There are several instances where the gold contacts on PCBs need to be replated. IPC A-610 discusses several of these cases. One of the more common defects is when solder "splashes" onto the contact during the wave or selective soldering process, thereby contaminating it. This requires that the operator strip away the solder, first by wicking off excess solder and then to a greater degree via mechanical and chemical stripping means. This is then followed by a replating of the contacts. Another defect is when "pitting" occurs in the contact area. This pitting is a result of a defective plating operation. Many times, this defect can be buffed out and then it can be replated. Other times, the contact area is scratched, which similarly requires the buffing out of the scratch followed by replating.
Gold replating of these contacts is a process that requires setup, special chemistry that requires advanced PCB repair tech skill, and the correct materials and training.
The operator skills required for gold contact replating are similar to those required for micro device rework and PCB repair. Patience, dexterity and the ability to improvise correctly if the standard process followed does not go as planned are all attributes of a PCB rework and repair technician. The most experienced and skilled techs are the right ones for being trained to perform PCB repair techniques such as gold finger replating.
Figure 1: Stripping the remnants as part of preparing the contacts for replating.
Numerous materials are required for the proper repair of gold fingers per the industry standard IPC-7721 4.6.3, which in many cases can be had in a complete replating kit. Several solutions are required for the replating process, including the gold stripping and the nickel and the gold plating solutions. Applicator tips are put onto the end of the electrodes and come in a variety of sizes to accommodate the different gold finger geometries. A variable power supply will supply the current for the plating process. In addition, there are polishing materials including the polishing compound and water to rinse the remnant compound off the board. Finally, a grounding wire is used to form the bus to all the contacts requiring replating. As a warning, the replating area should be properly ventilated and care must be taken for this process as the solutions are hazardous in nature.
Figure 2: Replating the contact with gold solution.
There are numerous ways personnel can be trained in this PCB repair procedure. One method is via the IPC-7721 PCB repair program. This industry certification program can accommodate the training of this repair technique at the CIS level. Otherwise various training companies or gold replating suppliers can provide training in this repair technique.
The basic replating process is referenced in IPC-7721 4.6.3 or in the manufacturer’s instructions as part of the gold replating kit. If the gold finger is severely damaged, a gold-plated circuit frame and the pad repair process as outlined in IPC-7721 procedure 4.6.1 or 4.6.2 can be utilized to replace the finger lieu of the replating process.
If you are outsourcing this as a service, inspect the replated area per the most recent IPC-A-610 guidelines (Revision G, Section 10.1.1). The bond strength of the plating material can also be measured using the peel tape test found in IPC TM-650.