In a message dated 12/23/98 5:59:31 AM Pacific Standard Time,
[log in to unmask] writes:
<< Dear Technetters,
I am having a problem with SMT adhesive sticking to some types
of solder mask. Does someone out there have experience with using SMT
adhesive on glossy boards vs. matte finish boards. I am using Loctite
3609 or Epibond 7275 or Epibond 7275-1 with a cure cycle of 145-150
degrees centigrade for 4-5 minutes. Is there an SMT adhesive or cure
cycle that works better with glossy boards? We seem to have a problem
with adhesion at room temperature and at wave solder temperatures. We
run a standard wave solder profile and have even increased the wave
speed to 5.75 feet/minute from our norm of 4.5-5.0 feet/minute. This
helps but we run the risk of voids and poor solder connections on some
components.
If I cure the boards and then use a 'push' tester to measure the force
required to remove the 'glued' components, is there a standard for
minimum and maximum expected force for removal? I have heard of some
people using a custom socket and a torque wrench to measure the strength
of the cured adhesive. Is there a standard specification for this test
method? Which test method do you think is best? The tests mentioned
above are done at room temperature. Is there a way to measure the 'hot'
strength of the adhesive bond?
Thanks in advance.
Wade Oberle>>
Hi Wade!
To tell you the truth, I've never measured the force that the epoxy
provides to a part after curing...about as far as I've gone is to wait until
the board has cooled from curing and then taken a pair of tweezers and (I have
a pair of fine pointed ones I carry with me all the time) and tried to torque
the part off...I can't get much force on the part without bending the tips of
my tweezers, but if passes that test, I've never had a problem with losing
parts in wave...and that "test" if you will, really doesn't torque the part
that much, but goes to show that a lot of strength really isn't required for
wave.
The times that I have lost parts in the wave have been caused by three things:
1. Incomplete cure. On the thicker, denser boards, where I needed to slow the
conveyer down to allow everything to reach cure temps and ensure that it was
there for the time recommended by the epoxy vendor.
2. Old epoxy. The epoxy had reached, or was very close to the shelf life date
on the tube...at almost $300 a tube, it's somewhat understandable that those
dates can be pushed to the limit sometimes...
3. Board bowing across the wave and actually scraping the bottom on the wave
former in
the pot...this has happened to me a time or two when I've had to wave boards
without a fixture, and they won't fit right in a universal fixture (Proto's,
you know the one'sies and two'sies...)
This may be a dumb question, but are you SURE you're losing them in the wave?
The reason I ask, is because there has been many times that I've had QA come
to me and tell me that we're losing parts in the wave because they're missing
when they look at them after wave, and they've inspected them after cure in
SMT and know the parts were there and cured good, so it's assumed the parts
came off in the wave. But actually they we're knocked off being handled while
masking and stuffing the thru-hole.
The locations where you have parts missing, is there any epoxy remaining on
the PCB? Or is it wiped slick like none was ever there? If you see some cured
epoxy there, then it's not the mask. That's also something that I've not
experienced, I've used both loctite and Ciba-Geigy epoxy (the same formulas as
you have) and never have had a problem with different mask types. There may be
others out there that can testify to the same thing.
I hope this may help a bit...
-Steve Gregory-
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