Connector Design/Materialsand

Connector Reliability

by:Robert S. Mroczkowski

© Copyright 1993, by AMP Incorporated.All lnternational Rights Reserved.

AMP is a trademark.

INTRODUCTIONConnector reliability has taken on new significanceas electronics, especially computers and telecom-munications equipment, become more important inboth business and everyday life. The reliability of aconnector depends on two factors. First, theapplication, which determines the requirements -both environmental and functional - which theconnector must meet. And, second, the design andmaterials of manufacture of the connector, whichdetermine the degradation mechanisms. This paperwill provide an overview of how these two factorsinteract to determine connector reliability in today’selectronics equipment.

To provide a perspective on connector reliabilitysome field history will be reviewed and a laboratoryapproach to estimating connector reliability will beproposed.

CONNECTOR OVERVIEWThe discussion begins with a simple question.

“What Is a Connector?”

This question can be answered in two ways, functionallyand structurally. First a functional definition.

Connector FunctionA connector provides a separable connectionbetween two elements of an electronic system with-out unacceptable signal distortion or power loss.

There are two important parts to this definition, the“separable connection” and the “unacceptable” perfor-mance. Both depend on the connector application and itselectrical and environmental requirements.

The separable connection is the reason for using a connectorin the first place, to provide easy repair, upgrading, mainte-nance or interconnectability. Requirements on the separableinterface include mating force limitations and meeting aspecified number of mating cycles.

“Unacceptable” performance includes a large range ofcharacteristics, but this discussion will concentrate on theresistance the connector introduces into the electronicsystem.

To provide an understanding of connector resistance,consider a “structural definition” of a connector, its designand materials of manufacture.

Connector StructureFigure 1 provides a schematic illustration of a connector.Every connector includes:

l two permanent interfaces, the connections to thesubunits which are to be connected,

l the contact springs in each half of the connector,

l the separable interface and

l the connector housing which maintains the locationof the contacts and isolates them from one anotherelectrically.

The insets in the figure illustrate the contact finish and thestructure of the separable interface on a microscopic level.A brief description of each of these connector componentsis in order.

The Separable Contact Interface

The separable contact interface is the place where thetwo halves of the connector meet. For this discussion, itis sufficient to note two characteristics of the interface.First, the interface surfaces are rough on the microscaleat which contact occurs as indicated on the lower insetin Figure 1. Second, a resistance, called constrictionresistance, is introduced simply due to the restriction incontact area which occurs at the separable interface as

Figure 1. Schematic illustration of a typical connectorindicating the major structural components. Insetsillustrate the contact interface and contact finish.

Figure 2. Schematic illustration of constrictionresistance. Constriction resistance is a geometriceffect.

illustrated in Figure 2. For a detailed discussion ofcontact interface structure see Williamson (1) andMroczkowski (2).

An additional resistance may occur due to films at the inter-face. Films may affect contact resistance by introducing aseries film resistance or by reducing the area of metalliccontact as will be discussed.

One of the major objectives of connector design is toprevent film formation or disrupt existing films on matingof the connector. In addition, effective film management isa major criterion for selecting a contact finish because filmformation is a source of connector degradation as will bediscussed.

The Contact Finish - There are two reasons to apply acontact finish:

l to protect the contact spring from corrosion and

l to simplify film management.

The two major classes of contact finishes are:

l noble metal (gold, palladium and alloys of thesemetals) and

l non noble (primarily tin or tin/lead).

Both classes provide corrosion protection for the base metalsprings. They differ, however, in the requirements for filmmanagement. Noble finishes minimize film formation,while for tin finishes the surface oxides are easily disrupted.Film management for noble metals requires preserving thenobility of the finish from external sources of degradation.For tin finishes mechanical displacement of the tin oxide isrequired and mechanical stability of the interface must bemaintained to minimize the potential for reoxidation.Additional discussion of these concepts will be provided ina later section.

The Contact SpringThe contact spring provides both electrical and mechanicalfunctions. Electrical requirements are minimal, basically thespring must be conductive. Mechanically, the spring pro-vides the contact normal force which develops theseparable interface and maintains interface stability overthe application life of the connector. The contact springmust also provide a mechanism for the permanent connec-tions of the connector to the subunits of which it is a part asmentioned previously.

The Connector HousingThe connector housing also performs electrical and mechan-ical functions. In this case however, the electrical functionis insulative. The connector housing insulates the contactsin the connector from one another. Mechanically the hous-ing locates the contact springs, to facilitate mounting andmating of the connector, and supports the springs mechani-cally. An additional function of the housing is to protectthe contacts from the operating environment and frommechanical abuse.

Connector ResistanceTo complete this brief overview of connector structureconsider the sources of resistance in a connector.

As indicated in Figure 3, the connector resistance consistsOf :

l permanent connection resistances,

l the bulk resistances of the contact springs and

l the resistance introduced by the separable interface.

In a typical connector, the magnitudes of resistance for eachof these contributions is of the order of:

l tens to hundreds of microohms for the permanentconnections,

l a few to a few tens of milliohms for the spring bulkresistance and

l a milliohm or so for the separable interfacecontribution.

Note that the connection resistances are small compared tobulk contributions of the contact spring. The big difference,however, is that the connection resistances are variable.Degradation of connector resistance occurs at the separableor permanent interfaces due to loss in contact area by sever-al mechanisms including corrosion, wear and loss in contactforce.

