I started with one of the stepper-motor-driver boards, because they're the smallest. I'm not entirely sure what I did wrong: either not enough solder paste or not enough heat, but the main lesson I leant was "test the joints before you plug it in!" Several of the joints weren't properly made, including one of the sense resistors, and I blew one of the channels in the driver chip when the poor thing tried to produce infinite current. Then I damaged the pcb tracks getting the old chip off. After I'd patched the thing back together with a new chip and bits of copper wire to replace the lifted tracks it looks like this:
The rest of the boards all went perfectly, and all seem to work, so here are my tips for the beginner:
- Be a bit generous with solder paste. Solder balls come off easily, and desoldering braid fixes bridges without trouble. If you have to add more solder by hand, you'll probably end up with too much, so you'll have to use the braid anyway. Might as well be generous in the first place (but not too much!).
- Shut the door, open a window and turn the extractor on full. You really don't want to breathe too much of the fumes: a friend-of-a-friend ended up in hospital with breathing difficulties after doing research on solder paste. I hold my breath when leaning close to examine the board as it heats up.
- Don't be too worried about heat. Surface-mount chips are designed to be cooked, and will probably survive unless you burn the board. I use a large aluminium frying pan on one of the smaller burners, and I cook it for about three minutes on a fairly low heat to warm everything up, then turn the heat up pretty high until the solder melts, which is usually about three more minutes. Then I turn it off and leave it to cool in the pan.
- Examine it carefully! I use a large magnifying glass for peering, an old hat pin for prodding, and a pair of bent-nosed pliers for tweaking, and I check that everything is on its pad with shiny solder before letting it cool. You don't want to have to desolder anything.
- If you have to desolder a chip, cut through all the legs then desolder them individually. Unless you are a real expert, there simply isn't any way to undo that many solder joints at once. The chip is already dead: don't bugger the board as well.
- Removing solder bridges is just part of the job. For the finest pitch QFPs you might end up with the whole side bridged together, but even if it takes several goes, desoldering braid will fix it. Put the braid on the bridge, heat it up until the solder melts, cut off used braid, repeat until bridge is gone.
- TEST BEFORE USE! Look at the schematic, and use a multimeter to check that each pin on each chip is connected to what it should be, and not to its neighbours (unless it's meant to be!). The audible continuity setting is good for this, but it can't distinguish between a 0.25Ω resistor and a short. It also lights up the LEDs, which is disconcerting if you're not expecting it.