Steve,
The dryfilm soldermask as tenting is old technology. I am surprised your pad
height is not greater than .003. Lackwerke Peters, DuPont and several others
make a tenting paste that eliminates pad height problems but still gives you
a tented hole. Your supplier may not be aware/experienced with the process.
Tony Steinke
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey (EHCOE)
Sent: Wednesday, June 17, 2009 12:45 PM
To: [log in to unmask]
Subject: Re: [TN] SolderMask Question
This is known as tenting. Usually the final height will be below 0.003,
but no one will want to be held to that limit. If there is a functional
reason you are incorporating a via protection strategy, then there are
only 3 accepted scenarios that will pass a Class 3 or 3/A requirement
for long term reliability in an Aerospace or high-rel environment. Their
descriptions can be found in IPC-4761 and the 3 types are as follows:
* Type II-b
* Type VI-b
* Type VII
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vargas, Stephen M
Sent: Wednesday, June 17, 2009 9:22 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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