This method is used to replace solder mask or coatings on circuit boards at BGA locations. BGA locations provide unique challenges due to their inaccessibility after reflow, exposure to high temperatures, and requirement that the BGA pads only are surrounded by mask. Vias and circuits that are exposed may cause shorting or BGA solder joint starvation.
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Minimum Skill Level - Intermediate
Recommended for technicians with skills in basic soldering and component rework, but may be inexperienced in general repair/rework procedures.
Conformance Level - High
This procedure most closely duplicates the physical characteristics of the original, and most probably complies with all the functional, environmental and serviceability factors.
Ideal for absorbing cleaning solvents, and applying color agents and epoxies.
Precision microscope with stand and lighting for work and inspection.
Use to dispense a minute amount of adhesive and for micro-positioning of small objects.
General purpose oven for drying, baking and curing epoxies.
Nonabrasive, low-linting wipes for cleanup.
Images and Figures
Soldermask Required at BGA Site
Figure 1: Scrape down to the copper surface to create a barrier to solder flow between pad and via.
Figure 2: Apply the replacement coating. A wood stick or small dispenser may be used.
Figure 3: "Dog Bone" area coated with soldermask
Clean the area.
Surfaces to be coated must be thoroughly cleaned prior to coating to ensure adequate adhesion, minimized corrosion, and optimized electrical properties.
Remove all excess solder from BGA pad and the via (top and bottom side) using solder braid or desoldering tools. BGA pads should be flat and the vias clear of solder.
Vias with solder trapped inside may "volcano" underneath the reflowed component. It is important to remove the solder if possible.
Scrape solder off the "Dog Bone" section between the BGA pad and connecting via. (See Figure 1)
Note This is to prevent solder from flowing down the "Dog Bone" path to the via location.
Mix the epoxy or replacement coating.
Apply the replacement coating to the board surface as required.(See Figure 2) Apply a thin coating of epoxy to prevent interference with component placement. Adding a color agent to the epoxy is not recommended since it may slightly reduce the epoxy strength or adhesion.
Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling. Provide a full cure time to ensure maximum epoxy strength.
Caution Some components may be sensitive to high temperature.
Visual examination for texture, color match, adhesion and coverage.
Electrical tests to conductors around the repaired area as applicable.