A Look Into PCBA Rework and Repair

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“On a through-hole board, that’s certainly the most common problem that we have in terms of soldering,” continues Freedman. “The barrel fill is always an issue. I dealt with very thick boards for a very long time; server boards and telecom boards that are well in excess of 0.093”. They’re just a bear to repair in all regards. The first thing that is spotted generally for repair is the through-hole solder joint itself. We see nice barrel fill on the signals, of course, and marginal solder joints on powers and grounds. Those are always seeing touches. We have operators calling that out because of either visual or X-ray results, and trying to fill things that are already more than strong enough to last the lifetime of the product.”

Laura Ripoli_9Sep17.jpgRipoli says they quite often find that people who are trying to remove things from the board—especially bottom termination components—and don’t have the proper equipment to do that, tend to rip up pads on the boards. “It ends up in our hands to do pad repairs as well as placing that BGA back down. We have people with over 20 years of experience dealing with those types of repairs,” she notes.

The majority of BEST’s rework and repair involves bottom termination components, says Patten.

“More so than the through-hole,” adds Ripoli. “Definitely, the QFNs, LGAs, BGAs are a large percentage of the work we do. We see a lot of those coming in with excessive solder voiding or solder shorts.”

Price agrees that bottom termination devices are the most prevalent rework requirement. “Often, what drives larger quantities of component rework, is when there’s some issue with a component, whether the manufacturer has determined there’s a bad date code on a component that is populated on hundreds or thousands of boards that needs to be reworked, or there are other issues such as mixed chemistry. An example would be a RoHS lead-free component placed into a leaded soldering process, therefore it doesn’t get to proper reflow temperatures. There are a variety of things that drive large quantity rework applications like that. By and large, BGA, LGA and QFN rework is very challenging to anybody. The rework itself is difficult, as is the inspection process. There’s not a lot of trained eyes that can examine and understand and look at an X-ray with certainty when it comes to ‘head in pillow,’ or ‘solder open’ and other defects, it is very challenging.

“I would say that is probably the most challenging rework application out there for the general population of contract manufacturers that are doing rework in-house, especially when you get into the board designs mentioned earlier with increased thickness of boards and tighter spacing of adjacent components. Preventing collateral issues when you’re doing a hot air rework is difficult, the hot air doesn’t want to stay where it is directed, it wants to spread out, and it does. It will spread out through the board and it will spread out to adjacent components and it will create conditions outside of the target zone that is called collateral damage. Overheating adjacent component, reflowing solder joints and other issues can occur. Add the conformal coating element to it and under fill at that particular rework site, well, now you’ve elevated the degree of difficulty to the rework application. It is extremely difficult to have 100% success rate with all of those factors.”

When it comes to solder voiding, Ripoli says there are many things that can contribute to that issue: one being the customer using mixed alloys when assembling a board. “We’ll find evidence of that going on. It could be that the paste is not the right thickness. It could be the profile on the original assembly that’s causing the issue. To fix the problem, it’s back to the customers to figure out what step in their process is causing an issue,” she says.

Dan Patten_9Sep17.jpg“They’re more likely to have the data or the tribal knowledge and the cause of the problem and we might just be able to have the equivalent expertise to solve the problem,” says Patten. “We don’t always get involved in a recommendation. Sometimes, we do and we certainly can, but to learn that entire process for that build and the profiles and so forth doesn’t always go through our hands.”

“We have some customers that work with us while they’re doing their proto stage of assembly,” says Ripoli. “They’ll come back to us and send us a board with connectors and have us measure the solder fill percentage, the X-ray. We’ll give them that feedback, they’ll go back and make changes to their process, send us another batch of boards, and we go through that same process again until they can tweak and find the right solutions to getting that proper fill.”

One of the biggest issues when it comes to rework and repair is the temperature cycles.

“The problem is when you have to rework something, it has to go through four or six cycles of heat or something like that. Then it’s a problem and they didn’t predict that. Nobody predicts reworks on their boards,” says Price.

“Not only that, but you won’t necessarily know that the board has been repaired multiple times before it gets to you,” says Patten.

Most of the time, the rework specialists will never find out how many times a customer—the contract manufacturer/PCB assembler—touched the product before they get it. “It is a big mystery,” says Patten, “and it’s often a cause of unpredictable failures.”

But some PCB assemblers/contract manufacturers do track the number of rework/repair cycle that a board goes through.

“When we get larger quantities of rework or repair, we don’t see that as often; but when we have the one really special custom board, we get that a lot. They tried and they failed, so they give it to us. We don’t know what they’ve tried or how they failed. We don’t get that history usually,” says Patten.

To read the full version of this article, which appeared in the September 2017 issue of SMT Magazine, click here.


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