Because most of my designs are made (too) 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. And 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 most (or all) of the following tools:
- soldering iron with temperature control,
- several soldering tips,
- hot-air gun,
- desoldering wick,
- tweezers (straight and/or curved),
- thin wire (i.e. wirewrap),
- spare parts (like resistors and capacitor kits),
- knife (I prefer medical scalpels as they have multiple shaped knifes),
- PCB vise or other fixture for the PCB,
- hot glue gun (love and hate relationship with it).
Today we going to fix forgotten pullup resistors.
As mostly a SOIC chip is involved (whether the microcontroller or the memory chip) one can easily solder the required pullups directly to the legs of one those chips. Add flux to the pins and apply a fair amount of solder to both of them and form a little solder ball, but try not to short to the pins next to it. Next step is to solder one side of the resistor to this ‘solder ball’ and repeat for all the pullups. Now add some more flux to the resistors, and gently tin the other end of the resistors. Probably you need to hold them down with your tweezers. Strip some insulation from the wire and tin the exposed end. Put the tinned wire to the end of the resistor and try to heat both and form a nice shiny solderjoint. Probably you need to hold everything down with the tweezers while letting it cool down. SMD components transfer the heat quickly from one side to the other and thus melting the other solderjoint very easily too. When using larger components (use 0805 if possible) will give you a bit more time. Next solder the other end of the wire to a nearby bypass capacitor or powersupply pin (keep the wire short). Bend the wire in straight turns for a proffesional look. However I’ll settle for functionality though 😉 And removing the flux residu (it’s no-clean) would break the fix.