4.2.3 Conductor Repair, Welding Method

This procedure is used to repair short breaks or opens in circuits on circuit boards. A parallel gap welder is used to weld a jumper ribbon across the damaged conductor.

Caution                                                                                                                                                                         Welding current and voltages may affect component reliability on assembled circuit boards. The repaired section must not reduce the circuit width, spacings or current carrying capacity below the allowable tolerances.

Caution
Sub miniature parallel gap welding is extremely sensitive to proper placement of the electrode tip. Inconsistent welds can occur due to poor electrode tip placement, incorrect tip size, or improper weld settings.

Prior to using any welding equipment, certain precautions should be taken. The equipment should have electrodes cleaned, aligned and set for the proper board thickness.

Test samples that have similar circuit widths, spacing, thickness, surface finish, contour, etc. Observe and test the weld quality, alignment, discoloration, fusion and the appearance of the base material in the area of the weld. Readjust the weld equipment settings and repeat until acceptable results have been achieved.

The alignment of the welded ribbon to the circuit pattern should be within 0.050 mm (.002"). The weld bond strength should exceed the circuit/base material bond strength.

Tools and Materials
Cleaner
Epoxy
Flux, Liquid
Knife
Kovar Ribbon
Microscope
Parallel Gap Welder
Ribbon - Gold Plated Kovar
Solder
Soldering Iron
Tweezers
Wipes

Procedures 

1. Clean the area.
   
2. Select a section of Kovar ribbon of the same width as the conductor pattern being repaired ± .050 mm (0.002").
   
3. Cut the ribbon approximately 3.0 mm (.120") longer than the section being repaired
   
4. Clean the ribbon conductor and base material surrounding the repair area.
   
5. Place the center the ribbon over the section to be repaired leaving equal ribbon end lengths on each side and parallel to the circuit pattern.
   
6. Place the circuit board under the weld electrodes so that the electrodes are depressed to the area of the weld.
   
7. Hold the ribbon in place with the Precision Tweezers until the weld is completed.
   Weld in place using settings based on the accepted test samples.
   
8. Clean the area.
   
9. Carefully inspect the joint for weld quality and alignment.
   
10. If required, apply a small amount of flux and tin the entire area with solder.
    
11. Clean the area.
    
12. Coat the repaired area with epoxy if needed.

Evaluation

1. Visual examination, dimensional measurement of conductor width and spacing, and electrical continuity measurement.