I know I'm late to the game, but I can't resist asking this question:
From my experience, the tin is applied AFTER soldermask, so
you have mask over bare copper, and tin over exposed copper.
The tin in PHOTO2.JPG in the exposed area looks beautiful,
so isn't the question (ignoring the whiskers for the moment)
"How can bare copper erupt through the mask?"
Unless I missed one of your previous posts, it seems to me that
any speculation about copper poking through the tin finish is
irrelevant. I'm only addressing Question 2 below, but you mentioned
introducing a nickel barrier, and that will not be plated under the mask
either, will it? only on exposed circuitry...
just wondering,
Jack
-=-=-=-
*Subject:* Need clever comments *From:* Hernefjord Ingemar <
[log in to unmask]> *Reply-To:* TechNet E-Mail Forum <
[log in to unmask]>, Hernefjord Ingemar <[log in to unmask]> *
Date:* Mon, 7 Sep 2009 13:21:54 +0200 *Content-Type:* text/plain
Hi all, need some professional backup regarding MIL quality boards.
Objects: FR-4 Class III double-sided multi-layer boards, populated
with SOICS, BGAs,and a lot of passive components.
Observation 1 : the non soldered board have lots of Tin whiskers on
inside of the PTH barrel. My thought is this: if whiskers can grow
long before the board is assembled, then ain't it likely that even CAF
can be generated? See photo 1.
Observation 2: Copper has somehow penetrated the solder mask. This
can be found everywhere along the conductor traces. You need a very
good light microscope and a SEM to see it. See photo 2.
Board data: Copper with 0.8 micrometer Immersion Tin. No nickel
barrier. Solder mask thickness not specified.
Application: Typical MIL-883 environment
Q1: What is your opinion about that thin Tin directly on copper? I
dislike the concept. Copper is very mobile at high temperatures, and
combined with humidity, there can be leakage currents and corrosion
issues. Even if the boards are CCd, there is a risk with copper .
Q2: I gave the advice to introduce a nickel barrier, but our customer
claimed, that they can't because of pressfit connectors and pressfit
test pins on the board. Furthermore, they had heard that one cannot
have nickel platings when pressfitting, because the nickel will crack
and oxidize and cause electrical disfunction. Is this your opinion
too? Are there any relevant testing behind such statements?
Thanks in advance
Inge
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