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Repeated reflows

Rush Fan


Repeated reflows | 24 August, 2004

I was notified that one of our operators took it upon themselves to run a board assembly through our reflow oven multiple times when he discovered what looked like incomplete reflow. Based on this being done, its been asked of me how, if any, has the reliability of the board been affected? Until now I hadn't really given it much thought as it was just something we never did as a part of normal production. Anyone have any thoughts on the consequences of running a board assembly through a reflow oven multiple times? How are the joints affected? Maximum number of heat cycles that a board assembly should see? The assembly had been previously profiled so the recipe is a robust recipe yielding a good profile curve. (Peak of around 210 with 65 -70 seconds over 183).

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Repeated reflows | 24 August, 2004

There is danger in doing this based on theory. Chances of the solder joints becoming cold or having poor wetting due to the fact that during the first run all the flux vehicles had evaporated and with the second run they're no longer present to help build good solder joints.

But I've experienced this many times, running PCB's twice in the reflow oven as long as the board had reached room temp and not while it's still hot. And the result is okay and our QC didn't see any problem, including our AOI. The paste that we used was No-Clean. Until now I still could not understand why?

I hope that some experts will be able to explain this.

regards, Dreamy

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Robert Ack


Repeated reflows | 24 August, 2004

Being a surface mount department manager for well over 6 years I have seen this countless times. While we run mostly water soluble I havent seen any negative issues while running boards through the oven twice. Sometimes we even spray an additional amount of flux if we ran into cold solder on the initial attempt. Sometimes it works while other times not but it isnt something I would recommend.

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Repeated reflows | 26 August, 2004

We do perform multiple reflow only for reliability testing. 90% of the time we have not seen any change in the solder joint and no change in electricals. If you do see failure you might want to see the solderability of the surface finish too.. just a thought.


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C Lampron


Repeated reflows | 27 August, 2004


I believe that the area's of concern are primarily component related. Some component are rated for a temp (usually 260 degrees max) for some length of time. Tant Caps come to mind as being heat sensitive. The other concern would be the inermetallic of the solder joint. The more times the solder joint is reflowed, the more the intermetallic layer grows. This is mechanically the weakest part of the joint. So there will be a degradation of mechanical strength but you should not see any catistophic failures with 2 or 3 reflows. After that, I would be a litle concerned.

Good Luck


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