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Reflowing two sided PCBs

Reflowing two sided PCBs

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I need to reflow a two-sided PCB with SAC 305 solder. The bo... - Oct 24, 2024 by klauss

You should build the bottom side first. The other side has c... - Oct 24, 2024 by DucHoang

I guess if anything 2nd side would need more heat since more... - Oct 24, 2024 by PeterM

Bottom side first, use same temp setting both sides. ... - Oct 25, 2024 by Linux

Reflow bottom side first, reduce heat slightly for second re... - Oct 25, 2024 by pawan tanwar

You could try using a water-soluble component masking to hol... - Oct 25, 2024 by Auriga2001

If you have an edge pin conveyor oven and a good profile the... - Oct 25, 2024 by dontfeedphils

I would (and actually I do :) use 2 different pastes SAC fo... - Oct 26, 2024 by madisreivik

what you mean by 'edge pin'? ... - Oct 30, 2024 by klauss

Edge pin conveyor = normal conveyor where the PCBA is held o... - Oct 30, 2024 by dontfeedphils

There are some people giving you advice on here that don't r... - Nov 01, 2024 by Sr. Tech

klauss

#91508

Reflowing two sided PCBs | 24 October, 2024

I need to reflow a two-sided PCB with SAC 305 solder. The bottom side has a BGA128, QFN32, and some small USON packaged components, while the top side has larger components like a QFP100, SOIC20, connectors, and DPAKs. I'm unsure which side to reflow first. If I reflow the bottom first to prevent the larger components from falling during the second reflow, the BGA will heat up twice, which might cause it to shift. On the other hand, if I reflow the top first, the larger components might fall during the second reflow.

And, when I set the oven profile for both sides of the PCB, I use the same settings. However, should I reduce the heat for the second reflow?

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DucHoang

#91509

Reflowing two sided PCBs | 24 October, 2024

You should build the bottom side first. The other side has connectors, if you build first, that would be hard for printing solderpaste for the second side. No problem with the BGA if you use rails in oven. Do not send the boards into oven directly on mesh. I have the same boards and build them for over 10 years. No problem. Use the same oven profile or you might need to change a little for the second side.

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PeterM

#91510

Reflowing two sided PCBs | 24 October, 2024

I guess if anything 2nd side would need more heat since more mass, but if same as first side works, go for it. And agree, as long as there is rails the BGA should stay put.

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Linux

#91512

Reflowing two sided PCBs | 25 October, 2024

Bottom side first, use same temp setting both sides.

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pawan tanwar

#91513

Reflowing two sided PCBs | 25 October, 2024

Reflow bottom side first, reduce heat slightly for second reflow.

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Auriga2001

#91517

Reflowing two sided PCBs | 25 October, 2024

You could try using a water-soluble component masking to hold the big parts in place.

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dontfeedphils

#91518

Reflowing two sided PCBs | 25 October, 2024

If you have an edge pin conveyor oven and a good profile then you'll be fine, really shouldn't be a problem.

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madisreivik

#91519

Reflowing two sided PCBs | 26 October, 2024

I would (and actually I do :) use 2 different pastes

SAC for "simple" side LMPA for "populated" side

LMPA has much lower peak temp, so SAC does not remelt and as a result, no glue etc are needed.

Yesterday did a board, one side - some 0805 comps and RJ45 connectors, other side - submodules, TQFP80 and BGA components. First assembled the side with RJ45 and 0805, SAC pasta, peak 260 deg

Then LMPA to the other side, peak was about 210 deg.

All was fine (exept too steep temp rise for SAC, which resulted in some splatter, but this was my fault)

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klauss

#91532

Reflowing two sided PCBs | 30 October, 2024

what you mean by 'edge pin'?

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dontfeedphils

#91533

Reflowing two sided PCBs | 30 October, 2024

Edge pin conveyor = normal conveyor where the PCBA is held on the edges by a few mm.

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Sr. Tech

#91540

Reflowing two sided PCBs | 1 November, 2024

There are some people giving you advice on here that don't really understand reflow.

Double sided reflow is something that happens everyday all day and is nothing to get scared about.

Typically the side that has the largest components runs last. If you find that you have components dropping off side 1 when you run side 2, you can usually secure them with some chipbonder.

As long as your reflow profile falls within the solder paste and component manufacturers spec there is no reason to have different profiles for top and bottom.

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