Capacitor Replacement Procedure

A 330 µF 10 V surface-mount electrolytic (C142), whose dark top segment marks the negative side, beside an empty footprint (C165) with the positive pad marked + on the silkscreen.

Capacitor replacement is the bench work of taking old electrolytic capacitors off a board and fitting new ones of suitable rating, the right way round. Most of the risk is to the board. The pads under a capacitor tear easily if the part is levered or pulled before its solder has melted, and leaked electrolyte may already have attacked the joints and tracks around it. Nichicon's advice for removing any soldered capacitor is to melt the solder enough that no stress is applied to the leads.

How to tell that a capacitor has failed, and why particular ratings matter, is covered in Capacitor Failure Symptoms. The values for individual machines are in the guides in Category:Capacitor Replacement Guides.

When to use it

• A capacitor has been shown to be faulty; see Capacitor Failure Symptoms.
• A machine has capacitors of a type known to leak. On the MSX HIC-1 video board, sd_snatcher advises replacing C10, C14 and C22 even in machines kept in storage, because the damage is done when they leak.
• Electrolyte has already leaked onto the board. Take the failed parts off first and then clean the board; see Battery Explosion, Capacitor or Corrosion Damage.
• A tantalum capacitor has gone short-circuit across a supply rail. Minuszerodegrees finds the tantalum on the +12 V line the usual cause of a short on that line on IBM 5150 and 5160 motherboards.

Safety

• Unplug the machine. Discharge each capacitor before removing it; Nichicon says to discharge it through a 1 kΩ resistor, and that a voltage can reappear between the terminals of a capacitor that has been in use. On a machine with a picture tube, follow the CRT Discharge Procedure first.
• A capacitor whose vent opens releases hydrogen and vaporised electrolyte at over 100 °C. Keep your face away from a capacitor that is venting, and switch off. Wash electrolyte off skin with soap and water, and flush eyes with water.
• An electrolytic fitted the wrong way round can pop, swell or burst and then act as a short circuit, and a reversed solid tantalum can be permanently damaged or destroyed.
• For the heat, fume and lead hazards of desoldering, see Through-Hole Desoldering.

Tools and materials

• A soldering iron and desoldering tools; see Through-Hole Desoldering.
• For surface-mount parts, hot tweezers, a second iron, a hot-air station or low-melt removal alloy; see Surface-Mount Rework.
• Flush or side cutters, for cutting off old parts and trimming new leads.
• A sharp craft knife.
• A 1 kΩ resistor with insulated leads, for discharging.
• Isopropyl alcohol, swabs and a brush. Nichicon lists isopropyl alcohol among the cleaning agents that are safe near aluminium electrolytics, and warns against halogenated cleaners.
• The replacement capacitors, chosen as below.

Choosing replacements

Choosing a replacement capacitor

Property	What to check
Capacitance	The value in the machine's parts list or capacitor guide. Nichicon asks for the ratings to be confirmed before a capacitor is fitted.
Voltage	At least the rating of the part removed; see Capacitor Failure Symptoms. Nichicon also requires the DC voltage plus the ripple peak to stay below the rated voltage.
Temperature and life	A 105 °C part outlasts an 85 °C part of the same endurance at the same temperature; see Capacitor Failure Symptoms.
ESR	niga2 recommends low-ESR electrolytics for the switching supply of the Panasonic FS-A1F. Next to a linear regulator, check the regulator's data sheet first. Older low-dropout regulators need an output capacitor whose ESR lies within a range: TI's TPS76050 rang repeatedly after a load step with a 2.2 µF ceramic capacitor, a sign of instability, and settled once 1 Ω was added in series.
Size and lead spacing	The new part has to fit the space, and its lead spacing has to match the hole spacing. Where it does not, bend the leads before soldering, without straining the capacitor. Nichicon asks for at least 2 mm of clearance above the vent of a can 6.3 to 16 mm across, and 3 mm for 18 to 35 mm.
Type	Small electrolytics can sometimes be replaced by ceramic or tantalum parts. For the HIC-1 board, sd_snatcher gives 3216 (1206) ceramics for the 10 µF positions and a 4532 (1812) ceramic or 6032 tantalum for 100 µF, all rated 16 V or more. Check the ESR point above before changing type near a regulator.

