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I have designed a two-sided (components on both side) 4-layer SMD PCB that I will reflow with my solder oven. To do so, I am planning to use high-temperature leaded solder (MG Chemicals 4860P, ~250 C) on one sided and then reflow the other side with low-temperature silver-based solder (Chipquick SMDLTLFP15T4, ~165 C). Reference to the idea: https://www.instructables.com/id/Making-double-side-boards-in-reflow-oven/

I noticed earlier that the Chipquick solder does not want to reflow or stick to the PCBs I normally order from PCBWay or JLCPCB with HASL (leaded) finishes (default). The leaded one reflows extremely well.

I was wondering what type of surface finish would be optimum for both types of solder. Surface finishes offered are: HASL with lead, HASL lead-free, Immersion gold(ENIG), OSP, Hard gold, Immersion silver(Ag), ENEPIG and None(Plain copper).

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  • \$\begingroup\$ MG Chemicals 4860P is just regular 63/37 solder. Where is 250C coming from? You could use lead-free instead of the Chipquik Bismuth stuff. Also, have you considered just relying on surface tension to keep the underside components from falling off? At least for the smaller lighter ones? I saw a forum where even a QFP-64 did not fall off. \$\endgroup\$
    – DKNguyen
    Commented Sep 2, 2020 at 19:02
  • \$\begingroup\$ 250C is the peak reflow temperature. I am not sure what you mean by holding components with adhesives. I don't use any adhesives when I use 4860P during reflowing regardless of component size and it reflows fine. Unless you meant to say hold the components on bottom side with adhesives so that both sides can be reflowed together with same solder inside the oven. \$\endgroup\$
    – Swapnil Saha
    Commented Sep 2, 2020 at 19:04
  • \$\begingroup\$ I mean rely on surface tension to keep the components from falling off the underside (and a tack adhesive for larger components). Reflow the underside first, flip it over, then populate and reflow the top side. That way you are never handling the PCB in a state where the components are upside down and can fall off. Or, if your hands are steady and the solder paste is sticky enough, you could populate both sides and reflow at once and rely on the paste to keep it stuck on the underside. \$\endgroup\$
    – DKNguyen
    Commented Sep 2, 2020 at 19:05
  • \$\begingroup\$ ^ Will that not melt the reflowed side if I use the same solder? Sorry I am not familiar much with two sided reflowing so maybe you could point me to that forum link. \$\endgroup\$
    – Swapnil Saha
    Commented Sep 2, 2020 at 19:07
  • \$\begingroup\$ If you use the same solder, yes it will, but the surface tension should keep it on the PCB. I read that people have done so with QFP-64 chips and they still don't fall off. Remember, this is liquid metal. It has a very strong surface tension. Get a little SMD protoboard and experiment. Reflow it, open up the oven and tap it of pick it up and see how strongly it hangs on. \$\endgroup\$
    – DKNguyen
    Commented Sep 2, 2020 at 19:09

1 Answer 1

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If you really want to go with your route of using two different solders, consider using lead-free solder instead of the low temperature bismuth solder if you are having trouble with it wetting.

But the better way to do it is to just use the same solder on both sides and rely on surface tension of the molten solder to keep the component on the PCB while it is upside down. I read around and it seems that even QFP-64 will be just fine.

Reflow the bottom first, flip it over, then populate and reflow the top side. That way you are never handling the PCB in a state where the components are upside down but not soldered or with molten solder and can fall off.

You can also use Kapton tape or high temperature reflow tack adhesive (tough to find) to keep the larger components on the board when they are upside down while the top side is reflowing. But the better way is to lay out your PCB so the heavy components are on the top side and the light components are on the bottom side, the light side first.

If you do use tack adhesive on the bottom for larger components, apply the adhesive AFTER it has been reflowed so that the adhesive doesn't interfere with the reflow.

Or, if your hands are steady and the solder paste is sticky enough, you could populate both sides and reflow at once and rely on the paste to keep it stuck on the underside. I wouldn't trust myself with this though.

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