PCB rework

Den tirsdag den 27. oktober 2015 kl. 01.53.31 UTC+1 skrev krw:

quite few ideas here:

-Lasse

Well, you need a soldering iron with a needle-fine point. You don't say the pitch of the BGA. I have tacked #30 AWG wire wrap wire to the top of leads on 0.4mm pitch quad flat packs, but it takes a steady hand and a microscope. I suspect you could do the same to the ball, but you'd have to be careful to not blob two balls together.

Jon

Only that few to fix, silver ink pen ?

Years ago I had to to a board repair on one trace and I used that copper expoxy designed for repairing the defroster wire on your windows. It worked but I wouldn't suggest it as a production fix!

Jamie

Yes, there are some really good ideas there. Thanks! Of the alternatives, I like the self-stick copper trace repair kits. I may get some of them anyway.

I was hoping there was someone who does this sort of thing. Our CM can replace BGAs reliably (or so they say). I can probably get new parts from the manufacturer without too much trouble so we won't have to mess with damaged balls or reball the parts. I'll probably try slipping a wire under the package to hit the ball but I'm pretty sure a #30 wire won't even come close to fitting under the package. I don't have anything smaller, either. We do have an XRAY machine so I can see what I've done.

It's only .8mm pitch but getting a wire under the package is the problem. #30 wire is .255mm, so it might work if it fits under.

Thought of that. Just looking for all the good ideas before trying a few.

Pretty big stuff and I don't know how it would survive reflow.

Den tirsdag den 27. oktober 2015 kl. 03.34.39 UTC+1 skrev krw:

you should be able to find much thinner wire, like a single strand from a multi strand or some magnet wire

-Lasse

The contract assemblers I use are very good at repairing PCB traces. They use the self adhesive foil with good results. So it can be done, I just don't know how much practice it takes or what equipment is required. When I say equipment, I mean I don't know if their $5000 magnifiers are essential or just very useful. I know I couldn't do it with the optics I have available if at all.

Why not let your assemblers take care of it?

I was looking for magnet wire online and saw 0.1mm polyester insulated wire suggested for use in trace repair on mobile phones and PCs.

I generally use 200um for mechanically robust components and 120um for more flexible bodge wires so that the bending moment of the wire doesn't break surface mount parts, but it is still strong enough to have some resilience againse accidental snapping. If you need thinner wire in small quantities with very easy availability, then un-winding a 24V or

Here is a (rather fragile) Cypress FX2 board that I used as a USB2 logic analyser with sigrok:

It pleases me that given sufficient patience, using magnet wire I can exceed the interconnect density of just about any commercial PCB or flex-pcb. I have never managed to re-solder any broken bond-wires on a bare die, (though I know someone who has).

Chris

Can you drill a tiny hole from the bottom side of the board, through the pad you need to connect? If so, wire size is not so big a factor in getting "under" the chip. Glue wire on bottom side, so it doesn't fall out of the hole when you apply heat...maybe?

Reminds me of a white wire fix made to a PGA package many years ago. Not only did we have to drill the board, we had to put a sleeve on the pin after soldering to keep it from touching the edge of the hole where the power plane was. I think this was Vcc pins connected to the ground plane, lots of room for sparks, lol.

Not sure how you would solder the wire through the hole to the BGA though. I guess the iron can come from the side and wire from the bottom?

I think it would be easier paste the board, stick a thin wire on the pad, maybe tack it to the board with some super glue, and then place and solder as normal

-Lasse

We always bring a few unused balls out to test points, for situations like this one.

If you need to pull down a specific pin, doesn't it go somewhere else on the board? Or is it some unconnected configuration mode pin or something?

The problem is the BGA. They're afraid of any foreign material under the BGA interfering with the self-alignment.

They're willing to try sneaking a wire under there. I'll probably try one myself before taking it to them.

I've had trouble soldering magnet wire in the past. I ordered some #32 solid and #32 seven-strand (7/40) wire from DigiKey today. The #30 wire does fit under but it's too stiff.

That would be my last choice. We've done it when there was no choice but there are too many things that can go wrong.

It's a bandgap reference pin for the USB port (driver I assume). It needs a resistor and cap to ground.

I bring all the unused I/O to (non-placed) 0402 resistors but this pin got forgotten. The datasheet gives some indication what it is (a reference) but absolutely no information about what to do with it. I'd just put the problem off until the final documentation came out (it still hasn't) and forgot about it when I released the board. Their answer today was to point me to a different product's datasheet. "Oh, that's obvious. "

Den onsdag den 28. oktober 2015 kl. 01.08.53 UTC+1 skrev krw:

I guess I should have said enameled wire, we have spools from I think 0.05mm and up insulation burns off when you dip it in molten solder on the tip of an iron

-Lasse