More about pogo pins, and a note about beryllium

20.12.2019 12:42

Back in November I wrote about reliability problems with a bed-of-nails test fixture I've made for an electronic circuit. The fixture with 21 pogo pins only had around 60% long-term probability that all pins would contact their test pads correctly, leading to a very high false alarm rate. I did a quick review of blog posts about similar setups and scientific literature I've found on the subject. Based on what I've read it seemed that such severe problems were rare. From my own analysis I concluded that likely causes were either dirty test pads or bad contacts inside the pogo pins themselves, between the plunger and the body of the pin. The pogo pins I was using were on the cheaper end of the spectrum, so the latter explanation seemed likely.

Recently I got hold of a set of more expensive pins and, as it happened, also a new digital microscope. I was wondering how the mechanical design of the new pins compared to the old ones, so I looked at them under the microscope. This lead to some new clues about the cause of the problems I was investigating:

Pogo pin tip comparison under a microscope.

Pogo pin tips pictured above from left to right:

a) Harwin P19-0121, new (23.00 € for 10 pieces). Tip material is gold-plated steel.

b) P75-B1 type of uncertain origin, new (4.46 € for 10 pieces).

c) and d) two examples of P75-B1 removed from the test fixture after approximately 1500 mating cycles.

The more expensive Harwin pins show a significantly sharper point than the ones sold by Adafruit. Even when new, the cheaper pins have a slightly rounded tip. Over many mating cycles with a test pad the tips end up being even more flattened. The c) and d) pins above have been used with a flat test point surface on a lead-free HASL-finish PCB (the test setup described in my previous post). I couldn't find any specification of the longevity of the P75-series of pins. Harwin P19 are specified for 100k cycles, so it seems surprising that P75 would wear down so much after less than 2% of that amount. This evaluation by OKI shows that contact resistance of probes for wafer testing starts to rise somewhere after 10k cycles.

These flattened tips do explain somewhat the problem I'm seeing. Compared to sharp ones, dull or rounded contacts have a worse chance of piercing surface contamination on a PCB, like oxide or flux residue. Hence why my analysis showed that the failure rate was related to each production batch. Each batch had a slightly different amount of residue left on the boards and none was perfectly clean. First results show that replacing the pins did have a positive effect on the test reliability (I imagine it's hard to get any worse than that 40% fail rate), but I'll have to wait to get some statistically significant numbers.

In the context of using more expensive pogo pins, another issue came up. Some of the more expensive pogo pins use heads made from beryllium-copper alloy. None of the pins pictured above do, but other head shapes from the same Harwin P19 product line do in fact use beryllium according to their datasheets. Beryllium has some health risks associated with it, especially when it's in particulate form. I was wondering, if I switch the test setup to such pins, how much beryllium would be released into the environment due to parts wearing down in the way I've seen?

First paragraph from the Exposure Assessment Guide.

Image by Beryllium Science & Technology Association

From the microscope photographs above, I'm estimating that approximately 140000 μm3 of material was lost from one pin after 1500 cycles. This value is based on the volume of a cone that's missing above the flattened tips of pins c) and d) and probably overestimates the true amount. Given a BeCu alloy density of 8.25 g/cm3 and assuming beryllium content of 3% by mass, this comes out as approximately 0.04 μg of pure beryllium released into the environment. One figure I found for the recommended beryllium exposure per inhalable volume of air is 0.6 μg/m3.

This means that all accumulated dust from the wear of 15 pins would need to be distributed in a single cubic meter of air to reach the maximum recommended density for breathable air. Considering that the amount of wear shown above happened during a time span of months it seems unlikely that all of it would instantaneously end up gathered in a small volume. I don't know if the missing material would end up in the form of dust around the pins, or would be slowly carried away, smeared little by little on test pads. In any case, based on this back-of-the-envelope calculation beryllium contacts seem reasonably safe to use, even if the amount of beryllium lost isn't completely negligible compared to published exposure limits (but of course, I'm not any kind of a workplace safety expert).

I don't think this result is surprising. Finished products using beryllium are generally considered safe. BeCu alloys have been used for mundane things like golf clubs and musical instruments. Harwin doesn't publish any MSDS documents for their products. Also as far as I'm aware, beryllium use isn't covered by RoHS, REACH and other such regulations. But in any case, it can't hurt following some basic precautions when working with electronic components that incorporate this kind of materials.

Posted by Tomaž | Categories: Analog

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