Hand soldering a WLCSP package

23 Jun 2018
Progress: Complete

I admit that when I made the video of me soldering an MSOP-10 package, I did it because I needed to use the chip right away, and didn't have time to order a breakout board. But this time, I am just doing this for fun.

MSOP is smaller than TSSOP which is smaller than SOIC which is smaller than DIP. Although the pin pitch is larger, the physical size of the ATtiny9 in the charliestar project is smaller than the MSOP-10 package. Most ATtiny parts are also available in QFN format, which has a smaller rectangle with pads around the edges. But there's a fundamental limit to how close you can put adjacent pins and still have them reflow correctly.

The next level is BGA, or Ball Grid Array. The solder balls mean assembly isn't limited by the resolution of your solder stencil, and the 2D array means the pins can be packed much more densely.

But there's a 'package' that's even smaller than BGA, and it's called WLCSP, or Wafer-Level Chip Scale Package. This stands out because there isn't really a package to it, it's just a bare silicon wafer! The ATtiny20 is available in a 12-ball WLCSP package that measures 1.555 by 1.403mm, and is 0.538mm thick. I ordered a few.

So, without further ado:

The kapton tape is sticky-side up in the middle, held down by the outer two strips. The protoboard is 0.1 inch (2.54mm) pitch.

The 30AWG stranded wire is an excellent source of tiny copper wires, 0.1mm thick. They're not enamelled so there's no need to strip the coating this time. It is still essential to tin the wire first though. Watching back the video, it definitely seems that the balls are easier to solder to once they've been melted once. This must be to do with the flux in the solder I'm using, and perhaps I should have covered the whole chip in a liquid flux beforehand.

The power and ground pins are definitely much harder to solder to than the others, even at this scale they noticeably suck more heat.

Putting the top corner marker over the through hole really makes a big difference, I no longer have to worry about remembering the orientation.

Filming

The opening shots are taken with a Nikon 60mm 2.8 AF-D Macro (my favourite lens) at closest focus, at 1.3x crop on a DX frame. Even this wasn't close enough though, so I switched to a reversed kit-lens (18-135mm) at 18mm mounted with a reversal ring and left at minimum aperture. I had to stick 12 watts of LED lighting right up against it to get a usable picture. It's funny, at that zoom level the camera is getting a better picture than I was through the microscope.

Soundtrack

To avoid 15 minutes of silence, I added some piano music. Feel free to mute it if it's annoying. The song is an original composition entitled "Hand soldering a WLCSP package".

Conclusion

I honestly think that the MSOP-10 soldering video is the most boring thing I've ever uploaded. And yet, confusingly, it's my most viewed (and most disliked) video on youtube. Let's see how this compares...

Anyway I powered up the chip and it appears to be fully working. I haven't decided what to do with it yet.