Electrical test probes and methods of making the same

Details Electrical test probes and methods of making the same Electrical test probes and methods of making the same

2002-09-24

2003-12-23

Cuneo, Kamand (Department: 2829)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

Reexamination Certificate

active

06667629

ABSTRACT:

BACKGROUND
1. Technical Field
The present disclosure is directed to a test probe plunger, a test probe including the plunger and methods of making the same and, in particular, to a test probe plunger having an outer layer of a palladium-cobalt alloy.
2. Related Art
The technology related to test probes for integrated circuits is highly complex, involving the careful balancing of many considerations including resistance requirements, hardness requirements, as well as structural limitations. Generally, such test probes are formed from a relatively hard base material such as steel which is overplated with a very low resistance and highly conductive metal.
One problem associated with testing ICs is the formation of an oxide on the exterior surface of the probe plunger. Such surface oxides may influence the contact resistance of the test probe, which then may influence the test characteristics of the IC which is being tested. For example, surface oxides may interfere with the function of the probe such that the overall contact resistance may be high, which will detrimentally affect the measured characteristics of the IC. Therefore, it is desirable to coat test probes with metals that do not oxidize or oxidize only minimally when exposed to ambient conditions in order to minimize or eliminate the effects of such surface oxides.
Gold, silver, platinum, palladium, iridium, rhenium, mercury, ruthenium, and osmium are sometimes referred to as “noble metals” because they are generally less reactive than other metals, or non-reactive, when exposed to ambient conditions. In addition, the so-called noble metals generally have highly desirable physical and electrical properties, such as low contact resistance, and they generally maintain their color, luster, and aesthetic properties. For the foregoing reasons, noble metals are generally used to coat test probes.
With the exception of gold, all metals react with oxygen in the air to form oxides (including the other noble metals, albeit to a lesser degree). The surface oxides formed by metals have varying degrees of conductivity, thickness, and hardness. Thus, gold has gradually become the industry standard for coating test probes and for forming electrical interconnects for ICs due to its non-reactivity and because it does not form an oxide. When gold is used to plate probe plungers, a very small amount of another metal such as cobalt, nickel or iron may be co-deposited (typically as a cyanide-metal complex) along with the gold in order to increase the lubricity and hardness of the deposit, and reduce the co-efficient of friction of the deposit. Plated gold deposits that include such co-deposits are known as “hard” gold. The co-deposits are generally present in concentrations of less than 1%, typically about 0.2-0.3%.
Another problem associated with testing ICs is the formation of oxide on solder. In general, when a test probe is used to test an integrated circuit, the probe plunger comes into contact with the solder-plated surface of an IC lead or a solder ball. The surface of the solder generally has an oxide skin covering the pure soft solder, which may be problematic because the solder oxides are harder and much less conductive than pure solder. When the probe plunger touches the surface of the solder, the tip first must push through the surface oxide to make contact with the pure solder underneath. In order to facilitate puncturing the surface oxide on the solder ball or the solder, test probes generally have spring-type plungers which exert a high normal force, enabling the tip to puncture the surface. The spring inside the test probe allows the testing device to move the probe a predetermined distance, which produces a specific normal force. Once the probe is released, a small amount of the solder and its oxides generally remains attached to the plunger tip.
The solder that remains attached to the plunger tip is problematic because it may metallurgically react with the surface metal of the plunger. The exact reactions between the solder and the metal on the surface of the plunger will depend on the characteristics of the surface material, as well as the ambient temperatures and pressures.
In addition, the tin in the solder may diffuse into the plated coating on the test probes and form inter-metallic compounds. If the solder successfully reacts with the plated coating, it will form a strong bond of attachment.
In the case of a gold plated plunger, the solder is usually successful in forming a strong bond, even at normal room temperatures and relatively low pressures of the test probe spring forces. Additional cycles of the test probe will pull off additional small pieces of solder and, if the surface of the plunger is already covered with strongly attached residue from previous solder touches, the additional small pieces of solder easily will stick to the surface. Over time, the buildup of the solder and inter-metallics on the probe plunger will increase the resistance of the probe plunger, eventually causing high enough resistance values that the probe must be replaced and discarded.
There have been many attempts made in the industry to solve the problem of solder build-up on probe plungers. One attempt involved mechanically scrubbing off the solder buildup after a given number of test cycles, typically using a brush, and then if possible, continuing to use the test probe for additional cycles. However, mechanical scrubbing is disadvantageous because the probe plunger may be damaged. Another attempt to remove solder-build-up involved chemically cleaning the probe plungers, which is disadvantageous because the chemicals may leave undesirable residues. In either case, all of the solder could not be removed. Thus, tin would remain on the surface of the plunger and would eventually diffuse into the gold to form gold-tin inter-metallics. Gold-tin inter-metallics generally have higher resistance values than the original gold layer on the exterior of the probe plunger. And again, over time, the probe plunger would acquire a mechanical buildup of solder and an increasingly thick surface inter-metallic coating on top of the gold plating. The gold plated layer of the test probe would be buried underneath and become ineffective or less effective than the original gold plated layer. These problems are particularly pronounced in gold plated plungers because there is a high degree of attraction between the gold and the tin.
Other attempts made in the industry to overcome the problem of solder adhesion to the test probe have involved changing the structure of the probe plunger to minimize the contact area between the probe plunger and the solder.
Another attempt involved coating test probes with a coating that includes PTFE.
There remains a need in the art for an improved test probe for integrated circuits that has reduced affinity between the plated metal outer layer on the exterior of the probe plunger and the tin solder on the leads of the integrated circuits and that have low resistance values compatible with present testing equipment.
SUMMARY
The present disclosure is directed to a test probe plunger (hereinafter “probe plunger” or “plunger”) that forms part of a test probe for testing ICs. The probe plunger is plated with an outer layer of material that has a reduced affinity for solder and which does not easily form inter-metallics with the solder. The preferred material from which the outer layer may be formed is selected to have, among other things, a relatively low co-efficient of friction, good ductility, relatively small grain size, and low contact resistance, all of which may combine to provide, among other things, reduced affinity for solder. Preferred embodiments of the probe plunger combine all of the preferred characteristics.
In one embodiment, the disclosure is directed to a probe plunger. The probe plunger includes a base having an exterior surface and a barrier layer overlaying and in direct contact with the exterior surface of the base. A palladium-cobalt alloy outer layer overlays and is in direct contact with the exterior su

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