Back in the 80's I was working with what was considered "high density" boards (Burroughs Corporation memory boards) with Sn/Pb traces. Where the soldermask wrinkled on parallel traces, occasionally a small fissure occurred below the surface allowing a fine whisker of solder to cause a short. This wasn't CAF as it was detected immediately (<5 min.) after wave/clean. SMOBC (very difficult to find a supplier capable of this process at the time) solved the problem.

Moral of the story: Be sure the electrical clearances are adequate. 

Ed Popielarski
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ahne Oosterhof
Sent: Monday, May 05, 2014 9:58 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask over Tin Lead Plating

That is 'correct'.

Ahne.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: 5 May, 2014 05:15
To: [log in to unmask]
Subject: Re: [TN] Solder Mask over Tin Lead Plating

This was common practice in the days before SMT.  The masks used at the time would ripple where the tracks were reflowed, but didn't crack.  Still made it through the solder wave just fine, and no problem with hand soldering either. 
I sometimes still see this today with single sided wave soldered boards such as cheap consumer audio and power supplies.

"Correct" is what works for your application.  Inherently, there is no reliability issue with this construct.

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ramakrishnan Saravanan
Sent: Sunday, May 04, 2014 4:18 AM
To: [log in to unmask]
Subject: [TN] Solder Mask over Tin Lead Plating

Hi,

We received few PCBs from an European PCB manufacturer for Aerospace Industry. 
I am surprised to see Solder Mask is applied over tin lead plated tracks. My understanding is if u do reflow , the solder under the mask will melt and affect the mask. The correct method is applying Soldermask over bare copper isn't?. Kindly advise. regards,

R.Saravanan

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