Burnt PCB Repair

A step-down converter IC that has burnt on a multilayer laptop board. The package is charred and split, the laminate beside it is blackened, and copper shows where the surface has burnt away (photo: Profpcde, CC0).

Burnt PCB repair deals with circuit board laminate that has been scorched, carbonised or blistered by an overheating part, an arc or a short circuit. The charred material is cut or milled out down to clean laminate, the cavity is filled with epoxy, and the tracks that ran through the area are rebuilt or re-routed round it. A board burnt right through needs a new piece of laminate let in, which is specialist work.

The burnt material has to come out because carbonised board conducts. Tracking, the surface breakdown measured by the Comparative Tracking Index, starts when heat from arcing carbonises an insulating material; the carbonised areas conduct better than the clean insulator, pass more current, heat further, and eventually the insulation becomes completely conductive. SGI Octane repairers clean charring on a supply's primary side down to the copper before power is applied, because carbon conducts at mains voltage.

Repair the damage only after finding what caused it: a part that failed, a part that runs too hot, a leak of electrolyte, or conductors too close together for the voltage between them.

When to use it

Two burned J1 yoke connectors from Macintosh analogue boards; Larry Pina notes that the damage is always on pin 4, the horizontal sweep connection (Macintosh Repair & Upgrade Secrets, Figure 5-18).

• The board is brown or black under a part that ran hot. On the Macintosh Color Classic analogue board, the paralleled 18 V zeners DL21 and DL22 overheat and scorch the board under them.
• A short or a failed part has burnt a track open. On a DEC 5411086 power line monitor, a shorted bridge rectifier burnt the AC input traces at the edge connector.
• Leaked electrolyte has shorted parts and burnt the board, as in Sharp X68000 power supplies.
• An arc has tracked between two conductors. On a VOX AC15 amplifier board, a conductive path formed between two terminals with a 370 V rail very close to ground.
• The laminate has blistered or delaminated from heat without charring; Circuit Technology Center repairs that by injecting epoxy into the blister.

Measure the resistance between conductors across the marked area, with the parts that bridge them removed; carbonised board shows a resistance where there should be none. After chipping out the burnt board on the VOX amplifier, the repairer found no conductivity left between the terminals.

If the damage is a single burnt track on sound board, PCB Trace Repair or Jumper Wire Repair may be all that is needed.

Safety

• Unplug the machine and wait for supply capacitors to discharge, then measure them. The primary side of a Sharp X68000 supply still holds more than 100 V DC after the plug is pulled. On equipment with a picture tube, follow the CRT Discharge Procedure.
• Power a repaired supply for the first time through a current limit. One repaired SGI Octane supply exploded at its first power-up and tripped the house breaker; repairers use a current-limiting lamp. A current-limited bench supply does the same job on a low-voltage board.
• Scraping and milling the board can generate static charge; take anti-static precautions.
• Mix and cure epoxy as its maker directs, and do not heat-cure at a temperature the parts on the board cannot tolerate.

Tools and materials

• A sharp knife and a hardened scraper, and for larger areas a small drill with ball mills.
• Isopropyl alcohol, to flood the area and show up damaged fibres.
• Clear two-part epoxy, with a colour agent to match the board if wanted, and polyimide (Kapton) tape for masking.
• A foam swab to texture the epoxy surface over a large area.
• A multimeter on its highest resistance range. The VOX repairer checked a cured sample of his epoxy with a megohmmeter: over 2000 MΩ at 500 V, and much lower at 1000 V.
• Wire, copper foil and replacement pads for rebuilding the circuit; see PCB Trace Repair, Jumper Wire Repair and PCB Pad Repair.
• Replacement parts for whatever burnt, uprated where the original ran hot. Console5's kit for the Macintosh Color Classic analogue board replaces DL21 and DL22 with a single 1N5355B 5 W 18 V zener and fits 5 W resistors at RL62, RL22 and RF11.

Procedure

Record and remove

1. Photograph the area and copy down the tracks that run through it, from the board or the schematic. Circuit Technology Center has diagrams or photographs of the original circuits to hand before it removes damaged base material, because surface tracks in the area may have to be replaced.
2. Find the cause of the burn and the parts it took with it. On a Sharp X68000 supply, leaked electrolyte had shorted a zener, R33, R34 and a transistor, the power transistor below them had burst and the fuse had blown; all of them were replaced with the capacitors.
3. Remove the burnt parts and any part in the area that would get in the way of the repair; on the VOX amplifier the connector was desoldered so that the burnt board under it could be drilled out.
4. Clean the area.

Cut out the charred material

The charred laminate (1) is removed down to clean board (2), then the cavity is filled level with epoxy and the circuit rebuilt across it (3). After Circuit Technology Center procedure 3.5.1.

