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2.4.3 Coating Replacement, Solder Mask, BGA Locations
Outline
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.

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.

Acceptability References
IPC-A-600 2.0 Externally Observable Characteristics
IPC-A-610 10.0 Laminate Conditions
Procedure References
1.0 Foreword
2.1 Handling Electronic Assemblies
2.2 Cleaning
2.5 Baking And Preheating
2.7 Epoxy Mixing and Handling
Tools and Materials
CircuitMedic Part
Alcohol Swab
Self-saturating Isopropyl alcohol swab. Squeeze handle to release alcohol.
Cleaner
Cleaner
General purpose cleaner for removing contamination.
CircuitMedic Part
Cleaning Brushes
Disposable brushes for use with solvents for cleaning and application of coatings.
Color Agent
Color Agents
Use to tint the color of epoxy used for solder mask repair or circuit base board repair.
CircuitMedic Part
Circuit Bond Packs
Clear, superior strength epoxy in two-compartment plastic packages
CircuitMedic Part
Foam Swab
Ideal for absorbing cleaning solvents, and applying color agents and epoxies.
Microscope
Microscope
It is a challenge to undertake precision repair without a good microscope.
CircuitMedic Part
Micro Probes
Use to dispense a minute amount of adhesive and for micro-positioning of small objects.
Oven
Oven
General purpose oven for drying, baking and curing epoxies.
Wipes
Wipes
Nonabrasive, low-linting wipes for cleanup.
Images and Figures
2.4.3 Coating Replacement, Solder Mask, BGA Locations
Soldermask Required at BGA Site
2.4.3 Coating Replacement, Solder Mask, BGA Locations
Figure 1: Scrape down to the copper surface to create a barrier to solder flow between pad and via.
2.4.3 Coating Replacement, Solder Mask, BGA Locations
Figure 2: Apply the replacement coating. A wood stick or small dispenser may be used.
2.4.3 Coating Replacement, Solder Mask, BGA Locations
Figure 3: "Dog Bone" area coated with soldermask

Procedure
  1. Clean the area.

    Caution
    Surfaces to be coated must be thoroughly cleaned prior to coating to ensure adequate adhesion, minimized corrosion, and optimized electrical properties.

  2. 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.

    Note
    Vias with solder trapped inside may "volcano" underneath the reflowed component. It is important to remove the solder if possible.

  3. 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.

  4. Mix the epoxy or replacement coating.

  5. 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.

  6. 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.
Evaluation
  1. Visual examination for texture, color match, adhesion and coverage.

  2. Electrical tests to conductors around the repaired area as applicable.

Procedure for reference only.