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I recently purchased a clone 858D SMD rework station and would like to get familiar with de-soldering and re-soldering SMD chips.

I was able to de-solder fine, but when trying to re-solder I can't seem to get all the legs to solder onto the PCB. My question is, what could be the cause of this?

Here's the steps I took before re-soldering:

  1. Added flux to the pads on the PCB
  2. Cleaned up/added solder to the pads
  3. Add a bit of solder to the legs of the SMD chip

One big thing I noticed is that before re-soldering, the chip didn't sit flat at all on the pads.

This was the PCB before soldering:

PCB before soldering

The outcome after soldering:

PCB after soldering

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  • \$\begingroup\$ Looks like the PCB isn't reaching a temperature high enough so the solder isn't melting. Also, be very generous with flux. (There is no need to add solder to the legs of the component) \$\endgroup\$ – Wesley Lee Jul 10 at 0:29
  • \$\begingroup\$ More flux. Why are you tinning the component leads? These aren't BGAs. \$\endgroup\$ – DKNguyen Jul 10 at 1:00
  • \$\begingroup\$ Clean up the board with desoldering braid before putting a new IC on \$\endgroup\$ – Scott Seidman Jul 10 at 1:08
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    \$\begingroup\$ Clean old solder off with eg desoldering braid as Scott says. THEN position IC on pads and tack solder ONE corner pin in place. Finalise position, press down on IC to seat it level on PCB then tack solder the opposite corner pin. The IC is now correctly positioned and level. \$\endgroup\$ – Russell McMahon Jul 10 at 1:33
  • \$\begingroup\$ @DKNguyen I felt I added quite a lot of flux. The black colour around the pads you see in pic 1 is all the burnt flux. I use a gel based flux so it's hard to only use a little. \$\endgroup\$ – Naxin Jul 10 at 1:37
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You need to get the PCB up to temperature evenly enough that the solder melts all the way around -- if you don't get all the solder melted all at once, then the solid stuff will always hold the chip up and it'll never work.

For a hot-air station, this means either getting a tip that blows hot air evenly over all of the chip, or playing the tip in an oval around the chip, with some extra attention to the board side (because the board will wick away heat).

It can help a lot to take a piece of tin can and bend a 'U' or 'L' shaped piece that surrounds the chip and sort of holds the hot air in the vicinity of the chip.

An alternative with any sort of package that has legs sticking out like that is to clean the PCB off until the pads are just tinned. Then tack the chip down in two corners, and solder it on leg by leg. If it's too fine a pitch to do that, just get everything soldered, even if you get solder bridges -- then go back and clean off the solder bridges with solder wick. The more magnification you can get when you're doing this, the better.

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  • \$\begingroup\$ Tbh I was concerned I was burning the chip. I was at 250C with medium-high airflow. I was focusing on one side at a time though. \$\endgroup\$ – Naxin Jul 10 at 1:47
  • \$\begingroup\$ Can you clarify what you mean by 'The more magnification... the better'. Do you mean use a magnifying glass while working on SMD? \$\endgroup\$ – Naxin Jul 10 at 1:49
  • \$\begingroup\$ 250C is generally not hot enough, you usually need 300, sometimes more. Keep in mind most things you'll get your hands on are put together with lead free solder. \$\endgroup\$ – Chris Stratton Jul 10 at 1:54
  • \$\begingroup\$ I use a 10x binocular microscope. \$\endgroup\$ – TimWescott Jul 10 at 2:20
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    \$\begingroup\$ Oh, I'm an idiot! It's 10x when it has the extra objective that I never use. It's more like 3x or 5x normally. Still, I find it indispensable -- I can't go smaller than 0805 with the naked eye, I can do 0402 parts with the microscope. \$\endgroup\$ – TimWescott Jul 10 at 3:10

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