| | | DEAR TECHNETTER'S, | | | When profiling for the reflow process, we are given a recommended profile to use for the solder paste (no-clean). They give us a range of times to be in each zone. Ex. <90 sec ramp up to 150 degrees C. 60-90 sec in the soak zone. 60 +/- 15 sec in time above liquidus. This can be a large difference in the total time. Is there a target time we should look for in each zone. I would think the most critical time would be time above liquidus because of the intermetallic. Any additional information would be appreciated. | | | | Forget intermetallics. Concentrate on solder joint quality | | and reliability. Again, solder joint formation is defined | | as the joining of two metals, with a specified solder | | medium, without difussion or intermatillics. | | INTERMETALLICS ARE FORMED AFTER ACCEPTABLE SOLDER JOINT | | FORMATION, or not, and continue from there. | | I would like to help more and thank you for your question, | | Earl Moon | I agree with Earl - Concentrate on joint quality and reliability first. A lot depends on the particular product, and common sense needs to be applied. The seemingly wide open windows exist to help with product/process related problems, like pins tied to heavy ground planes, or areas of high/low thermal density, etc. | Some of the things I look for in a profile are the convergence of temperature differentials in the soak zone. If you can get your thermals to converge in 60 or 70 seconds, there's no real reason to hold your board there for another 20 or 30. Same with the spike. If you can get everything spiked and cooled with small deltas across your board in 45 seconds (perfect world!), why hold them for 75? My logic here is that we're subjecting the PWB's to temperatures above their glass transition | temp which is kind of okay, but not great. I think it's my personal prefernce to run on the cool side of the mean, because I went to work for a place that was nuking the boards and they all looked like taco shells after reflowing and it was a lot of work to reprofile them all. | Don't forget - If you're running with palladium-nickel plated leads you have to get your spike temp above 215 or you may not have a reliable joint!
Chrys, Outstanding. All we want is an assembly meeting initial quality acceptance criteria and, then, one meeting long term reliability requirements (customer contract validation and satisfaction. As far as Tg, or any other critical factor is concerned during thermal excursions, and solderability is concerned, we must get back to PCB basics concerning design rules as material selections and MLB constructions. Even the minimal Tg epoxy types work well within clearly defined process management parameters. Always look to the first conceived concepts as design rules, component and PCB solder termination area coating selections (concerning solderability), solder termination area wetting, and thermal process management (as you have clearly defined) during the soldering process. It all comes down to cause and effect. This means deriving useful statistical process information fed back to process management procedures so improvement constantly is made. Take one area at a time while understanding cause and effect relationsips contributing to solder joint quality and system reliability. Earl Moon
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DEAR TECHNETTER'S,
When profiling for the reflow pr...
- Apr 29, 1998
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