Re: Soldering bypass caps to QFP possible?

Spehro Pefhany wrote:
On Sat, 26 Jan 2008 08:56:37 -0800, the renowned Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Sat, 26 Jan 2008 01:58:57 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Sat, 26 Jan 2008 01:00:17 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Joel Koltner wrote:
"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message news:Grvmj.664$R84.277@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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 :-(
Something I've done in the past is to make cover the top of the chip in solid copper foil tape, soldering the outer tape edges to all the ground pins you have available (using 30 ga. wire). Then, solder a cap to this new ground plane on one side and run a 30 ga. wire from the other side down the few mm to the power pins (or any other pin you're looking to bypass).

This also makes for an excellent ground point if you're using, e.g., FET probes with a ground "leg" that needs to be within a few cm of the probe tip because you actually want 1GHz response or whatever.

I thought about a copper tape lash-up. Unfortunately this stuff is used in a rather hot and humid climate and I am afraid the glue on that tape won't hold up.
Copper tape will stick forever.

But the main issue is soldering to those 0.5mm pitch pins.
Make a tiny PCB that has bypass caps, and glue it on top of the chip,
and drop down ground and vcc wires.

That's a good idea, could be epoxied down. The wires would be a royal pain in the neck though. I know catheter assemblers who could do that but they are all cranking overtime as it is because there aren't enough of them. Pretty much all my clients with catheter manufacturing have permanent ads out.
Really, it wouldn't be hard to solder wires to those pins.

Hey, how about this:

ftp://66.117.156.8/Break2.jpg

ftp://66.117.156.8/OnBoard.jpg

Some Maxim SO-8 comparators started failing, so we made a pc board
with a different, smaller part on it, plus some schottky diodes and
resistors. It solders down into the SO-8 footprint. That saved roughly
a megabuck worth of boards.

Nice! Unfortunately this already is a fine-pitch part, a 100-QFP with 0.5mm pitch. To avoid this capacitor stuff I could also muffle things from an ESD/RFI point of view if the front panel overlay in front of a large LCD was conductive. But it ain't. Placing another (conductive) film below would work but its glue side would face up and eventually stick to the current overlay. IME that turns yucky really quick.

Looks like one of those up the creek situations.

I would have no problem with doing one or a few for field tests, but
actually shipping a whole bunch of those things with that kind of
rework.. especially if the environment is not benign in the extreme
(which the conformal coating would tend to indicate is NOT the case)..
they should evaluate the cost per failure (from this point in time,
water's already under the bridge) compared to redoing the boards
salvaging whatever makes sense. The end users may be quite patient
with a sure fix, but if they are shipped something else, which then
fails, it may be the last time your client hears from them (maybe
indirectly they might hear from their legal dept, but probablyh not
AP). That's more of a business decision.. cost and risk management.


It's not just that, also turn-around time. A re-layout and new run realistically takes many weeks. Rework is actually quite common. When I took apart an aircraft (!) radio from a mainstream manufacturer in that market I was initally shocked by the amount of rework in there. Especially when considering the production volume. Then a service tech told me that this is quite "normal".


If you absolutely have to-- maybe strip the conformal coating, glue an
0805 or 0603 part to the top with a similar kind of cement as is used
for gluing SMT parts to boards (Locktite XXX) and run a couple of fine
wires (with some give in them) a few mm to the pins. Inspect the hell
out of it, then conformally coat again to glob the wires and cap down.
It still won't be as good as having a proper layout.


Either that or a 01005 cap soldered directly.

--
Regards, Joerg

http://www.analogconsultants.com/
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