Consider these degradation mechanisms in more detail.

Figure 3. Schematic illustration of the componentsof connector resistance. Contributions from thepermanent connection(s), spring bulk resistance andthe contact resistance of the separable interface areindicated.

Connector Degradation MechanismsIn this section, connector degradation mechanisms will bediscussed in the context of connector materials and design.

Three degradation mechanisms will be considered:

l Corrosion

l Wear

. Loss of contact normal force.

Discussion of corrosion and wear involves contact finishproperties, while loss in normal force depends on contactspring material selection and mechanical design.

Corrosion relates primarily to the contact interface and thecontact finish. Corrosion increases contact resistance by twomechanisms:

l a series contribution due to films at the interface and

l a reduction in contact area due to penetration ofcorrosion products into the interface.

The series contribution occurs when mechanical disruptionof the films is incomplete. The contact interface in suchcases consists of parallel resistances of metallic and filmcovered regions. Loss in contact area can result fromincomplete film disruption or from corrosion productsencroaching into the contact area as the interface moves dueto mechanical or thermal driving forces.

Three general types of corrosion must be considered:

l surface corrosion,

l corrosion migration and

l pore corrosion.

Surface Corrosion - Surface corrosion refers to formationof corrosion films over the entire surface of the contactsuch as tin oxide and oxides and chlorides on palladiumand palladium alloys. Surface corrosion can cause contactresistance increases through either of the mechanismspreviously mentioned depending on how effectively thefilms are disrupted on mating and the mechanical stabilityof the contact interface.

Corrosion Migration - Corrosion migration refers to themovement of corrosion products from sites away from thecontact interface into the contact area. Such sites includecontact edges and defects in the contact finish. It is impor-tant to note that corrosion migration is very sensitive to theoperating environment. Abbott (3) has shown that corrosionmigration is of concern predominately in environments inwhich sulfur and chlorine are present.

Pore Corrosion - When the defect site from which corro-sion migration occurs is a pore, a small discontinuity in thecontact finish, the corrosion mechanism is referred to aspore corrosion. Pores themselves do not affect contact resis-tance. Only if the pores become corrosion sites iscontact resistance degraded.

C o r r o s i o n t h e F i n i s hThe effects of operating environments on contact resistance

depend on the contact finish. For precious metal finishes allof the mechanisms described above are active. For tin con-tact finishes, surface corrosion is the dominant mechanism,but with a specific kinetics, fretting corrosion. The finisheswill be considered separately.

Precious Metal Finishes - Figures 4 through 10, afterMroczkowski (4), summarize the effects of corrosion, dueto mixed flowing gas exposures, on contact resistance forprecious metal plated coupons. The test environment usedwas intended to simulate exposure to an industrial environ-ment of moderate severity (3). These data are NOT fromconnectors, but from coupons exposed to the environmentand then probed with a hemispherical soft gold probe.Three precious metal platings were evaluated, gold, palladi-um and an 80 palladium - 20 nickel alloy. The preciousmetal plating thickness was nominally 0.75 microns for allfinishes. All coupons had a nickel underplate of nominalthickness 2.5 microns.

Figure 4 shows the effects of the mixed flowing gas envi-ronment on the gold/nickel sample, the curves shown arethe average of nine individual probe readings. The test envi-ronment results in significant degradation in contact resis-tance in 48 hours despite the fact that gold is inert to thegases in the test environment. The effects of the corrosionprocesses on contact resistance can be understood from con-sideration of Figures 5 through 10.

Figure 4. Plot of Contact Resistance versus NormalForce for gold/nickel coupons exposed to a testatmosphere intended to simulate an industrial environ-ment. Data for 0, 48 and 100 hour exposures are pre-sented. These data were obtained by random probingof the coupon with a hemispherical soft gold probe.

Figure 5 is a photomicrograph of the gold surface afterexposure to the test environment. The migration of porecorrosion products is evident as haloes around the poresites, the pore density on these coupons is higher than thattypical of connector finishes. Figure 6 shows the individualprobe point contact resistance data from the 100 hourexposure in Figure 4. Note that 3 of the 9 individual curvesshow little effect of the corrosive environment on the goldsurface. These data support the claim that the gold is inert,contact resistance degradation occurs due to corrosionmigration, in this case from pore sites.

Figure 5. Photomicrograph (100X) of pore corrosionhaloes on the coupons producing the data in Figure 4.One of the probe sites is visible in the photomicro-graph.

Figure 6. Plot of Contact Resistance versus NormalForce of the individual readings of the 100 hourexposure in Figure 4. These data indicate the effectsof pore corrosion on contact resistance.

Figures 7 and 8 show data for palladium/nickel coupons.These data are similar to that for gold. Figure 9, containsdata obtained by probing the sample selectively to avoid, asmuch as possible, the effects of pore corrosion. Figure 9a,for gold exposed for 100 hours, indicates the relative inert-ness of the gold, the contact resistance distribution is similarto that of the as received coupons. The palladium data,Figure 9b, shows the effects of general surface corrosion ofpalladium after 100 hours exposure. The average contactresistance has increased and the distribution has broadened.Palladium is not as inert as gold. For this reason, among

Figure 7. Plot of Contact Resistance versus NormalForce for palladium/nickel coupons exposed to a testatmosphere intended to simulate an industrial environ-ment. Data for 0, 48 and 100 hour exposures arepresented. These data were obtained by randomprobing of the coupon with a hemispherical soft goldprobe and are the analog to Figure 4.

