Affordable board repair epoxy

[HHaase]

Well, not sure where else to turn, so I figured this forum would be the place to go.

I recently got a pinball board in for repair with a burned up transistor, unfortunately in this case i was badly over-fused and after cleaning all the carbon off I'm looking at a transistor sized hole in the board.

So of course this means that I'd need to build back up the base material of the board before having any hope of fixing the electronics issues. Problem is, I can only find a few sources for epoxies and none of them affordable.

Anybody have a suggestion for a source?

-Hans

 

[cr0sh]

Why can't you use some cheap 2-part epoxy (non metal-bearing) for this (along with maybe some fiberglass matting or something similar)?

 

[HHaase]

Probably worth a shot, the board's otherwise a paperweight anyway. Never used the stuff, so I wasn't sure if it was conductive or not.

-Hans

 

[KeepItSimpleStupid]

I'm thinking something like PC-3 or whatever it is. Available at a hardware store. Epoxies in general are non-conductive Conductive ones say so and usually contain silver or carbon.

It might take a while to find, but there are some PC re-work materials that use conductive copper and conductive pads.

Even without the special re-work kits, there is ays copper tape: http://search.digikey.com/us/en/products/1181 X 1/4"/3M1181A-ND/30708 They generally won't be conductive on the sticky side. Swaged eyelets can also be used for repair.

You can use teflon from www.amazonsuppply.com to mold the epoxy to your likling. Teflon (PTFE) will not stick to epoxy unless you use the bondable teflon which is sually brown on one side. I probably have one of these: **broken link removed**

If you need a small piece like 2" x 2" or so, I can probably mail you some. I forget the thickness, but it's probably < 0/010" and is bondable on one side. So you could fasten it to a block of wood if you had to and place the vise, PCB, epoxy, teflon, wood, vise. I would just have to check my stock. PM me.

You could use fiberglass too.

Use a Dremel to shape and/or sand.

Use copper tape to make the traces.

Find eyelets to make hole repairs if need be.

 

[cr0sh]

Probably worth a shot, the board's otherwise a paperweight anyway. Never used the stuff, so I wasn't sure if it was conductive or not.

Some is, some isn't - gotta read the package and do some research. You could also try a fiberglass repair kit (for automobiles/boats), although you'll end up with more leftover than you'll probably need. If you go with just regular epoxy, try to find some loose fiberglass filler (wear gloves and a mask, and maybe eye protection when you work with it); build up a "backer" on the board with a taped up piece of plastic/cardboard with maybe some vaseline or something (to keep the epoxy from sticking to the backer); mix your two part plus filler, then "trowel" it in to the hole. There shouldn't be much shrinkage after curing, but if there is, add another layer. Once completely cured, carefully remove the backer, and you might want to smear another layer of epoxy mix only, overlapping the patch with the pcb. Take your time, and it should work ok (hopefully - I've never done something like this, but it seems like it would be fairly workable). Something else to keep in mind: Go with the long-cure (24 hour) stuff; sure, it will take longer, but it will be a better result, and give you more time to work with it. Good luck!

 

[HHaase]

Sounds like a good plan to work on. I have some molding materials from model building work, such as mold release and various masking materials. I also have some West Marine epoxy resin and fiberglass matting as leftovers from another project..... and I regular use eyelets to repair thru-holes and burned off solder pads. I just never went to this extent of re-building a board that's been this badly burned up before, I normally decline the repairs. I'm just feeling bold today.

When I said "I've never used the stuff", I probably should have been more specific that I've never used fiberglass epoxies and resins for electronics work. I may also do a test batch real quick to check for conductivity.
I'll probably do a thin core first, just to get something to use as a backing, then do the front and rear surface finish individually for better cosmetics. (I wonder if I still have my resin dyes or not, hmmm). Right now I've got all the bad material hacked out of the hole, it's almost big enough to fit a TO-220 transistor through it completely, but not quite.

-Hans

 

[KeepItSimpleStupid]

Instead of starting from scratch, take a look here for lots of ideas: **broken link removed**

Plenty of guides and examples using their materials.

 

[jpanhalt]

Why does the hole have to be filled? Look up "dead-bug" and "Manhattan" construction methods. I would just clean up the hole -- maybe even drill it bigger so it looks like it was meant to be that way -- and construct the missing circuitry in that space.

I don't think you will find an epoxy that will stick well to just the edge of the board, unless you plan on overlapping some glass or kevlar on the board. Then you have made a small project into a bigger one.

John

 

[jpanhalt]

@KISS, I have used their stuff, particularly the rivets, which I bought several years ago. Last time I looked, their prices and minimum orders had increased quite a bit.

