This kind of double mask practice is not very common however a good process
if done right. It is much cheaper than epoxy filled (Peters, SAN-EI, etc)
and a lot more reliable than LPI only. Make sure your vendor understand you
mask height requirements and use 1.5 mil version of Dryfilm soldermask
instead of 3 or 4 mils. The size of Dryfilm mask should be smaller than the
pad size. The tenting must be from both sides.

Jason

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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vargas, Stephen M
Sent: Wednesday, June 17, 2009 11:22 AM
To: [log in to unmask]
Subject: [TN] SolderMask Question

All:

 

We have a Class III PCB, that we are building using leaded technology
(non-RoHS), HASL finish, where my customer is specifying double masking
over vias for "via protection". Dry film is initially being placed over
the vias only, then the board is covered by LPI. Our problem is that it
creates problems with the solder stencil not properly gasketing to the
PCB during screen-printing resulting in solder defects. We are seeing
raised solder mask as high as .003". Is this (double masking)
commonplace? Are there more "manufacturing friendly" options for optimum
via protection? I've asked about possibly using dry film mask only (no
LPI on top), but was told that the mask feature sizes are too small for
dry film. Any insight would be appreciated. 

 

Regards,

Steve Vargas

 

Polaris Contract Mfg Inc


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