SMT rework is the operation of bringing the printed circuit board back into compliance with the original inspection criteria using the original materials and processes. The objective of a removal and replacement of a component onto the PCB is to emulate the original manufacturing process albeit in the location only.
SMT rework generally consists of the following process:
Removal of the existing component
Preparation of the site where the device has been removed
Cleaning of the site
Inspection of the site
Replacement using a new device
Cleaning of the site (if necessary)
In the removal process, the existing component needs to be removed without damage to neighboring parts. Depending on the location this may require the removal of other neighboring components.
In site preparation the solder and perhaps other coatings or flux residue needs to be removed prior to replacement of the device. Here the SMT rework technician must be careful to not damage pads or solder mask.
Site cleaning is important so that the proper BGA site inspection can take place.
Inspection of the now cleaned site requires that the SMT rework/ repair technician should be well-versed in the inspection standards. Many times the site is re-inspected by a different operator after the completion of the rework process.
The replacement of the component or components is to be such that it emulates locally the profile of the original manufacturing process.
After placement, cleaning may be required depending on the flux chemistry required. No clean flux chemistries require no cleaning processes but water soluble flux chemistries require a water wash and others perhaps with a saponifier added into the wash cycle.
Inspection after SMT rework may require automatic optical inspection depending on the severity of the rework with manual inspection taking place based on the inspection criteria.
SMT rework generally is accomplished by soldering techs with more experience and a higher degree of skill than the assembly line soldering technicians. The reasons for this is that the boards, by the time they require SMT rework to be performed on them, are near their highest. The work itself generally requires a higher degree of skill as well as requiring that the rework operator be familiar with and skilled in using a variety of different tools. Some of the tools of the SMT rework trade that the technician should be familiar with include hot air rework stations, micro tweezers, x-ray systems, endoscopic inspection equipment and other specially-configured equipment. Many times the SMT rework technician is also skilled in some basic troubleshooting techniques.