Legend and markings repair: boards with damaged legends or markings aren't necessarily destined for the scrap bin--three IPC-approved methods can help.

Numbers, letters, legends and nomendature are common features on printed circuit board (PCB) assemblies. When they become damaged, or partly or wholly obliterated, should they be repaired or replaced? Is repair possible?

The answer to each of these questions is, of course, yes. At times, letters and numbers must be added to a circuit board. Often, these characters are damaged during the assembly process. For example, the legend for component R7 may be damaged and appear to read R1 instead. Will R7 be accidentally removed if an engineering component order (ECO) later requires the operator to replace R1? This scenario may not be entirely plausible, but many instances exist where markings and legends on a PCB must be clear and intact to avoid unnecessary repair or replacement.

Nomenclature is typically screen printed onto boards during initial manufacture, using epoxy inks that are subsequently cured. Information, such as part numbers, serial numbers and revision levels, is critical and should not be lost or obliterated. Three procedures have been approved by the IPC (Northbrook, IL) for the repair of legends and markings.

Stamping

The stamping method, the most commonly used, uses epoxy ink and an ink stamp to place legends on the board surface--a process similar to the finger printing process used by police departments. Begin the procedure by scraping off the remaining character or legend with a knife. The area should then be cleaned thoroughly.

Next, select the appropriate characters from the peg stamp set, or have a special stamp created. Then, mix the epoxy ink--white is the most common color. Spread a thin, even coating of the epoxy ink on an ink plate or other smooth surface, such as a pane of glass or a glass slide. Continue by pressing the peg stamp into the epoxy coating to coat the character surface. Gently press the peg stamp onto the desired location on the PCB's surface. Complete the process by curing the epoxy ink according to the manufacturer's instructions.

Peg stamps are inexpensive and can usually be obtained from a local office supply store. The epoxy ink used is specifically designed for legend application. I once was confronted by a change to an etched part number on 146 assembled PCBs valued at $96 each. The new part number had several different characters from the existing number, and the entire old number had to be removed or replaced. These boards were headed for the scrap bin if the number could not be corrected.

The old part number was removed by applying heat from a soldering iron that allowed each character to be peeled off with a knife. A one-piece peg stamp, including all nine characters of the new part number, was created. The rework charge was $14 per board. In the end, the costs involved spoke for themselves--the total value of the boards totaled $14,016, and the rework costs totaled $2,044. Overall, savings totaled $11,972.

Hand-Lettering

A sharpened wooden stylus is often used with the hand-lettering method, since even an artist's brush can be too thick. Unfortunately, this method can generate inconsistent and irregular results. The method is also time-consuming and requires a high degree of manual dexterity. After the board is cleared and cleaned, the technician dips the sharpened point of the stylus into the ink and draws the characters onto the board.

Stenciling

The stencil method is appropriate for high volumes, but much practice is needed. Few boards possess the space needed to allow the use of even the smallest of stencils. To complete the procedure, a small roller, saturated with ink, is rolled over the stencil to make a print. Smearing of the ink is a constant threat. The epoxy used is a two-part solution and features a shelf life of approximately one year. One might think that using a simple felt-tip pen might work, but the ink certainly cannot compete with a two-part, curable epoxy ink. Components can also be re-marked using the same technique.

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Jeff Ferry is president of Circuit Technology Center, Haver hill, MA; [phone]; www.circuittechctr.com.