Desoldering Braid: Removal Control in Rework

Desoldering braid is a flat woven strip of fine copper that is pressed onto a joint with a soldering iron, and it removes the solder by capillary action. It is the simplest way to clear a pad or a through hole, and it is also one of the easiest ways to damage one. The method is used on both through-hole and surface mount joints, and the risks it carries are different in each of the two cases.

The braid is a consumable and it is meant to be cut away and discarded as it fills. The removal itself is a brief thermal event, and the quality of the result depends on how that short event is controlled rather than on how hard the iron is pressed. A removal done in one clean pass leaves a pad that needs no repair, which is the standard to aim for.

What Desoldering Braid Does

The braid is placed over the joint, the iron is applied to the braid rather than to the joint, and heat travels through the copper to melt the solder. The molten solder is then drawn into the weave by capillary action and held there. Braid should be cut fresh for each joint rather than reused, because a saturated wick has nowhere left to hold solder.

Because the braid carries heat well it also spreads it, so a small braid on a heavy joint will cool the iron rather than melt the solder. Getting the heat into the joint is the whole difficulty of the technique. The width of the braid should be matched to the pad, since a braid wider than the pad will heat the mask around it.

Wick Types and Sizes

Braid is supplied in a range of widths and in fluxed and unfluxed forms. A narrow braid suits a fine pad and a wide one suits a ground plane, and the width should match the joint rather than the operator habit. Flux should be applied to the joint as well as to the braid when the unfluxed type is used, so that the oxide is broken down.

Fluxed braid contains a dry flux that activates as the braid heats, while unfluxed braid needs flux applied separately. The unfluxed type gives more control over the flux, and the fluxed type is faster on a simple joint. A tip that is too small for the joint will need a long dwell, which is the very thing that damages the pad.

Flux and Heat Transfer

Flux is what makes the solder flow into the braid rather than sit on top of it. Without flux the solder oxidises as it melts and the wick picks up very little, which leads an operator to press harder and add dwell. Tip temperature should be set for the alloy being removed, and a lead-free joint needs more heat than a tin-lead one.

The flux should be from the same family as the one used in the original assembly, so that the residue left behind is one the cleaning process already handles. Rework heating is described in rework heating profiles notes. A pad that has lifted slightly can sometimes be re-bonded with adhesive, but the repair should be documented and inspected.

Iron Tip and Contact

The tip should be large enough to store the heat the joint needs, and it should be clean and tinned so that it transfers that heat into the braid. A tip that has oxidised will not transfer enough heat however long it is held in place. Preheating is the single most effective way to shorten the dwell, because it removes the heat sink effect of the plane.

Contact should be made with the flat of the tip and with light pressure. Pressure does not improve heat transfer, and on a fine pad it is the main cause of lifted copper and of a damaged solder mask edge. Hot air with a small nozzle can clear a heavy joint without touching the pad, and it is often the better choice on a ground plane.

Dwell and Pad Damage

Dwell is the time the iron is in contact, and it is the number that should be controlled. On a through hole a dwell long enough to clear the barrel is long enough to weaken the bond between the pad and the laminate. Wick fibres left on a pad look like fine hairs under magnification, and they should be brushed away before the joint is re-soldered.

The second and third attempts at the same joint are what cause most of the damage, because the pad has already been heated once. A joint that will not clear should be re-fluxed and allowed to cool before another attempt. Barrel damage is described in through-hole rework notes. A pad that has been overheated often looks dull and grey, and its solderability is reduced even when the shape is correct.

Working on Through-Hole Joints

A through hole needs more heat than a surface pad, and it needs the solder to be molten all the way through the barrel. Preheating the assembly reduces the heat the iron has to supply and shortens the dwell considerably. The operator should be able to state how many attempts a pad has seen, which is why the count belongs on the record.

Where a hole is connected to a ground plane, the plane acts as a heat sink and the joint will not clear with an iron alone. Those joints belong on a preheated stage or with a hot air assist rather than on the bench. Joints on a ground plane should be identified at the layout stage, so that the rework plan can call for preheating.

Cleaning and Inspection After Removal

After the solder is removed, the pad and the hole should be cleaned of flux residue and inspected under magnification. A pad that has lifted shows as a bright line at its edge, and a hole that has been overheated shows a dull or discoloured barrel. Where the same joint is reworked repeatedly across a lot, the cause is usually in the original process rather than in the removal.

Wick fibres can also be left behind on the pad, and they interfere with the next soldering operation. Removal technique is described in desoldering rework work. A joint cleared without lifting copper is evidence that the technique is being followed, and it is worth recording as such.

Records and Verification

Rework records should note which joints were reworked, how many attempts were made and whether the pad was still intact afterwards. Repeated attempts at the same joint are the signal that the removal method needs to change.

Acceptance of the reworked joint follows the criteria in the published IPC documents. A joint that looks sound but has been heated three times is a candidate for a future field failure.

FAQ

Should the iron touch the joint or the braid? The braid. Heat reaches the joint through the copper weave, and pressing the iron on the pad itself is what lifts copper.

Why does the braid not pick up solder? Usually because there is not enough flux or the tip cannot deliver heat, so the solder oxidises instead of flowing into the weave.

How many attempts are acceptable on one joint? As few as possible. Each attempt adds dwell and heat, so a joint that will not clear should be re-fluxed and allowed to cool first.