Polarity marks

Aluminium electrolytics with liquid electrolyte, in surface-mount (vertical chip), axial, radial and snap-in styles: the mark identifies the negative terminal.

Solid-electrolyte chips (aluminium polymer and tantalum) and a tantalum bead: here the mark identifies the positive terminal.

• An aluminium electrolytic marks its negative lead with a minus sign or a coloured stripe along the can, and a new radial part may have a longer positive lead.
• On a surface-mount aluminium electrolytic, a dark segment printed on the top of the can marks the negative side, as in the photograph at the top.
• A solid tantalum capacitor marks its positive terminal with a stripe, and sometimes with a bevelled edge as well. The stripe therefore means the opposite on a tantalum chip to what it means on an aluminium can.
• Boards often mark the positive pad or hole with a + on the silkscreen, as in the photograph at the top. Do not assume a layout is the same on every revision of a board: minuszerodegrees found that the three-hole tantalum positions on IBM 51xx motherboards and cards are wired in two different ways.

Procedure

Prepare the board

1. Take the board out and note the value, voltage and orientation of each capacitor before removing it.
2. Discharge each capacitor through a 1 kΩ resistor.
3. Keep the iron off the bodies of capacitors that are staying on the board.

Remove a through-hole capacitor

1. Add fresh solder to the old joints if they are dull or hard to melt.
2. Desolder both leads with a sucker, braid or desoldering station; see Through-Hole Desoldering.
3. Melt the solder fully before pulling. Nichicon's rule is to melt it enough that no stress reaches the leads.
4. Lift the capacitor out once both leads are free.

Where the board does not take repeated heating well, minuszerodegrees suggests a method for IBM 5150 boards, whose power and ground lines are hard to solder:

1. Cut the old capacitor off above the board, leaving its legs standing in the holes.
2. Solder the new capacitor's legs to the old legs, with the same polarity.

Minuszerodegrees also removes a non-essential failed tantalum from the later 5150 motherboard by simply cutting it off with side cutters.

Remove a surface-mount electrolytic

Surface-mount electrolytics sit on pads that lift easily, and the sources differ on the safest method.

1. Heat both joints at once, with hot tweezers or with two irons, one on each end.
2. Lift the capacitor away while both joints are molten.
3. Without two irons, cut the capacitor's legs carefully with a craft knife and then unsolder each stub on its own.

Other methods:

• Hot air or low-melt removal alloy; see Surface-Mount Rework. On the PowerBook Duo, Fizzbinn melted Chip Quik with plenty of flux into each joint and lifted the can off with a single iron (see the worked examples).
• RetroTechie cuts through the solder itself with a sharp knife, not the track beneath, until the part can be pulled away; the board is not heated until the new part goes on.
• niga2 twists the can off. He grips the waist of the can with side cutters, cutting parallel to the line of the two terminals, which in his experience lifts fewer pads, and wrenches the top off. He then removes the rest of the can and the seal, cuts the legs, and lifts away the plastic base.

The twist-off method is the one the sources disagree about. niga2 notes that pads still lift sometimes, Frank Gründel warns that removing a surface-mount capacitor any way other than with two irons or by cutting its legs can easily rip the pads off the board, and Nichicon's general rule is that a capacitor's leads should not be stressed during removal.

Clean the board

1. Clean off the leaked electrolyte with isopropyl alcohol, keeping to Nichicon's limits on time and temperature; see Cleaning up after a leak.
2. Check the joints around each old capacitor. niga2 found that joints which had turned dull, or smelt of electrolyte when heated, had been attacked.
3. Scrape corroded tracks back to bright copper with a craft knife and resolder them; niga2 found the copper under the corrosion was often still sound.
4. Test the tracks and plated-through holes around each position for continuity before fitting anything; see Continuity Testing and Signal Tracing.
5. Repair broken tracks, pads or vias; see PCB Trace Repair, PCB Pad Repair and Via and Through-Hole Repair.
6. Clear the old solder from the holes or pads; see Through-Hole Desoldering and Surface-Mount Rework.

