> Our problem is that at times not only are parts skewed before curing but
> they are missing because the glue dot did not contact the part properly
> (according to our mfg. people pad height is the suspected problem). We use
> Universal GSM equipment for placement.

Hi Steve --
I'll assume that your P&P is calibrated and qualifies for the
repeatability aspect.  One more thing you may want to address as far
as the placement equipment is the pressure used to achieve vacuum and
release the part.  We've had some difficulties with our auto
glue/auto place process due to these pressures "shifting" from the
ideal.  Your placement machine may be calibrated for
placement (coordinates) and be off in its pressure settings.  I've
run sample first run boards (to test a program) on double stick taped
boards, which placed perfectly, only to see problems when placing in
solder paste (due to the pressure settings needing adjustment).

As far as the gluing process, does your gluer deposit "dots" of
epoxy?  We have found that the ideal glue dot size is .025 - .030
diameter.  (The pads on our discretes are about .030 apart).  What we
did to achieve this size dot was take a .030 (OD) syringe and modify
it to fit our gluing equipment (the one that they supplied made a
.050 dot).  For an 0805 component we'd use 1 dot, maybe a 1210 would
have 2 dots across the width of the placement location.  When there
is too much glue, the parts shift.  When there is inadequate glue,
the part may fall away at wave.  How is your gluer as far as
consistancy and small size dot abilities?

Here's one other factor:  How does your glue perform in different
temp's humidity, etc?  We use Loctite and I know they suggest
refridgerating and then removing 24 hrs in advance of use (to allow
it to warm up to room temp).  We have found about 2 hrs after it's
been removed from refridgeration, it performs best.  (Has the
viscosity to allow us to manipulate it to the size dot we want).
Depending on the day (temp,etc), it may perform "well" for us for 4-6
hrs.  When it gets finicky, we get a new tube.  We probably go thru
more, but spend less time trying to adjust.

While your pad height may be a problem, assuming no gross difference
is present pad to pad, I'd look to these issues.

Keep me posted!

Regards,
Lainie Loveless
General Atronics Corp
Wyndmoor, PA
(215) 242-7336

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