When boards lose their edge: when the corner of a circuit board breaks or is damaged, the problem cannot be ignored.

One of the most easily, and most frequently, damaged areas of a circuit board is the edge or corner. Corner damage often occurs when employees mishandle or drop the board. Since circuit boards are generally rectangular, corners are particularly vulnerable to damage. Corners may feature crucial board elements (mounting holes) or be connected to the housing, or body, of the final assembled product. When damage occurs, the corner must be replaced with materials equivalent to the original structural strength.

Developing a Repair Method

Over the years, I've encountered such a repair scenario thousands of times. In fact, this type of mechanical damage is so common, similar repairs are made practically every day. When I first encountered broken corners, no established or reliable repair methods existed. As a result, over time, and after trying a variety of different approaches, I discovered that, by borrowing certain techniques from other trades or disciplines, such techniques could be applied to circuit board repair.

With corner repair, the tried and true technique of tongue and groove joinery used in woodworking and cabinetmaking was the solution. This technique is now an IPC-recommended method and can be applied to circuit boards that are at least 1.27 mm (0.050 in.) thick, and yield a joint as strong as the original. The method may be used on single-sided, double-sided or even multi-layer boards or assemblies.

In this procedure, the missing corner material is completely replaced by a new piece of material of equal thickness. In many cases, depending on the depth of the damage (distance from the edge), circuitry may not be involved. If circuitry is involved, it must also be replaced and restored to the original condition and functionality.

Performing the Repair

Begin by removing all broken material, cutting away the damaged board material using a small saw or milling cutter. All delaminated material must also be removed--cutting down to undamaged board material. Cut a straight, clean edge and file the edge to ensure flatness. The new edge must be at a 90[degrees] angle to the board surfaces. Clean the area before and after cutting and filing.

Next, install a circular carbide saw into an appropriate micro-drilling system. The saw will be used to cut a groove along the center of the fresh edge, the area where the new piece of board material will be installed. Set the speed to maximum and machine the groove. The groove must be centered in the edge to ensure the new piece will fit properly. The groove width should be approximately one-third the circuit board thickness. The groove depth should be approximately 0.762 mm (0.030 in.) deep.

Now, cut a piece of replacement board material that is the same thickness and type as the piece removed. The replacement piece may be oversized slightly--the excess material will be removed after the replacement piece has been epoxied in place. Keep in mind that trimming down excess is easier than making up for a piece that is cut too small!

The next step is to machine a tongue onto the entire mating edge of the replacement base material. This step is accomplished using an appropriately sized end mill and a milling machine. The dimensions of the tongue should equal the size of the milled groove.

If performed properly, the new piece will fit snugly into the grooved mating surface of the original board. Once the new piece fits nicely, mix together appropriate proportions of two-part epoxy, coat both the tongue and groove surfaces, and then fit the two pieces together. Wipe away any excess epoxy and let it cure fully, according to specifications. Excess epoxy can be scraped off; the edges of the new piece should be milled flush with the edges of the board. Apply an additional thin coating or sealant to any scraped, abraded or open areas of the board material.

Complete the repair by drilling holes or slots, or adding circuitry as required, and replacing any solder mask or conformal coating. When finished, the dimensions of the area replaced should be checked to ensure they conform to specifications.

Jeff Ferry is president of Circuit Technology Center, Haverhill. MA; [phone]; www.circuittech.com.