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Vapor Phase and Backward Compatibility
Board Talk
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TranscriptPhil And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, coming to you from ITM Consulting. Jim, this is exciting. We have a vapor phase question. Who better to answer this question but Dr. Vapor Phase, Mr. Jim Hall himself? Jim What is vapor phase, Phil? Phil Blow the cobwebs from your mind there. Jim Oh yeah, I used to do that. Phil Yes, you’re responsible because I have seen your name on some of the patents. Not getting off that easy, old-timer. This is from S.O. S.O. writes, for our surface mount boards, we solder with a vapor phase soldering machine. We usually use lead-free solder paste that has a melting point of 217 degrees C, along with 230 degree C Galden, which is the medium that we use these days. In this case, they are using the Galden LS 230. We can mention Galden by name because I think they are the only ones that make this stuff these days. We have a new project that wants us to use tin-lead solder. Obviously, he is talking SN 63. This paste has a lower melting point of 183 degrees C. Presumably, we should also change the Galden down to a 200 degrees C Galden LS 200 to suit the tin-lead solder paste. What is the correct approach here? Also, most components these days are tinned with lead-free solder. How does that work out with our tin-lead solder? Jim Let’s talk about vapor phase and backward compatibility. We actually have covered backward compatibility using lead-free parts in tin-lead solder. Go into the Board Talk crypt and you will find at least one session that addresses that backward compatibility issue. You are certainly right. You want to use a lower temperature fluid if you are going to solder tin-lead. As I always say, you never want to get the board any hotter than you have to. The only reason you have to get it hot is to promote good wetting. We’re going back thirty-five, forty years ago when I worked in vapor phase. The only game in town was 3M’s SC 70, which boiled at 215 C, which gave us very good soldering. 200 is kind of low for general tin-lead soldering. If you look at the spec sheets of your tin-lead solder paste you will see that most of them have a minimum temperature like 205, 210. You want to check with the manufacturer and do some of your own testing. Speaking theoretically, we have certainly soldered tin-lead at 200 degrees C on specific applications. The question is are you going to get adequate wetting and adequate flux activation and adequate flux deactivation if you are using a no-clean. You want to check with your manufacturer. Looking at it from the theoretical side, Dr. Lee of Indium tells us that in ideal conditions you can solder seven or eight degrees above the full liquidous temperature of your alloy. Well as you pointed out, your full liquidous is 183 for tin-lead. So, 200 is above that. I could argue that vapor phase is a pretty ideal soldering situation because there is very uniform heating and it is oxygen free. I think you have a pretty good chance of getting good success with that. But again, you want to confirm with your solder paste vendor and do some of your testing. On the really positive side, it may be very advantageous to go to 200 degrees for soldering it because you can limit warping of the board and components and minimize head-in-pillow and non-wet open defects. If you look at the literature that is being published on low temperature solders now, a number of published papers talk about keeping reflow below 200 degrees because below those temperatures your warpage of components and PCBs is reduced to the point where you can eliminate or minimize head-in-pillow and non-wet open defects. So, I think there are some positive reasons to go to 200 degrees. Phil But as Jim emphasized, you have to check with your solder paste manufacturer. Specifically ask what their experience is going to this temperature, or any temperature, using vapor phase. Vapor phase has been around long enough that it has an established base, so they should have that kind of data for you. Jim All of the major solder manufacturers have very competent, well-trained, experienced people to help you. Any of your issues relating to your solder paste, you should be contacting them and taking advantage of that knowledge which they all will share very enthusiastically with you. Phil Thanks for your questions S.O. And to all of your out there, you have just wasted some of your apparently not all that valuable time listening to Board Talk with Jim Hall and Phil Zarrow. We thank you. Believe me, whatever you are doing whatever temperature your using, please don’t solder like my brother. Jim And please don’t solder like my brother. |
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