Figure 8. Plot of Contact Resistance versus NormalForce of the individual readings of the 100 hourexposure in Figure 7. These data indicate the effectsof pore corrosion on contant resistance.

others, palladium, and palladium alloy, contact finishesgenerally include a gold topcoat, a tenth micron or so, as aprotective surface.

Figure 9. Plot of Contact Resistance versus NormalForce for gold/nickel, Figure 9a, and palladium/nickel,Figure 9b, coupons exposed for 100 hours to a testatmosphere intended to simulate an industrialenvironment. These data were obtained by selectiveprobing to avoid the effects of pore corrosion haloes,shown in Figure 5, on contact resistance. The dataindicate the different corrosion susceptibility of thegold and palladium in the test environment.

Figure 10 contains the same type of data for the preciousmetal alloy 80 Palladium - 20 Nickel. In this case thecontact resistance degradation curves are similar regardlessof whether the probing is random or selective. This is due tothe reaction of the nickel in the alloy with the test environ-ment resulting in general surface corrosion. Alloying of pre-cious metals can impact environmental performance whenalloying constituents include base metals.

In summary, contact resistance degradation of preciousmetal finishes arises from a combination of surface corro-sion and corrosion migration. For gold finishes, corrosionmigration is dominant while for palladium and palladiumalloy finishes surface corrosion is also active.

Figure 10. Plot of Contact Resistance versus NormalForce for palladium-nickel (80-20)/nickel couponsexposed to a test atmosphere intended to simulate anindustrial environment. Data for 0, 48 and 100 hourexposures are presented. These data were obtained byrandom probing of the coupon.

Corrosion and the Connector Housing - Before leavingthe subject of corrosion in precious metal finishes, com-ments on the importance of the connector housing inprotecting the contacts from the environment are in order.The data discussed in the previous section referred tocoupon data and indicate the effect of the environment onthe materials of the contact system. Corrosion mechanismsaffecting contact materials are active in typical connectoroperating environments. Connector degradation throughcorrosion, however, is not common. Figure 11 (4) showscontact resistance performance for gold over nickel (1micron gold over 2.5 microns nickel) plated connectorsexposed, unmated and mated, to the same environment asthe coupons discussed previously. Note that the unmatedexposures result in significant contact resistance degrada-tion, illustrating the effect of the environment on the contactmaterials. Mated exposures, however, show little effect oncontact resistance. This difference in performance isattributed to the shielding effect of the housing in restrictingaccess of the environment to the contact interface.

Tin Contact Finishes - As mentioned previously, tinfinishes are subject to a different corrosion degradationmechanism, fretting corrosion. Tin is resistant to surfacecorrosion in that a protective oxide film forms on the tinsurface which limits further corrosion of the tin. This oxidefilm does not affect contact resistance since it is easilydisrupted on mating of the connector. The disruption of tinoxide is schematically illustrated in Figure 12a (2). The tinoxide, being thin, hard and brittle, fractures under theapplication of contact normal force. The load is transferredto the tin which, being soft and ductile, flows to enlarge thecracks in the oxide and extrudes through the cracks toestablish the desired metallic interface. This mechanismexplains the utility of tin finishes despite the presence of asurface oxide.

Figure 11. Contact Resistance versus Exposure timefor gold/nickel plated connectors exposed to a testatmosphere intended to simulate an industrialenvironment. Data for unmated and mated exposuresare shown. Some of the connectors were durabilitycycled as indicated in the !egend on the plot.

Unfortunately the oxide forming tendencies of tin remainactive, and if the contact interface moves, as shown inFigures 12b through 12d, contact resistance increasessteadily. This degradation mechanism is known as “frettingcorrosion”. “Fretting” refers to the small motions, hun-dredths to tenths of a millimeter, which occur randomly dueto mechanical disturbances or thermal expansion mismatch-es. “Corrosion” refers to the reoxidation of the tin surface asit is exposed during the fretting. Fretting corrosion canresult in rapid contact resistance degradation and is thedominant degradation mechanism for tin contact finishes.

Corrosion is an important degradation mechanism inconnectors. The two basic classes of contact finishes, pre-cious metal and tin differ in the kinetics of corrosion. Goldfinishes degrade through ingress of corrosion products fromvarious sources on the contact spring to the contact area.

For corrosion migration to be active, however, the operatingenvironment must contain sulfur and chlorine. Palladiumand palladium alloys are more sensitive to general corrosionthan is gold.

Tin finishes degrade through fretting corrosion, whichrequires oxygen and motion and, therefore, can occur in anyoperating environment. The key factor in environmentalseverity for tin is the driving force for contact motion.Differential thermal expansion is the most common driverfor fretting.

Figure 12. Schematic illustration of the mechanismof contact formation in tin finished connectors,Figure 12a. Figures 12b through 12d illustrate themechanism of fretting corrosion.

WearThe second degradation mechanism to be discussed is wear.It is important to note that the significance of wear is that itincreases corrosion susceptibility by removing the protec-tion provided by the contact finish.

Wear as a degradation mechanism in connectors takes itsimportance from the separability requirement on theconnector. Cycle life requirements on connectors rangefrom a few cycles to several hundred in typical applicationswith a few thousand being required in some special cases.Wear is a result of the connector mating and is dependenton the design and materials of manufacture of the connectorand the required number of mating cycles the connectormust support.

