Because most of my designs are made late at night I tend to make lots of silly mistakes. In this series I’ll talk about how to fix them. I have several tools/consumables in my workshop that helps me to fix errors. I can’t stress too much how important it is to have the right tools for the job. In my opinion each workshop should at least have some, but preferable all off the following tools:
- soldering iron with temperature control
- several soldering tips
- hot-air gun
- desoldering wick
- tweezers (straight and/or curved)
- thin wires (i.e. wirewrap)
- spare parts (like resistors and capacitor kits)
- knife (I prefer medical scalpels as they have multiple shaped knifes)
- PCB vise or thirdhand
- hotmeltgluegun (love and hate relationship with it)
Today we going to fix swapped bussignals like TXD/RXD, SDA/SCL and MOSI/MISO.
This one is a bit trickier as it can involve cutting traces and soldering wires to bare tracks. This will put stress on the pcb tracks and can be easiliy ripped off after the repair or during future use. Try when possible to lift pins of chips and use airwires to swap the bus, but also don’t forget the pullups ;). Try to relieve the mechanical stress with (hot)glue if possible (or needed).
Use the hotair gun to desolder the chip. I use these settings for the hotair gun: 350-400 degree C and about 30% airflow. Circle patiently with the nozzle around the pins of the IC and wait untill the solder gets shiny, now gently lift the chip with your tweezers from the pcb. Always try to avoid mechanical stress on the part and PCB. Place the chip on a safe spot on your workbench and let it cooldown. When the component is cooled down use your pliers to gently bend the legs that needs to be swapped up.
To solder the chip back in there are two options: using hotair or your soldering iron. If you’ll plan to use the soldering iron you need first to suck away the leftover solder on the pads with desoldering wick. Apply flux to the pads and the wick and heat it up using your soldering iron and watch it suck up the excess solder. Place the chip after the PCB has cooled down and solder it in placec
The other way is using the hotair gun (should be still hot). Apply some flux and melt the (excess) solder, and put the chip back on. Keep heating a bit more to let the chip sucked in place by the surface tension. I prefer the last method as solderwick isn’t my friend and doesn’t always work well for me. Connect a thin wire to both of the lifted legs.
Cutting traces and soldering wires to them is quite hard and you’ll need a steady hand and good vision to do it. First scrape carefully the soldermask from the involved tracks with a knife to expose the bare copper. Try not to scrape any soldermask of adjacent track/groundpours as this soldermask prevents shorts and unwanted solder connections. After exposing the copper carefully cut both tracks with the knife and check if the track is cut properly and not shorting with other tracks/groundpours. I use the continuity check of my DVM for this. Next use flux and use some solder to thin the tracks and wire, then use the tweezers to position and hold the wire parallel to the tracks and carefully heat both with the soldering iron. Let the solderjoint carefully cooldown and repeat if necessary. Sometimes heat will warm an adjacent joint and make it loose, try lowering themperature of the soldering iron or heating it less. It can take some time to master it, but this skill is a joy for ever.
Since all these connections are a bit fragile and easily torn loose it is a good idea to fix them with some hotglue, but first check if the repair works before hotglueing it!! Since I don’t know of any means to remove it cleanly :/