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I have designed and then manufactured a a PCB that is basically a WiFi thermometer with MQTT capability. The PCB hosts several components, plus an 18650-type battery holder capable of holding 2 batteries and it should also host a 2S battery management system (BMS) for managing the batteries. The BMS is actually a "prefab" PCB such as this one.

Here is a picture of my PCB (black) with the prefab 2S BMS PCB (blue).

What do I need to do?

I need to solder the blue PCB directly to the black one on the corresponding pads (B+ with B+, B- with B-, etc.,) but I am having some issues.

What have I tried?

Firstly, I have melted some soldering wire on the pads of the black PCB (B+, B-, BM, P+ and P-) then tried to solder the blue PCB to the black one with a "classic" soldering iron (bad idea.) Secondly, I tried doing the same but used a hot-air gun for soldering (somewhat better results but very prone to damaging the components of the blue PCB or the tracks of the black PCB.)

In the end, I simply didn't install the prefab BMS.

  • What would you recommend for successfully soldering the two PCBs together?
  • Do I need to resort to a SMD reflow oven or SMD hot plate?

I have found a similar post (this one), but it makes use of "castellated holes" (sort of) and thus it is of no use to me.

A couple of closing notes:

  • Sorry for not posting the picture of my PCB directly on this post, I have received a server error from Stack Exchange side.
  • I know that the 2S-BMS PCB should be soldered with a spot-welder to nickel strips, but I didn't want to do so on my PCB.
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    \$\begingroup\$ What exactly is the problem with a regular soldering iron? Are the pads on your PCB too small and you don't have enough space to make a proper connection with the iron? Are you just asking about this as a one of project fix or as a fix for larger scale production? \$\endgroup\$ Commented Jan 30 at 18:35
  • \$\begingroup\$ The distance between the two PCBs is too little for (my) soldering iron. I just couldn't melt the solder on the pads when the PCBs are one on the top of the other. Maybe with a different and smaller tip, the regular soldering iron would work out, I could try that as well. Just asking for this and future projects, not for larger scale as I am an hobbyist. \$\endgroup\$
    – mrcsrplln
    Commented Jan 30 at 20:14
  • \$\begingroup\$ Since PCB to PCB is rarely ever a good solution, what are the requirements here? How much stacking height can you tolerate? \$\endgroup\$
    – Lundin
    Commented Jan 31 at 9:04
  • \$\begingroup\$ @Lundin - Yep, I agree on that, I should defintely learn (or at least learn to replicate) the 2s-BMS circuitry and place the components directly on my own PCB. This also applies to microcontrollers, buck-boost converters etc... but it takes time to learn. Anyways, in this case I don't really have much issues with stacking the 2 PCBs, as I will 3D-print a case and I can adapt it to my requirements (also, the 18650 battery and its holder are much higher than the 2 stacked PCBs). \$\endgroup\$
    – mrcsrplln
    Commented Jan 31 at 13:56

2 Answers 2

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I think I would try a hot plate or IR heating underneath the large PCB and either a soldering iron or hot air gun.

With large power planes the heat wicks out too much so you need to heat the whole PCB to 100-150C, then soldering should be doable.

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  • \$\begingroup\$ Thank you, I think it makes sense and will try this out. \$\endgroup\$
    – mrcsrplln
    Commented Jan 30 at 20:09
  • \$\begingroup\$ Another thing that has been helpful for chip removal is using a hot air gun on the bottom and an iron or another hot air gun on the top. But I think your heatsinks from the planes are too big and will require a plate. \$\endgroup\$
    – Voltage Spike
    Commented Jan 30 at 21:25
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It is difficult to solder a component by hand when it is as big as its pads. For future projects you could increase the pad size on your base PCB to be able to solder with just a regular soldering iron. Make the pads large enough for a big bevel tip for sufficient heat transfer:

enter image description here

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  • \$\begingroup\$ Excellent idea! Thank you. \$\endgroup\$
    – mrcsrplln
    Commented Jan 31 at 6:49

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