This paper will provide only a brief overview of connectorwear. A simple equation, due to Rabinowitz (6), to predictwear is

(4)

where V is the wear volume, the volume of materialremoved, k a wear coefficient, L the load, x the weardistance and H the hardness of the surfaces in contact.

With respect to connector design the parameters in thisequation translate to the following:

l L, the contact normal force

l H, the “finish hardness”

l x, the engagement length of the connector

l k, a “wear coefficient” which includes finishcharacteristics and the state of lubrication of themating surfaces

l V, wear volume. The distribution of the wearvolume is dependent on the geometries of the matingsurfaces.

The role of normal force is straightforward, wear increaseswith normal force. This is one reason for minimizingnormal force as will be discussed in a later section.

The finish hardness is more complex, especially in preciousmetal finished connectors. The effective hardness dependson both the precious metal hardness/thickness and that ofthe nickel underplate. The nickel underplate can significant-ly enhance connector durability as discussed by Antler andDrozdowicz (7).

The engagement length is also straightforward, although itis important to consider localization of the wear such asoccurs in a receptacle to post connector, the wear on thereceptacle is localized while that on the post is distributed.

The “wear coefficient” is complex in that it includes thesurface roughness and the state of lubrication, both of whichare variable. Surface roughness may change significantlyduring the wear process. The state of lubrication varies fromatmospheric “contamination” to the application ofspecial contact lubricants. Lubrication is possibly the mostimportant means of improving durability of connectors.

A discussion of wear mechanisms and kinetics is beyondthe scope of this paper. For detailed discussion of connectorwear and lubrication see the review article by Antler (8).

SummaryNoble metal finishes provide much higher durability lifethan tin finishes. This is due to two main factors, higherhardness for the precious metals, and a lower normal forcerequirement. Normal forces for tin connectors are typicallyhigher than for precious metal to provide the mechanicalstability necessary to minimize tendencies towards frettingcorrosion.

Loss in normal forceLoss in contact normal force is the final degradationmechanism to be discussed. But first, it is important toprovide a basic understanding of the function of normalforce in a connector.

Contact Normal ForceNormal force is important in two distinct functions,establishing the original contact interface and maintainingits stability over the application lifetime of the connector.The normal force required to establish the contact interfaceare relatively small. Figure 13 (9) shows a plot of contactresistance versus normal force for gold contact finishes.Note that a contact resistance of the order of 3 milliobmscan be achieved with normal forces in the range of 25 to 30grams. The majority of the normal force, typically of theorder of 100 grams, is to ensure mechanical stability of thecontact interface. Insufficient normal force allows fordisturbance of the contact interface which renders it suscep-tible to the ingress of corrosion products and resultantincreases in contact resistance.

Figure 13. Contact Resistance versus Normal Forcefor gold/nickel and wrought gold contact interfaces.

In general, high normal force is undesirable since it increas-es mating forces, stresses on the contact springs/housingsand wear (9). For these reasons establishing a minimumnormal force requirement is an important design considera-tion. If normal force is to be minimized, however, ensuringthe stability of normal force becomes important, which iswhy loss of normal force becomes an important degradationmechanism.

Contact normal force is generated by deflection of thecontact spring as schematically illustrated in Figure 14.The important material and dimensional parameters affect-ing normal force are illustrated in the following equations.

Figure 14. Schematic illustration of a cantileverbeam and equations for calculating the normal forcegenerated by deflection of the beam.

For a cantilever beam the force deflection equation takes theform

where F is the force resulting from a beam deflection D, Eis the Young’s modulus of the spring material, and W, Land T represent the width, length and thickness of the beamrespectively. Many contact spring configurations fit orapproximate this cantilever geometry.

A second equation relates the stress in the beam to itsdeflection.

where S is the stress and the other variables are the same asin Equation (1).

Combining Equations (1) and (2) we obtain:

This equation shows that the normal force, F is determinedby the stress induced in the spring due to the deflection.

of Normal ForceEquations 1 and 3 provide a means for understanding thetwo major mechanisms leading to loss of normal force,permanent set and stress relaxation.

Permanent Set - Permanent set refers to a permanentdeflection of the contact beam due to overstressing thebeam. Equation 1 illustrates the importance of permanentset as a mechanism for degradation of normal force.Normal force varies directly with deflection. A set in thespring will decrease the deflection and, therefore, thenormal force.

Permanent set is a result of overstressing the contact spring.The overstress may be a result of improper design, thedesigned in beam deflection results in plastic deformationof the spring, or mating abuse, improper mating angleresults in plastic deformation. In either case, normal forcewill be reduced by the reduction in beam deflection onsubsequent matings. Proper materials selection and contactdesign can minimize or eliminate plastic deformation underproper application conditions. Abusive mating can bemoderated by design features in the contact spring andconnector housing, but proper application instructions andtrained users are critical to minimizing abuse.

Stress Relaxation - Equation 3 illustrates the effect ofstress relaxation on loss in normal force. Normal force isdirectly proportional to the stress in the beam. Stressrelaxation is defined as a time/temperature dependent lossin stress under constant deflection. In other words, the stressin a deflected contact spring will decrease with time/tem-perature resulting in a loss in normal force. The effect ofstress relaxation on connector performance iscontrolled by materials selection and consideration of appli-cation requirements, in particular operating temperature.Stress relaxation is a straightforward process and easilypredicted as discussed by Bersett (10) and Horn (11).Selection of the contact spring material on the basis of theexpected time/temperature profile of the connector applica-tion is sufficient to eliminate stress relaxation as a degrada-tion mechanism.

