A Noble Metal at Your Service: Palladium Cobalt and Test Probes
October 9, 2012
Rika Denshi America offers a broad array of test probes (spring probes) for tester interfaces, test and burn-in sockets, microprocessor, PCB and wafer testing. The following is a brief overview of the what, how, and why of using palladium cobalt and how it prevents solder buildup on the tip of a test probe. Visit Rika Denshi America online to view the full booklet.
Why Palladium Cobalt Makes a Difference on Test Probes…
How did palladium-plating of connectors start?
About 25 years ago, there was a big push to look for a lower-cost plating alternative to gold. Gold had just gone to market pricing and shot up to about $800/oz., and there was a panic in the plating industry. Palladium was a reasonable choice because it was considered a noble metal, and the price was lower than gold at the time.
Palladium Nickel and AT&T in the 1980s
Initially, palladium plating was covered with a flash of gold. This new plating combination replaced traditional gold-only plating in many connector applications. During the 1980s, AT&T (Bell Labs) alloyed the palladium with nickel. This new plated alloy worked very well — and it had a lower cost than gold — so many AT&T connectors were changed over to palladium nickel (PdNi). Over the past 25 years, there have been millions of connectors made, used, and still in use with PdNi plating.
Evolution of Palladium Nickel to Palladium Cobalt
Even as PdNi was accepted, AT&T continued to look for improved plating materials. In 1993 they started the R&D work on palladium cobalt (PdCo). It was introduced to the public in 1997. AT&T, AMP, and Molex all started to promote PdCo and the new plating material looked like it would be widely accepted in the connector industry. But the market price of palladium increased dramatically between 1998 and 2000, and PdCo plating momentum stalled.
Since 1998, palladium cobalt plating design wins have been based on its material properties, not the incentive of lower cost. A typical example of use in the test industry is the contact pad of a DUT board. Plating the contact pads of a DUT board with PdCo reduces the wear from the sharp tip of a spring probe or the wiping action of a socket pin and increases the useful life of the DUT board.
Palladium prices have come down significantly in recent years and the overall interest in PdCo plating increased once again for many applications.
What causes solder buildup on the tip of a test probe?
A solder ball or a solder-plated surface of an IC lead has an oxide skin covering the pure, soft solder. The solder oxides are harder than pure solder and are not highly conductive. When a test probe tip touches a solder surface, it has to push through the surface oxides and make contact with the pure solder underneath. That’s why test probe tip sharpness and spring forces are so important. Upon release, some small amount of the solder, and its oxides, stays attached to the plunger tip, like butter on a knife.
The attached solder will immediately start a metallurgical reaction with the surface metal of the plunger. The exact reactions will depend on the characteristics of the surface material and the ambient temperatures and pressures. Tin is a very active metal commonly found in solders. Its aggressive traits are often modified by other elements in the solder formula. When no-lead solder is used, the destructive tendencies of tin are let loose. It will try to diffuse into the plated material and it will also try to form an intermetallic compound. If the tin successfully reacts with the plated material, it will form a strong bond of attachment.
In the case of a gold-plated plunger, the solder is usually successful in forming a very strong bond even at normal room temperatures and the relatively low pressures of the test probe’s spring forces. Additional cycles of the test probe against the solder surface will pull off additional small pieces of solder. If the surface of the plunger is already covered with strongly attached residue from previous solder touches, the additional small pieces of solder will easily stick to the surface.
If the plunger tip is plated with a slippery and hard material that is not prone to solder attachment, such as PdCo, then the new solder pieces will just wipe away and drop off the plunger tip.
Solder adheres to the tip of a test probe plunger by a combination of many mechanisms. The choice of a plating material, the chemical formula used in the solder, or the temperature experienced at the plunger tip can all influence these mechanisms and the degree of adherence.
Why would two dissimilar metals NOT stick to each other?
Generally, by keeping temperatures and pressures low, two dissimilar metals will not adhere to each other. The definition of “low temperature” is a relative term. Each metal has a unique melting temperature. The closer the metal is to its melting point, the “hotter” the process will be for the events that influence the tendencies of the metals to adhere to each other. When a metal starts to reach its melting point, the atoms start to move around with more and more energy. This atomic movement facilitates diffusion and the formation of intermetallic compounds.
Gold and solder have low melting points. Palladium and cobalt have much higher melting points. Materials with high melting temperatures tend to not adhere to other material at room temperatures.