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Limits for HASL with Complex Products
Board Talk
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TranscriptPhil And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers Pick and Place. We are here to talk with you about processes, particularly related to the assembly of circuit boards, including methodology, materials, and all that good stuff. Jim, what is today’s question? Jim It comes from R.F. We have been using HASL finish PCB with reasonable success. What are the limits for the future with more complex products? Phil We tend to get a little bit of controversy on this question. Basically, HASL and the actual HASL process, hot air solder leveling, has been with us since the mid-70s. The original methodology was a vertical methodology where the board is vertically immersed in a pot of molten solder. It is coming out using hot air squeegees to hot air the solder paste. It works, it is great. There was a different version which does handle the board in a horizontal methodology. However, the numbers that we have seen on this is that the surface finish itself can be anywhere from 1.77 to 5.08 microns. That is quite a spread, and that is quite a bit of topography. On the one hand, with regard to HASL, regardless of whether you are using tin-lead, using SN100, tin-copper, or whatever you are using for your plating surface with the HASL, one advantage is nothing solders like solder. It is a very, very solderable surface, it has a pretty good shelf life, and it is cheap, it is inexpensive. Plus, the fact that most fab shops can do HASL, it has been around a long time. However, most of the HASL being done by the fab shops is the vertical type because the machines are a lot cheaper compared to the horizontal machines. There are probably a few more horizontal machines out there. But it is still the idea of using a hot air knife as a squeegee. So again, you are going to get this variation. Now, if you have through-hole, who cares, not sensitive to surface topography. But most of us have surface mount or at least mixed technology boards. So again, that spread that I mentioned, 1.77 to .08 microns, well, that’s a lot. These days, we are extremely concerned about coplanarity issues both in the component and the board, things like warpage, your solder paste, uniformity in terms of height and volume, and things along those lines. You know what, if your application is based on the pitch of your components, things like that, if you can get by with HASL and horizontal, good luck riding the dinosaur. But Jim, what about the other issues involved with the HASL process? Jim R.F. specifically says about the future and more complex products. My concern is thermal shock to the internal structure of modern PCBs. These more complex products are getting smaller and smaller details if you think about the internal layers. The blinds and spaces are getting closer together, and when we get into HDI with multiple laminations, micro-vias, and so forth, we have a very delicate, fine-pitch structure in the internal layers. The idea of taking that brand new board and dumping it in a 270-degree, or even 240-for tin-lead, it just scares me. With the expansion, you already talked about warping, but Z-axis expansion, again non-uniform because of the vertical process; I'm just very concerned about the potential degradation of the internal structure of your printed circuit boards with the modern printed circuit boards with finer pitch details in there. All the other finishes do not involve high temperatures. High temperature does nothing good for that PCB. Phil Again, we have talked about other surface finishes in the past. There is information out there, and there are quite a few finishes out there, but as Jim said, none of them are exposing the substrates to these high thermal challenges, so to speak. And again, the other thing is all the ones I can think of have a much flatter topography of the surface itself than HASL presents. But they all come with their limitations and things like that, whether it is a process being used, shelf life, cost, or things along those lines. Again, HASL does have the advantage; nothing solders like solder. Can you live with the aspects that we have mentioned today? Knowing there are a lot of fanboys out there, maybe a few fangirls, of HASL, I’m sure we will be hearing from you. But in the meantime, we wish you the best. This is Phil Zarrow and Jim Hall, The Assembly Brothers, wishing you a good day. Whatever surface finish you are soldering to, just don’t solder like my brother. Jim And don’t solder like my brother. |
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