Through-Hole Desoldering

Desoldering from the solder side of a board with a hand-held desoldering gun, a dic SC-7000.

Through-hole desoldering is the removal of the solder from the joints of a part whose leads pass through holes in the board, so that the part can be lifted out without damage to the board. It comes before most other repairs on a vintage machine: replacing a failed chip or capacitor, fitting a socket, or taking a part out to test it. It is also where boards are most often damaged. Adafruit's soldering guide notes that a lifted pad most often happens when a part is being desoldered, and on a double-sided board with plated-through holes a lead that is pulled while any solder still holds it can tear the plating out of the hole, or break a track that is then hidden under the new part.

All the methods below do the same job: melt the solder in each hole, take it away, and free each lead before any force is put on the part. The choice between a hand pump, braid and a powered desoldering tool depends on how much solder there is, how much heat the joint can take and how many joints there are. A cheap part that has already been proved faulty can be cut out instead, which puts much less stress on the board.

When to use it

Desolder a part when it has been proved faulty, when a socket is to be fitted in its place, or when it has to come out of circuit to be measured. Before starting, decide whether desoldering is needed at all:

• A chip that is already in a socket is pulled, not desoldered; see IC Removal and Socketing.
• A surface-mount part needs different tools; see Surface-Mount Rework.
• A joint that is cracked or dull but otherwise sound is reflowed, not removed; see Through-Hole Soldering.
• A cheap logic or RAM chip that has been proved faulty is safer to cut out than to desolder whole, as described below.
• A pad, track or hole that is already damaged needs repair before a new part goes in; see PCB Pad Repair, PCB Trace Repair and Via and Through-Hole Repair.
• For an electrolytic capacitor, see Capacitor Replacement Procedure, which also covers the surface-mount types.

How much heat a joint needs depends on the board. Hakko's instructions for its FR-301 desoldering tool give the lowest setting for single-sided boards, a higher one for boards with plated-through holes, and the highest for multilayer boards. On a double-sided board the solder has to be molten on both sides of the board at once, and the power and ground pins, which sit in large areas of copper, draw heat away fastest.

Safety

• A soldering iron element can reach 400 °C. Hold the work with tweezers or clamps, keep the iron in its stand, and switch off and unplug it when finished. The nozzle of a desoldering tool is hot whenever the tool is on, and the solder that collects in its filter is still extremely hot when it is emptied.
• Wear eye protection: solder can spit when it is heated.
• Lead from solder enters the body mainly by mouth, from hands that were not washed before eating. Wash with soap and water after the job; gloves are not a substitute.
• Rosin (colophony) flux fume irritates the eyes, throat and lungs, and repeated exposure can cause asthma. Use fume extraction.
• Unplug the machine before taking the board out. Discharge a capacitor before removing it. On a machine with a picture tube, follow the CRT Discharge Procedure first.

Tools and materials

Desoldering tools

Tool	How it works	Notes from the sources
Hand solder sucker (spring-loaded pump) and a separate iron	The iron melts the joint; the button on the sucker releases its plunger, which draws the molten solder up the nozzle	Ray Carlsen prefers a large sucker, such as the Edsyn Soldapullt, for its higher vacuum, and finds it easier to keep clean than an all-in-one vacuum iron. Bob Wettermann of BEST Inc. advises against a manual pump for clearing pads, because its suction is not continuous while the joint heats and cools.
Desoldering braid (wick)	Copper braid that soaks up molten solder	Chemtronics sells Soder-Wick in widths from 0.8 mm (size 1) to 5.3 mm (size 6), in rosin, no-clean and unfluxed versions. Carlsen uses braid and a larger tip for the stubborn power and ground pins.
Desoldering station or gun	A heated hollow nozzle with a vacuum pump behind it; the trigger draws the solder through the nozzle into a filter	Hakko's FR-301 runs at 350 to 500 °C and comes with a 1.0 mm nozzle and cleaning pins. Carlsen found that vacuum irons clog easily unless they are the expensive kind with an electric pump and a chamber for the waste solder.
Hot air	Warms the whole part	For loosening a part once all of its pins are already clear of solder.

