Post by thread:Surface mount soldering

[Harold commented...] Years ago I recall seeing a vapor phase soldering system for SMT that I thought was pretty neat. I haven't seen it since. It consisted of a Pyrex container with condensor tubes around the top. You'd put some liquid in the bottom and run water through the condensor tube. the liquid would be heated to boiling, but the condensor kept it from escaping. You'd lower the SMT board into the vapor, and everything would be instantly soldered (the parts were held in place with solder paste). It's probably been 5 or 10 years since I saw that machine. No one else seems to have seen it. Anyone know if it's still produced? [and Herb added...] I don't how easy this would be to implement but a medical electronics manufacturer I worked for used the "Vapor Phase Process" for surface mount circuits. This involved using two chemicals, one had a lower boiling point but a higher density than the other. These chemicals would be heated in a vat. The chemicals would vaporize but one would stay above the other like a thermal blanket retaining the heat of the other chemical vapor in the vat. The circuit board with solder paste silk screened on it would have the components placed on it and lowered into the vat of hot vapor which would melt the paste. The circuit boards would be of ceramic cladded metal which could stand up to the high vapor heat. The solder paste was finely ground tin/lead solder mixed with rosin. Variations would include using hot air instead of the chemical vapors. [And my point is....] I think vapor phase reflow has been pretty much abandoned these days; it used flurocarbons, which we've since discovered are not-so-nice for the ozone layer. (Or so it's said by some, anyhow.) A board would be populated with parts after solder paste screening, then lowered into the vat of hot vapor. The vapor would condense on the board, giving up its heat and dripping to the bottom of the tank where it was re-boiled. After a dwell time, the board emerged from the tank and went through a cooldown cycle. It was not necessary to use a special board; plain old FR-4 works just fine. The metal- core boards are usually used for performance or RF reasons, not because of manufacturing method. These days, most everybody I know is doing hot-air reflow. It's a simple conveyor oven, with several temperature zones for preheat, soldering, ramp-down and cooling. It's important not to thermally 'shock' the board, since you can crack ceramic parts or even a trace if they're small. One variant on hot-air reflow is hot-nitrogen reflow, where a bottle of nitrogen gas provides a low-level continuous purge of the oven. This cuts down on oxidation and improves flux wetting. Some people are still doing radiant oven reflow, but the hot-air process produces more repeatable results, I'm told. If you can lay your hands on an old convection oven (check garage sales or Goodwill) they work great. It's just an oven with a high-temperature fan blowing the air around inside. Put it in, set it for 30 degrees or so above the melting point of your solder, and let it sit until the solder melts. I just hand-solder SMT's with an iron. It's not really that hard; get a head-mounted magnifier and a Metcal iron with the 600 degree 1/64" tip. You also want to use an RMA flux solder (Kester "44" rosin works best, Multicore is awful) in the finest gauge you can get. I use the 63/37 alloy, in a .010 diameter. A dab of RMA rosin flux in an insulin syringe also helps a lot. Using this setup I've soldered my prototype runs (a dozen or so boards sometimes) with parts as dense as 208-pin QFP's on 20 mil centers. It's not really that tough, once you get the hang of it. If you go with the oven approach, you'll still need the hand-work equipment, since you'll be chasing down bridges on your QFPs. If it's just SO and PLCC packages, you'll be fine, but watch out for bad joints on PLCCs; they'll tend to flow out and fail to wick up the leg. I try to avoid them for that reason when I'm designing a board and have a choice. If you want to try vapor phase soldering, my CRC handbook lists Dodecane [CH3-(CH2)10-CH3] with a boiling point of 216C. 63/37 solder melts at 183C, so that would work. Just be aware that it's flammable (it's a long-chain hydrocarbon) so any ignition sources would probably put an early end to your experiment. I'd suggest you don't do this, but I include it for reference. If you die trying it, don't blame me. :-) Mark G. Forbes, R & D Engineer | Acres Gaming, Inc. (541) 766-2515 KC7LZD | 815 NW 9th Street (541) 753-7524 fax spam_OUTforbesmTakeThisOuTpeak.org | Corvallis, OR 97330 http://www.peak.org/~forbesm .....mforbesKILLspam@spam@acresgaming.com "There has been an alarming increase in the number of things I know nothing about." ---Anomalous

First, thank you Mark -- your email answered a *lot* of questions for me. On a related note, I happened to be browsing around in the bookstore in the train station on my way into work this morning, and noticed that the cover of the Summer 1998 Communications Quarterly magzine had a teaser along the lines of "make your own hot-air soldering station". On (I think it was) page 96, there is a short article which describes the author's sucess in modifying a basic Weller station to do hot air soldering. He had a EC1201A handpiece, with I'm not sure what base. He used a Pace hot-air tip, modified to mount into the Weller handpiece. He also drilled out an air port in the side of the handpiece and hooked it up with a nylon airhose driven by a $20 aquarium pump. I didn't particularly feel like paying the $10 for the magazine, so I don't have a copy of the story. If you are interested and can't find a copy of the magazine (it does not appear to have a website, but is published by CQ Magazine, which has a website at http://members.aol.com/cqmagazine/), I'll try to find out more. --Bob On Mon, Nov 30, 1998 at 10:11:18PM -0800, Mark G. Forbes wrote: {Quote hidden}> [Harold commented...] > > I just hand-solder SMT's with an iron. It's not really that hard; get > a head-mounted magnifier and a Metcal iron with the 600 degree 1/64" tip. > You also want to use an RMA flux solder (Kester "44" rosin works best, > Multicore is awful) in the finest gauge you can get. I use the 63/37 alloy, > in a .010 diameter. A dab of RMA rosin flux in an insulin syringe also > helps a lot. Using this setup I've soldered my prototype runs (a dozen > or so boards sometimes) with parts as dense as 208-pin QFP's on 20 mil > centers. It's not really that tough, once you get the hang of it. > > If you go with the oven approach, you'll still need the hand-work equipment, > since you'll be chasing down bridges on your QFPs. If it's just SO and > PLCC packages, you'll be fine, but watch out for bad joints on PLCCs; they'll > tend to flow out and fail to wick up the leg. I try to avoid them for > that reason when I'm designing a board and have a choice. -- ============================================================ Bob Drzyzgula It's not a problem bobKILLspamdrzyzgula.org until something bad happens ============================================================