Seven Techniques for Printed Circuit Board Reworking

Printed circuit board reworking consists of the repair, improvement, or upgrading of existing boards. Board reworking has as objects the replacement of malfunctioning components or the addition of functions through design modifications, and involves precise soldering operations. Through such reworking, the lifetime of a printed board can be extended, and cost reductions and effective use of resources are made possible. This article explains reworking methods.

Printed Circuit Board Reworking Techniques

Circuit design is a series of trial-and-error processes. Should a mistake be made in even a single location of a print pattern or a circuit diagram, the circuit will not operate properly. Even in the absence of errors, if insufficient attention has been paid to noise or heat generation, the expected performance and long-term stable operation will not be attained. Upon such occasions one is inclined to peremptorily discard many problem boards and start again from scratch, but this adds to cost and man-hours. In such circumstances, by using the tools and techniques at hand to change out components or repair wiring, many problems can be resolved.

Technique 1: Removing components

Hold a soldering iron against a mound of solder, to warm and melt it (Figure 1). Then, use a suction apparatus to remove the melted solder (Figure 2). Eliminate solder until the terminals are loose.

The key here is to adequately melt the solder. If the solder cannot be sucked away adequately, rebuild the solder and try once again.

Figure 1. Removing a transistor with leads: heating with the soldering iron

Figure 2. Removing solder. The resin at the tip of the suction apparatus does not melt.

Technique 2: Removing a transistor with leads from a through-hole board

Lead wires of a component that has been mounted in through-holes (penetrating holes) of a multilayer board are firmly attached to the board by solder that has entered the holes and is on the inner walls. Hence the solder cannot be completely removed, and will not come off easily.

A motorized automatic desoldering pump can be used to forcibly remove solder easily by suction (Figure 2).

In the case of components with few leads such as discrete transistors, the soldering iron is held parallel to the leads and brought into contact, heating all the leads at once to melt the solder, and pliers are then used to grasp the transistor body and pull it away at once before the solder hardens. Then, a soldering iron and a solder suction apparatus are used to remove solder that has entered into through-holes. The resin tip of the solder suction apparatus will not be melted by the soldering iron. When all the solder has been sucked out, the lead wires of a component will move freely. The surrounding board area may be soiled by flux; use ethanol and a cotton swab to clean it (Figure 3).

Figure 3. Suck out solder until lead wires move, and use ethanol and a cotton swab to wipe soiled surfaces.

Technique 3: Removing chip components

Chip resistors and chip capacitors are removed using two soldering irons to simultaneously heat the two pads and remove the component (Figure 4). Bring the soldering irons into contact parallel to the chip component, and heat both pads at the same time (Figure 5).

Figure 4. Heat two pads simultaneously using soldering irons

Figure 5. When the solder has melted, shift the lead sideways from the pad

Move leads horizontally away from the pads without lifting the component (Figure 6). Using tweezer-type irons (“hot tweezers”), this task is easily performed.

Figure 6. Remove the chip from the pads

Once the component has been removed, use a solder wick to remove solder on the pads (Figure 7) in preparation for re-mounting. This task can be performed effectively by causing the solder wick to absorb a very small amount of solder in advance.

Figure 7. Use a solder wick to remove solder on pads

After the solder has been suctioned away, use ethanol to wipe away any flux adhering to the board that had been present in the solder wick (Figure 8).

Figure 8. Wipe away adhering flux using ethanol

Technique 4: Removing surface-mounted ICs

Removal of an IC that has only two rows of pins is comparatively easy. One masses a large amount of solder at the pins on both sides, and then uses two soldering irons to heat the solder and remove the IC. Rather than lifting the IC upward, it should be moved horizontally so that the pins are moved away from the pads.

Removal of a QFP (Quad Flat Package), having pins on four sides, is somewhat more difficult.

First, place large amounts of solder along the four rows of pins, without worrying about solder bridging. Using soldering irons, apply heat such that all of the solder at the four rows of pins melts. When the solder has melted, slide the IC away from the pads.

Solder tends to cool quickly and harden, so caution is required. It should be easier to begin by using solder with a low melting point.

Here re-use of the IC is not being considered, so after using a knife to sever the IC pins and removing the package, a solder wick is used to remove the pins and solder.

