Sherman
There are a couple of points I'd like to make:
1. gold-plated Kovar is sheer hell (perhaps I should say shear hell!) for stress
corrosion. I had some PCBs with gold-plated Kovar trannie leads (TO18) where the Kovar
had totally corroded away where the leads had been formed, the connection being
maintained by the gold, until the trannie fell off. This was in adverse climatic
conditions. We got round this problem by solder dipping the leads up to 1 mm from the
case after forming. I'm not saying this is the cause of your problems, which may be
triggered partially by the presence of gold-tin intermetallics, but it's worth knowing.
2. RTV is about the worst kind of coating there is. Its chemistry is highly ionic and
it requires atmospheric humidity absorbed to "complete" its polymerisation. Many types
depend even on strong acetic acid to work. Also, ammonia + water = ammonium hydroxide,
an aggressive alkali. Furthermore, although it is water repellent, it is the most
humidity-porous of all coatings, some 20 times worse than epoxy coatings so, if you
have any hygroscopic contaminant under the RTV, it will get nicely damp and stay damp,
just ideal for creating corrosion, especially if it were ionic in nature, as could be
with astoichiometric or incompletely cured RTV.
My guess is that you need to qualify your process to your requirements.
Brian
Sherman Banks wrote:
> Hi Technetters:
>
> I have recently joined this company and a legacy product has been having a
> few field failures. The nature of the failure is a fractured solder joint.
> It is a lap configuration of a Kovar (gold plated) lead onto a HASL SMT pad.
> The fracture seems to originate at the point of the joint where the exposed
> base metal is from clipping. The solder looks especially grainy although it
> has not seen significant temperature cycles or time at temperature.
> The solder joint is encased in RTV 128 as a conformal coating. According to
> GE tech support, RTV 128 outgases ammonia as it cures.
> My question is: Has anyone any experience using RTV in this situation? Has
> anyone seen or heard about RTV attacking solder joints to this extent?
>
> As this is sent late on Thanksgiving eve, if you see it before Thursday,
> Happy Thanksgiving, if after, I hope you had a Happy Thanksgiving.
>
> Regards,
> Sherman Banks
> [log in to unmask]
>
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