Soldering bypass caps to QFP possible?

[Joerg]

Ok guys, got a client's system here, to see what can be done to save the
day. Among other (easier) things I discovered a serious layout booboo.
The usual, opened up the Gerbers and my toe nails curled. Too late for a
relayout at this point, must be reworked :-(

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? Shorts,
opens, etc. I mean, can a good rework house do that reliably, in your
experience (not by what they'd tell me)? To throw another curve, these
boards are clear-coated.

And no, I did not do that layout ;-)

 

[Ben Jackson]

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...

 

[Spehro Pefhany]

Ok guys, got a client's system here, to see what can be done to save the
day. Among other (easier) things I discovered a serious layout booboo.
The usual, opened up the Gerbers and my toe nails curled. Too late for a
relayout at this point, must be reworked :-(

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? Shorts,
opens, etc. I mean, can a good rework house do that reliably, in your
experience (not by what they'd tell me)? To throw another curve, these
boards are clear-coated.

And no, I did not do that layout ;-)

Sometimes you just have to say NO.


Best regards,
Spehro Pefhany

 

[Joerg]

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 :-(

 

[Joerg]

Sometimes you just have to say NO.

Yep, this one might just not be possible.

 

[Terry Given]

Ok guys, got a client's system here, to see what can be done to save the
day. Among other (easier) things I discovered a serious layout booboo.
The usual, opened up the Gerbers and my toe nails curled. Too late for a
relayout at this point, must be reworked :-(

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? Shorts,
opens, etc. I mean, can a good rework house do that reliably, in your
experience (not by what they'd tell me)? To throw another curve, these
boards are clear-coated.

And no, I did not do that layout ;-)

Hi Joerg,

not entirely related, but at the turn of the century I visited a CM in
HN. They showed me one of their workers (a nice Chinese lady) adding a
wire to a 512 pin BGA. expensive chip, 12 layer PCB, trace left off.
Apparently she was the only staff member who could do it.....

Cheers
Terry

 

[Joerg]

Hi Joerg,

not entirely related, but at the turn of the century I visited a CM in
HN. They showed me one of their workers (a nice Chinese lady) adding a
wire to a 512 pin BGA. expensive chip, 12 layer PCB, trace left off.
Apparently she was the only staff member who could do it.....

That's exactly what I want to avoid. Like in a company where I worked as
a student to make some money. Only Antonio could weld those delicate
aluminum pieces. Everyone else who tried just ended up making holes.
Then one day Antonio and his wife decided it was now time to move back
to Spain and nothing could convince them to stay ...

 

[Joel Koltner]

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.

---Joel

 

[Joerg]

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.

 

[John Larkin]

Ok guys, got a client's system here, to see what can be done to save the
day. Among other (easier) things I discovered a serious layout booboo.
The usual, opened up the Gerbers and my toe nails curled. Too late for a
relayout at this point, must be reworked :-(

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? Shorts,
opens, etc. I mean, can a good rework house do that reliably, in your
experience (not by what they'd tell me)? To throw another curve, these
boards are clear-coated.

And no, I did not do that layout ;-)

Does it need bypass caps? For emi compliance, or just to work?

Can't they be kluged on the board somewhere?

I'm guessing that some skilled person could solder 0201 caps across
those pins. I think maybe even I could. I've been soldering some
really outrageously tiny stuff this week. The coating would have to be
scraped off, of course.

You could certainly solder wires and run them up to 0402's right on
top of the chip.

How many boards did they build?

John

 

[John Larkin]

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.

John

 

[Joerg]

Does it need bypass caps? For emi compliance, or just to work?

Both, but mostly because the micro often dies during thunderstorms.

Can't they be kluged on the board somewhere?

Nope. The next pair of vias and the lone bypass cap is two furlongs down
the fence.

I'm guessing that some skilled person could solder 0201 caps across
those pins. I think maybe even I could. I've been soldering some
really outrageously tiny stuff this week. The coating would have to be
scraped off, of course.

Possibly, although 01005 may be better in order for the solder to wick
up properly. But I'd need some serious capacitance there, at least
0.1uF. 0201 comes in that capacitance range and 6.3V is enough, even 4V
would.

You could certainly solder wires and run them up to 0402's right on
top of the chip.

Maybe we have to, as long as the reowrk shops are able to.

How many boards did they build?

About 2500 :-(

 

[Joerg]

Copper tape will stick forever.


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.

 

[John Popelish]

Copper tape will stick forever.


Make a tiny PCB that has bypass caps, and glue it on top of the chip,
and drop down ground and vcc wires.

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.

 

[Joel Koltner]

Both, but mostly because the micro often dies during thunderstorms.

<< chuckle >> And here I thought a PLL losing sync when someone touches it
with their fingers was bad...

(We have some synthesizer ICs from a well-known manufacturer that exhibit spur
levels anywhere from about 0 to -60dBc and we're at the point where we're
about 99% convinced that we're getting bad ICs... a major part of the
indictment being that, when you put the PFD's output into tri-state, the VCO
runs off like a rabbit from a dog rather than just drifting away relatively
slow as you'd expect...)

 

[Fred Bartoli]

Joerg a écrit :

Both, but mostly because the micro often dies during thunderstorms.



Nope. The next pair of vias and the lone bypass cap is two furlongs down
the fence.



Possibly, although 01005 may be better in order for the solder to wick
up properly. But I'd need some serious capacitance there, at least
0.1uF. 0201 comes in that capacitance range and 6.3V is enough, even 4V
would.

Use 0204 format caps.
For ex:
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=490-4310-2-ND

Plus they'll be much easier to solder.

Maybe we have to, as long as the reowrk shops are able to.



About 2500 :-(

Ouch!
I never tried it but maybe this works:
- just scratch the varnish on the pins top surface (let it stay between
the pins, as a regular solder mask would do on a PCB)
- apply a *tiny* blob of solder paste across both pins
- pick the cap and roughly center it on the blob
- gently hot air reflow all this.

-see what happened...

 

[dalai lamah]

Un bel giorno Joerg digitò:

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?

If the pin are adjacent, it should be quite easy and clean if you use hot
air and very small capacitors.

Anyway, if you really need 0.1 uF caps, it's not necessary to solder them
directly on the pins. It's enough to solder them below 2-3 cm from the
chip. There is a very nice application note on the Xilinx site about bypass
caps:

http://www.xilinx.com/support/documentation/application_notes/xapp623.pdf

 

[Leon]

Un bel giorno Joerg digitò:


If the pin are adjacent, it should be quite easy and clean if you use hot
air and very small capacitors.

Anyway, if you really need 0.1 uF caps, it's not necessary to solder them
directly on the pins. It's enough to solder them below 2-3 cm from the
chip. There is a very nice application note on the Xilinx site about bypass
caps:

http://www.xilinx.com/support/documentation/application_notes/xapp623...

There used to be a company here in the UK called Precison Repair
Services that could probably fix them, but they don't seem to be
around any more. They've done some comparatively simple rework for me
in the past, but some of the things they used to do were amazing.

Leon




Leon

 

[Baron]

Both, but mostly because the micro often dies during thunderstorms.



Nope. The next pair of vias and the lone bypass cap is two furlongs
down the fence.



Possibly, although 01005 may be better in order for the solder to wick
up properly. But I'd need some serious capacitance there, at least
0.1uF. 0201 comes in that capacitance range and 6.3V is enough, even
4V would.



Maybe we have to, as long as the reowrk shops are able to.



About 2500 :-(

I don't know if you have any space where you can go right through the
board and put caps in the holes. I've done this with decoupling caps
on microwave amps.

HTH

 

[Joerg]

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.