Issue link: https://iconnect007.uberflip.com/i/869081
September 2017 • SMT Magazine 15 A LOOK INTO PCBA REWORK AND REPAIR and the reliability testing, you'll find that even a 20% solder joint in a thick board has more than enough reliability. I'm not advocating that we go down to 20%, but I'm saying let's get real about what's actually required. I think we need to move out of the middle ages of soldering in terms of through-hole and consider doing more work in terms of modeling and reliability of the through-hole joint specification." "On a through-hole board, that's certain- ly 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 re- gards. 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 op- erators 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." Ripoli says they quite often find that people who are try- ing to remove things from the board—especially bottom ter- mination components—and don't have the proper equip- ment 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 in- volves bottom termination components, says Patten. "More so than the through-hole," adds Rip- oli. "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 void- ing or solder shorts." Price agrees that bottom termination devic- es are the most prevalent rework requirement. "Often, what drives larger quantities of com- ponent rework, is when there's some issue with a component, whether the manufacturer has determined there's a bad date code on a com- ponent that is populated on hundreds or thou- sands of boards that needs to be reworked, or there are other issues such as mixed chemistry. An example would be a RoHS lead-free com- ponent placed into a leaded soldering process, therefore it doesn't get to proper reflow tem- peratures. 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 exam - ine 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 chal- lenging. "I would say that is probably the most chal- lenging rework application out there for the general population of contract manufac- turers that are doing rework in-house, especially when you get into the board designs mentioned ear- lier with increased thickness of boards and tighter spacing of adjacent components. Pre- venting collateral issues when you're doing a hot air rework is difficult, the hot air doesn't want to stay where it is direct- ed, it wants to spread out, and it does. It will spread out through the board and it will spread out to adja- cent components and it will cre- ate conditions outside of the tar- get zone that is called collateral damage. Over- heating adjacent component, reflowing solder joints and other issues can occur. Add the con- formal coating element to it and under fill at that particular rework site, well, now you've el- evated the degree of difficulty to the rework ap- plication. It is extremely difficult to have 100% success rate with all of those factors." When it comes to solder voiding, Ripo- li says there are many things that can contrib- ute 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 Laura Ripoli