coat also works as a nice base to further electroplate if desired.

Music wire is best used for smaller springs. It comes in wire sizes from .004 inches to .125 inches in increments of .001 of an inch. Wire is drawn in many sizes, which allows a
designer more options in spring design. Temperatures over 1210C (2500 F) can cause the spring to relax, depending upon the loading. For example: if a spring is loaded to 90,000
PSI 1210C (2500 F) can cause a 5 % drop in spring force. Manufactures can provide data
that can aid designers in this area.

The elastic limit of music wire is usually 65 to 75% in tension and 45 to 65% in torsion. The electrical conductivity is between 8 to 12% of copper. The conductivity can be enhanced considerably through plating. The modulus of elasticity of wire is affected by the diameter it
is drawn to. It can vary from 30 million PSI for small wires to 28 million PSI in larger wires. In torsion it can vary from 12 million to 11.5 million PSI. Heat treatment can be used to reduce stresses built up in winding.

Stainless steel is very similar to other steels except it contains chromium, from 12 to 20%.
It may also contain nickel up to 10 %. Chromium and nickel effect the steel in such a way
that it makes it corrosion resistant. Passivating is also a process used to enhance the anti-corrosiveness of stainless steel. Stainless steel is commonly used for springs that will be exposed to environments that are corrosive in nature.

The American Iron Steel Institute has set standards for different grades of stainless steel
and designated type numbers so they can be easily specified. There is a 300 and 400
series. The 300 series has chromium and nickel at percentages of 18 and 8 respectively.
Heat-treating can not harden these types. They are hardened from cold working. Steels can
be ordered ¼, ½, ¾, and full hard. The modulus of elasticity of stainless steel can vary from
28 million to 30 million PSI. Tensile strength can vary from 100,000 to 220,000 PSI. Most manufacturers can provide data on each grade they manufacture.

Brass is an alloy of copper and zinc. However saying the word brass is a great generality because many things can be added to brass to achieve different types of mechanical and electronic properties. There are many different types of brass and manufacturers go to great effort not to reveal the exact compositions. Tin, Lead, Aluminum, Silicon, Iron, Phosphorous, Nickel, Beryllium, Manganese, Chromium, Antimony, Tellurium, and Selenium can be
added in different amounts to get different properties. Brass is hardened through working
the material. It can be bought in the form of wire rods, sheets and coils. Brass can also be bought in ¼, ½, ¾, and full hardness. Most brass can be welded, soldered, and plated.
Stamping is a common method used to make parts out of brass. It is easily formed, and
bent and cut. Copper base alloys have excellent resistance to corrosion.

Spring Brass is usually defined as 70 % copper and 30 % zinc. It is the cheapest of the
brasses and used in applications where cost is a factor. It can be easily formed and drawn.
It can only be hardened by work hardening. It is good in applications of low stress and low fatigue. It should not be used in applications over 790C (1750 F.) The modulus in tension is around 15 million PSI and in torsion is 5 million PSI. Tensile strength can vary between 70 thousand and 95 thousand PSI depending upon hardness.

Phosphor bronze is a copper alloy with little or no zinc but possesses 4 to 10 % tin and
some phosphorus. It is corrosive resistant. It has better mechanical and electrical
properties than spring brass. It has a higher cost but also has a higher modulus at 16 to 17 million PSI for tension. It may be stressed 30 to 50 % higher and offers higher fatigue life.
It is best not to exceed 1070C (2250 F.) This material is the most extensively used stamped spring material due to its cost and performance.

This material is made out of 98 % copper and 2 % beryllium. Unlike other copper alloys it
can be heat treated for hardness. Its forming properties are excellent. There is no difference
in its mechanical properties based on cold rolling directions unlike the other copper alloys.
Its electrical conductivity is twice that of Phosphor bronze. It can with stand high stress and
has a long fatigue life. Its modulus in tension is between 17 million and 20 million PSI. It
also can be used in temperatures up to 1490C (3000F.) This material is used to make high quality stamped electronic contacts. It also has the highest cost out of any of the copper
alloys.

Contact performance is based on many factors and conditions. Contact force, the area of
contact and the quality of contact is a large factor. The type of plating materials used for both
of the mating surfaces also effects the contact performance. The base material and the conductivity of the base material have a very large effect on contact performance. The effects
of corrosion due to atmospheric conditions and other corrosive conditions can severely deteriorate contact performance. Oxide build up on contact surfaces can also deteriorate
contact performance. The shape of the contact mating surfaces and how they interconnect
with each other is another factor.

