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Soldering bypass caps to QFP possible?

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 ;-)

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg
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Sometimes you just have to say NO.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Yep, this one might just not be possible.

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
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.....

Cheers Terry

Reply to
Terry Given

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 ...

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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

Reply to
John Larkin

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 :-(

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

YIKES!

What WERE they thinking?

Definitely make a small pc board to glue on top of the chip, with little wires hanging down, to solder to the ic pins.

Sort of like this one:

ftp://66.117.156.8/883A.jpg

John

Reply to
John Larkin

Use 0204 format caps. For ex:

formatting link
?name=490-4310-2-ND

Plus they'll be much easier to solder.

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...

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Thanks,
Fred.
Reply to
Fred Bartoli

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

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Best Regards:
                     Baron.
Reply to
Baron

You have your own R&D plus manufacturing so designs have to exceed the Larkin quality threshold before they go into production. So you know it's all good. But when you are consulting stuff like this is a fairly common thing you'd see :-(

Sometimes I have clients who want a new design from scratch. Others come with existing ones that plain don't work. Consulting is like architects and building contractors in one person. We get to design/build someone's dream home but we also have to restore flood or fire damage. In many cases we have to serve a company specialized in their field but not electronics. And if problems creep up they often don't after some substantial field exposure. For example storms only happen during a very short season and the usual ESD pistol tests at TUEV or UL won't reveal too many problems up front (it passed all those).

I might have to :-(

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Yes. I want to also try that with a 01005 cap since it would nicely "ride" the gap. Some paste, some flux, then heat.

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

The top is full of traces but there aren't any useful GND vias anywhere near anyhow.

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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