Operation Gold Finger

All That Glitters

Gold edge contacts can become worn, damaged or contaminated by solder and must be either repaired and restored, or replaced.

Examples of solder contaminated and damaged gold edge contacts.

Generally, two options are available to confront problem gold edge contacts: replace the contact or re-plate with gold. Both procedures result in a high conformance level of repair that is reliable and long lasting.

Sizing up the Options

Re-plating is a reliable, yet complicated, process requiring a considerable amount of setup, special chemistries that are hazardous (such as potassium cyanide solution) and, in general, a high level of operator skill and training. Gold re-plating is the best choice when a significant volume of re-plating is needed to justify the setup, when minor physical damage (chipping or scratches) exists and even when no physical damage is present on the contact.

Replacing the contact is faster and simpler, lacking the setup and chemistry of re-plating, but should only be used for low volumes—five or six contacts, or less, per board. If damage to the base material of the board exists, the base material must be repaired before the contacts can be repaired or replaced.

Image shows a gold contact replaced using the Film Adhesive Method.

Contact Replacement

The contact replacement procedure involves replacing a damaged edge contact with a new, dry-film, adhesive-backed gold plated edge contact. The new edge contact is hot bonded to the circuit board surface using a bonding iron or bonding press. The board surface must be completely flat and undamaged. The replacement edge contacts are fabricated from copper foil and have a dry-film adhesive coating on the back. Edge contacts are available in hundreds of sizes and shapes (custom, too) and are generally supplied plated with nickel or gold.

To replace a contact, remove the defective edge contact and a short length of the connecting circuit—heating the area with a soldering iron to make the old contact easier to remove. After old epoxy, residue and contamination are scraped from the board surface, the area should be cleaned. Flux and tin the connection area with a small amount of solder and a soldering iron. Select a new, appropriate contact, with adhesive backing, and trim from the contact foil sheet and put in place—with the overlap solder connection a minimum of twice the circuit width. Scrape the adhesive backing from the overlap connection area. Then, with a piece of Kapton tape as a protective layer, hot-bond the new contact into place. With a small amount of flux, solder and the careful application of a soldering iron tip, solder the overlap area together to form a lap-soldered connection.

Image shows gold contacts being replated using the Plating Method.

Contact Re-Plating

Gold edge contacts may need re-plating if contaminated with solder spatter or reflowed solder paste. Such contamination requires that the operator strip away the solder completely, by both mechanical and chemical means, and re-plates the contacts.

Since the procedure involves the use of stripping and plating chemicals, prudent care must be taken—and proper handling procedures followed—to protect the health and safety of workers. The plating solutions are hazardous chemical solutions and must be disposed of properly. For example, gold plating solution contains potassium gold cyanide, nickel-plating solution contains nickel sulfate and electro-cleaning solution contains sodium hydroxide.

First, clean the circuit board assembly. Then, apply plating tape to the problem area to protect adjacent areas and components from the plating and stripping solutions. Next, flow solder over the entire area of any contaminated contacts—the stripping solution will act upon the area uniformly. Remove as much solder as possible from the contact(s) with a soldering iron and copper solder wicking braid. The remainder of the solder can be removed with stripping solution.

Once the solder has been stripped away and the surface properly rinsed and prepared, the contacts may be re-plated. This electroplating process uses a DC power supply. Connect one lead to the connector edge to be plated. Connect the second lead to the plating probe. The plating probe has an anode fastened to the tip that is wrapped in absorbent material. Dip the anode into a high-speed proprietary plating solution. Swab the saturated anode across the circuit board connector edge contacts—the metal contained in the solution is plated wherever electrical contact is made. Form a conductive bus to all contacts that need plating. A reliable bus connection is the most important step in successful plating.

When finished, rinse the area with water. Remove and discard all plating tape and thoroughly rinse the area with water again. Dry the area using wipes. Remove the wire, or conductive paint used to bus the contacts, protecting the contacts from further contamination by masking them with high-temperature tape while removing the bus connection.

For precise thickness requirements, the final thickness of the plating should be verified with proper measuring equipment. The plating bond may also be checked by performing a peel test, using peel-testing tape. Lastly, visually examine the rework area for color and luster.

Conclusion

Make sure that all technicians involved in the plating process wear protective eyewear, gloves and clothing. Follow the step-by-step procedure for plating carefully—this includes making the cathode and cable connections, and setting the correct output current on the power supply and voltage/time settings according to the equipment manufacturer's specifications. Whether replacing or re-plating gold edge contacts, be precise, be safe and be methodical. Follow the procedural steps religiously—the results are reliable connections and a new life for repaired boards.

Several members of the Circuit Technology Center team contributed to this feature story.