This month’s Demo/Lab reviews the basic steps in performing a successful printed wiring board (PWB) repair. A through-hole, ½-watt-resistor in a circuit of a power supply module severely overheated and eventually burned out. During the component removal process, the land pattern separated from the board surface due to breakdown of the annular ring’s adhesive bond. The land connection to the plated hole was also destroyed. This created two repair conditions:

1. Restore the land pattern connection to circuitry
2. Replace the plated through-hole connection.

Addressing the lifted land pattern, IPC 7721, 4.5.2 “Land Repair, Film Adhesive Method,” was chosen as the highest level of conformance and the most efficient procedure to follow. This procedure utilizes land patterns punched from solder-coated foil, with a thermally-activated adhesive backing. While the procedure lists all of the repair steps in detail, following are the critical steps:

1. Remove the damaged/lifted land and adjacent solder resist from the conductor path and apply a thin “tinning coat” of solder.
2. Select and prepare the proper replacement land pattern, removing the adhesive from the area where the land-to-conductor solder connection will be formed.
3. Using polymide tape or other high-temperature tape, affix the new land pattern to the board surface, overlapping the tail of the land pattern with the circuit path at least two conductor widths.
4. Leaving the tape in place, use the appropriate thermal bonding tool and correct downward pressure. Bond the land pattern into place.
5. After the repaired area has cooled and the adhesive has set, remove the tape and clean the area. Reapply tape only to the new land area, in order to prevent solder overflow. Apply flux and solder the connection. After the connection has cooled, clean the area.

The first portion of the repair task is now complete, but now the plated hole has to be restored. To complete this step, refer to IPC 7721 Procedure 5.1, “Plated Hole Repair, No Inner Layer Connection.” The procedure details all of the repair steps in correct order and involves the use of an eyelet to replace the plated hole. The most important step here is the selection of the correct size eyelet. The following must be considered during the eyelet selection process:

1. Inside diameter – which should be .075 - .5 mm greater than the lead diameter in order to allow proper solder fill and wetting.
2. Length under flange – the length of the eyelet barrel should be .630 - .890 mm greater than the board thickness to provide adequate available material for the “formed flange” produced by the setting process.
3. Flange diameter – should be small enough so as not to interfere with adjacent circuitry or reduce design electrical clearance.
4. Outside diameter – dictates the size of the clearance hole, which should not exceed .125 mm of the eyelet’s outside diameter.

Following are the critical steps of the restoration process:

1. Select the correct drill size and drill out the damaged hole wall. Be careful not to damage the newly installed land pattern.
2. Insert the eyelet into the hole. (It is a good practice to install the “factory flange” on the side of the board with the new land pattern.)
3. Select the proper “eyelet setting tools,” and while evenly applying downward pressure, set the eyelet.
4. Apply a small amount of external flux to the connections and “fuse solder” the eyelet to the circuitry (apply additional solder if necessary).
5. Thoroughly clean the soldered connections and eyelet installation and inspect for proper solder wetting and acceptable eyelet assembly.
6. Apply a small amount of epoxy (a color agent may be added) to the area of the connection and around the newly installed land pattern. Allow it to cure properly.

The detailed steps of the two repair procedures can be found in IPC 7721 Procedure 4.5.2, “Land Repair, Film Adhesive Method” and Procedure 5.1, “Plated Hole Repair, No Inner Layer Connection.” It is imperative that all of the detailed steps in both procedures be followed in the order prescribed and that considerations be made in order to accommodate both repair procedures.

These procedures are only two of the numerous and varied repair techniques presented in the hands-on training course – IPC-7711/7721 “Rework and Repair of Electronic Assemblies,” offered at the EMPF Electronics Manufacturing Learning Center. Courses are available in our classroom or on-site at your facility. For additional information, class schedules, or to register for a course, please contact the Registrar at (610) 362-1295 or via email at registrar@empf.org.