Scientists have spent considerable time studying and documenting this subject. A great reference guide on this subject is Electrical Contacts Principle and Applications edited by
Paul Slade and published by Marcel Decker. This text explores in great detail the above
subjects.

As stated in previous sections, each electrical signal has a minimum contact force
associated with it. Pressure is defined as the force per square inch. Each interconnection
has an area of contact that the electrical signal passes through. This area is dictated by the shape of the interconnections and the roughness of the plated or non-plated surfaces.
Even though a contact surface may visually appear smooth, on a microscopic level the
surface may be very rough. This can cause point contacts in the contact area. If the contact
force is not high enough this point contact could produce high contact resistance and
arching. Arching can then further deteriorate the contact surface, which can cause pitting,
and further corrosion. Arching also produces oxides, which can also increase contact resistance. Raising the contact force can cause more of these high points to make better contact. Adequate contact force can flatten out these contact points and lower the contact resistance.

Circular dimple type contact areas are usually good practice to enhance contact pressure.
With stamped contacts a circular dimple can easily be added during the stamping process.
The size and curvature will vary with each application. The effectiveness of the dimple used
must be tested and modified to yield the desired results for each unique application. If the dimple is too small and too sharp the dimple could score the matting surface. If the dimple
is to big its effect will be minimal.

Fig-32

These drawings show a typical stamped contact with a dimple type feature to optimize
contact pressure.

Fig-33

Designing a wiping action into the mating condition can also enhance contact pressure.
The wiping action tends to flatten out these high spots in the mating surfaces. The contact
force must be kept to safe levels such that the wiping action does not erode the plating if
plating is used. Curved cantilever beams lend themselves perfectly for wiping actions. As
the contact is deflected the contact point usually shifts slightly which causes the wiping
action on the mating surface. Wiping actions can also wipe away oxides that can form on
contact surfaces. These oxides are caused by reactions between the surface material and gasses in the atmosphere. The presence of oxides can act as an insulator that raises the contact resistance. Over a period of time oxides can completely block the electrical signal. Helical springs do not lend themselves to creating dimple features like stamped contacts. Creating a wiping action with a helical spring is also not easy.

Conductivity is the ease at which an electrical signal can flow. Copper has the highest
known conductivity. Electrical conductivity is related closely to thermal conductivity. So a
material that is electrically conductive is usually thermally conductive. Electrical conductivity
is expressed as a percentage of the International Annealed Copper Standard (%IACS) measured at 200C for material in annealed temper. Pure copper has an electrical
conductivity of 101 IACS. As elements are added to copper the conductivity decreases, but
the alloys created have superior mechanical properties. The conductivity of a material is
usually inversely related to the mechanical properties of the material. So, the designer must trade off between conductivity and mechanical performance. The conductivity of copper
alloys can vary between 5 and 101 IACS depending upon what alloy is used.

Fig-34

Fig-35

The conductivity of most steels are between 10 and 15 IACS. The conductivity of Gold is 77
and Silver is 100 IACS. Gold and Silver have excellent conductivity. That is why they are used
for plating. However these elements are very soft so they are usually alloyed with other
materials which drops their conductivity but raises their resistance to wear.

Plating of base spring and contact materials is used to enhance conductivity of base
materials and also coat the base material to protect from corrosion. There are two main
ways to add precious metal to a base material, plating and cladding. Inlaying or cladding a plating material into a base material can be done by skiving a groove into the substrate
material into which a strip of plating material is placed. Rolling the material to the desired thickness also bonds the material together. Inlaying can also be done for wire by pressing
the substrate material in the form of rods into tubes of the plating material. The composite
is drawn through dies to the desired wire diameter. Inlayed materials are very expensive however, they remove the need to plate after the spring or contact is manufactured. It is
similar to paying for the plating up front in the stock material. Common cladding materials
are pure Gold, 75Au25Ag, 80pd20Ni, 60Pd40Ag, 69Au25Ag6Pt and Palladium. Plating after
the contact or spring has been formed can be done by electrolytic and non-electric methods.
The following materials can be used for plating; Tin and Tin Lead alloys, hard Golds, Gold
flash, Silver, Silver Gold alloys, Nickel and Nickel alloys. Most of these platings are added
in very thin films between 1um to 5 um.

Hard Gold is the best finish for most contact applications. Noble metals are less prone to
film build-up. Gold is subject to pore corrosion. Because of the high conductivity, it can be
used to drop the contact force needed to transmit the signal desired.

