> 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 :-(
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. But the main issue is soldering to those 0.5mm pitch pins.
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.
If heroic measures are called for, you might use welded or ultrasonically bonded wires to tie the cap to the pins. I'm picturing some of the wired stuff I have seen in hybrid (chips wire bonded to surrounding traces) construction.
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.
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.
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.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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".
I agree. Fixing the layout is the best thing to do. 2500 boards are peanuts, unless they are big mult-layers boards.
The next best thing to do is to solder an identical QFP lead-frame on top of it, with the passive components custom bonded to the lead- frame. How many pins are there for the existing QFP?
We can build the custom QFP for you. You don't even have to send us your boards.
I was hoping for 48 pins, since we are ordering that. I'll check if they have 100 pins in stock. If not, usually they need to make 10,000 pcs (any pins multiple of 4). With some arm twistings, they might do
5,000.
Just one cap? Or do you want to add more parts in it.
Well, I am really up the creek now. The cap helps but not a lot (because they also split the ground off). The next available node for either GND or VCC is miles away :-(
On Sat, 26 Jan 2008 00:48:38 GMT, in sci.electronics.design, Joerg bloviated:
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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