Loss of normal force is a degradation mechanism in thesense that it reduces the mechanical stability of the contactinterface making the connector more susceptible to corro-sion. Loss of normal force through plastic deformationshould be addressed in the design of the connector to ensurethat spring deflections are not excessive and to minimizesusceptibility to abuse, especially on mating. Stress relax-ation is addressed through material selection to ensure thatthe spring has sufficient stress relaxation resistance for theoperating environment. Operating temperature is the majorfactor in stress relaxation.

Degradation Mechanisms andConnector Design/MaterialsIt may be useful to restate some of the key issues ofconnector degradation in terms explicit to connectordesign/materials selection.

The Contact FinishThe contact interface, and, therefore, the contact finish arethe key elements in connector degradation since all increas-es in contact resistance result from loss of contact area insome manner. The contact finish plays its primary role ininfluencing the corrosion behavior of the contact interface.

For precious metal finishes, gold - and to a lesser degreepalladium and its alloys - is inert in typical connectoroperating environments. Connector design is directed

towards preventing corrosion from elsewhere in the connec-tor from reaching the contact interface. Pore corrosion andcorrosion from exposed base metal at stamped edges are ofparticular concern. Corrosion of precious metal finishedconnectors has been observed. Its effects depend on theoperating environment, especially with respect to sulfur andchlorine. Shielding by the housing moderates the potentialfor corrosion.

For tin finishes, the major degradation mechanism is fret-ting corrosion which can occur in any operating environ-ment. The major factor in fretting corrosion is the drivingforce for fretting motions. Differential thermal expansion isthe most common driver. Connector design to minimizefretting susceptibility centers around high normal forces toprovide sufficient friction force at the contact interface toprevent motion from occurring. Contact lubricants are alsoavailable to reduce oxidation susceptibility.

The Contact SpringFrom a connector degradation viewpoint, the major springmaterial selection criterion is stress relaxation resistance.Loss of normal force through plastic deformation is pre-dominantly a design exercise to ensure that spring stressesare not excessive. The variation in yield strength for thecommonly use copper alloys is a secondary factor in thisregard. Stress relaxation resistance, however, does varysignificantly across the copper alloy range. Berylliumcopper is the most commonly used alloy when stress relax-ation is a major concern. Brasses should be avoided andphosphor bronzes are suitable for most applications.

The connector housing has received little attention in thisdiscussion. This is primarily due to the emphasis on contactresistance degradation. Connector housing design, to shieldthe contact interface from the environment is the majorcontribution of the housing to connector reliability from acorrosion viewpoint.A few comments on the connector housing in a context ofother reliability issues is in order. The operating tempera-ture, in use and in assembly, is a major factor in housingmaterial selection. Surface mounting requirement and hightemperature applications may dictate material selection.Chemical stability, primarily for assembly/cleaning opera-tions may also be important. The range of materialsavailable and suitable for connector housings is large andgrowing. Frequently some major characteristic, such as tem-perature capability or mold flow will dictate the materialselection process.

This concludes the discussion of reliability in terms ofconnector structure, design and materials. Attention nowreturns to connector function and applications.

CONNECTOR APPLICATIONS/FUNCTIONSConnector applications and functions are two ways todescribe how connectors are used. Connector applicationswill be described in terms of the subsystems which are

being connected, a Levels of Packaging approach, followingGranitz (12). Functionally connectors will be considered interms of signal or power distribution. Consider applicationsfirst.

Connector Applications:Levels of PackagingIt is important to note that the Level of Packaging is definedby the points in the system which are being connected, notthe connector itself. In fact, many connector types are usedin more than one level of packaging. The six Levels ofPackaging, and an associated connection/connector type,are:

1. Chip pad to package leads, eg. wire bonds

2. Component to circuit board, eg. DIP socket

3. Circuit board to circuit board, eg. card edgeconnector

4. Sub assembly to sub assembly, eg. ribbon cableassembly

5. Sub assembly to input/output, eg. D sub cableassembly

6. System to System eg, coax cable assembly.

Figure 15 schematically illustrates the six levels ofpackaging.

Figure 15. Schematic representation of an electronicsystem indicating the Levels of Packaging.

Some general comments on the individual levels are inorder.

The characteristics of Level 1 interconnections are:

l Most often made by highly automated methods.

l Are very specialized.

l Are usually not separable or repairable.

l Are enclosed by the device package.

l Must be extremely reliable.

Interconnections at this level:

l Usually must withstand soldering environments.

l Are relatively small in size and usually do not needmounting hardware.

l Have contacts that are not individually repairable.

l Have low mating cycle requirements.

l Are serviced by trained personnel.

Level 3Level 3 interconnections are the first level at whichseparable connectors appear. Advances in microelectronicstechnology have led to high performance, high I/O printedcircuit boards which lead to the following requirements onlevel 3 connectors:

l High pin counts, over 1000, and high densityconnectors, 0.100 inch centerlines with 0.050 rapidlycoming on.

l Mating forces become important due to high pinouts,with guiding hardware and keying also being needed.

l Mating cycle requirements are in the tens tohundreds.

l High speed capability to support board processingspeeds, nanosecond switching and controlledimpedance are becoming important.

l Repairability is required.

l Trained assembly personnel are used on the systemlevel, but application by users is increasing sorobustness becomes a consideration also.

