Outline
This method is used to repair minor damage to a key slot or other cutout in a printed board or assembly. The area is repaired using high-strength epoxy.
Caution: Care should be taken to limit the application of epoxy to the specific areas desired and to avoid damage to the conductive patterns, contacts and components.
Minimum Skill Level - Advanced
Recommended for technicians with soldering and component rework skills and exposure to most repair/rework procedures, but lacking extensive experience.
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.
Micro Probes
Sharp probes for dispensing adhesive and positioning small objects.
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.
Oven
General purpose oven for drying, baking and curing epoxies.
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: Mill away the damaged board base material using a Micro-Drill System and ball mill.
Figure 2: Apply epoxy to the edges of the key slot using a mixing stick or micro-probe.
Figure 3: Completed key slot repair.
Figure 4: Micro-Drill System.
Procedure
Clean the area to be filled, including the edges.
Mill away the damaged board base material using a micro-drill system and ball mill. All damaged base board material must be removed. No fibers of laminate material should be exposed at the surface of the key slot. (See Figure 1)
Note: To clearly see that all damaged material has been removed, flood the area with alcohol or solvent. Damaged internal fibers of the base material will show up clearly.
Caution: Abrasion operations can generate electrostatic charges.
Remove all loose material and clean the area.
Apply high-temperature tape to the surface of the circuit board adjacent to the slot. The tape should protect any adjacent contacts or components. Note: The circuit board may be preheated prior to filling the area with epoxy. A preheated circuit board will allow the epoxy to easily flow and level out. Epoxy applied to an unheated circuit board may settle below the circuit board surface as the epoxy cures.
Mix the epoxy. If desired, add a color agent to the mixed epoxy to match the circuit board color.
Apply a small amount of epoxy to the edges of the slot. A mixing stick sharpened at the end or micro-probe may be used. (See Figure 2) Note: A slight overfill of epoxy may be desired to allow for shrinkage when epoxy cures. Note: The circuit board may be turned on its side to keep the epoxy in place while it cures.
Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling. Caution: Some components may be sensitive to high temperatures.
After the epoxy has cured, remove the tape.
If needed, use a knife or scraper and scrape off any excess epoxy.
If precision is required, machine the edges of the slot using a milling machine or precision drill system and appropriate milling cutter. Use great care to correctly relocate the slot.
Note: If needed, apply an additional thin coating to seal any scraped areas.
Clean the area.
Evaluation
Visual examination and measurement of key slot location and dimension.
