Sometimes co-planarity problems are actually paste slump or hot slump
problems.

Regards

Mike Fenner
Indium Corporation



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Russ Nowland
Sent: Thursday, March 24, 2005 2:37 PM
To: [log in to unmask]
Subject: [TN] QFP w/heatspreader co-planarity


Hello,

I have a 44 pin .8mm pitch QFP which one would typically think no big
deal.  This device however have the heat slug underneath the device to
dissipate the heat into the PWB.  This slug has to be soldered but to keep
the device from floating we quarter it to do a ~60% reduction on the pad
area.  The problem is we occasionally get several leads with the foot-in-
the-mud defects.  Considering all of the other pins wetted well and only a
couple pins are sitting on top of the solder my experiences tell me co-
planarity.  We talked to the device manufacturer and they claim the parts
are checked prior to being packaged in trays and should be good.  We sent
two batch to a programming house to have the leads checked and they failed
~2%.  At this point the device manufacturer is disputing/verifying the
failures.

My question is:  Does anybody know what industry standard for lead co-
planarity applies to this type of device?

Everybody wants to use the a standard 4 mils but no one can tell me where
that requirement is documented.  My other concern is the fact that the
bottom of the device and not the leads controls the seating plane.  I am
told that the 4 mils is based on the lowest and highest plane on the
leads.  I did not get the feeling the bottom of the device was taken into
account.  Even if it was I am concerned that if the leads are above the
seating plane of the body the leads my not fully contact the paste and end
up sitting on top.

Any clues?

Thanks in advanced,

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