Jack,
SMOBC is the common industrial standard, as you pointed out, however, there
ARE some fabricators that apply the solder mask after Tin/Lead-ing the
copper traces. The later method has an obvioius disadvantage, see below
quoted from an article written by US Environmental Agency:
" This method predominates for several reasons. Copper is a surface that
lends itself to rigorous cleaning, which is essential for solder mask
adhesion. Tin-lead under solder mask will liquefy during soldering and may
cause the mask to blister and peel. The hot air solder leveling process
generally produces less waste water and introduces less lead into the waste
water stream than tin-lead plating and reflow. Despite these advantages,
well-known disadvantages also exist. The shelf-life of hot air solder
leveled circuits is short and solder thicknesses on pads and hole barrels is
notoriously difficult to control. For these reasons, a small minority of
specifications continue to call for tin-lead plate and reflow or other
alternati air solder leveling, nomenclature screening, and finally, gold
edge plating if necessary. "
I think that is what happened to our boards....." cause the mask to blister
and peel"...
Another paper describes Tin under solder mask this way:
" Facility F initially was concerned with the soldermask breakdown where the
Tin leaches underneath the soldermask....etc"
Quoted from EPA (United States Environmental Protection Agency.
When I started the investigation (had just some hours to spend before
reporting the result!), I was fully convinced that these boards were SMOBC,
but our customer said they used tinning before soldermask. I have asked for
a confirmation from the board fabricator, but got no answer.
Thanks for your comment, good critics.
Inge
----- Original Message -----
From: "Jack Olson" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, September 09, 2009 4:44 PM
Subject: [TN] Need clever comments
>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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