3.5.2 Base Material Repair, Area Transplant Method

Outline

This procedure is used to repair mechanical or thermal damage to the circuit board base material. This method is used when extended areas of base material must be completely replaced. This method may be used on single-sided, double-sided, or multilayer circuit boards or assemblies.

Caution: Surface circuits may need to be replaced in the damaged area. Be sure that the appropriate circuit diagrams or photographs reflecting the original circuits are available so that they may be replaced after repairing the base board material. Damage to internal circuits or planes may have to be restored using surface wires.

Minimum Skill Level - Expert

Conformance Level - High

Tools and Materials

Ball Mills, Abrasives, Cutting Tools Ball mills, abrasives and cutting tools for working on circuit boards.

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.

End Mills Designed for end cutting and hole boring.

File High carbon steel needle file perfect for all kinds of detail work.

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

High Temp Tape Discs High temperature polyimide tape discs, .50" diameter.

Knife with #16 Blades A must-have tool for precise cutting, scraping and trimming.

Micro Drill System Versatile power tool for milling, drilling, grinding, cutting and sanding.

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

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

Precision Drill Precision drill press for accuracy and controlled depth drilling.

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

Scraper, Curved Tip Hardened stainless steel tip for scraping solder mask and removing defects.

Wipes Nonabrasive, low-linting wipes for cleanup.

Procedure

1. Clean the area.
2. Mill away the damaged board material using the Micro-Drill System and ball mill. Remove all evidence of the damaged material. No fibers of laminate material should be exposed. At the surface, file the edges to ensure the opening is rectangular or uniform. (See Figure 1) 
   Caution: Abrasion operations can generate electrostatic charges.
3. Clean the area.
4. Bevel the edge using the Micro-Drill System and ball mill or a file. (See Figure 2) Install an end mill into the chuck of a Precision Drill System. Set the speed to maximum and machine a step or lap joint on the edge of the circuit board where the new base material will be installed. The depth and width of the step should be approximately 1/2 of the circuit board thickness. (See Figure 3) 
   Caution: Exercise care to avoid damage to any internal conductors. If any internal conductors are damaged, surface wires may be required.
5. Cut or machine a piece of replacement base board material of the same thickness and type as the piece removed. The replacement piece must also be the same size and shape as the opening, including the step joint.
6. Install an end mill into the chuck of a Precision Drill Press. Machine a step onto the entire mating edge of the replacement base material. The step's dimensions should match the step's size in the circuit board milled groove. (See Figure 4)
7. Where required, apply High-Temperature Tape to protect exposed parts of the circuit board bordering the prepared area.
8. Check the fit to be sure the new base material properly mates with the step in the circuit board.
9. Mix the epoxy.
10. Coat both the tongue and groove surfaces with epoxy and fit together. (See Figure 5) Remove excess epoxy.
11. Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.
12. After the epoxy has cured, remove the High-Temperature Tape.
13. If needed, scrape off any excess epoxy using a knife or scraper.
    Note: If needed, apply a thin coating to seal any scraped areas.
14. Clean the area.
15. Complete by drilling holes, slots, etc., or adding circuitry as required.

Evaluation

1. Dimensions of the area replaced should be checked to conform to the specifications required.

Images

Base Material Repair, Area Transplant Method

Mill away the damaged base material using the Micro-Drill System and ball mill.

Bevel edge using the Micro-Drill System or file.

Mill a step into the edge of the circuit board.

Mill a step onto the edge of the replacement base material.

Bond replacement piece in place.

Completed repair.