Typically ENIG is done after soldermask, so the soldermask will be SMOBC.

I have seen a product that was specified as all ENIG, then soldermasked. I 
believe it was an attempt to overspecify to increase protection from 
electromigration risk on traces under the soldermask.  In this case 
soldermask over ENIG was no problem to process. This finish does not melt 
and cause the wrinkling seen years ago when soldermask over HASL 
definitely caused lifted wrinkled soldermask.



Roger Mack  P.Eng. 
Manufacturing Specialist
Parker Hannifin Canada
Electronic Controls Division
1305 Clarence Avenue
Winnipeg, MB  R3T 1T4 Canada 
direct 204 453 3339 x373
[log in to unmask]
www.parker.com/ecd







From:
R Sedlak <[log in to unmask]>
To:
<[log in to unmask]>
Date:
08-09-2012 03:50 PM
Subject:
Re: [TN] SMOBC process
Sent by:
TechNet <[log in to unmask]>



Gerald:
I respond partly because your message is confusing, but then I am easily 
confused these days.
SMOBC means Solder mask over bare Copper, so there should be nothing under 
the solder mask but Copper, no solder, ENIG, etc.  Your message implies 
that the solder under the mask causes problems.
My naive mind suspects that if you are getting mask lift, it may be due to 
moisture turning to steam, and the HASL treatment serves to minimize the 
contained moisture.

My 2.5 cents.
Rudy Sedlak
RD Chemical Company

--- On Thu, 8/9/12, Gerald Bogert <[log in to unmask]> 
wrote:

From: Gerald Bogert <[log in to unmask]>
Subject: [TN] SMOBC process
To: [log in to unmask]
Date: Thursday, August 9, 2012, 6:57 AM

August 9, 2012

 

Some of our OEMs manufacture PWBs using the HASL process and they use
solder mask over bare copper (SMOBC) to prevent re-melting of the
tin/lead from the traces during subsequent assembly level soldering
operations.  If SMOBC is not used, the re-melting of the solder causes
blisters under the solder mask coating which can result in peeling or
flaking of some of the solder mask from the traces.

 

We have one OEM who will be manufacturing an ENIG board with solder
mask.  My question is with this finish, will the gold melt during
assembly soldering to cause the same issue as when using HASL?  Also,
does anybody use SMOBC over ENIG?  My understanding that it can be done
but the results are not cosmetically appealing. 


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