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Is HASL a Good Choice for Surface Finish?
Board Talk
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TranscriptPhil Welcome to board talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers. Jim, what's today's question? Jim It comes to us from D.S. We are using lead free HASL (Hot Air Solder Leveling.) Is this the best choice for a surface finish? We have several bottom-terminated components on our board and our production is low volume and the PCB's are often stored for a year or more. Phil Once you mention surface mount specifically, bottom terminated components, the answer is no it's not. HASL was originally invented back in the days of through-hole in the mid '70s and it was a welcome change from methodologies we were using before like IR fusing and hydro squeegee and all those other fun methodologies of putting a tin-lead coating on. But once surface mount came around what we found is unlike through-hole we are concerned about the topography and the smoothness of the board, especially after we started to get finer and finer pitches. Look how much we critique solder paste stud, deposition and solder paste height, how important that is. So if you're starting off with a lumpy, bumpy finish you're going to have problems. And that was even before the advent of bottom terminated components, well before they became so popular in area arrays. So it becomes even more critical. Even with the lead-free versions, some of the mythology out there is that lead-free HASL is smoother than tin-lead HASL and Jim and I have attended enough seminars and propaganda sessions and stuff like that to say no it's not. If they're still using the vertical methodology for a hot air solder leveling, whether using an SN100 alloy or tin copper, it's basically from what we've seen not going to be any smoother than tin-lead HASL. So HASLis definitely, we don't believe that's the way to go. What are some of the alternatives? Well there's OSP (Organic Solder Preservative.) Jim, what are some limitations with OSP? Jim What jumps right out, shelf life. Nobody recommends that you store OSP coated boards over a year so based upon the data given here in the question, eliminate OSP. OSP works great for single soldering applications but most people don't like to store to for more than six months or so. It's very inexpensive. It's the least expensive. We don't know if their cost is an issue here. The other question that people will raise, are there multiple soldering operations? Does it maintain solderability through multiple heat cycles? Most specifically, if you have a board where you have to reflow both sides and then do a wave soldering or selective soldering operation, can you get reliable hole fill after the barrels of those holes have gone through two reflow cycles. Phil You want the advantages of OSP and it's come a long way in that it is the cheapest finish, but as I said there's a lot of caveats. Then we get into some of the immersion finishes, immersion tin, immersion silver. The jury's still out on those and with regard to some of the pros and cons. There is shelf life concern with immersion silver. Certainly it's a lot more forgiving and it's longer duration than OSP, but it also I would say from assembly standpoint, you better have your act together as far as handling. You know there's still oxidation issues and there's tarnishing issues. Jim I disagree. I feel that first off immersion tin is not an acceptable finish. It just doesn't solder well. It suffers many of the problems that OSP does with multiple heat cycles. It's used a lot in Europe but most people in the U.S. don't like it and most people I've heard that have tested it say it doesn't solder well which is surprising for tin. I feel that although the limitations that my brother outlined with immersion silver are true, many people feel comfortable for specific applications. Certainly not if you're going into a high sulfur atmosphere because it is the most prone to creep corrosion of all the surfaces. But some of the server people and other high rel people use immersion silver and they are comfortable with it so I feel that it is a valid choice depending upon your application. Phil I would tend to lean to tried and trusty ENIG (Electroless Nickel Immersion Gold) surfaces. Been around awhile, it's time tested. Certainly storage concerns are most nominal because oxidation's not really that much of an issue. But again the only caveat with ENIG is make sure you got a good fab shop. We've talked about black pad and some of the other concerns. Make sure they've got the process under control and particularly as it pertains to things like gold thicknesses and nickel thicknesses, all should be in conformance with the IPC specifications. So I tend to favor ENIG but again it's your choice. But I think my brother and I definitely concur with regard to HASL. Run away, run away, HASL is not the way to go particularly for surface mount with bottom-terminated components, area arrays and certainly any pitch below 20 mil and below. Well we thank you for listening to Board Talk and regardless of the surface finish you use, whatever you do don't solder it like my brother. Jim And don't solder like my brother. |
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