Level 4Level 4 interconnections are characterized by a large varietyof connector types and often include cables. Requirementsfor level 4 connectors include:

l Special features to facilitate cable applications.

l Mating cycles in the hundreds.

l Robustness due to exposure to untrained users.

l Locking and latching features are common.

l Requirements for shielding are increasing.

L e v e lLevel 5 interconnections are the connections to the outsideworld, I/O connectors. They share many of the requirementsof level 4 with a few additional.

l Because one half of the connector is outside thesystem, standardization is important forintermatability.

l For the same reason, robustness, ease of use andcosmetics are important.

l Shielding, filtering and interference considerationsbecome important.

L e v e lThe variety of level 6 interconnections is also large andincludes cabling. Many of the level 4 and 5 requirementsremain important.

l Robustness increases in importance.

l Mating cycle requirements may be high, severalhundred.

l Shielding and filtering may be more important due tolonger exposed lengths.

l Standardization is a major consideration.

These comments are sufficient to provide a context for con-nector applications and indicate the variety of requirementsconnectors must meet. Attention now turns to connectorfunction.

Connector Function:Signal and Power DistributionConnector requirements for signal and power distributionare different in fundamental respects. In some cases, how-ever, the same connector will be used for both applications.Such applications require a basic understanding of thedifference between signal and power contact requirements.

Signal distribution requirements center around maintainingthe integrity of the signal waveform. For high data ratesystems this may involve controlled impedance connectordesigns and careful attention to signal to ground ratios. Ingeneral, however, this level of sophistication is not neces-sary and consideration of the connector resistance alone issufficient.

The magnitude of the required connector resistance isstrongly dependent on the devices in the circuitry the con-nector must interconnect. For many devices high connectorresistance, hundreds of milliohms, can be tolerated. This“relaxation” of connector requirements does not simplifyconnector design. Contact resistances of this magnitude areinherently unstable due to the fact that they represent a verysmall contact interface area and are, therefore, very sensi-tive to degradation through corrosion or mechanical distur-bances. The ability of a system to tolerate higherconnector resistance, however, does impact on connectorreliability as will be discussed in a later section.

Power Distribution RequirementsResistance requirements for a contact or connector in-tended for power distribution are much more sensitive toresistance. This is a result of the Joule heating whichaccompanies current flow. Joule heating, which isproportional to the connector resistance, can result inincreases in the connector operating temperature, a majorfactor in connector degradation, and create conditions forthermal runaway. In many cases the current rating of a con-tact is determined by a 30 degree Centigrade temperaturerise, T- rise, criterion so minimizing resistance maximizescurrent rating. Both magnitude and stability of contact resis-tance are critical for power applications. In addition to T-rise, there are fundamental limits to allowable currentthrough a contact based on interface melting considerations.This subject is beyond the scope of this paper, see Cormanand Mroczkowski (13) for additional discussion.

Power distribution in connectors can be carried out in twodifferent modes, dedicated power contacts and signalcontacts in parallel. Design and circuit considerations differin the two cases (13).

Connector requirements are dependent on the application inwhich the connector is used. Two approaches, Levels ofPackaging and Signal/Power have been discussed. Thelevel of packaging impacts connector design with respectto durability and resistance requirements as well as thenecessity for accounting for the potential for abuse of theconnector by the user. Signal/power considerations high-light the criteria for contact resistance, both magnitude andstability.

CONNECTOR RELIABILITY:FIELD AND LABORATORYIn the Introduction it was stated that connector reliabilitydepends on the requirements the connector must meet andon the degradation mechanisms to which the connector isexposed. The previous discussion has provided a context forthe following discussion of connector reliability in the field,and a proposal for methods to estimate connectorreliability through laboratory testing.

A simplistic definition of reliability for a connector is:

The probability of maintaining a specified range ofconnector resistance under specified operatingconditions, for which designed, for a specified time.

As discussed, the connector application determines theresistance requirements, and the operating conditions,which in turn determine the degradation mechanisms whichmay be active.

It is useful to consider degradation mechanisms in thecontext of whether they are intrinsic or extrinsic. Intrinsicmechanisms are related to the design and materials of

construction of the connector. Extrinsic mechanisms arethose which are related to the application.

Examples of intrinsic degradation are corrosion, loss ofnormal force through stress relaxation, and Joule heatingleading to temperature related degradation.

Examples of extrinsic degradation are contamination andfretting corrosion. Each of these conditions is dependent onthe application of the connector, both in manufacturing andusage in the final system. Such degradation mechanisms canbe qualitatively assessed, but, in general, are difficult, if notimpossible, to quantify for use in a determination of con-nector reliability.

Examples of other degradation mechanisms, which areoutside the scope of connector reliability, include using theconnector outside its rated temperature range (both ambientand enclosure related), applying currents in excess of theproduct specification (in both single and distributed modes),and improper mating practices (mating at excessive angles,pulling on cables, etc.) leading to contact abuse. Suchdegradation is outside the scope of connector reliability inthat the connector is being used outside the “specifiedoperating conditions for which designed” section of the reli-ability definition and, therefore, is misuse of the connector.

Connector manufacturers have control over intrinsic degra-dation, but not over extrinsic factors. Extrinsic degradationcan be controlled only by proper specification of productperformance by connector manufacturers and proper use ofthe available information by the user. This joint responsibil-ity ensures that the “under specific conditions for whichdesigned” section of the definition is met. In this case,“user” is intended to include connector and electricalequipment manufacturers as well as the ultimate user of theequipment.

