Just finished repairing a badly messed up PCB . The fanta-tech end user read somewhere somehow that the PCB could be the culprit , no wonder it didn't work, then read that ".... THE ROM MUST BE SWAPPED"... and the result was a complete mess. Delaminated PCB, cut tracks and surrounding microscopical SMD components "removed".
The problem was elsewhere and this time the PCB must have been repaired to get data, but I would have saved a lot of unnecessary PITA (= Pain In The Ass).
Once for all, these are the facts about PCB rework :
0) many PCBs are multilayer. This means in a few words (leave the theory and how are they made) that if you damage them enough, the entire board is screwed.
1) BGA parts (for the layman : the ICs with NO pins surrounding, only small "balls" under) require IR (infreared) dedicated workstation and eventually REBALLING (re-fitting on the IC the microscopic solder balls that get damaged during the removal process). If you just want to remove a BGA chip for fun and see how it is under / how it is soldered, other methods *may* work in some way, you'll destroy something for sure but the BGA can be removed - don't expect for it to work again. Have seen it but I don't recommend it. Reballing is not easy and require a lot of practice, the use of chemicals for fluxing / cleaning etc. and a special "oven" . Re-soldering is "simpler", require precise alignment and the mentioned IR station, and a lot of practice.
Not for end-user at all and even for many intermediate tech people (just because the tooling cost a lot of money). Every time I have to change a BGA chip on motherboard or other circuits ... I rather want to do something else :S
2) SMD chips like SOIC E2PROMs and similar are a little bit easier, BUT :
- they can easily be destroyed mechanically when removing if you don't know how to remove them (they are more robust than it seems but quite fragile if you don't follow the procedures).
- modern PCBs use lead free solder that has a specific melting temperature, require use of appropriate flux AND previous solder residues need to be COMPLETELY REMOVED before resoldering.
- a thin tip soldering iron with controlled temperature, possibly grounded or galvanically isolated should be used. Tip must be kept clean and ACTIVATED (otherwise it gets de-wetted).
- Use the appropriate tin wire.
- solder the joint not apply on tip then deposit on joint.
- If needed, use SMD flux. Good quality one. No, it's not the soldering paste .
- Desoldering braid is an useful tool, but require some practice. Get good quality and it comes in different thicknesses for a reason... And don't forget : some braid leave some dirt that should be removed after...
- To remove residue, there are specific flux remover products.
- Tracks and clads tend to rip out from the PCB if temperature is not appropriate or too much pressure applied or iron is improperly used. Also PCB can delaminate.
- Don't touch the surrounding components unnecessarily and avoid bridging pins. Double check before turning on again.
- Tin and flux and flux remover and other chemicals are not Viagra or have pleasant effects (OK, modern stuff have less impact on health), so avoid unnecessary / long term exposure to fumes and vapors and handle them with caution.
3) The best way for SMD removal is hot air station. A lot of practice and experiments are required to master it ! NO, hairdryers, paint strippers and co. can make only disasters (well, the paint stripper gun may be used to COMPLETELY BLOW AWAY all components from a PCB
) despite Internet.
If practice is not enough to make feel self-confident , that's why the professionals are made for. "A CIASCUNO IL SUO". Don't feel safe ? DON'T DO IT. Next lesson this week.
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