4.7.2 Surface Mount Pad Repair, Film Adhesive Method
Outline
This method replaces damaged surface mount pads with new adhesive-backed dry film pads. The new pads are bonded to the circuit board surface using a bonding iron.
Caution: It is essential that the board surface be smooth and flat. If the base material is damaged, see the appropriate procedure.
Note: This method uses replacement surface mount pads. The new pads are fabricated from copper foil. They are available in many sizes and shapes and are generally supplied solder plated.
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.
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.
Tweezers
Multiple sizes and tip configurations of tweezers for various small parts handling needs.
Wipes
Nonabrasive, low-linting wipes for cleanup.
Bonding Iron Tips
Bonding Iron Tips fit into the handheld Bonding Iron. The bottom surface of each Bonding Tip is used to apply heat and pressure to bond adhesive-backed replacement lands, pads, and edge contacts to a circuit board surface. The pressure/force listed is the recommended load in pounds to apply to the top surface of the replacement adhesive-backed pads, lands, and conductors. The load is based on the Bonding Tip surface area to meet the recommended load for Circuit Frames at 200 - 400 psi.
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 Frames
Circuit Frames have a dry-film adhesive backing that is heat-cured in 30 seconds. Use Circuit Frames to repair and replace damaged surface mount pads, lands and conductors without the mess of liquid epoxy, with a bond strength equal to the original, in just a few minutes. The dry-film adhesive backing makes this delicate repair procedure easy, fast, and highly reliable. Circuit Frames are available with a bright tin, tin/lead and nickel/gold plating finishes. This reliable IPC recommended procedure meets the highest conformance level for this type of repair. For over 30 years Circuit Frames have been used by thousands of commercial, medical and military manufacturers around the globe. Below are examples for some Circuit Frame patterns.
Images and Figures
Damaged Surface Mount Pad
Figure 1: Remove the defective pad and remove soldermask from the connecting circuit.
Figure 2: Select a replacement pad that matches the missing pad.
Figure 3: Scrape off the adhesive bonding film from the solder joint area on the back of new pad.
Figure 4: Cut out the new surface mount pad. Cut from the plated side.
Figure 5: Place the new surface mount pad in place using Kapton tape.
Figure 6: Bond the new pad using a Bonding Iron.
Figure 7: Completed repair.
Procedure
Clean the area.
Remove the defective pad and a short length of the connecting circuit. (See Figure 1)
Use a knife and scrape off any epoxy residue, contamination, or burned material from the board surface.
Scrap any solder mask or coating from the connecting circuit. (See Figure 1)
Clean the area.
Apply a small amount of liquid flux to the connection area on the board surface and tin with solder. Clean the area. The length of the overlap solder connection should be a minimum of 2 times the circuit width.
The area for the new pad on the board surface must be smooth and flat. If internal fibers of the board are exposed, or if there are deep scratches on the surface, they should be repaired. Refer to the appropriate procedure.
Select a replacement surface mount pad that most closely matches the surface mount pad to be replaced. If a special size or shape is needed, they can be custom fabricated. (See Figure 2)
Note: New surface mount pads are fabricated from copper foil. The foil is plated on the top with solder, and an adhesive bonding film is applied to the bottom.
Before trimming out the new pad, carefully scrape off the adhesive bonding film from the solder joint connection area on the back of the new pad. (See Figure 3)
Caution: Scrape off the epoxy backing only from the joint connection area. Avoid touching the epoxy backing with your fingers or other materials that may contaminate the surface and reduce the bond strength when handling the replacement contact.
Cut out and trim the new pad. Cut out from the plated side. Cut the length to provide the maximum allowable circuit overlap for soldering. Minimum two times the circuit width. (See Figure 4)
Place a piece of High-Temperature Tape over the top surface of the new pad. Place the new pad into position on the circuit board surface using the tape to help in alignment. (See Figure 5)
Select a bonding tip with a shape to match the shape of the new pad. See the bonding tip chart in the replacement parts section of the manual provided with the repair system or repair kit.
Note: The tip used for bonding should be as small as possible but should completely cover the entire surface of the new pad.
Position the circuit board to be flat and stable. Gently place the hot bonding tip onto the High-Temperature Tape covering the new pad. Apply pressure as recommended in the manual of the repair system or repair kit for 5 seconds to tack the new pad in place. Carefully peel off the tape. (See Figure 6)
Caution: Excessive bonding pressure may cause measling in the circuit board surface or may cause the new pad to slide out of position.
Gently place the bonding tip directly onto the new pad. Apply pressure as recommended in the manual of the repair system or repair kit for an additional 30 seconds to bond the pad fully. After the bonding cycle, remove the tape used for alignment. The new pad is fully cured. Carefully clean the area and inspect the new pad for proper alignment.
If the new pad has a connecting circuit apply a small amount of liquid flux to the lap solder joint connection area and solder the circuit from the new pad to the circuit on the circuit board surface. Use minimal flux and solder to ensure a reliable connection. Tape may be placed over the top of the new pad to prevent excess solder overflow.
Note: If the configuration permits, the overlap solder joint connection should be a minimum of 3.00 mm (0.125") from the related termination. This gap will minimize the possibility of simultaneous reflow during soldering operations. Refer to 7.1 Soldering Basics.
Mix epoxy and coat the lap solder joint connection. Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.
Note: Additional epoxy can be applied around the perimeter of the new pad to provide additional bond strength.
Caution: Some components may be sensitive to high temperatures.
Apply surface coating to match prior coating as required.