1. Scrape away the damaged board and its solder mask with a knife, or mill it out with a ball mill. All the damaged material has to go.
2. Flood the area with alcohol. Damaged fibres inside the laminate show up clearly; remove them too.
3. Go beyond what looks clean. On the VOX amplifier, another contributor judged that carbon was still left after the owner's first attempt, and marked a larger section to drill or grind out.
4. Remove all loose material and clean the area.
5. Measure across the cleaned area between the conductors that were affected. There should be no conductivity.

Fill the cavity

This is Circuit Technology Center's base material repair, epoxy method (procedure 3.5.1), for damage that goes deep into the board but not through it.

1. Mask exposed parts of the board round the area with polyimide tape.
2. Warm the board if the parts allow it. A preheated board lets the epoxy flow and level; on a cold board, epoxy can settle below the surface as it cures.
3. Mix the epoxy, adding a colour agent if wanted.
4. Fill the cavity up to the board surface, leaving a slight overfill for shrinkage, so that no laminate fibres are exposed. Spread it with a sharpened mixing stick; for a large area, dab it with a foam swab to texture the surface.
5. Cure the epoxy. Circuit Technology Center cures its epoxy for 24 hours at room temperature or 4 hours at 65 °C.
6. Remove the tape, and scrape the epoxy down level with the surrounding board.
7. Clean the area and, if needed, seal any scraped areas with a thin coat.

Where the damage goes right through the board, Circuit Technology Center mills out the whole area, machines a stepped joint about half the board's thickness into the edge, and bonds in a matching piece of laminate of the same thickness and type with epoxy, before drilling holes and adding circuits (procedure 3.5.2, expert skill).

Rebuild or re-route the circuit

AC input traces burnt at the edge connector of a DEC 5411086 power line monitor after its bridge rectifier shorted (photo: Jonathan Chapman, Glitch Works).

1. Rebuild tracks across the filled area with wire or copper foil lapped onto sound copper at each end, as in PCB Trace Repair.
2. Where the area is too damaged to carry tracks again, run insulated jumper wires round it to the next sound points on each net, as in Jumper Wire Repair. On the DEC 5411086, Jonathan Chapman cut back the damaged edge-connector traces and soldered a short AC pigtail to the board, with an in-line 15 A fast-blow fuse.
3. Inner-layer connections lost in the damage have to be restored with surface wires.
4. Replace pads that burnt away; see PCB Pad Repair.
5. Fit the new parts. Where a part ran hot, stand it off the board on its leads so that it can cool, as recommended for the Color Classic's replacement zener and resistors.

Delaminated or blistered board

For a blister without charring, Circuit Technology Center drills at least two small holes into opposite edges of the blister, clear of tracks, bakes the board to drive out moisture, and injects low-viscosity epoxy into one hole while the board is still warm, so that it is drawn through the void. Gentle pressure from the fill hole towards the vent hole, or a vacuum on the vent, helps it fill.

Checking the repair

• Measure resistance between each pair of conductors that ran through or beside the burnt area. There should be no conductivity across the repair.
• Check continuity along every rebuilt track and jumper.
• Check that the epoxy is level with the board, fully bonded and not chipped, and that none of it covers pads or other circuits.
• Power the board for the first time through a current limit, and check the supply rails before connecting anything else.
• Test every function that passes through the repaired area.

Common mistakes

• Leaving carbon in the board. Any remaining char can conduct and the fault can return, as the second repairer warned on the VOX amplifier.
• Replacing the burnt part without finding why it burnt. Sharp X68000 repairers are told to find the cause before fitting a new fuse.
• Refitting a part that runs hot flat against the board. One Color Classic owner's 1 W replacement at RL62 smoked again during adjustment.
• Filling over loose material or contamination. Circuit Technology Center removes all loose material and cleans the area before filling.
• Filling a cold board, so that the epoxy settles below the surface as it cures.
• Powering a repaired supply straight from the mains.

Worked examples on this wiki

• PDP-11/34 Power Supply Restoration: AC traces burnt by a shorted bridge rectifier, cut back and replaced with a fused pigtail.
• SGI Octane Troubleshooting Guide: burnt load resistors and charring in Lucent supplies, cleaned to the copper before power-up, and a repaired supply that exploded at first power-up.
• Macintosh Color Classic Troubleshooting: zeners and resistors that scorch the analogue board, and the uprated parts that replace them.
• Sharp X68000 Troubleshooting Guide: electrolyte that burnt the supply board and shorted a transistor and fuse.
• Macintosh Plus Flyback Transformer Replacement: a burned board round Q3 and burning at flyback pin 4 after a shorted flyback.
• Panasonic FS-A1GT Troubleshooting Guide: a burnt-out track from the supply rectifiers bridged with wire.

Related pages

• PCB Trace Repair
• Jumper Wire Repair
• PCB Pad Repair
• Via and Through-Hole Repair
• Finding Short Circuits
• Testing Diodes, Transistors and Regulators
• Capacitor Failure Symptoms
• Battery Explosion, Capacitor or Corrosion Damage
• CRT Discharge Procedure