Also needed:

• A temperature-controlled iron. NASA's soldering standard required irons that hold the set temperature within ±5.5 °C, and did not allow soldering guns. SparkFun recommends a larger tip for joints on ground planes and large plated holes.
• Fresh flux-cored solder for re-wetting old joints. NASA specified Sn60 or Sn63 alloy with R or RMA flux. Kester gives Sn63Pb37 a melting point of 183 °C and Sn60Pb40 a melting range of 183 to 190 °C.
• Extra flux, as liquid, gel or paste.
• Fine tweezers, a strong light and a magnifier, and small flush cutters.
• Isopropyl alcohol and a brush or lint-free wipes for cleaning off flux.

Procedure

Prepare the board and the joints

1. Unplug the machine, take the board out, and note which way round the part is fitted. A chip's pin 1 is marked by a notch or a dot at one end; an electrolytic capacitor's negative lead is marked by a stripe or a minus sign.
2. Clean the iron tip and tin it with a thin coat of solder.
3. Set the temperature. The figures vary with the tool and the job: Chemtronics suggests about 315 °C for braid, adjusted as needed, and its braid data sheet gives 600 to 750 °F (about 315 to 400 °C); Hakko's FR-301 scale covers 350 to 500 °C, set lowest for single-sided boards and highest for multilayer boards. A higher setting wears a desoldering nozzle faster.
4. Add a little fresh solder and flux to the joint. Hakko suggests this to form a heat bridge that helps the joint heat up.

With a solder sucker

A spring-loaded hand solder sucker, with its release button on the barrel and a white nozzle.

1. Heat the joint for about two seconds, until the solder is molten on both sides of the board.
2. Use the tip of the iron to ease the lead towards the centre of its hole.
3. Put the nozzle of the sucker over the joint and press the button on the plunger to draw the solder away.
4. Check that the lead now moves freely in the hole. If it is still stuck to the wall of the hole, wiggle it with tweezers or press it sideways, towards the body of the chip, until it pops free.
5. If solder is still left in the hole, add fresh solder and desolder the joint again.
6. For the pins on large copper areas, use braid and a larger tip, or a second iron on the other side of the board.

With desoldering braid

Desoldering braid after use. The middle section has soaked up solder and turned silver; the ends are still copper coloured.

These steps follow Chemtronics' instructions, which are based on IPC-7711 method 3.1.5.

1. Choose a braid about as wide as the solder to be removed. Chemtronics says equal to or slightly wider than the pad; Wettermann, describing the braid method for surface-mount lands (IPC-7711 method 4.1.3), says slightly narrower than the pad.
2. Choose a tip about as wide as the braid.
3. Work from the solder side of the board.
4. Add flux to the joint or to the braid if the braid is unfluxed or the wicking is slow.
5. Lay the braid over the junction of the lead and the pad, and press the iron tip onto the braid directly over the solder. Take care where you hold the braid: heat travels along it.
6. Watch the braid change from copper to silver as it fills with solder.
7. Lift the iron and the braid together as soon as the colour stops spreading. Heat held on for longer only adds to the risk of lifting the pad.
8. Do not drag the braid across the pads while it is hot. Wettermann warns that this can lift pads and damage the solder mask, and NASA's surface-mount standard warns that braid moved about during heating will in most cases loosen or remove the land.
9. Cut off the used length of braid before moving to the next joint.
10. On a part with many leads close together, skip and alternate between joints so that heat does not build up in one place.

With a desoldering station or gun

These steps follow Hakko's instructions for the FR-301.

1. Fit a nozzle that suits the job. Wettermann recommends matching the tip to the width of the pad, because a tip larger than the pad can burn the laminate.
2. Switch on, wait for the nozzle to reach temperature, and coat its end with solder.
3. Place the nozzle over the lead and heat the lead and the solder, not the pad. Hakko warns that a nozzle placed directly on the land may make it peel off.
4. With the nozzle still in place, move the lead gently. If it moves easily, all the solder in the joint has melted.
5. Pull the trigger to draw the solder away.
6. If solder is left, re-solder the joint with new solder and desolder it again.
7. Clean the nozzle often: wipe it like a soldering tip, push the matching cleaning pin through its bore while it is still hot, and empty the solder that collects in the filter tube.
8. Replace a worn nozzle instead of turning up the temperature, which only speeds up the wear.

Freeing and lifting a multi-pin part

1. Check every pin before lifting anything. Wiggle each one with tweezers to make sure it is free.
2. Push sideways on the top of the chip one way and then the other; pins that were stuck can be heard clicking free.
3. If a lead was bent over (clinched) when the board was made, straighten it first. IPC-7711 has separate removal methods for partly and fully clinched leads.
4. Once every pin is clear of solder, a little hot air on the part can make the last step easier and less risky.
5. Pry up one end of the chip gently with a small screwdriver, then the other. If any pin sticks, stop and free that pin before going on.
6. Do not force the part out, even when all the pins look free.

