I use a heatgun set at 315 degrees C set at low airflow to remove/attach heatsinks and chips.
Attaching heatsinks on S17 boards is pretty tricky. For S17 and S17 pro, I've been removing most residual solder from the heatsink and adding what I think is the correct amount of solder to the top of the ASIC. I put some tacky flux on the top of the chip, then heat the heatsink with my heatgun for ~20 seconds and set it down on the chip. There is then a small window of time where you can adjust the placement of the heatsink. If you are not quick enough, the solder holding the chip to the board can melt and then you can shift the chip.
On the S17+, that technique didn't work for chips with the smaller heatsinks. So on those, I prepare the chip and heatsink the same way, but then just place the heatsink on the chip and heat it directly until the solder flows.
For attaching chips, I remove the solder from the board with solder wick, apply tacky flux to the board and place the chip in position, then apply heat for ~45 seconds. For new chips, I use a stencil to apply solder paste to the chip and melt the solder before placing it on the board. Nearly every time I do this, all connections are not made, so I add solder paste with a small syringe to the edge of the chip, reflow, add more flux, reflow again, and clean up any excess solder with an exacto knife while the solder is still liquid. Pretty tedious process.
Flux I use:
https://www.chipquik.com/store/product_info.php?products_id=310003solder paste I use:
https://www.chipquik.com/store/product_info.php?products_id=470006solder wick:
https://www.chipquik.com/store/product_info.php?products_id=640002Heat gun I use:
https://www.masterappliance.com/proheat-1500-lcd-programmable-professional-heat-gun-kit/
Started working on another S17+, this one is a real mess. One hashboard had 9 heatsinks fall off, some with chips still attached.
The heatsinks that fell off look like they came off with the film from the top of the chip. Without the film on the chip, it is impossible to get solder to flow evenly on the top of the chip, so impossible to get a good contact with the heatsink. I saw the same issue on some chips on another S17+. Looks like a manufacturing issue with the chip packages. Or maybe it just breaks down if it gets too hot for too long.
To save the chips, I'd have to use a thermally conductive adhesive instead of solder to attach the heatsinks. Not too excited about that idea. A lot of the chips got hot enough to melt the solder holding them to the board and slid down towards the bottom of the miner. Pretty sure some/most of them would be toast anyway so might just replace them all.
Pretty sure this board is going in the "maybe later" pile.
These chips would normally be covered in solder, or would be an amber color if a heatsink was never attached.
Most of the film from the chip is still stuck to the heatsink.