I'm having some boards made, and the assembly house needs to do some rework on them that involves adding a couple wires to the bottom of the board. The end user is likely to apply double sided tape to the bottom of the board however and I am concerned about the wires pulling loose if they decide to pull the board off whatever they've stuck it to.
Currently, I use jumper wires for breadboards to make these repairs. They're pretty sturdy. But I assume the board house has their own wire and it's easier/cheaper to use that than to have them cut up jumpers as I did.
I asked the board house if they adhere the wires to the board in any way and they said they don't, but that they're flexible and can do whatever I want.
The problem is... I don't know what I want. Or rather, I have no idea what sorts of adhesives or tape or whatever would be typically used for this sort of repair. Digikey sells some adhesives but I have no idea what the right one would be. Kapton tape springs to mind. I think I've seen that used but I have never used it and don't know how strong it is.
I suppose I could just tell them to tack the wires in place using superglue but that doesn't seem like what the professionals would do.
Hot glue would make a mess of the board. I've used it in the past where I attached wires for battery terminals to a board to prevent them from flexing to the point that they break. I found epoxy works a lot better for that. But it takes time to dry and the mixing nozzles quickly get clogged up.
Is there any way to swerve the issue by routing the wire so that it avoids the flat areas where users are likely to try to stick things onto the board, or is protected by other components on the board?
If not, perhaps you could put a not-very-sticky sticker on the board that covers the vulnerable areas and prevents anything from adhering to the wire. You were looking for somewhere to put that "warranty void if removed" sticker, weren't you?
Perhaps you could add a protective base below the wires so that the user has something proper to put their sticky pads onto. But I've no idea whether a small piece of plastic would blow your budget.
I'm going to be making the repairs myself after all. The manufacturer wanted $10 per board to attach a couple wires (a job which takes me all of two minutes) and they didn't offer any suggestions for how to bond them to the pcb.
We always use the above to secure reworks to our PCB's. It seems to work well for securing fine wires and also SMD resistors if we only have one pad to solder to etc.
I'm a big fan of the hot glue gun way of doing this rework. Unless you are trying to tack down the entire length, I usually put a blob that covers the wire every 3" (Or one in the middle if less). Otherwise I would get the big jars of 5 minute epoxy and mix it yourself, and do all the boards at once. You definitely need to tack down those wires though, or they will break after a while.
I like the Acrylic too, how viscous is it?
Edit: On further thought, you could probably use superglue if you can make sure the wires are pressed against the PCB so there isn't any gap.
mirith:
I like the Acrylic too, how viscous is it?
It's hard to say, personally i think its perfect - small blobs don't spread too far, and the more you apply the further the spread. If that makes sense! I have used this method lots of times, we coat all our PCB's in acrylic conformal coating (same as the bottle but in aerosol format) but any reworks get protected with the bottled stuff first. I use it on loads of other projects for securing things - even seems to work well for strain relief on cables - I use a lot of ribbon cables that don't usually last long if moved about.
in the photo below you can just about see it on the kynar wire and the 0805 resistors on the back of the LCD module to the left
The problem with hot glue is it's often hard to get the tip of the glue gun in close enough to the wires you need to tack. Also, in this instance, the blobs of hot glue would make the underside of the PCB very uneven, and the user needs to be able to mount it on a flat surface with double sided foam tape.
Luckilly, I'm going to do the job myself so I don't need to worry about it anymore. I'll be using breadboard jumper wires, and those are sturdy enough for this to be a non-issue.
This is what it looks like when I do it myself:
The kynar wire and acrylic is an interesting solution though. If the route the wire had to take were more convoluted I might go for that. But these jumpers are really convenient and fairly inexpensive.
I was lucky to have broken out USART1 for serial debugging. Those wires are an SPI bus for my LED drivers which were on the same bus as my DAC. Unfortunately it turned out I could not send data to only one or the other and there was insufficient time between sound samples to update the leds.
conformal coating is also excellent for sealing the entire board once a project is "complete" and requires no further enhancement. it's ideal for ever-changing climates where the board is somewhat exposed to the elements of things like condensation within the cabin of a motor vehicle. a lot, if not all OEM's use these type of coatings on their ECU/ECM/PCM to prevent short circuits from things like moisture attacking the PCB, and it needs removed if one wishes to modify the board! very good products indeed.
Careless:
conformal coating is also excellent for sealing the entire board once a project is "complete" and requires no further enhancement. it's ideal for ever-changing climates where the board is somewhat exposed to the elements of things like condensation within the cabin of a motor vehicle. a lot, if not all OEM's use these type of coatings on their ECU/ECM/PCM to prevent short circuits from things like moisture attacking the PCB, and it needs removed if one wishes to modify the board! very good products indeed.
Pretty much why we use it on our automotive products. The worst thing about it is trying to seal up the connectors when coating using spray. We tried many but found acrylic to be the easiest to use, you can solder through it and it just melts away and then apply more over the reworked area with the brush. It's also easy to remove using IPA in a ultrasonic bath.
We used to use a two part "tackpack" in production very similar to the http://uk.rs-online.com/web/p/surface-mount-silicone-adhesives/1987144/ The ability to accelerate the bonding means that it's set in its final position before the tech goes on to the next board. I have seen techs spray the wire and put the blob on the board when we didn't want the accelerator on the board itself (analog or RF), and other techniques.
The alternative is we used to use a small blob of RTV silicon, it has the advantage that it can be removed with a razor on future rework.
Care should be taken with conformal coating a board, I have seen it creep into electromechanical parts such as socket and switches.