Two field studies on connector reliability will be reviewedin this context.

Field Reliability StudiesThe first study was conducted by Grau (14) in the late 70s.It included nine connector types as used in telephone centraloffices. Four central office sites with different severities inenvironmental categories were studied. The sample size wasvery large, over 50,000 contact pairs were measured orexamined.

Some of the key conclusions from the study include:

l No circuit failures due to connectors wereexperienced during the study.

l Resistance values higher than expected were found,100 ohms was the “max” recorded.

l Factory contamination was a major contributor to thehigh resistance values.

l No evidence of pore corrosion was found.

These conclusions are interesting in that they indicate thathigh values of connector resistance do not necessarily leadto circuit failures, at least in the telecommunications equip-ment studied. Extrinsic degradation, factory contamination,was a major contributor to high resistance. Pore corrosion,felt by many to be a major source of degradation, was notfound. This supports the previous comment that the envi-ronment plays a key role in whether corrosion migrationwill occur.

A second study, after Yager and Nixon (15), covered abroader range of applications, test instrumentation andcomputer equipment, and environments. In this study the“samples” consisted of printed wiring board assemblieswhich had been returned for repair. The study was one offailure analysis rather than reliability as such. Since, in gen-eral, the source of the failure was not identified, the authorsrefer to “defects” rather than “degradation mechanisms”.The terms are related but not identical. The sample included90 assemblies incorporating 12 different connector types.

Some of the conclusions from this study include:

An organic film was the predominant defect on thePWB fingers.

A variety of defects were identified, both intrinsicand extrinsic, representing the variety of applicationenvironments sampled.

Intrinsic defects observed included pore corrosion,edge corrosion and corrosion creep.

Extrinsic defects observed included an organic film,dust and debris and scratches.

The study also noted the effect of shielding of theconnector from the environment by the housing.

The authors categorized the defects in terms of the percent-age of samples exhibiting the defect and the severity, on ascale of one to five, five being the most severe. The mostcommon defect, observed on 97 percent of the samples withalmost 50 percent at a severity level of 4, was an organicfilm, a contaminant. Corrosion effects were non uniform.Pore corrosion was commonly observed at low to moderateseverity. Edge corrosion and corrosion creep were infre-quent and at low severity. Once again, extrinsic defectswere found to predominate.

While limited, the data from these two studies suggest thatextrinsic degradation mechanism are more common thanintrinsic. If true, this fact has significant implications forconnector reliability, in particular for estimating connectorreliability.

Avoiding extrinsic degradation is a joint task of connectormanufacturer and user. Connectors can be designed to beresistant to contamination and abuse, but only attention toproduct instruction sheets on the use of connectors canminimize such degradation.

With respect to estimating connector reliability, a pre-dominance of extrinsic degradation complicates the issue

considerably in that it is difficult to model or simulate suchdegradation mechanisms. The significance of this limitationwill be more apparent after a laboratory approach to con-nector reliability is discussed.

Laboratory Estimation of ReliabilityThere are two approaches to estimating connector reliabilitythrough laboratory exposures, comparative and statistical.Each has advantages and limitations.

A comparative program would involve using a connector ofknown reliability as a reference through a series of laborato-ry exposures intended to simulate application conditions ofinterest. Comparative data, say contact resistance, for thereference and test connector after exposure could be used toassess the relative reliability of the test connector. Therelevance of the data would be dependent on the confidencewith which the reference connector reliability wasdetermined and the confidence level that the laboratoryexposures appropriately simulate application conditions ofinterest.

To determine connector reliability using a statisticalapproach we must address at least the following issues:

l The active degradation mechanisms must beidentified and categorized with respect to theirimportance in the application of interest.

l Appropriate tests, acceleration factors and exposuresmust be known, defined, or determined, for thesedegradation mechanisms.

l Failure criteria appropriate to the application ofinterest must be established.

l The statistical approach to determining, orcalculating, reliability values must be agreed upon.

The following comments on each of these issues will sufficefor the purposes of this paper. For additional discussion seeMroczkowski and Maynard (16).

Degradation MechanismDegradation mechanisms have been discussed in previoussections. The subject here is categorizing them with respectto their importance. This is not necessarily a trivial task anddepends on both the operating environment and the systemoperating requirements. As mentioned, connectordesign/materials determine the potential degradationmechanisms, and application conditions determine whichmechanisms are active.

Test Factors.There are two major issues here. The first is ensuring thatthe test exposure simulates the application conditions. Arelated, but even more complex issue is determining anacceleration factor. Since the definition of reliabilityincludes performance for a specified time and accelerationfactor for the test exposure is required. In simple terms, theobjective is to be able to document that A days exposure totest B is equivalent to X years of operation in environmentY. There are few tests for which such an objective is met.

Failure Criteria:What is the appropriate value of contact resistance to use inestablishing contact reliability? There are two possibilities,the product specification value, which is somewhat generic,and an application related value.

Fundamentally, the product specification contact resistanceis not the proper choice. This value has a “reliability” aspectto it in the sense that the manufacturer has tested thePRODUCT DESIGN with respect to ensuring that thespecified contact resistance maximum will be maintained inits intended RANGE OF APPLICATIONS. In this respect,the product specification value includes a “safety factor” toaccount for the range of possible applications for theconnector.

