4.2.3 Conductor Repair, Welding Method

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Outline

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

Preparation: 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.

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.

Procedure References
1-0	1.0 Foreword
2-1	2.1 Handling Electronic Assemblies
2-2	2.2 Cleaning Procedures
2-7	2.7 Epoxy Mixing and Handling

Tools and Materials

Abrader
Mild abrasive for removing oxides and contaminants.

Circuit Bond Epoxy
Clear, superior strength epoxy in two-compartment plastic packages.

Cleaner
General purpose cleaner for removing contamination.

Cleaning Brushes
Disposable brushes for solvent cleaning and application of coatings.

Flex-Rack PCB Holder
Sturdy rack for PCBs used for rework and positioning.

Knife with #16 Blades
A must-have tool for precise cutting, scraping and trimming.

Precision Tool Set
Nine precision-crafted tools for detailed circuit board work.

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.

Procedure

Procedure

Clean the area.
Select a section of Kovar ribbon of the same width as the conductor pattern being repaired +/- .050 mm - 0.002".
Cut the ribbon approximately 3.0 mm (.120") longer than the section being repaired.
Clean the ribbon conductor and base material surrounding the repair area.
Place and center the ribbon over the section to be repaired, leaving equal ribbon end lengths on each side and parallel to the circuit pattern.
Place the circuit board under the weld electrodes so that the electrodes are depressed to the area of the weld.
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.
Clean the area.
Carefully inspect the joint for weld quality and alignment.
If required, apply a small amount of flux and tin the entire area with solder.
Clean the area.
Coat the repaired area with epoxy if needed.

Evaluation

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

Images

Conductor Repair, Welding Method