Palladium and Palladium-Nickel alloys are used in place of Gold where cost is a concern. Palladium is about 1/3 to half the cost of Gold. It however can develop films in polluted environments.

Tin and Tin Lead Alloys are relatively inexpensive but due to softness are not durable. Tin
can also grow what is known as Tin whiskers and fretting corrosion.

Nickel is also an inexpensive material but develops thick oxide coatings which must be
broken by high contact force and whipping action of the contact. Since Nickel is used to plate most batteries Nickel is an obvious choice for the mating contact. Using the same material removes the possibility of galvanic corrosion and wear challenges. Using the same plating
on mating surfaces creates similar wear.

Plating hardness also plays a roll in picking adequate plating. High conductivity is desired
but high conductivity is usually related to softness, which in turn means low wear resistance. Again we see there is a trade off between conductivity and hardness. The following table
(Figure 36) shows relative hardness values for electro-deposited plating alloys.

A designer is challenged with choosing a base material that can yield the necessary
mechanical properties required for contact force. At the same time he must be aware of the conductivity of this base material. He can enhance conductivity by plating the base material.
He can then introduce dimple features to improve contact pressure that lowers the contact resistance. Plating can enhance the interconnection however it can introduce added cost, oxidation and corrosion problems. Each interconnection problem is different and must be treated so.

Cost is a combination of many elements; base materials, tooling used to manufacture the
base material into the spring, manufacturing time needed or the machine running time, secondary operations, plating and volume all need to be addressed before deciding on a
spring design.

Base material in the form of drawn wire for wound springs or rolled sheets for stamped
contacts are affected daily by the cost of fair market value of the base materials, especially
when you are dealing with the more precious type metals. Markets control these metals just
as the prices of stocks are controlled by the stock market. Supply and demand is the basic
rule. The more exotic a material is the higher the cost. Alloys including many elements
usually require more sophisticated processing with higher quality control. These alloys
usually have higher mechanical properties, so there is a direct relationship between performance and price. This performance could be strength, corrosion resistance,
resistance to strength relaxation and so on. The more a material is worked the higher it
costs; for example, full hard material is more expensive than half- hard. The thickness of the stock material and the tolerance required by the designer can affect the cost. A tighter
tolerance can drive up cost. A tolerance of (+) or (-) .0005 will have a greater cost than (+)
or (-) .001. Cladding a material or adding an inlay into a base material will drive up material
cost considerably. This cost can be out weighed by the absence of plating. Paying up front
for inlaying can sometimes be a cost savings however, it can also drive up your
manufacturing cost. Manufacturing with inlaying and cladding can be tricky and require
greater inspection and quality control. Scrapping a lot of contacts made with inlayed
material is more expensive than scrapping a lot of non-inlayed material. It is safer to stamp
or wind than inspect and then plate, since a dimensional mistake can be detected before
the plating.

Tooling for wound springs include a spring winding machine and the arbor needed to wind
the spring. The volume needed on a daily basis will dictate how many machines are needed
or how many shifts to run. If it takes 10 seconds to wind a spring then 6 springs can be
wound per minute and 360 springs per hour. Calculations like these must be made to determine the number of machines or number of shifts. The number of springs made per
hour dictates the piece part price because there is a machine time cost per hour. This cost
is based on the people needed to set up the machine and check the finished product, as
well as maintain the machine. For most needs contracting spring winding to spring manufacturing companies is the best option. This eliminates the need to purchase
machines and train people to set them up, run them and maintain them. It is also easier to design a spring in such a way that it may be bought off the shelf. There are many
companies that make and stock certain sizes that are commonly used for known
applications, which would lower the cost dramatically.

Tooling for stamped contacts is more complicated. This requires a stamping press as
well as the design manufacture of a stamping tool. Further costs include the maintenance
of that tool and the set up cost to install the tool in the press when parts are needed. Tools
can be designed with multiple capacity. They could be designed with one contact per strip
or as many as four contacts per strip. A one capacity tool is less expensive than a two or four capacity tool
because each stamping station must be duplicated so many times. Each tool has blanking
stations and stamping stations. The strip is fed into the tool and is carried by carriers
through the die. Each time the die opens and closes the strip moves to a different station.
The blanking stations remove material not needed in the part. The forming stations place
the bends and forms needed. There is a tool made for each blank and form. Each blank and forming tool must be made and heat-treated. The harder the base material the more
expensive the tool will be because it will require a more sophisticated tool steel and heat
treating to stand up to the work required. The larger the number of forms and blanks and the more complicated each form and blank is the more expensive the tool is. The tolerance associated with each blank and form also drives up the cost. The speed of the die and the number of stations dictatethe time it takes to make each part. The more parts that can be
made per hour the lower the cost. Once the tool is paid, your costs will be the machine time,
the material needed and the maintenance of the tool. This is usually included in the piece
part price. Just as with spring winding, most of the time it is better to farm out this work to a stamping house because they have the expertise and the presses. Once the design is
complete a stamping house can quote the price of the tool and a piece part price for the part.