▸ Removing an IC with rear-surface pads

ICs having heat-dissipating pads on the rear surface, which have been increasingly common of late, are removed for rework by heating with a heat gun. However, because damage to the IC and board is considerable, this method is not recommended. One trick here is to apply a somewhat hard ointment-form flux and then apply heat.

Technique 5: Sever a printed pattern

A method in which a printed circuit board (printed pattern) is severed is explained.

A printed pattern that runs over the top surface and the rear surface can easily be severed using a utility knife. Cutting is performed at a location chosen so as to avoid dense print patterns, in order to avoid damaging other pattern areas (Figure 9).

Figure 9. Sever the pattern where the density is low

The utility knife is used to make incisions from two directions, forming a V shape (Figure 10). The most common glass epoxy boards (FR-4) are hard, and so one must grow accustomed to applying force. The incisions may be made somewhat deep.

Figure 10. Making incisions in a V shape with the utility knife

After severing the pattern, the white substrate below the copper should be visible (Figure 11). In addition to visual inspection, use a tester to confirm that there is no longer electrical continuity.

Figure 11. When white substrate is visible, severing is successful

Technique 6: Pulling out wiring from midway in a printed pattern

If wiring can be soldered midway in a printed pattern, the signal can be passed to a different signal path.

For IC and connector terminals and the like, pads can generally be found somewhere on a wiring path. But in rare cases, it may not be possible to find a place where wiring can be soldered. There are also cases in which one may want to put wiring on a rear surface with no components in order to improve the appearance of the top surface.

As wiring, single-strand “wrapping wire” with a thickness of AWG30 is recommended; it is sold as “30 AWG ETFE wire”.

First, a place on the board where the wiring pattern is not dense is selected as the place to add the wire (Figure 12).

Figure 12. Add the wire at a place where the pattern is not dense

Use the tip of a utility knife (not the blade) to scrape the board surface and remove the resist (Figure 13), exposing the copper (Figure 14). The copper will not be reduced in thickness merely by scraping.

Figure 13. Scrape the resist using the tip of a utility knife

Figure 14. Expose a sufficiently large area of copper

Peel away the insulation of the wire and wet it with solder (Figure 15). This is what is known as “pretinning”.

Figure 15. Peeling the insulation away and wetting the wire with solder

The exposed copper of the board is also wetted with solder (pretinning) (Figure 16). At this time, a large amount of solder is deposited in a mound (Figure 17).

Figure 16. Wetting the exposed copper with solder

Figure 17. Depositing solder on the copper forming a mound

Use tweezers to grasp the wire, and hold it in place until the solder has hardened (Figure 18).

Figure 18. Hold the wire in place with tweezers

Once the solder has hardened (Figure 19), confirm that it does not come loose even when pulled.

Figure 19. Using a magnifying glass for visual inspection, confirm that the wire does not come loose when pulled

Technique 7: Connecting terminal to terminal using wire material (wiring)

A method for connecting two points, pad A and pad B, with wire material is explained.

As the wire material, single-strand AWG 30 wire is used.

Use nippers and a wire stripper to remove from 30 to 50 mm of the insulation of the wire material (Figure 20). Deposit solder on the two pads that are to be wired together (Figure 21).

Figure 20. Remove 30 to 50 mm of the insulation of the wire material to be added

Figure 21. Wet the pads for wiring with solder

Solder the wire material to one of the pads (Figure 22).

Figure 22. Solder the additional wire material to one of the pads

Cut a length of insulation adequate for wiring (Figure 23), and use tweezers to move the insulation length to the end of the wire (Figure 24). A wire stripper is normally used to cut the insulation, but when the work area is cramped, the abovementioned small nippers can be useful here.

Figure 23. Cut the insulation in a length necessary for wiring

Figure 24. Move the cut insulation length to the wire end using tweezers

Solder the jumper wire to the other terminal (Figure 25), and use the nippers or a utility knife to cut the wire (Figure 26).

Figure 25. Solder the additional wire material to the other pad

Figure 26. Cut the wire

The wire material should be either stretched taut, or else bent 90°.

When the wiring distance is great, the resin product “Hackle”, which can be heated and melted using a soldering iron, is used to fasten the wire material to the board. A glue gun or epoxy adhesive could also be used. A glue gun is used in places where it may be necessary to again remove the wire; an epoxy adhesive is used when there will never be such a need. There is also the method of using polyimide tape (Kapton tape) to simply bind the wire to the board.