Outline
This method is used to modify or repair a key slot or other cutout in a printed board or assembly. A replacement piece of matching board material is epoxied into the area needing repair. A new cut is then machined into the repaired area if needed.
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 - 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.
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
Images and Figures
Damaged Key Slot
Figure 1: Mill out the damaged area using a milling machine or Precision Drill System.
Figure 2: Cut a groove into both sides of the key slot using a Micro-Drill System and carbide saw.
Figure 3: Machine a tongue onto both sides of the replacement base material.
Figure 4: Insert the replacement piece into the slot.
Figure 5: Cut off excess material and file to match edge.
Figure 6: Micro-Drill System.
Procedure
Clean the area to be filled, including the edges.
Mill out the damaged area using a milling machine or Precision Drill System and carbide end mill. (See Figure 1)
Clean the area.
Install 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 approx. 1/3 of the circuit board thickness. The groove depth should be approx. double the groove width. (See Figure 2)
Cut a piece of replacement base board material that is the same thickness and type as the circuit board. The replacement piece should be longer than the length of the slot to allow for ease of handling. Install an end mill into the milling machine. Machine a tongue onto both sides of the replacement piece. 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. /li>
Carefully check the fit of the replacement piece and then clean both the replacement piece and the slot. The replacement base material should fit firmly into the slot so that it will not move or fall out when epoxied in place.
Apply High-Temperature Tape to the surface of the circuit board adjacent to the slot. The tape should protect any adjacent contacts or components.
Mix the epoxy.
Apply a small amount of epoxy to the edges of the replacement piece and to the inside edges of the slot.
Insert the replacement piece into the slot. Check alignment. Remove excess epoxy. (See Figure 4)
If needed, apply additional epoxy to the edges of the slot. A mixing stick sharpened at the end may be used to apply the epoxy.
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. Note: If needed, apply additional thin coating to seal any scraped areas.
Cut off the excess length of replacement material and file to match the contour of the existing edge. (See Figure 5)
If a new slot is needed, machine using a milling machine and appropriate milling cutter. Use great care to correctly relocate the slot.
Clean the area.
Evaluation
Visual examination and measurement of key slot location and dimension.