Plating can be done a few ways. Barrel or batch plating can be done with contacts that will
not tangle or can be easily untangled. Barrel plating is also used where the whole contact is plated. This type of plating is excellent for wound springs. A de-tangling feature can be
designed into the spring by having some dead coils in the middle. Dipped plating is done
when the contacts are still on the stamping strip and they are dipped into the plating and
only a selected area is plated. This is commonly done in the electronic connector industry.
The contacts are left on the strip. They are made on the strip in the same spacing or pitch
that they are assembled into the insulator. After plating they are pressed into the insulator
and the strip is broken away.

The cost of plating is dictated by the cost of the precious metal used, the time it takes to
plate and the cost of that time. Cost of plating materials such as contact material can vary
day to day. The more sophisticated the plating alloy is the higher the cost. The plating
demands sometimes require a base coat and a flash coat. This is obviously an added cost. Barrel plating is usually less expensive because it requires less prep time and many parts
can be done simultaneously, however it has added cost if the contacts need to be
de-tangled. Dip plating uses less plating but requires more handling costs and set up time.
This process is very time consuming because there is a limit to how many contacts can be dipped at once, thereby driving up the cost even further.

Again this process is best left to companies that are experts in this area. There are many
environmental requirements to deal with if one wishes to do their own plating. Once the
plating requirements are known and the spring or contact design is complete a plating
company can supply a piece part quote.

Secondary operations aside from plating can be costly because they usually require
handling and fixtures. This should be avoided but sometimes it cannot. Any time a human
being must handle a part it gets more expensive. Secondary operations can be cutting
contact strips into lengths, separating the strip from the contact, or bending contact tails after assembly. For springs, secondary operations could be grinding the ends of springs and
welding or soldering elements to the spring.

Volumes are the biggest variable in cost. The higher the volume the lower the cost of a part.
When prototyping a stamped contact a sample charge could be astronomical compared to
the high volume price of the contact. This is so because it could take hours of a toolmakers
time to produce all the blanks and bends necessary to make the contact. Sample charges
of wound springs are usually much less. This usually requires a set up charge comparable
to a few hours of running time.

The tooling and the set up charge is the largest up front cost. The longer a tool is run the
smaller the part price becomes because it eliminates the human factor to set the tool up. If
a tool can be run without human supervision or with minimal supervision this can also
reduce part cost. A spring machine can only produce one spring at a time. To up the capacity, more spring machines must be purchased. A stamping die can be made with one to four
part capacities. This cuts the cost dramatically because it is doing the same work and
spending the same time to produce more parts. This, however, may require a bigger
machine with higher force capacity. It can also require more quality control costs and higher maintenance. Volume needs dictate the part capacity of the tool. It would be more cost
effective to build a one-part capacity tool for small volumes; higher volumes would demand higher capacity tooling.

A spring or stamping supplier should not only supply a part price but there should be price
breaks based on different levels of volume. The higher the volumes the less the price. The
designer should also get several quotes from different suppliers. His decision should not
only be based on price but also on the quality of work and reliability of the supplier to meet
deadlines and supply parts that meet specification. The price will increase if the parts have
to be scrapped, returned or reworked. Sometimes paying a little more for a part from a more
reliable supplier is actually more cost effective.

Definitions List

AC: or sometimes as A.C. – the abbreviation for alternating current.

Alloy: Metal prepared by adding other metals or nonmetals to a basic metal to secure
desirable properties. Typically, lower melting points, decreased electrical conductivity and increased hardness are characteristics.

Annealing: The process of maintaining a material at a known elevated temperature to
reduce discoloration, vacancies, and other metastable conditions, e.g. steel or glass. In
ferrous alloys the metal is held at a temperature above the upper critical temperature for a variable time and then cooled at a predetermined rate, depending on the alloy and the
particular properties of hardness, machinability, etc. which are needed. The term is usually qualified, e.g. quenching, annealing, isothermal annealing, graphitizing.