Cutting out a part that is being scrapped

Ray Carlsen recommends cutting the pins off small, cheap chips such as RAM, because replacements cost little and the board is not put at risk; the larger custom chips should be desoldered intact unless they are certainly faulty. Dave Curran of Tynemouth Software did this on a VIC-20 board when he could not get all the pins of a RAM chip free at the same time, after first proving the chip faulty; with the legs cut, every leg came out cleanly.

1. Prove that the part is faulty before cutting it. Once its legs are cut it cannot be tested or used again.
2. Cut through each leg and lift the body of the part away.
3. Remove the leg stubs one at a time.
4. Clear the solder from each hole.

Carlsen notes that a new chip can instead be soldered onto the old stubs, but removing them and fitting a socket makes a neater job.

Clearing the holes

1. Touch each hole briefly, for about half a second, with a clean iron. Any solder left in the hole moves out to its surfaces.
2. Alternatively, push a large pin gently into the hole and move it round; solder is soft and moves out of the way.
3. Hold the board up to a light to check that every hole is clear.

Checking the repair

1. Look over both sides of the board with a strong light and a magnifier, for broken tracks near the holes and for splashes of solder that could short neighbouring pins.
2. Look at the pins of the part that came out. A copper-coloured blob on a pin is plating torn out of the hole, and the connection between the two sides of the board at that hole is broken. Repair it before fitting anything; see Via and Through-Hole Repair.
3. Check the board against NASA's rejection criteria for printed wiring boards: no conductor pattern separated from the board (a lifted pad or track), no burns, no discoloration that bridges separate conductors, and no delamination.
4. Check continuity from each pad to the next point on its track, and check neighbouring pads for shorts. On a ZX Spectrum board, 1024MAK advises testing all the tracks and plated-through holes after the chips are out and before sockets are fitted. See Continuity Testing and Signal Tracing.
5. Clean off the flux with isopropyl alcohol. NASA's standard called for flux residue to be removed within half an hour of soldering, and did not allow ultrasonic cleaning of assemblies that carry components.

Common mistakes

• Lifting the part while a joint still holds. The stuck pin breaks off, or the plating comes out of the hole with it.
• Holding the iron on the joint for too long, or pressing hard on it. Both lift pads and tracks.
• Pressing a desoldering nozzle onto the land instead of onto the lead and the solder.
• Dragging hot braid across a row of pads.
• Carrying on with a desoldering tool whose pump has stopped working. Hakko says to clean the nozzle and heating element at once and to replace the filter if needed.
• Turning up the temperature to make up for a worn nozzle.
• Fitting the new part without inspecting the board first. Carlsen warns that a broken track hidden under a new chip or socket is very hard to find.
• Reworking the same joints again and again. Carlsen notes that each round of heating degrades the board, which is one reason to fit a socket after removing a chip.

Worked examples on this wiki

• Commodore 64 PLA Replacement: the soldered PLA at U17 desoldered pin by pin with Carlsen's method, then replaced with a socket.
• Commodore VIC-20 Blank Screen Repair: chips proved faulty cut out by their legs, and the legs removed one at a time.
• Atari 800XL RAM Replacement: a failed RAM chip cut out before a socket was fitted.
• Sinclair ZX Spectrum Z80 CPU Replacement: a soldered 40-pin CPU removed from a board whose tracks, pads and plated-through holes are easily damaged.
• Commodore PET 4000 Series RAM Replacement: boards whose chip legs were cut short and bent over at the factory, which makes removal slow.
• Super Nintendo Entertainment System Voltage Regulator Replacement: Nintendo's service manual lists the regulator as a repair for a soldering iron and a hand desoldering pump.
• Panasonic FS-A1F Capacitor Replacement Guide: through-holes damaged when a chip was removed with a 30 W iron and a hand solder sucker.

Related pages

• Through-Hole Soldering
• IC Removal and Socketing
• Surface-Mount Rework
• Capacitor Replacement Procedure
• PCB Pad Repair
• PCB Trace Repair
• Via and Through-Hole Repair
• Continuity Testing and Signal Tracing
• Battery Explosion, Capacitor or Corrosion Damage
• Recommended Tools