Outline
This method is used to replace damaged or missing circuits on internal layers of multilayer circuit boards.
Caution: The circuit widths, spacing and current carrying capacity must not be reduced below allowable tolerances.
Caution: The overlap joint used in this method may cause problems with high frequency circuitry.
Caution: This procedure is complicated and should be attempted only by properly skilled repair personnel using the best tools and equipment.
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.
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.
Microscope
Precision microscope with stand and lighting for work and inspection.
Oven
General purpose oven for drying, baking and curing epoxies.
Soldering Iron
Properly maintained soldering iron and properly sized soldering iron tips.
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
Circuit Tracks
Circuit Tracks are rectangular-shaped conductors made of 99.9% pure CDA 11000 copper. These rectangular ribbons closely conform to the original conductor dimensions. The replacement Circuit Track is bonded in place using epoxy.
115-5530 Circuit Track, .005" x .030" (.127 mm x .762 mm)
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: Milling into multilayer circuit board to expose the damaged internal circuit.
Figure 2: Micro-Drill System used for precision drilling and grinding.
Figure 3: cut through the base material, one layer at a time, until the desired inner layer has been reached.
Figure 4: Circuit Track in place ready to be soldered.
Figure 5: If spacing is critical or the circuit board uses high frequency circuits, bevel the joint as shown.
Figure 6: Coat the top and sides of the new circuit with epoxy. Add epoxy until flush with top surface.
Figure 7: Completed repair
Procedure
Locate and determine the coordinates where the repair is to be made. Use films or master drawings of the board as needed.
Note: Obtain as much information as possible on the conductive and non-conductive layers prior to starting the procedure.
Remove components from the immediate area if necessary and clean the area.
Use the microscope and Micro-Drill System and cut through the base material, one layer at a time, until the desired inner layer has been reached. (See Figures 1 and 2)
Caution: Great care should be taken to prevent further damage to internal circuits.
Each internal circuit should have a flat section exposed to allow the new circuit to be soldered in place. (See Figure 3)
Note: To reduce damage to the internal circuit, complete the final exposure of the internal circuit using a knife. (See Figure 3)
Remove all loose material. Clean the area.
Apply a small amount of liquid flux to the ends of the internal circuit. Tin the exposed end of each circuit using solder and a soldering iron.
Clean the area.
Select a Circuit Track to match the width and thickness of the circuit to be replaced. Cut a length approximately as needed. The Circuit Track should overlap the existing circuit a minimum of 2 times the circuit width.
Gently abrade the top and bottom of the Circuit Track with the buffer to remove any protective coating and clean.
Note: If needed, the ends of the Circuit Track may be tinned with solder prior to lap soldering in place.
Place the Circuit Track in position. The Circuit Track should overlap the existing circuit a minimum of 2 times the circuit width. (See Figure 4)
Note: If the spacing is critical or the circuit Board uses high-frequency circuits, bevel the joint.
Caution: This bevel joint method may cause problems with circuit Boards exposed to extreme temperature fluctuations.
Apply a small amount of liquid flux to the overlap joint.
Lap solder the Circuit Track to the exposed internal circuit using solder and a soldering iron. Make sure the new circuit is properly aligned.
Clean the area.
Note: The circuit Board may be preheated prior to filling the area with epoxy. A preheated PC 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 epoxy. If desired, add a color agent to the mixed epoxy to match the circuit board color.
Coat the top and sides of the replaced circuit with epoxy. The epoxy bonds the new circuit to the base board material and insulates the circuit. Continue adding epoxy up to the top surface of the circuit board or to the height of the next internal circuit. (See Figure 6)
Note: A slight overfill of epoxy may be desired to allow for shrinkage when the epoxy cures.
Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.
Caution: Some components may be sensitive to high temperatures.
Add additional Circuit Tracks if needed and coat with additional epoxy.
Continue completing all layers until the top surface of the circuit board is reached. (See Figure 7)
Clean the board as required.
Apply surface coating to match prior coating as required.
Evaluation
Visual examination for alignment and overlap of the new circuit.
Visual examination of epoxy coating for texture and color match.
