4.2.3 Conductor Repair, Welding Method

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
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.

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.

Acceptability References
IPC-A-600 2.0	Externally Observable Characteristics
IPC-A-610 10.0	Laminate Conditions
Procedure References
1.0	Foreword
2.1	Handling Electronic Assemblies
2.2	Cleaning
2.7	Epoxy Mixing and Handling
IPC7721 4.2.3	Conductor Repair, Welding Method

Tools and Materials

Abrader
Mild abrasive for removing oxides and contaminates from conductive surfaces.

Cleaner
General purpose cleaner for removing contamination.

Cleaning Brushes
Disposable brushes for use with solvents for cleaning and application of coatings.

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

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

Soldering Iron
Properly maintained soldering iron and properly sized soldering iron tips.

Tweezer, Point Tip
Fine point tweezers for precision work.

Wipes
Nonabrasive, low-linting wipes for cleanup.

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.

Procedure for reference only.