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Hot plate reflow soldering and solder beads

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tounho:
Hello,

I have made my own hot plate PCB using the JLCPCB aluminum PCBs and a simple Arduino (for now) + MOSFET + Thermistor control circuitry.
I have been trying around a lot but I keep having problems with solder beads.
Initially I had been using many years old, never refrigerated Chipquick Sn42Bi58 paste. Since the ageing might be my problem, I recently bought new MG Chemicals 4902P Sn42Bi57Ag1 paste (from Amazon, maybe a mistake), but I am still having problems.
I am applying the paste directly onto the pads per hand via the syringe. I tried different amounts of paste and different temperatures, going up to 200C.
My latest attempt's temperature curve looks like this:
(a)
The target was 180s at 120C and then 200C for 60s. I know that might be excessive for 138C paste. 160C from the manufacturer's datasheet don't work either.
Using the new paste, with the temperature curve above, the result looks like this:
(b)
I tried using more or less paste at different reflow temperatures (180C, 200C) but nothing works.
I also tried using the old paste with the same temperature curve and it weirdly looks better (even so I used way too much paste)!? (only pay attention to C5)
(c)
A video of the attempt from (c) can be found here (d)
All above are 603 components.
At this point I don't know what to try next. I have seen other people using the hotplate method before, so in principle it should work.
The MG Chemicals 4902P paste has the batch number 21248Y. Is the MFG coded into that by any chance? Maybe it was lying in a warm Amazon warehouse since 2021?

I would be incredibly thankful if anyone here had some ideas what I am doing wrong or what I could try next.

asmi:
Floating solder balls usually means that there is too much paste, and can indicate that it's held in a molten state for too long for the viscosity of the alloy. I don't know physical properties of bismuth alloys, but I find that the less viscous is the solder in a molten state, the more often I tend to see these balls all over the place (especially nasty if they are under BGA because they can short something eventually if not washed away). In my experience, I get those balls a lot more often with leaded paste (Sn-Pb) than with lead-free SAC (Sn-Ag-Cu). I'm not a materials expert though, so take it for what it's worth.

thm_w:
https://www.eevblog.com/forum/manufacture/are-these-balls-because-of-soak-or-did-i-squirt-too-much-paste-(reflow-help)/

tooki:

--- Quote from: tounho on May 23, 2023, 07:29:23 pm ---I would be incredibly thankful if anyone here had some ideas what I am doing wrong or what I could try next.

--- End quote ---
Your paste is fine. The problem is that you are applying way, way, WAY too much solder paste. I’m guessing at least 5 times as much as you need.

It’s really hard to apply paste in controlled amounts by hand using a syringe (there’s literally special equipment just to help with that), so either accept that you will need to rework it afterward, or just make your life easier and use a stencil.

What will help with syringe use is very fine conical tips. They require much less force than needle tips. (I’d recommend getting some 25ga (0.26mm) conical tips.)

Take a look at some photos and videos of solder paste applied by stencil. You’ll see it’s a thin layer applied to less than the full area of the pad. (A typical stencil is 0.125mm thick or less!) That should give you an idea of how little solder paste you actually need.

tounho:

--- Quote from: asmi on May 23, 2023, 08:38:43 pm ---Floating solder balls usually means that there is too much paste, and can indicate that it's held in a molten state for too long for the viscosity of the alloy. I don't know physical properties of bismuth alloys, but I find that the less viscous is the solder in a molten state, the more often I tend to see these balls all over the place (especially nasty if they are under BGA because they can short something eventually if not washed away). In my experience, I get those balls a lot more often with leaded paste (Sn-Pb) than with lead-free SAC (Sn-Ag-Cu). I'm not a materials expert though, so take it for what it's worth.

--- End quote ---

My hotplate has a slightly too high resistance because I forgot to untick the plating thickness box in the Saturn PCB Toolkit. Now it is ~1.4Ohms instead of the 1Ohms I wanted. Because it heats up very slowly above 180-200°C, I haven't tried "normal" solder paste yet and stuck to the 138°C one. I don't want to cause damage to components because it is spending too much time in that 180°C+ region.
I will order a new set with <1Ohms the next time I order stuff at JLCPCB and then I can try "normal" temperature paste like the SAC you recommend.
I will also try better application of the paste on the pads too.
Thank you very much!


--- Quote from: thm_w on May 23, 2023, 10:37:35 pm ---https://www.eevblog.com/forum/manufacture/are-these-balls-because-of-soak-or-did-i-squirt-too-much-paste-(reflow-help)/

--- End quote ---

Thank you for the link. So it is just too much paste or paste not properly on the pads.


--- Quote from: tooki on May 24, 2023, 07:19:16 am ---Your paste is fine. The problem is that you are applying way, way, WAY too much solder paste. I’m guessing at least 5 times as much as you need.

It’s really hard to apply paste in controlled amounts by hand using a syringe (there’s literally special equipment just to help with that), so either accept that you will need to rework it afterward, or just make your life easier and use a stencil.

What will help with syringe use is very fine conical tips. They require much less force than needle tips. (I’d recommend getting some 25ga (0.26mm) conical tips.)

Take a look at some photos and videos of solder paste applied by stencil. You’ll see it’s a thin layer applied to less than the full area of the pad. (A typical stencil is 0.125mm thick or less!) That should give you an idea of how little solder paste you actually need.

--- End quote ---

Okay, it's good to know my paste is fine. The syringe is for T3, so not very fine. I will try the conical tips with a smaller opening. My hands are very shaky so I have troubles applying paste perfectly without tools. I have avoided stencils since they are more expensive than a full set of PCBs. I will try them next time I order a batch of PCBs.
Thank you very much!

I have also thought maybe I can use some type of stand for the syringe. I have seen there are expensive, fully automatic paste application machines like this.
Would anything speak against putting the syringe in a stand like this https://www.amazon.de/-/en/Universal-Adjustable-Professional-Microscope-Magnifying/dp/B0797PPX8D? I could move the PCB under it to the right position using my microscope, lower the syringe a bit, apply paste and move on.
The syringe came with an adapter to put in a compressed-air hose. I don't know what to google yet to find what I am looking for, but I am assuming if the stand was too unstable and pushing on the piston moved it too much, I could put in some cheap, hand powered air pressure device there. Something like a hose going into a bellows/food-pump/hand-pump to push the piston without touching the syringe.

Thanks again to everyone replying, it was all very helpful.

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