Jim,
In addition to some of the other responses,
I agree with those who say use elongated stencil apertures over the solder
mask, this has worked very well for me in the past, and most of the newer pastes
I've tried this with have fairly descent "wick-back" properties.
After doing the calculations per Phil's directions, I would suggest using a
test pattern and trying apertures larger then the calculated, to optimize paste
volume and to learn where the max size is, since there is a fine line that will
start to produce solder balls with too much paste.
I would also suggest slowing down the print speed and increasing pressure
slightly to help fill the holes (assuming metal blades, rubber blades you will
be defeating the purpose (scoop-out)). Having the blades at an angle <45 degrees
has also shown improvement, but you may not need to go to this extreme. You have
the right ingredients with minimized lead protrusion. If the board is .062"
thick, you should be able to achieve good results. Of course, the thicker the
board, the more paste volume needed and the tougher to get 100% fillets.
Good Luck
Jeff Hempton
United Technologies Electronic Controls
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Subject: [TN] Pin-in-paste for 50 mil part
Author: Jim Gryga <[log in to unmask]> at Internet
Date: 12/15/98 2:03 PM
Hey Techhies,
Is there any body out there using pin-in-paste (intrusive reflow, whatever
you want to call it) on parts with a lead pitch of 50 mils? I have a 38
pin and 30 pin 50 mil pitch through hole right angle header I need to
solder on a double reflow board. I can't slelective wave solder because
they sit on right angles to one another (I'm sure one would have solder
bridges), I don't want to hand solder because of the added cost, and I
prefer not to use a fountain solder (we've had bridging and insufficient
hole fill fountain soldering other 50 mil connectors). The pth diameter is
about 0.022". The leads are 0.015" square. There is minimal lead
protrusion through the board, about 0.025". We've done some experimental
p-i-p boards with a 6 mil stencil but only acheive acceptable (75%) hole
fill on about 50% of the connections. We also tried 0.025" holes with
worse results. I have 20 mil QFP's on the same side as the connectors so
I'm hesitant to go with a thicker stencil. Next step will be to try a step
down stencil.
If you have any experience or recommendations on stencil aperture
configurations, stencil technology, paste types, etc., etc, I'm all ears.
Jim Gryga
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