In a particular application, on the other hand, a user willhave established a value of contact resistance at which thesystem of interest will cease to function. The value may be100 milliohms, or more, in a signal application or 0.5milliohm, or less, for a power contact. The “failure” criteri-on for contact resistance should be based on this applicationspecific value and not the product specification contactresistance. The desired requirements on confidence limitand reliability should then be applied with this resistancevalue as the upper limit of acceptability.

Many of the issues concerning statistical treatment of thedata obtained from individual contacts are straightforward.Relating contact data to connector performance does,however, raise some issues. For example, the contact dataare obtained on contacts in a particular connector. The data,therefore, are influenced by the connector. How thisinfluence is to be quantified or included in the connectorreliability calculation merits attention.

SummaryThe purpose of this discussion was to present some of theissues and considerations which are pertinent to determiningand calculating connector reliability via a laboratory testingprogram. A reliability evaluation program consists of thefollowing steps:

1. Determine an application specific contactresistance acceptance criterion. A criterion willalso be required for any other failure mode whichis to be included in the evaluation program.

2. Develop a test program to address the expecteddegradation mechanisms operative in theapplication. Ranking of failure modes may beconsidered in this process.

3. Derive acceleration factors, when possible, for thetests to be specified. When this cannot be done noreliability prediction can be made. In such casesonly comparative performance capabilities can beprovided.

4. Decide on the statistical treatment appropriate to thedata generated in the evaluation program.

5. Calculate the component reliability.

It must be emphasized that both the connector manufacturerand the user should agree on the content, approaches andvalues to be specified in these steps individually, and in theevaluation program in general. In particular, mutualengineering judgements must be made to select appropriate“acceptance/failure” criteria and acceleration factors. Areliability evaluation program according to these procedureswill allow estimation of the intrinsic reliability of a connec-tor. It is important to note, however, than such a programdoes not, and cannot, address extrinsic degradation modes.

CONCLUSIONThis paper has reviewed some of the interactions betweenconnector materials/design and application environmentsand requirements as they impact connector reliability.Approaches to evaluating or estimating connector reliabilityhave been discussed. Reliability estimation for intrinsicdegradation mechanisms is feasible, within limits, butextrinsic degradation mechanisms cannot be accounted forin a laboratory environment.

To achieve the highest reliability in a connector, theintrinsic connector reliability, determined by connectordesign/materials selection, must be preserved by conscien-tious attention to connector specifications, to ensure opera-tion of the connector within its designed area of application,and avoidance of extrinsic degradation processes such ascontamination in assembly and application. Connectors canbe designed to resist, but not totally eliminate susceptibilityto, such degradation.

REFERENCES1. Williamson, J.B.P., “The Microworld of the Contact

Spot”, Proc. 27th Ann. Holm Conference on ElectricContacts, 1981.

2. Mroczkowski, R.S., “Connector Contact Surfaces:Where the Action Is”, Presented at Indicon, 1983.

3. Abbott, W.H., “The Corrosion of Porous Gold Platingsin Field and Laboratory Environments”, Proc. 13thInternational Conference on Electric ContactPhenomena, 1986.

4. Mroczkowski, R.S., “Corrosion and Electrical ContactInterfaces”, Paper #328, NACE, Corrosion 85, 1985.

5. Mroczkowski, R.S., “Materials Considerations inConnector Design”, Proc. 1st Electronic Materials andProcessing Conf., American Society for Materials, 1988.

6. Rabinowitz, E., Friction and Wear of Materials, JohnWiley and Sons, Inc., New York, 1965.

7. Antler, M., and Drozdowicz, M.H., “Wear of GoldElectrodeposits: Effect of Substrate and of NickelUnderplate”, Proc. 9th International Conference onElectric Contact Phenomena, 1978.

8. Antler, M., “Wear of Contact Finishes: Mechanisms,Modelling, and Recent Studies of the Importance ofTopography, Underplate and Lubricants”, Proc. 11thAM. Connections and Interconnection TechnologySymposium, 1978.

9. Whitley, J.H., and Mroczkowski, R.S., “ConcerningNormal Force Requirements for Precious Metal PlatedContacts”, Proc. 20th. Ann. Connections andInterconnection Technology Symposium, 1987.

10. Bersett, T.E., “Back to Basics: Properties of CopperAlloy Strip for Contacts and Terminals”, Proc. 14thAm. Connections and Interconnection TechnologySymposium, 1982.

11. Horn, K. W., and Zarlingo, S.P., “UnderstandingStress Relaxation in copper Alloys”, Proc 15th Ann.Connections and Interconnection TechnologySymposium, 1983.

12. Granitz, R.G., “Levels of Packaging”, Instruments andControl Systems, August, 1992.

13. Corman, N.E. and Mroczkowski, R.S., “Fundamentalsof Power Contacts/Connectors”, Proc. 23d Ann.Connections and Interconnection TechnologySymposium, 1990.

14. Grau, T.G., “A Field Study of the ElectricalPerformance of Separable Connectors”, Proc. 28th.Elec. Comp. Conf. 1978.

15. Yager, T.A., and Nixon, K., “A Field Study ofConnector Reliability”, IEEE Trans. CHMT, Vol. 7,No. 4, 1984.

16. Mroczkowski, R.S. and Maynard, J.M., “EstimatingConnector Reliability”, IEEE Trans. on Reliability,1991.