Steve,
Solder mask over Tin/Lead is indeed an ancient stuff IMHO.
I can think of two reasons to do that:
  1.. As far as I remember, it was favored by those who used Tin/Lead fusing instead of HASL during board production and applied the soldermask afterwards. 
  2.. Some "very strict military" customers also prefer to have all copper on the board covered by Tin/Lead (being rather corrosion  resistant) and the soldermask on top of it.
Dryfilm soldermask should have a fair adhesion to Tin/Lead but nontheless, it tends to peel off during assembly. In some cases it's just annoying but in others it could cause the board to be scrapped The main pain being during rework where even if the soldermask application was the best - it'll start peeling off. Now, imagine doing a rework of BGA on such a board........

Good luck with it.
Yehuda

Yehuda Weisz
Tel: (972)-3-6342045
Cel: (972)-53-556897
----- Original Message ----- 
From: "Steve Gregory" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, March 31, 2004 11:41 PM
Subject: [TN] Solder mask over Tin/Lead...


> Hi All!
> 
> I'm just wondering if I'm missing something. Is there any performance reason
> whatsoever, that requires solder mask over Tin/Lead instead of using SMOBC
> (Solder Mask Over Bare Copper)?
> 
> Got a board in receiving inspection that I got called back to look at because
> the mask was failing tape test. When I saw the board, it was solder mask over
> Tin/Lead, just like it was called out on the drawing. The mask had already
> lost adhesion in a few places, but we hadn't done anything to it yet
> assembly-wise...
> 
> I see didn't see this particular boards drawing before we went out and bought
> it, otherwise I would have thrown-up a red flag then. What suprised me
> though, is that our fab vendor didn't say anything before they built it. Is anything
> other than SMOBC being used regularly. I thought solder mask over Tin/Lead
> was ancient stuff, and that everybody learned not to do that anymore...
> 
> -Steve Gregory-
> 
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