Re: Soldering bypass caps to QFP possible?

On Sat, 26 Jan 2008 00:48:38 GMT, in sci.electronics.design, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> bloviated:

Ben Jackson wrote:
On 2008-01-25, Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:
Is it reasonably possible to solder a bypass cap to adjacent pins on a
0.5mm pitch QFP chip or would that just open up a can of worms?

I've reworked commercial FPGA prototype boards with QFP packages by
soldering 30ga wire to pins (including adjacent pins) but it's a pain.

If you just need bypassing you are probably better off straddling smt
caps between nearby vias (package size chosen to match your vias!). It's
not necessary to put them on the pins -- most QFP designs I've seen have
the bypassing on the opposite side of the board anyway. And keep in
mind that the bondout wire from the die to the pin is going to be longer
than the distance to your cap...


Except that there aren't any vias. The next one is about 1-1/2 inches
away and that's also where the lone bypass cap dwelleth. Ground is
almost as long and takes a different path :-(

Through hole caps, cut & solder the leads to the pins, small amount of
sealant or double stick tape to adhere the body of the cap to the top
of the QFP.

It is slow and painful process but quite doable.

What type of Conformal coating was used? Some can be removed in small
sections with a glass bead airgun.



.

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