Nothing good can come from a freak idea after midnight !
I was tired and should have gone to bed. But when I get too tired I can’t fall asleep anymore and turn into a zombie, looking for something stupid to do. Sadly at that time my idea seemed brilliant.
So I started testing how low a voltage is tolerated by my 64-pixel LED controller. Going below 3.3V made all the blue LEDs shut off, but it the rest kept working properly. The brown-out detection kicked in at about 2.6V and reset the ATmega168 chip. I repeated this test a few times and it was reproducible. Then for some reason I managed to type “260” in my power supply’s keypad instead of “026” and immediately hit the “ON” button as well. The current limiter was set to 40mA and kicked in immediately, but too late. Instead of 2.60V I fed 26.0V to the circuit.
So this was a perfect excuse to upgrade my first PCB prototype of the controller to new chips. It’s sad that I killed the ATmega168 chip, but the 74HCT595 (work OK, but can’t source/sink enough current) shift registers needed to be replaced with 74HC595 (better) ones anyway. A good excuse to test my recently acquired ChipQuik test set and some practice for the pending ATmega64 replacement adventure on my sick Jyetech oscilloscope.
The paste flux works great, but smells really strange. It reminds me of white wine and cork. It also creates a lot of smoke and is very sticky. You’ll have to insert the plunger into the syringe yourself. Make sure you do that before putting the ‘needle’ on or you’ll squirt all of it across your workplace. The Bismuth based alloy is very brittle. If you bend it, it breaks. Bismuth is non-toxic! It takes very little of it to remove a chip.
Here’s the procedure that worked for me:
- Apply paste flux to the pins
- Cut off a small piece of alloy
- Melt and mix with old solder
- Repeat for all sides
- Simply remove the chip
- Thoroughly wick away residual solder
- Clean the board with isopropyl alcohol
Once all sides are treated and heated sufficiently, the chip will slide off the PCB easily. There’s enough time to put down the soldering iron and use a pair of tweezers to remove the chip.
One of the interesting properties of Bismuth is its low thermal conductivity. It is just about 2% of copper ( 7.97 W/(m*K) vs. 401 W/(m*K) ). In practice this means it stays warm and therefore liquid much much longer than ordinary solder. You can poke it with a pair of metal tweezers and it stays liquid, whereas ordinary solder would solidify instantly.
Here are some pictures of how I removed two SOIC-16 and one TQFP32-7×7 chip:
And a video:
I highly recommend this product!