Fit a through-hole capacitor

1. Check the new part's ratings against the list.
2. Check its polarity against the board.
3. If the lead spacing does not match the holes, bend the leads before fitting, without straining the capacitor.
4. Insert the capacitor with its negative stripe on the negative side.
5. Solder both leads, keeping the iron off the body; see Through-Hole Soldering.
6. Trim the leads with flush cutters.
7. Do not tilt, lay down or twist the capacitor once it is soldered.
8. Clean off the flux. Nichicon warns that fluxes containing non-ionic halides can get into a capacitor and corrode it.

Fit a surface-mount capacitor

1. Check the ratings and polarity, as above.
2. Tin one pad.
3. Hold the capacitor in place with tweezers, the right way round, and reheat the tinned pad to fix that end.
4. Solder the other end.
5. Do not let the capacitor move once it is soldered; niga2 found the pads on the FS-A1F's HIC board lift very easily.
6. Clean off the flux, as above.

niga2 fitted ordinary leaded capacitors on the HIC board's surface-mount pads, and soldered one straight to an IC pin where its pad was too badly corroded. For the soldering methods, see Surface-Mount Rework.

Checking the repair

1. Check every new capacitor against the list for value, voltage and orientation.
2. Inspect each joint; see Through-Hole Soldering and Surface-Mount Rework.
3. Measure across each supply rail for a short before applying power; a shorted tantalum shows up as a short on its rail. See Finding Short Circuits.
4. Power up with the board in view, and switch off at once if a capacitor smokes or vents.

Common mistakes

• Fitting an electrolytic the wrong way round.
• Reading a tantalum's stripe as negative. On a tantalum it marks the positive terminal.
• Pulling or levering a capacitor before its solder has melted, which tears pads and tracks.
• Leaving leaked electrolyte on the board. Nichicon notes that electrolyte is conductive and can cause smoking or fire where it lies on a board.
• Cleaning with halogenated solvents, or leaving flux residue that contains halides.
• Bending or twisting a new capacitor after it has been soldered.
• Blowing hot air over electrolytics that are staying on the board. Nichicon warns that their sleeves can shrink, bulge or crack after more than two minutes at 150 °C, or under hot gas.
• Changing to a much lower-ESR capacitor at the output of an older low-dropout regulator without checking its data sheet.
• Reusing a capacitor taken from a board that has been powered. Nichicon advises against it, because a voltage can reappear between the terminals.

Worked examples on this wiki

• IBM PC (5150) Capacitor Replacement Guide: tantalums on the +12 V line, three-hole positions wired two ways on different board revisions, and the cut-off-and-graft method for the board's hard-to-solder power lines.
• Panasonic FS-A1F Capacitor Replacement Guide: niga2's twist-off method for the surface-mount electrolytics on the HIC-1 board, pads that lift very easily, and low-ESR parts for the switching supply.
• Macintosh Centris 610 Capacitor Replacement Guide: hot air or low-melt solder for surface-mount electrolytics, with a warning not to twist them off.
• PowerBook Duo Capacitor Replacement Guide: Chip Quik and flux to lift each can with one iron where the cans are packed too tightly to twist, and a via eaten through by electrolyte.
• Apple Newton MessagePad Capacitor Replacement Guide: two irons, or cut legs and separate stubs, for C1 on the Original MessagePad.
• Atari ST Book LCD Board Repair: the old capacitors removed by cutting their leads, Kapton tape round the work, and resistors damaged by leaked electrolyte.

Related pages

• Capacitor Failure Symptoms
• Battery Explosion, Capacitor or Corrosion Damage
• Category:Capacitor Replacement Guides
• Through-Hole Desoldering
• Through-Hole Soldering
• Surface-Mount Rework
• PCB Pad Repair
• PCB Trace Repair
• Via and Through-Hole Repair
• Finding Short Circuits
• CRT Discharge Procedure
• Nichicon Application Guidelines for Aluminium Electrolytic Capacitors
• Recommended Tools