3.5.3 Base Material Repair, Edge Transplant Method
Outline
This procedure is used to repair mechanical or thermal damage to 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
Recommended for technicians with advanced soldering and component rework skills and extensive experience in most 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.
Scraper, Curved Tip
Hardened stainless steel tip for scraping solder mask and removing defects.
Additional Items and Supplies
Cleaner
General purpose cleaner for removing contamination.
End Mills
Designed for end cutting and hole boring.
Microscope
Precision microscope with stand and lighting for work and inspection.
Milling Machine
Use for precision milling of circuit board materials.
Oven
General purpose oven for drying, baking and curing epoxies.
Precision Drill
Precision drill press for accuracy and controlled depth drilling.
Wipes
Nonabrasive, low-linting wipes for cleanup.
Circuit Bond Epoxy
Circuit Bond is a clear, low viscosity, superior strength epoxy precisely measured out into two-compartment plastic packages, so it's easy to use, and there's no measuring. For over a decade, this high-strength epoxy has been qualified and used by thousands of high-rel electronics manufacturers across the globe.
1.
Circuit Bond has a working pot life of 30 minutes. It should not be mixed until ready to use.
2.
To use Circuit Bond, remove the plastic clip separating the resin and hardener. Squeeze back and forth from one half of the package to the other to mix the contents.
3.
Cut a corner off the package and squeeze all the contents into a Plastic Cup. Stir the contents to ensure it is thoroughly mixed.
4.
Circuit Bond may contain bubbles from the mixing process. If needed, use a vacuum system to remove bubbles.
5.
Color Agent can be mixed in with Circuit Bond to match surface colors if desired.
6.
Apply using a Foam Swab, Micro Probe, or Mixing Stick as required.
7.
Cure Circuit Bond for 24 hours at room temperature or 4 hours at 65°C (150°F).
Specifications
Packaging
2 gram pre-measured packages
Mix ratio
4 parts resin to 1 part hardener
Mix Ratio by Weight (R/H)
100/25
Color
Clear, transparent
Pot life
30 minutes
Cure cycle
24 hours at room temp (25 °C) or 4 hours @ 65°C
Thixotropic Index
1
Specific Gravity
1.20
Percent Solids
100%
Viscosity (after mixing)
2000 cps
Operating temperature range
-55°C to 135°C
Hardness
88 Shore D
Lap Shear, Alum to Alum
1100 psi
Glass Transition Temperature, Ultimate
92°C
Coefficient of Expansion, cm/cm/°C
6 E-05
Dielectric strength
400 volts/mil
Dielectric Constant, 1KHz@25°C
4
Shelf Life
6 months minimum
Color Agents
Color Agent is a one-part, air-drying, semi-paste ink commonly used to print hard surfaces, including circuit board base materials. Color Agent can be used to tint the color of epoxy used for solder mask repair or circuit baseboard repair. To use, mix a small amount of color agent with 2 part epoxy as the epoxy is initially mixed. Add more color agents to increase the color intensity if needed.
Figure 1: Cut away damaged base material with a saw or milling cutter.
Figure 2: Cut a groove into the edge of the circuit board.
Figure 3: Mill a tongue onto the edge of the replacement base material.
Figure 4: Check fit of new base material. The tongue must mate with the groove in circuit board.
Figure 5: Trim the excess material with a saw or milling cutter.
Figure 6: Complete the repair by drilling holes and applying coatings as required.
Procedure
Clean the area.
Cut away the damaged board material using the Saw or milling cutter. Remove all evidence of the damaged material. No fibers of laminate material should be exposed. File the edge to ensure that the edge is flat. (See Figure 1)
Caution: Exercise care to avoid damage to any internal conductors. If any internal conductors are damaged, surface wires may be required to restore the electrical connection.
Clean the area.
Install a carbide saw into the Micro-Drill System. Set the speed to maximum and machine a groove in the edge of the circuit board where the new base material will be installed. The groove must be centered on the edge to ensure that the new piece will fit properly. The groove width should be approximately 1/3 of the PC board thickness. The groove depth should be approximately double the groove width. (See Figure 2)
Cut a piece of replacement base board material that is the same thickness and type as the piece removed. The replacement piece may be oversized, the excess material will be removed after the replacement piece has been epoxied in place.
Install an end mill into the chuck of a milling machine. Machine a tongue onto the entire mating edge of the replacement base material. The dimensions of the tongue should match the size of the milled groove. (See Figure 3)
Where required, apply High-Temperature Tape to protect exposed parts of the circuit board bordering the prepared area.
Check the fit to be sure the new base material properly mates with the groove in the circuit board. (See Figure 4)
Mix the epoxy.
Coat both the tongue and groove surfaces with epoxy and fit together. Remove excess epoxy.
Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.
After the epoxy has cured, remove the High-Temperature Tape.
If needed, scrape off any excess epoxy using the knife or scraper.
Note: If needed, apply an additional thin coating to seal any scraped areas.
Saw or mill off excess base material and file flush with the existing edge. (See Figure 5)
Clean the area.
Complete by drilling holes, slots, etc., or adding circuitry as required. (See Figure 6)
If needed, replace solder mask or conformal coating. (See Figure 6)
Evaluation
Dimensions of area replaced should be checked to conform to specifications required.
