I had a dilemna when I designed RetroShield. At one hand, I wanted to use thru-hole parts so it will be easier to assemble. On the other hand, CLK trace is critical and I wanted to use surface-mount components for signal conditioning. I spent about a week thinking about this in the background. Eventually, surface-mount components won, because i) it had to work reliably, ii) bigger parts, size 0805, should be relatively easier to work with.
These are how I solder surface-mount parts:
Do everything by hand. It doesn’t look as good but good enough for bring-up and prototyping.
- Make sure the pads are clean of solder. (flat)
- Add a blob of solder on only one pad.
- Hold the component with tweezers (microscope helps a lot).
- As you heat the solder blob, push the component flat on the board, and align it nicely.
- Let solder cool.
- If part doesn’t look right, melt the solder and realign.
- Once you are satisfied, add solder to the other pad. DO NOT PUSH on the component at all. Just add solder to the side. You don’t want to stress the part.
You are done!
Well, soldering one by one gets tedious after awhile. So I looked online and saw people used hot ovens and hot-plates. So I experimented. This is what worked for me:
Results-to-effort ratio was better than I expected. I wouldn’t be afraid to solder a complicated part like QFP from now on.
- hot warmer (go for low temperature)
- temperature gun
- low-temp solder paste
- I got a single heater from Target.
- Study the temperature-vs-time profile for low-temp solder paste.
- Turn on the heater and record temperature on pcb vs temp setting and try to get a profile of how temperatures rise and fall. You need to record temperatures at different locations every 15 or 30 seconds. Make sure you can take data several times up and down, and you can replicate the required temp profile consistently.
- Use solder paste syringe to drop tiny amounts of solder paste on all pads. (you may want to bend/squeeze the metal tip a little bit so paste comes out as thin as hair. Anything bigger is too much solder.)
- Place components with tweezers carefully. It helps to do one part on all boards so you don’t mix up the parts. Also watch out for directions for diodes,etc..
- Study the temperature profile of the solder paste. again.
- You are ready to start the heater.
- Place PCB (with parts on them) on your heater and start your temperature profile.
- I used a flash-light to keep an eye on the solder. Goal is to warm up everything before melting the solder. Then you increase the temperature to melt the solder and evaporate whatever is inside. Then you let the boards cool down.
Good job, you will have nice looking surface-mount parts. It is critical you warm up the parts enough (pcb + caps, resistors etc without melting the solder) so there is no stress on parts. Once the solder melts, parts drop to the pcb and align with the pads nicely.
For me, it took several tries to get the temperature profile consistently. Take good notes. Always monitor the pcb surface temperature. In other words, don’t go to bathroom while boards are getting cooked.
IC sockets and dual-row headers,etc.
These are easy but the key is to make sure they are flush with the pcb before you solder 2nd pin. Otherwise it will look ugly and it will be very difficult / impossible to fix it.
I normally solder one pin while pushing the part flush with the pcb. Then I solder a second pin at the opposite corner. Verify I’m happy with placement, then I solder two more. Then solder every other pin so plastic doesn’t melt.