What is the typical adhesion factor (Bonding Strength) for soldermask to
the bare copper surface?
Will there be a difference in the shear factor number?

I understand that the industry standard for adhesive bonding is 4 inch
ounces. How would this number affect the bonding strength of the soldermask
to the bare copper when this amount of torque is applied. I know that there
is a tape pull test in IPC-TM-650 for checking soldermask adhesion. I do
not think that the numbers garnered from that test is applicable in this
case, or is it?

Now I will attempt to tell what we are trying to achieve here. We are
applying a torque test on our chips after they have been glued to the PCB.
What we found in our first set of tests was that we were seeing 5-6 inch
ounces of torque being applied to the chip on a matte finished soldermasked
board before it would tear from the board. On some Glossy finished
soldermasked boards from a different vendor we were seeing 3-4 inch ounces
of torque before the chip would tear away from PCB. On the glossy finished
boards we were seeing chips falling off the boards prior to wave soldering
or IR soldering depending on what process was called out.  We made a hasty
decision to have the vendor change his process to give us a more matte
finish on the PCB's. After the change we found that the numbers had gone
down to 2 and sometime below 2 inch ounces. We also were seeing that the
soldermask itself was coming off the boards now when looked at under a
microscope you could see tiny pieces of soldermask attached to the glue
dot. When the board itself was check you could see a shinny pink dot where
the mask was. We went back to the vendor and they checked their process and
made some adjustments we believe will help this situation. After getting
our incoming inspection manager, wave solder/adhesive supervisor and PCB
Design together the thought occurred to me that it was possible that we
were doing a test that was outside the normal process window of the
soldermask itself.

I have sent specifications of the locktite adhesive to our vendors
soldermask supplier to see if there was any chance that the adhesive might
be breaking down the soldermask in some fashion but that was negative. the
vendor has replaced some pre soldermask scrubber brushes and checked his
curing ovens also.

Has anyone else encountered similar opportunities in the past and if so
what were the conclusions?

Steve Collins
PCB Design Supervisor

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