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Losing parts in reflow

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Losing parts in reflow | 13 June, 2007

Is there a sort of standard method to prevent part loss in reflow?

I've had a couple of situations come up recently where we've lost some parts during second-side processing. We're running an Heller 1800, with a mesh belt. Most items process fine, but, every now and then, we find that we lose some other side components.

We've attempted using aluminum foil underneath the boards, but I've just found through profiling that that's almost a 20 degree heat sink! I lost almost 35 seconds of TAL with the foil in place.

Any ideas?

thanks ..rob

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Losing parts in reflow | 13 June, 2007

Edited because I completely misinterpreted your post.

We used 2 lengths of Durapol with slots for the board edges and a shorty (free-floating, not attached) under the middle.

Not much more elaborate than your titanium shims but we had it laying around, so..... A Durapol fixture would work great but that stuff is pretty spendy.

Still, if they were shimmed before and parts fell off, they're probably just to heavy for the soldered area. I'd hand place a dot of chipbonder first if that's the case but that's not always feasible if you're running high volumes.

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Losing parts in reflow | 13 June, 2007

I experienced this with a high-mass capacitor. I was able to turn down the bottom-side convection (around 50�C lower than each of the top-side heaters), and able to NOT lose any parts. I got the bottom-side part just slightly over liquidus, and minimized the TAL too to keep it on the board through the topside reflow zone.

Also, there is a formula where parts mass, in grams, can be calculated to determine if the part may fall off during 2nd side reflow. It is based on the part's mass and total solderable area. Perhaps use this data to determine if you can "switch sides" (turn top-side into bottom-side and vice versa) if that's possible in your situation.

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Losing parts in reflow | 13 June, 2007

The parts were soldered in place on the first side.

Any recommendations on material for this kind of fixture? I've shimmed boards off the belt before with titanium wave plows...but, I'd love to find something slightly more elegant.

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Losing parts in reflow | 14 June, 2007

we had same problems with heavy loaded boards (40 x 10 leg transformer each side) we placed 2 glue dots (Fuji GL)under trafo and played with profile bottom side (thermateh lcv4 - 6 zones, SnPb process) that did the trick

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Losing parts in reflow | 14 June, 2007

It may be your screen printer my friend. You are using a MPM aren't you? I knew it. Try a DEK to rid yourself of these pesky problems.

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Losing parts in reflow | 15 June, 2007

I remember seeing that formula, but looking through my notes, I can't find it. My R&D group is starting to bigger and bigger parts on the bottom side. Could you please email or post that formula?

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Losing parts in reflow | 16 June, 2007

Search the fine SMTnet Archives, for instance:

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Losing parts in reflow | 18 June, 2007

Almost every board/panel we build is double sided and we use an MPM SPM screen printer with no problems. The screen printer is not the cause.

We have seen heavier parts fall off the first side when going through the reflow for the second side. We have done two things to cure the problem. 1) We have turned down the bottom side heaters in the final zone (TAL zone) 20C lower than the top zone. 2) If it is a real heavy part we solder it on by hand afterwards.

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