0.5mm pitch QFN DIY Soldering for FST-01

STM32F103TB, which is QFN 36-pin

For the second experimental version of FST-01, I use STM32F103TB, 36-pin QFN chip.

Articles on Surface Mount Soldering

I read (parts of) following articles.

My solution for QFN soldering

DIY reflow soldering station

I made my own soldering reflow station. It consists of three parts.

  • Electric heater (300W/600W)
  • Stainless steel plate (0.5mm (thickness) 100mm x 200mm)
  • MS8264 digital multimeter with thermocouple (for temparature measurement)

I put PCB (this is fake, with no parts on top) on the plate and measure temparature and watch reflow process.

mesurement of temparature

Reflow process is simple, turn on, wait, and turn off. To elaborate, it's like following:

  • Put PCB on the plate
  • Turn on the electric heater (full power, 600W)
  • Wait about three minutes for 190 degree (temparature sensor on the plate)
  • Watch carefully to see reflow of solder paste
  • Turn off the electric heater when it is 210 degree (and solder looks gleam)

Technical points

  • Positioning of IC is not that so critical if we compare TQFP hand soldering. With reflow soldering, QFN chip adjusts its position by itself.
  • Amount of solder paste should be very small like nothing. It is completely OK not to cover pad by solder paste, but put some (a bit) of paste on the pad. Especially, it is mostly none for the bottom pad of the chip (solder on the PCB pad (solder plating) is almost enough).
  • Solder paste is mostly requirement. I tried three times, but I didn't success without solder paste. For four times, I got successes with solder paste (with no failure).
W/ Solder paste W/O Solder paste
Four success No success
No failure Three failure
recommendation challenge

PCB and final result

As I don't have stencil for solder paste, I did reflow only for STM32F103TB, XTAL, and LED (which are difficult by hand soldering).

For other ICs, a resister, and capacitors, I did hand soldering.

Here is a picture of PCB and final result.

PCB and final result