Charles,
Does it appear that the leads are popping up during reflow or
afterwards? If so, it may be popcorning. We have seen this in the past
on .5 mm pitch 100 pin QFP's. Have some parts evaluated using acoustic
microscopy to determine whether they experienced delamination.
If not, it may be the lack of filleting on the solder joint. Look for
any physical representation in the joint which would indicate
mechanical failure after reflow. Increasing the pad dimensions will
help if this is the root cause, but you may have to gang mask the
resist. If so, use caution as your solder apertures may also have to
change.
Hope this help,
Allen Hertz
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(954) 846-5829
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Subject: Pins popping up
Author: [log in to unmask] at ftl03
Date: 11/12/96 11:20 PM
Technet:
I have a board about 13.5" x 4.5" (PC plug in card), .062" thick,
8 layer, FR4, 1 oz copper, 2 power planes, with 16 100 pin TQFP
packages (0.5mm lead pitch) spread across the board (2 rows of 8), as
well as many other PLCC and SOIC devices.
The problem we are having with these prototypes is that some pins
on the TQFP's pop up durring handling. The boards seem to be too
flexable, which probably causes the pins to break off the pads. Is there
a way to make the board more rigid besides using board stiffiners? In
other words can anything be done as far as changing the board stack up,
or will using thicker copper help this situation? Or are there different
types of FR4 available? Or is there something I am not looking at?
Also, what would be a good pad size for the 100 pin TQFP chip?
Currently I am using 1.2mm x .25mm, but I think the IPC recommends 1.6mm
x .35mm, but it doesn't look like it would allow any soldermask between
pads. Or does anybody even bother with soldermask between pads at .5mm
pitch? What is the middle ground?
Thanks in advance!
Charles Wills
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