Brinell Hardness Test: A method of measuring the hardness of a material by measuring the area of indentation produced by a hard steel ball under standard conditions of loading. Expressed as Brinell Hardness Number which is the quotient of the load on the ball in the
kgf (kilograms force) divided by the area of indentation in mm².

Cantilever Beam: A beam or girder fixed at one end and free at the other end.

Clad Metal: One with two or three layers bonded together to form a composite with e.g. a
corrosion resistant layer formed over a stronger core by co-rolling, heavy plating, chemical
deposition etc.

Cold-Drawing: The process of producing bar or wire by drawing through a steel die without
heating the material.

Cold-Working: The operation of shaping metals at or near atmospheric temperature so as to
produce strain-hardening.

Compression Spring: A helical spring with separated coils, or a conical coil spring, with plain,
squared or ground ends, made of round, oblong, or square-section wire.

Conductivity: The ration of the current density in a conductor to the electric field causing the
current to flow. It is the reciprocal of resistivity.

Contact Resistance: Resistance at the surface of contact between two conductors; mainly
determined by constriction of current in the materials near the point of contact. Contact area is
extended by pressure increasing this resistance.

Corrosion: The slow wearing away of solids, especially metals, by chemical attack.

DC: or sometimes as D.C. – the abbreviation for direct current.

Deflection: The amount of bending or twisting of a structure or machine part under a load.

Drawing: The operation of producing wire, or giving rods a good finish and accurate
dimensions, by pulling through one or a series of tapered dies.

Elastic Limit: The highest stress that can be applied to a metal with producing a
measurable amount of plastic (i.e. permanent) deformation. Usually assumed to coincide with the limit of proportionality.

Fatigue: In reference to metals: The phenomenon of the failure of metals under the repeated
application of a cycle of stress. Factors involved include amplitude, average severity, rate of
cyclic stress and temperature effect. The fatigue limit is that in which the upper limit of the
range of stress that a metal can withstand indefinitely. If this limit is exceeded, failure will
eventually occur.
Force: That which, when acting on a body which is free to move, produces an acceleration in
the motion of the body, measured by rate of change of momentum of body. The unit of force
is that which produces unit acceleration in unit mass; for example: pounds/in ².

Fretting Corrosion: Corrosion due to slight movement of unprotected metal surfaces, left in
contact either in a corroding atmosphere or under heavy stress. Galvanic Corrosion:
Corrosion resulting from the current flow between two dissimilar metals in contact with an electrolyte.

Hard Drawn: The term applied to wire or tube which has been greatly reduced in cross-
section without annealing.

Hardening: The process of making steel hard by cooling from above the critical range at a
rate that prevents the formation of ferrite or pearlite and results in the formation of martensite. May
involve cooling in water, oil, or air, according to composition and size of the article.

Hardness: Signifies, in general, resistance to cutting, indentation and/or abrasion. It is
actually measured by determining the resistance to indentation, as in Brinell, Rockwell,
Vickers and scleroscope hardness tests. The value of hardness obtained by the different methods are to
some extent related to each other and to the ultimate tensile stress of non-brittle metals.

Heat Treatment: Generally, any heating operation performed on a solid metal, e.g. heating
for hot-working or annealing after cold-working. Particularly, the thermal treatment of steel by
normalizing, hardening, tempering, etc. Also used in connection with precipitation-hardening
alloys, such as those of aluminum.

Hooke’s Law: For an elastic material the strain is proportional to the applied stress. The
value of the stress at which a material ceases to obey Hooke’s law is the limit of
proportionality.

Mechanical Working: Rolling, spinning, pressing, hammering, etc. a metal to change its
shape. Stamping can be considered to be a method for mechanical working.

Modulus of Elasticity: For a substance, the ratio of stress to strain within the elastic range,
i.e. where Hooke’s law is obeyed. Measured in units of stress.

Moment: Of a force or vector about a point, the product of the force or vector and the
perpendicular distance of the point from its line in action.

Moment of Inertia: Of a body about an axis; the sum Smr² taken over all particles of the body
where m is the mass of a particle and r its perpendicular distance from the specified axis.

Noble Metals: Metals, such as gold, silver, platinum, etc., which do not enter readily into
chemical combination with non-metals. They have a high resistance to corrosive attack by
acids and corrosive agents, and resist atmospheric oxidation. They can also be referred to
as base metals.

