I am trying to assemble a PCB that has very small components, an example datasheet is below. I am using tweezers, microscope, and a stencil to apply the solder paste. But this is no match for shaky hands.

Most of the difficult components have pins underneath the modules like the picture below so there is little room for rework after the soldering. Does anyone have any methods they've found especially useful for placing small SMD components?

Also since I'll be using a reflow oven, would like to ask if there is a way to make the reflow curve more forgiving like extending time at max temp to allow solder to flow longer.



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    \$\begingroup\$ For small parts, tweezers and a second hand to brace against work well, especially when you've had too much coffee. For larger, get a suction cup holder so you can precisely let go without smearing the paste off the pads. \$\endgroup\$ Commented Jun 21, 2021 at 0:26
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    \$\begingroup\$ @Feynman137 I use a cheap one of these for large SMD parts: amazon.com/dp/B00XSDHPEK They're not ideal, but serviceable for the larger parts where tweezers don't work well. For small stuff like that LDO, just tweezers and drop in. Reflow is very forgiving, it'll suck the part onto the pads even if you smear the paste around. \$\endgroup\$ Commented Jun 21, 2021 at 2:24
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    \$\begingroup\$ You shouldn't need to be overly accurate, surface tension should pull the part into the proper position. \$\endgroup\$
    – Ron Beyer
    Commented Jun 21, 2021 at 2:42
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    \$\begingroup\$ Look at the solder paste pattern under the microscope before placing the part. Wipe it off and repeat if there is the slightest defect, particularly too little solder paste. Using a higher temperature in the reflow process may be more forgiving but that's difficult when you are using lead-free solder because the margin is tighter before you start to endanger the components. \$\endgroup\$ Commented Jun 21, 2021 at 2:55
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    \$\begingroup\$ I suggest using a lead-free profile on leaded solder then. \$\endgroup\$ Commented Jun 21, 2021 at 3:26

1 Answer 1

  • Tip #1:

Use low temperature 138°C bismuth based solder, you can get it in wire or paste for reflow.

This has many advantages, for the reflow, you can use a much lower heat curve, which is more forgiving, especially if you have plastic connectors.

Small "hobby" reflow oven often have uneven & unprecise temperature and using low temperature solder will give you way more headroom.

Another advantage, is that for prototyping, it will be much easier to replace components, rework and tweak things around.

  • Tip #2:

Apply the paste with the stencil in one swipe, especially with tight count components, this will reduce bridging.

  • Tip #3:

You will always have bridging in tight pitch component. Use solder flux and cupper wick, coat the wick with the solder flux and use that to remove bridging.

  • Tip #4:

Avoid component with pads underneath, like BGA, because you will never know if they are properly soldered without an X-Ray machine.

Stay with minimum 0603 component size unless you are really size constraint.

As for the shaking, steady the side of your palm on the table and work with the tweezers and your fingers.

You don't need to place them very accurately, you can always, once placed & before reflow, go under the microscope and push them around with the tweezers until they are properly placed.

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    \$\begingroup\$ Another tip if you need to make very small adjustments after placing a part, is it to use the lever effect. Plant the tip of a tool very close to the part and tilt it towards it until it pushes it a little. \$\endgroup\$ Commented Jun 21, 2021 at 9:06
  • \$\begingroup\$ For the bismuth based solder, is your experience that it is pretty weak/brittle? \$\endgroup\$
    – Feynman137
    Commented Jun 21, 2021 at 12:48
  • \$\begingroup\$ @Feynman137, I never had issue so far, usually I will do prototype in low temp solder and then do production with normal solder. I guess if you have a high vibration or high temperature transient environment that is could be a concern. \$\endgroup\$
    – Damien
    Commented Jun 22, 2021 at 7:25

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