What is Considered Acceptable PCB Rework and Repair?



What is Considered Acceptable PCB Rework and Repair?
An operator attempted to repair three damaged conductors using bus wire overcoated with epoxy. Do you consider this repair acceptable? Is there an IPC spec we should follow? Jim Hall and Phil Zarrow, The Assembly Brothers, address this question.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers. We are here to help you with your process, assembly, and other related questions and problems, and situations. Jim, this is an interesting one, acceptable conductor repair. R.M. writes an operator attempted to repair three damaged conductors using bus wire overcoated with epoxy. Do you consider this repair acceptable? If we start to repair a process over, is there an IPC spec we should follow?

Jim
Let’s define because there is a little picture included. Apparently, what has happened is that on the outer surface of a printed circuit assembly three traces of conductors got damaged. I assume with some sort of scratch or so forth. Now they want to repair them on the board, without throwing the board away. I assume they soldered some wires on there. They said with using bus wire overcoated with epoxy. They put some insulation over there.

You now have your conductors continue to make the connections that they are supposed to on the board and the conductors are insulated, as they would be on a normal board with solder mask. So that is where we are trying to get to. But going back to what I said, the question states do you consider this repair acceptable. It really doesn’t matter. The most important thing is what does your customer consider acceptable? This is so important we can’t emphasize it enough. There may be some IPC specs. Didn’t you have some specs that you thought would include some of this information, Phil?

Phil
Yeah, Jim. I believe 77 11 C / 77 21 C covers this. If it doesn’t, I’m sure we will hear from our readers.

Jim
I would like to give a shout-out to our wonderful readers who are very quick to fill in our lack of knowledge about IPC specs. The reality is that some people use this stuff every day. Which is a good point, repair of damaged printed circuit boards I think is called rework in IPC terms. I can never remember the exact differentiation for IPC of repair versus rework. I think repair means fixing the original and rework means changing and modifying the board. This is done all the time.

There are PCB repair houses that do it extensively. We have worked with Circuit Technology Center in Massachusetts. I have been amazed at the things that they can do, patching broken traces, even down multiple layers into the board with specific techniques. What it all amounts to is going in and replacing the copper and then replacing the dielectric or the insulating material. Obviously, professional places have very standardized techniques.

Phil
But regarding the acceptability of the repair, on the one hand, it is application-driven just like everything else in this industry. So it depends on what kind of environment this board is sitting in, what type of variables are acting on it and from there it is an agreement between you and the customer whether this is acceptable or not. That is really what it comes down to, taking all of those factors into account.

Jim
Right, the absolute question is, is it going to be reliable in the application that I am going to put this board into. But a lot of people in a lot of environments are also concerned about aesthetics and some people just don’t want to have patched wires and things like that on the circuit board, for whatever reason and that is something you need to work out with your customer. But certainly, it can be done.

I wouldn’t comment on anything other than that you have to make good integrity of the copper. If you are attaching the wires, they should be soldered down so that there is good, reliable contact and continuity. Then you need to insulate it with epoxy or any other material that is acceptable to you and your customer, and ultimately to the functioning of the final product.

Phil
I would be kind of curious how the conductors got damaged in the first place. But that is another story for another day. You have been listening to Board Talk. We hope we have provided some guidelines. Certainly, the underlying, fundamental rule as far as repair and rework acceptability goes. Whatever you are soldering and repairing whatever you do, please don’t solder like my brother.

Jim
And please don’t solder like my brother.

Comments

IPC 7711/21 has guidelines that are not absolute. As long as functionality is restored without a compromise to reliability; it's up to the customer to approve of course. A considering factor is wire length and gauge. Any time a wire is added to a board it can act like an antenna, which can alter timing in certain circuits, or cause a board that was FCC approved for emissions to lose that approval due to circuit modifications.
Bradley J Fern, Entrust
Damage to surface conductors, typically caused by a handling issue or rework operation gone bad, is a common type of defect that we repair here at Circuit Technology Center routinely. As Phil indicated, the IPC 7711/7721 is the guidebook for the Repair, Modification, and Rework of Electronic Assemblies; this book contains several procedures that detail repair methods of damaged surface conductors. The IPC procedures all recommend using lengths of flat, pure copper ribbon to restore damaged circuits. The lengths of ribbon, once soldered, get overcoated with a thin layer of high-strength epoxy to seal them to the board surface. Color agents can be blended into the epoxy to match the board solder mask for cosmetic purposes if desired.

While using buss wire to repair the circuits doesn't exactly match 7711/7721 recommended processes, one could certainly argue that this repair should be considered acceptable, as long as functionality has been restored. The end customer of course has the final call.
Bob LePage, Circuit Technology Center, Inc.

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