I've been playing around with the Soldered Inkplate 10 (see previous post) and on one of them I managed to insert the LiPo battery with the wrong polarity and got to watch the magic smoke emit from the tiny charge controller.
The device worked ok but wouldn't charge and would drain the battery. So that little component needed to be changed. Looking at the schematic it's an MCP73831T from Microchip.
Zooming in we can see the little five leg chip labelled KDY3.
The datasheet tells us KDY3 means it's a MCP73831T-2ACI/OT. That part is about 70 eurocents from Mouser. The package type is SOT-23 but with five leads. This is a pretty common format for transistors (with three leads) and is really small (just under 3 mm wide!).
Some time ago I bought myself a rework station and hadn't actually got to use it. This was the perfect first project. The steps were:
1. Remove the dead chip by gently heating the chip with the SMD rework set to 350C and the lowest fan speed. Two essential items are tweezers to remove the chip and some sort of magnifier so you can see what's happening. I have a desk mounted magnifier with built-in LED lighting.
2. Clean up the pads. To do that I used some solder braid. Just hold the braid onto the pads with the soldering iron for a few moments. The solder will flow into the braid and remove both the soldering iron and braid at the same time to make sure you don't accidentally rip the pads off.
3. Apply some solder paste from a syringe onto the pads. The magnifier is super helpful here. Once the solder paste in on the chip can be dropped into it. To solder it, I like to hold the chip with tweezers so it doesn't get blown away and use the rework air gun to melt the solder.
4. Give it all a clean with isopropyl alcohol to remove any flux. (Grain of white rice for scale).
And then it's done. Nothing revolutionary, just a satisfying little holiday project.