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Trouble With Skewed DPAK Components
Board Talk
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TranscriptPhil And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, who by day go as ITM Consulting. We're here to take your questions on process issues, components, materials, equipment procedures and try to help you resolve them. Today Jim we have a question from B.O. We are having trouble with surface mount DPAK components skewing during reflow. We have checked our profile and have tried a diagonal hatch aperture on our stencil. We are still having problems with skewing DPAKs. Any thoughts? Jim Let's take this opportunity to say why do components skew? Defined correctly it happens during reflow, specifically right at the point where the solder metal in the solder paste melts and wets the surfaces of the pad and the leads on the DPAC. We all know that if everything is balanced the surface tension forces will actually pull the component into correct alignment. Now skewing is when the component is not in the correct alignment so what has happened is that those same surface tension forces have somehow gotten out of balance. There are a number of reasons to do that. The best way to do it is to optimize the reflow profile. Make sure that the heating of all of that pad reaches soldering temperature and all of the paste melts. And all of that surface tensions around the perimeter of the big pad on the DPAK, which is what is causing you the problem, all that melts together. That the surface tension forces are balanced and do not skew the part. So you want to make sure that you have adequate pre-heat. You may want to add a soak. Perhaps the best thing would be to go to a shoulder profile, recommend by Dr. Lee of Indium. Where you actually in the profile use one zone just before the reflow right at the melting temperature to slow down the heating. You cool down the temperature of that zone. The profile would basically look like a straight ramp right up to just below the melting temperature. Not this typical soak temperature, 10 or 15 degrees, one or two degrees below the melting temperature. And then you slow the profile down just as it goes through that melting. Slowing down formation of surface tension, and minimizing the possibility that those nasty surface tension forces will get out of balance and skew your component. Phil Right and that is what about 30 seconds more or less? Jim Oh even less than that, 15 to 20 seconds. You need a big oven to do that because you have to use one zone. In my opinion you need at least a seven zone oven or that one-zone shoulder section will become too long. Phil As Jim mentioned, this float phenomenon we have seen, it has always been there. We've seen it in early reflow we used to see it in individual leads in parts, particularly two leaded parts. The way we used to control it, and still do is by balancing the pads and the apertures, in other words the overall volume of solder on both sides. However, with your DPAK you are dealing with the same problem that people have with the big BTCs and the QFNs. We have a very, very large area and it becomes much trickier to balance. As Jim said, shoulder profile can help. You should definitely experiment with some other aperture designs, some window paning. Keep abreast with whatever people are writing about these days with regard to BTCs, certainly might be applicable to your DPAK. Jim And my answer to that, having read a lot of what people have published, is that it is component specific. What diagonal cross section you are using might work on one style of component, might not work on another one. It all depends on the geometry and the paste and the thickness of the stencil and so forth. There are lot of different window paning pattern that you can try. From my experience it is going to be a trial and error process. Much better to solve it with the reflow profile. And Phil made some good points. Be sure that your board is designed in terms of the shape of the pad, the solder mask around it, any vias adjacent to it are masked off and so forth. So you don't have board geometry and construction issues that are causing these surface tension forces to get out of balance. Phil Very good. Well, I think that we hopefully covered it. I am sure our readers will have some input on some of their experiences. Jim Good point Phil. And thank you to all of you who comment, criticize us. We really appreciate it, and it is really a forum for sharing information. Phil With that said, whether you are soldering DPAKs, BTCs, whatever you do, please don't solder like my brother. Jim And don't solder like my brother. |
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