Jim,
     There is a potential hidden defect related to solder mask webs between 
fine pitch pads.  Not all suppliers and LPI combinations can successfully 
place mask webs between fine pitch pads.  The potential defect starts with 
the adhesion of the webs.   If they are not fully attached to the laminate, 
they will break off during handling and processing.  The broken web pieces 
are so thin and light that they stay on the surface of the board but of 
course in various new locations.  Any that come to rest on SMT pads can 
become a reliability issue.  Just one of these pieces laying within the 
solder area when an SMT component is placed can lead to an open or 
intermittent lead connection.  The web pieces are thick enough to raise the 
component's lead off of the pad.  Since solder mask is designed to repel 
solder, the web fragment does just that, leaving a poor or sometimes open 
connection on that pad.  A visual inspection of the bare boards by a skilled 
inspector can detect this potential.  These poorly attached webs will also 
fail the tape test.  Properly attached webs will always pass the tape test.
       There are suppliers who have LPI processes that can successfully 
provide mask webs between the fine pitch pads.
        Concerning the actual value of the webs, the experience here has 
been with a good paste screen printer, the webs are not necessary.  When 
properly included, the webs are nice to have but they are not needed to 
avoid solder bridging.

        Norm Dill
        [log in to unmask]
 ----------
From: TechNet-request
To: TechNet
Cc: spendow_jim
Subject: Soldermask webs
Date: Wednesday, May 08, 1996 11:49AM

     Some designers make an effort to maintain a web of soldermask between
     the solder pads on SMT devices, where pad spacing permits. When the
     spacing of fine pitch devices preclude the web, it is omitted. I have
     heard that the webs are used to reduce solder bridging.

     Thinking through this issue, I find webs being used where pad spacing
     is large enough so as to make solder bridging unlikely, while webs are
     omitted when the pad spacing is close enough to make solder bridging
     most likely.

     I am interested in any factual data that supports the reason for, or
     the value of, soldermask webs.

     Thank you in advance for your response.

     Jim williams
     [log in to unmask]