John

 

[Nigel Goodwin]

Why does the hole have to be filled? Look up "dead-bug" and "Manhattan" construction methods. I would just clean up the hole -- maybe even drill it bigger so it looks like it was meant to be that way -- and construct the missing circuitry in that space.

Exactly - you don't need to try and repair the hole, just work round it.

For larger holes (depending on exactly what it is) I often solder thick solid copper wire (earth wire out of 2.5mm T&E) across the hole to replace the larger destroyed tracks, and that can be used to support the components.

 

[HHaase]

In this case, I do need to actually build the board back up, as it's for a customer and not just a tinkering project.

Here's a couple photos of the hole in question. As you can see, it's burned out the entire mounting area for one transistor, and the TO-220's on this board are not well supported as it is.
I've already cleaned it back to the point I have clean cloth for fresh epoxy to soak into and bond to, and adding a bit more material is no problem at all. Just using some heavier gauge solid conductor won't be enough in this case.

Plus, I like the challenge of this repair, trying to restore it as close to original as possible.

View attachment 64398
View attachment 64399

 

[JimB]

In this case, I do need to actually build the board back up, as it's for a customer and not just a tinkering project.

In that case it all depends on the customers requirements and how much $$$ they want to throw at it.
In most comercial places those boards would be scrap and fixed with a new one.

If the equipment is obsolete and the boards are unobtainium, that changes things.

One technique which I have heard about but never seen done, the burnt section was cut out using saw, files or W.H.Y. and a close fitting insert for the resulting hole was made from blank fibreglass circuit board (no copper). The insert was then held in with normal epoxy resin.
Then the circuit connections were made good using wire wrap type wire.

How to mount the replacement transistor?
Maybe drill holes in the inserted section and press in "Vero-Pins" and solder the transistor to the pins.

JimB

 

[jpanhalt]

Nice cleaning job. When you need to make "butt" joint, it is best to taper both sides to convert it to a scarf/lap joint. You have already got a nice taper on the original board. I would take a piece of FR4 PCB, taper it with a bench sander so it fits almost flush, then just use any bonding epoxy. As this is not structurally critical, I would suggest any 1:1 mix that is relatively fluid so it will soak into the fibers. You can add Cabosil (fumed silica) as a thickener to keep it from running out of the joint and to act as a filler. You will only need a thimble full of the silica. I would stay away from the really fast setting epoxies, as a 30-minute or longer setting one will allow more soak and working time.
Surprisingly little epoxy goes a long way.

John

Oops, I write too slow. John

 

[HHaase]

I ended up using some fiberglass cloth that I had on-hand, using the existing taper from when I cleaned it all up, and put a small layer of cloth on both the front and back sides, and added some random loose fibers on top of that as well.

Used the west-marine epoxy resin that I had on hand, which is around a 12 hour cure time. It's still a bit soft for finish sanding though, I'll work on that tomorrow, and get some more photos then.

-Hans

 

[cr0sh]

I'm wondering if you shouldn't replace that resistor pack (?) above where it burned, as that part probably saw some heat and may have been damaged or derated in some fashion...? Restoring that one (?) track to the transistor will be a challenge, but some thick wire and a hefty amount of solder should work ok. Please post more photos as you progress...

 

[HHaase]

The resistor pack took a lot of carbon, but doesn't look like it took so much heat, and it checked ok with the meter.
I'm not totally happy with the final result on this one, but it's done, and I have a new technique that I want to keep improving on.
The fiberglass part wasn't too hard at all, and eyelets are eyelets, I use them once in a while. Though I do want to improve my accuracy when drilling.

The part I wasn't anywhere near happy with was the new traces I cut out. They just look like crap, I think I'll look into those photoetched ones to make life easier. IIRC some of them also have heat activated adhesive on them too.

I'm also too lazy to post it all again, so I'm just going to link to my blog instead, it's been a long day.

**broken link removed**

 

[cr0sh]

The resistor pack took a lot of carbon, but doesn't look like it took so much heat, and it checked ok with the meter.
I'm not totally happy with the final result on this one, but it's done, and I have a new technique that I want to keep improving on.
The fiberglass part wasn't too hard at all, and eyelets are eyelets, I use them once in a while. Though I do want to improve my accuracy when drilling.

The part I wasn't anywhere near happy with was the new traces I cut out. They just look like crap, I think I'll look into those photoetched ones to make life easier. IIRC some of them also have heat activated adhesive on them too.

I'm also too lazy to post it all again, so I'm just going to link to my blog instead, it's been a long day.

**broken link removed**

Considering what you started with, I think it turned out quite well!