This paper is an expanded treatment of the first part of the William Blum Lecture presented at SUR/FIN 1988 in Los Angeles by Dr. Morton Antler, 1987 AESF Scientific Achievement Award recipient. The subjects covered include friction, mechanical wear, fretting corrosion and frictional polymerization—all vital processes that affect the performance of contact finishes in electronic connectors. The dominant wear mechanisms (i.e., adhesion, abrasion and fretting) are also discussed. Substrate and lubrication effects on the tribology of contact finishes will be elaborated on in a second paper covering the final half of Dr. Antler’s Blum Lecture to be published in the NASF Report for March 2014. A printable PDF version of this first part is available by clicking HERE.
Introduction An electric contact is the heart of separable connectors and printed circuit boards, instrument slip rings, switches and other current-carrying devices. These components are designed to allow the engagement or disconnection of an electronic assembly or to permit one contact surface to be moved while it touches another. Because the contact resistance between mating surfaces must be low, gold and other noble metals are used extensively. Tin and tin-lead solder are exceptions because their deformability allows insulating surface oxide films to be readily broken when contact is established.
The main applications of contact-containing components have been for telecommunication equipment, computers, aircraft and aerospace devices, various instruments and military hardware. However, the use of consumer products that contain large numbers of separable contacts is growing rapidly, especially in the automotive and entertainment fields. In developing finishing technologies for the noble metals, the high cost, limited availability and general unsuitability of these metals as structural contact materials have been the major challenges.
In this paper, metal finishing is interpreted broadly to embrace the diverse processes of electroplating, electroless plating, cladding and a variety of physical coating and surface-modification methods such as ion plating and ion implantation. Finishing can be extended to post-plating passivation treatments for porous deposits and even to contact lubrication when the application of a lubricant is made on the plating line directly following metal deposition.
Tribology* is important because contact coatings are generally thin, and it is necessary to maintain the integrity of the deposit during sliding to minimize exposure of base underplatings and substrates. The requirement for low friction is becoming more critical as the numbers of contacts in separable connectors increases. While resistance to corrosion and tarnishing and the thermal stability of contact resistance have always been of concern in the selection of finishes, it is not often realized that the tribological requirements of coatings have been of equal or greater importance in molding metal finishing practices. As will be discussed for electrodeposits, the incorporation of cobalt or nickel in gold electrodeposits, the practice of using a gold flash on palladium, and the use of nickel underplates developed largely because it was realized that these practices would improve the tribological performance of the contact finish.
The objective of this review is to present the tribological background behind modern practices in the metal finishing of contacts and provide guidelines for selecting finishes for electronic connectors. Some thoughts on the needs and possible future developments are also given.
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Contact wear mechanisms Mechanical wear involves loss of material as loose particles from solid surfaces. In special cases, the following are also classified as wear: dimensional changes or roughening due to metal transfer in the absence of loose-particle formation, the cracking of brittle coatings, and mechanically induced flow without measurable transfer or loss (i.e., burnishing).
Adhesive wear occurs when surfaces undergo metal transfer. Adhesive bonds are formed between touching asperities that are stronger than the cohesive strength of the metal. This bond formation results in plucking of material, which then may be lost from the surface to which it transfers during subsequent traversals.
Many metal systems operate at widely different rates of adhesive wear, from mild to severe, and the scale of wear can change in a narrow range of load (Fig. 1). Below the transition load, wear debris is finely divided; above it, debris is coarse. Provided the surfaces are very clean, sliding with noble metals is in the severe regime even under small loads (e.g., 1 g for pure gold plate and 10 g for cobalt-gold alloy deposits). In practice, the transition loads are related to the removal during sliding of adventitious contaminants such as adsorbed organic air pollutants. Such materials can be strikingly protective,1 but, when repeat-pass sliding occurs too quickly for the contaminants to be renewed, the transition to severe wear will occur.
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