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Understanding Reflow for Metal Core PCBs
Board Talk
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TranscriptPhil Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, pick and place. We're here today to answer for free your process questions and dilemmas and problems. Let's see, what do we have today? Jim This comes from P.B. "How does the reflow process for a metal core PCB differ from an FR4 laminate PCB? We've never processed a metal core before, and I would be grateful for any useful advice." Your profiling procedures are going to become more difficult, and more complex. It would be wonderful to be able to solder the components on the surface without conducting a lot of heat into the metal core, but that's generally impossible because most boards that use metal core are designed to use the solder joints to help conduct the heat from the active components down into the metal core. Regardless of what technique you use, a lot of heat is going to be conducted down into that metal core, therefore your profile is going to be longer. You're going to have to heat slower. You may have to add a soak section before liquidus to allow the heat to penetrate into that metal core so that you can safely reflow all the components on the surface without too great a Delta T. That means achieving full liquidus temperature on all the solder joints without overheating any of the components and materials. It's been used extensively, but you have to anticipate longer profiles, slower ramp rates, and perhaps soak sections or longer soak sections. Phil Now this is probably going to bring a smile to certain people we know in Colorado and Minnesota, but wouldn't this probably be a good candidate for a vapor phase, Jim? Jim Absolutely. Metal core boards were one of the first assemblies we started processing in vapor phase and continued to process in the early 1980's when vapor phase was the principle reflow technique. Phil I remember in the early days of convection IR when we would see an application like this we would go, "Oh, just use vapor phase." But, again, if you don't have a vapor phase oven, hopefully you have a good convection oven with lots of zones. Jim Slow it down. Remember, you don't want to heat too rapidly because if you heat too rapidly, you can warp and delaminate these boards. We want to compliment our questioners. The fact that you've recognized that a metal core board is going to be different is a kudo for you. To some people, a board is a board and they just try to profile it normally and they get into trouble, so you were right, intelligent and astute to question us. Phil Remember, whatever you do, don't solder like my brother. Jim And keep the kids away from the flux pot. |
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