SMT Magazine

SMT-Sept2017

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14 SMT Magazine • September 2017 cur if the area is not prepared properly such as starved solder joints, solder shorts and other problems. Those are just a few general thoughts on some rework issues we confront." Price notes that board thickness is a chal- lenge. "Years ago, board thicknesses ranged from maybe 0.062" to 0.092". Now, we're rou- tinely seeing boards that are 0.125", and even thicker. There are ground planes involved in the construction of these boards that dissipate heat when you're trying to remove components. Then you add in the fact that you're dealing with a higher temperature re- flow with RoHS requirements. These are just challenges that we have to deal with. I don't see it going away. I see it becom- ing more and more challenging," he explains. "The prevalent issues that we see with circuit boards that are constructed like that re- late to the increased heating re- quirement. Boards require preheating and com- plete bake-out prior to rework. A lot more heat is needed in the rework area to achieve reflow temperatures. When more heat is necessary you increase the potential for physical damage to the board to occur. You may see baseboard dis- coloration or more significant base board dam- age such as delamination. Increased heat at the rework site can also result in lifted lands, sur- face mount pads and pulled plated through hole barrels. These are issues that anybody in- volved in rework will see, the most expert com- panies will see less of it due to experience but there are some designs that are very difficult to overcome." One of the problems that Freedman had seen very often was trying to repair things that don't need repair on boards, such as barrel fill that doesn't quite meet IPC spec. "One of the things that I had been driving for years when I was with Digital, Compaq and HP was trying to refine the barrel fill specifications. We have had some success in driving that movement with IPC. If you look at a through-hole solder joint, it's many times stronger than an SMT sol- der joint. Yet, we're happy to pass SMT solder joints with very little solder. When it comes to through-hole, people are so quick to take a soldering iron to it. It might just be the wrong thing to do," he points out. "What we want to do is minimize the number of heat cycles on a circuit board. Besides the number of heat cycles, we also want to minimize the number of touch- es. Any time you touch a board, you're like- ly to damage it, either through flex- ure or shearing of parts on the back side, or on the front side, with the soldering iron. Or, I see op- erators with picks all the time, poking and prodding to see if things are soldered. Like I said, anytime you can minimize the number of touches to a board, you're better off. I'm one that is much happier to pass a board that doesn't quite meet IPC than to have it go through a repair cycle. Chances are, the repaired board will be of a lesser reliability. "A through-hole solder joint is many times stronger than a surface mount solder joint. We get very particular about these massively strong solder joints and yet we're very willing to pass SMT solder joints that have almost no solder. I certainly understand that there are different mechanics involved in ex- pansion of contraction, etc. We turn a blind eye to SMT, whereas with through-hole we get overly particular. I said that there are two issues: number one is the number of heat cycles we ap- ply to a solder joint. That, in part, defines inter- metallic characteristics and reliability. The oth- er thing is the number of touches to a board. Any time a board is subjected to rework, you're more apt to create damage than if the board is left alone." For years, Freedman had done a lot of work on reliability of solder joints, including through- hole solder joints, as well as a lot of modeling and testing of solder joints to revise their speci- fication requirements and to also influence IPC. "And we have done that to some degree," he says. "If you have a very thin board, IPC's very happy with a minimum amount of solder joint. If you have a thick board, they want a very thick solder joint, and yet, if you do the modeling A LOOK INTO PCBA REWORK AND REPAIR Dan Patten

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