2.4.3 Coating Replacement, Solder Mask, BGA Locations

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Outline

This method replaces solder masks or coatings on circuit boards at BGA locations. BGA locations provide unique challenges due to their inaccessibility after reflow, exposure to high temperatures, and the requirement to surround the BGA pads with masks. The exposed Vias and circuits may cause shorting or BGA solder joint starvation.

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.

Procedure References
1-0	1.0 Foreword
2-1	2.1 Handling Electronic Assemblies
2-2	2.2 Cleaning Procedures
2-5	2.5 Baking and Preheating
2-7	2.7 Epoxy Mixing and Handling

Tools and Materials

Circuit Bond Epoxy
Clear, superior strength epoxy in two-compartment plastic packages.

Cleaner
General purpose cleaner for removing contamination.

Cleaning Brushes
Disposable brushes for solvent cleaning and application of coatings.

Color Agents
One part, semi-paste ink used to tint the color of epoxy and for direct printing on circuit board surfaces.

Flex-Rack PCB Holder
Sturdy rack for PCBs used for rework and positioning.

Foam Swab, Small
Swabs for use with solvents and application of color agents and epoxies.

Micro Probes
Sharp probes for dispensing adhesive and positioning small objects.

Microscope
Precision microscope with stand and lighting for work and inspection.

Oven
General purpose oven for drying, baking and curing epoxies.

Precision Tool Set
Nine precision-crafted tools for detailed circuit board work.

Wipes
Nonabrasive, low-linting wipes for cleanup.

Procedure

Clean the area.
Caution: Surfaces to be coated must be thoroughly cleaned before coating to ensure adequate adhesion, minimized corrosion, and optimized electrical properties.
Remove all excess solder from the BGA pad and the via (top and bottom sides) using solder braid or desoldering tools. The BGA pads should be flat, and the vias should be clear of solder.
Note: Vias with solder trapped inside may "volcano" underneath the reflowed component. If possible, it is important to remove the solder.
Scrape solder off the "Dog Bone" section between the BGA pad and the connecting via. (See Figure 1)
Note: This prevents 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's 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 temperatures.

Evaluation

Visual examination for texture, color match, adhesion, and coverage.
Electrical tests to conductors around the repaired area as applicable.

Images

Coating Replacement, Solder Mask, BGA Locations

Identify the area(s) with missing solder mask.

Scrape solder off the "Dog Bone" section between the BGA pad and the connecting via.

Apply the replacement coating to the board surface as required.

Cure the replacement coating per the manufacturers instructions.