Passivate: To mask the normal electro-potential of a metal. A treatment to give greater
resistance to corrosion in which the protection is afforded by surface coatings of films of
oxides, phosphates, etc.

Plating: Implies electroplating, in which an additional metal is applied to a substrate
material. It is most often used to render the workpiece more corrosion resistant. On
occasion, a soft base metal is plated with a harder metal to impart wear resistance;
likewise, the resultant composite material may be designed to exhibit different and/or
superior mechanical or other physical properties.

Relaxation: Exponential return of a system to equilibrium after sudden disturbance. Time
constant of exponential factor is relaxation time.

Resistivity: An intrinsic property of a conductor which gives the resistance in terms of its
dimensions.

Rockwell Hardness Test: A method of determining the hardness of metals by indenting them with
a hard steel ball or a diamond cone under a specified load and measuring the depth of
penetration.


Soldering: Hot joining of metals by adhesion using, as a thin film between the parts to be
joined, a metallic bonding alloy having a relatively low melting point.

Spring Rate: Ratio of load to deflection of a spring. Also called the spring constant.

Strain: When a material is distorted by forces acting on it, it is said to be in a state of strain or
strained. Strain is the ration of: deflection/ dimension of material and thus has no units. The main
types of strain are direct (tensile or comprehensive) strain: elongation or contraction/ original
length. Shear strain is: deflection in direction of shear force/ distance between the forces.

Strain-Aging: An increase in metal strength and hardness that proceeds with time, after
cold-working. It takes place slowly at room temperature and is accelerated by heating. It is
most pronounced in iron and steel, but also occurs in other metals.

Strain-Hardening: An increase in resistance to deformation (i.e. in hardness) produced by
deformation.

Stress: The force per unit area action on a material and lending to change its dimension;
i.e. cause a strain. The stress in the material is the ration of force applied to the area of
material resisting the force; i.e. force/area. The two main types of stress are direct or normal (tensile or compressive) stress and shear stress.

Tempering: The reheating of hardened steel at any temperature below the critical range,
in order to decrease the hardness. It is also called drawing. Sometimes it is applied to
reheating after rapid cooling, even when this results in increased hardness; e.g. in the case
of steels that exhibit secondary hardening.


Tensile Stress: The highest load applied to a metal in the course in a tensile test, divided
by the original cross-sectional area. In brittle or very tough metals, it coincides with the
point of fracture, but usually extension continues under a decreasing stress, after the
ultimate stress has been passed. It can also be called ultimate tensile stress.


Tensile Test: Two main forms of the test are: 1) those in which a static increasing pull
is applied until fracture results. From this a stress-strain curve may be plotted and the proof stress, yield point, ultimate tensile stress and elongation can be determined. And 2) those
in which a dynamic load is applied giving data on fatigue and impact.

Tolerance: The range between the permissible maximum and minimum limits of size of a
workpiece or distance between features.

Torsion: State of strain set up in a part by twisting. The external twisting effort is opposed
by the shear stresses induced in the material.

Wire-Drawing: The process of reducing the diameter of rod or wire by pulling it through
successively smaller holes.

Vickers Hardness Test: A method of determining the hardness of metals by indenting
them with a diamond pyramid under a specified load and measuring the size of the
impression produced.

Welding: Joining pieces of suitable metals or plastics, usually by raising the temperature
at the joint so that the pieces may be united by fusing or by forging or under pressure. The welding temperature may be attained by external heating, by passing an electric current
through the joint, or by friction. It can also be accomplished by joining pieces of suitable
metals by striking an electric arc between an electrode or filler metal rod and the pieces.


Work-Hardening: The increase in strength and hardness (i.e. resistance to deformation)
produced by working metals. It is most pronounced in cold-working, and in the case of
metals such as iron, copper, aluminum, and nickel. Lead, tin and zinc are not appreciably hardened by cold-working because they can recrystallize at room temperature.

Yield Strength/Point: The stress at which a substantial amount of plastic deformation takes
place under constant or reduced load. This sudden yielding is a characteristic of iron and annealed steels. I other metals, plastic deformation begins gradually and its incidence is indicated by measuring the proof stress, which, however, is frequently called the yield point.

Young’s Modulus: A modulus of elasticity applicable to the stretching of wire (or thin rod),
or to the bending of a beam. It is defined as the ratio of: tensile (or comprehensive) stress/ tensile (or compressive) strain. It is stated in the symbol E. It is also known as stretch or elongation modulus.