Well it's a strange thing how what goes around, comes around. Electroplated
tin lead was (and still is in some companies) the subsequent process step to
electroplate copper, and acts as the etch resist through ammoniacal etchant.
Before the days of HASL, this was then reflowed, and the surface finish of
fused tin lead was commonplace, so much so that all the accelerated ageing
tests were based on fused tin lead finish. With HASL, OSP, ENIG, Immersion
Tin, Immersion Silver finishes, the electroplated tin lead is stripped after
etch, hence the term soldermask over bare copper, and then the final finish
applied.
In my past, we converted a board finish from HASL to reflowed tin lead (but
3-5um, not 8-12um), so that the customer had a perfectly flat finish for
assembling RF cans, and it improved their first pass yield from 86% to 98%.
Unfortunately for the competition, they had thrown out their IR reflow, and
converted the tin lead baths to tin, so couldn't offer the finish !
Concerns over intermetallic growth prior to assembly were proven to be
unjustified, probably due to the short time between manufacture and
assembly - JIT works in some companies - and the commercial advantage was
significant as it reduced process steps, and increased assembly yield.
Dougal Stewart

email:  [log in to unmask]
phone: +44 1896 822204
mob:    +44 7984 629667


----- Original Message -----
From: <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, January 25, 2002 3:38 AM
Subject: Re: [TN] Electroplated Tin/Lead


> Lemme butt in with some theoretical stuff, that I have seen evidence of.
>
> The subject is galvanic corrosion, or what happens when you have two
metals
> (like Tin and Lead) electrically in contact with each other, and with the
> corrosive material.
>
> When this is the case, the more readily attacked of the two metals (in
this
> case Tin) is MUCH more readily attacked, while the less readily attacked
> metal is protected.  This is why galvanized pails do not rust as easily as
a
> bare steel pail would, that is why you "galvanize."
>
> However, in this case, you are trying to protect the entire exposed
surface,
> both Tin and Lead, and keep the surface solderable.  So, this is not such
a
> good idea, as the presence of the Lead should/would accelerate attack on
the
> Tin.  Or, what I am suggesting is that by this theory, Tin/Lead should be
> LESS protective than pure Tin.
>
> (All of the above advice goes out the window if you fuse or "reflow" the
> Tin/Lead deposit, in which case the plated metals form a true alloy,
instead
> of discrete crystals of Tin and crystals of Lead, and thus act like a
single
> metal.)
>
> Remember, free advice is often not worth what you paid for it....:-)
>
> Rudy Sedlak
> RD Chemical Company
>
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