5. The Impact of Contaminations, Protection by Nano Coating

5.1 The Tin-Gold Affinity

The so called RoHS directive is effective in the EU since 2002 (see Wikipedia). It prohibits the use of lead (and some other substances) as part of many composite materials, e.g. solder. Today, soldering with lead-free technology has been widely adopted. SnAg, SnCu, and SnAgCu alloys are used in various soldering technologies.

Regarding test engineering – and thus regarding the spring contact probe – the changeover from leaded solder for electronics to lead-free replacement alloys has been accompanied by striking negative effects on a smoothly running volume production. Service life of test probes in fixtures decreased enormously and the pseudo error rate virtually went through the roof. In other words: The first pass yield (FPY) heavily decreased. And that costs time and money and frays nerves.

At FIXTEST this highly problematic trend has quickly been given top priority and a R&D project was initiated. In a first step it was all about finding the exact root cause(s) for these phenomena. Due to our cooperation with a partner institute for nano technology we had analysis techniques and scientific competence at hand which proved particularly useful for tackling the given task. By means of chemical force microscopy (CFM) a certain affinity between pure tin and gold has been verified. With each contacting cycle ultra-small tin particles had been pulled out of the solder sample and sticked to the gold surface.

The new tin-silver-copper solder alloys are much harder than previous leaded solder alloys. With increasing storage time after the soldering process the hardness is further increasing. If e.g. a board test is run 48 hours after soldering, these solder joints are sort of “cured” and the tips of the test probes wear out much faster.

5.2 Effects on the application

In the second step an innovative new nano process technology has been used to develop our XXLonglife plating. This additional surface coating on top of the gold can be used in a broad range of contact applications. It prevents contact surfaces from contamination, mechanical wear and corrosion and thus inhibits loss of contact. All these improvements have been achieved while maintaining the electrical properties of the base material (e.g. Au).

While uncoated gold clearly shows an adhesive interaction with lead-free tin, the XXLonglife-coated surface behaves in a totally different manner: an even repulsive interaction with the lead-free solder can be observed. In daily practice this will achieve a massive increase in contact life-cycle-time and a long-term and trouble-free signal transmission – even when problematic lead-free surfaces with flux contaminations are contacted.

Pseudo-error rate decreases, the first pass yield is raised again to the desired level, and the entire test process runs smoothly. Due to cost reduction you will save ready cash. It’s really that simple!

Almost all FIXTEST contact products are optionally available with XXLonglife nano coating. Just add an “X” behind the first group of digits. E.g. will become 100X.118.09.20.