On 04:55 PM 4/17/01 -0400, Mike Reagan said:
>Andrew.
>Are you involved with developing flight boards?

No. I'm a "flightless" bird, so to speak, since I support Ground-Based 
Microgravity efforts solely at this time. Therefore, my understanding of 
the issue is somewhat the same as anyone else who isn't versed in the 
problem, ie, that of spectator. I keep requesting to take a flight 
certification course, but alas, my workload is too great, and I don't think 
"they" feel it's too important at this juncture...:(

However, I have also read various specs, as well as discussed the issue 
with several flight certified techs here at GRC with whom I am acquainted 
(one is building a PDM for me right now), and I believe that the basic 
thrust of the concern is that rapid and high-value thermal cycling of 
gold-plate with soldered component leads results in a brittle joint, in 
turn subject to rapid failure in the space flight regime ( lay-terms: 
really hot AND really cold AND really low air pressure)

As you suggest, the problem has been linked to gold migration into the 
solder during the soldering process and a resultant amalgam which offers 
poor response to rapid thermal cycling. At a guess, the coefficient of 
expansion for the gold and tin atoms is significantly different. Again, 
speaking only from a general understanding of solid-state physics, and at a 
greater "guess" value, the gold atom's coefficient of expansion is much 
larger than that of the tin. When it is subjected to rapid heating it 
deforms the tin matrix surrounding it, ie, causes the tin to crack.

>Interestingly, many component leads will be gold plated on flight approved
>hardware.  Go figure that one out

I agree. It's strange, but not uncommon. I do think that I remember 
something someone once told me about removing gold from critical elements 
prior to soldering flight material either by having to literally scrape it 
off the leads or by some other process, but I can't verify the accuracy of 
my memory...

Personally, my work requires no such worries, so I don't have much more 
than an academic interest at this point.

If I find something out more authoritative, I'll let "y'all" know...

regards,



Andrew J Jenkins. NCMR @ NASA-GRC
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