Hand Replacement of large non-BGA surface-mount IC's
2009-07-05 12:12 by Ian
This is another procedure I wrote while doing camera repair. Occasionally, there was an identifiable chip that would go bad, but which replacement was considered impossible at our small shop. The alternative was replacing the entire PCB that it was mounted on, which would be an order of magnitude more expensive.
The problem is, is that the bad part was 160 pin, thin-pitch, QFP monster that was placed by a robot with better-than-human vision. Oh yeah: A stereo microscope is not optional for this task.
Hand replacement of large non-BGA surface-mount IC’s (When you lack the right tools to do it conventionally):
- Donor board. Make sure it’s a PCB you don’t need, and that the desired part is good.
- Target board: This is a lot of trouble. Be sure you have a good reason to make the repair.
- Stereo microscope. Not optional for high-accuracy replacement of small-pitch SMD devices.
- Heat gun.
- Solder wick, flux, solder (of the same composition as the stuff on the original PCB), glue
- Sharp knife
1. Use the heat gun to heat the donor board until the solder melts.
2. With the heat still on the part, and being careful not to jar the PCB, slide the knife blade under a convenient corner of the part being removed. Once the blade is in far enough, lift up to free the part from the PCB. Set the part aside. Discard the donor board. You are done with it.
3. Repeat the procedure on the target board while keeping these things in mind:
– Remember the orientation of the part. – If you jar the PCB while the solder is wet, you will almost certainly dislodge lots of parts and/or bridge solder joints beyond feasible repair. DON’T JAR THE PCB. – For the same reasons, don’t slide the part during removal. Free-solder will bridge any contacts it hits. DON’T JAR THE PCB. – Don’t move other parts around. DON’T JAR THE PCB.
4. Once the target board has the defective part removed, use your iron and the solder wick to clean any residual solder from the pads. This is so the new part will seat correctly. Clean entire area with alcohol after this is done.
5. Under the scope, carefully place the new part on the target PCB, observing its orientation while you do so.
6. Once the part is nice and lined up (a tedious task), solidify your work by tacking down corner pads with your iron.
7. Liberally apply solder to each row of pins. Bridge everything until all pads have made contact with the solder.
8. Use the wick to remove the solder you just applied. If done correctly, enough solder will be left behind to hold each pin to its respective pad. For large parts, there will invariably be some touch-up work required when a pin doesn’t stick very well.
9. Clean well with alcohol and brush. CAREFULLY CHECK EACH PIN.
If part tests ok, you’re a bad ass. Have a beer.