Solder Wick and Rework Tools: Selection and Control

Solder wick is a braid of fine copper strands that removes molten solder from a joint by capillary action. It is the simplest desoldering tool in a rework station and the one that causes the most damage when it is used badly, because the same heat that melts the solder also stresses the pad, the mask and the laminate beneath it. Solder wick is therefore a tool that has to be used with a technique rather than with force, and the technique is worth teaching properly.

What Solder Wick Does

The braid is placed over the joint, heated with an iron, and the molten solder is drawn into the copper strands. The joint is left with very little solder, which allows a component to be lifted or a pad to be cleaned for a new joint.

Because the braid is copper, it conducts heat away from the iron very effectively. That is the source of most of the difficulty, because the operator responds by raising the iron temperature, which is exactly the wrong answer for the board. A higher setting does not deliver more heat to the joint, it simply raises the temperature of the tip and the risk to the laminate.

Wick Construction and Flux Content

The braid is made from fine copper strands woven into a flat ribbon, and it is supplied either flux free or with a flux coating. The strand count and the weave density determine how much solder the ribbon can absorb per unit length. A dense braid with many fine strands absorbs more solder and transfers heat more evenly than a coarse one of the same width.

Flux coated wick is more convenient, because the flux is released as the braid is heated. Uncoated wick requires the operator to add flux separately, which gives more control but adds a step and a variable. Where a flux coated braid is used, the residue it leaves has to be compatible with the cleaning process that follows.

Choosing the Right Width

The width has to match the joint. A narrow braid concentrates heat and is suitable for a single fine pad, while a wide braid spreads the heat over a larger area and is used for a row of pads or a large joint.

A braid that is much wider than the joint pulls heat from the surrounding area and from the mask, which increases the risk of damage. The rule is to use the narrowest braid that covers the joint in one pass. Keeping several widths at the station is more effective than trying to make one width do every job.

Technique and Heat Control

The iron should be applied to the braid rather than to the pad, and the contact time should be short. Repeating the operation with a fresh section of braid is far safer than holding the iron in place and waiting for the solder to flow.

The tip should be large enough to transfer heat efficiently at a modest temperature, because a small tip requires a higher setting to do the same job. A controlled temperature station with a calibrated tip is a basic requirement for this kind of work.

Flux and Residue After Rework

Rework leaves flux residue on the pad and around the joint, and that residue has to be removed before the new joint is made. A dirty pad will not wet properly, and a contaminated joint may fail later even though it looked correct when it was made. Cleaning with a suitable solvent and a soft brush, followed by a rinse, is the standard practice.

The cleaning has to be compatible with the mask and with the finish. An aggressive solvent that removes the flux may also attack the mask or leave the pad surface in a condition that resists soldering. Our solder defects guide describes how these conditions are classified.

Alternatives to Wick

A desoldering pump removes solder by vacuum and leaves less residue on the board, but it is less effective on a fine pitch row. A hot air station with a vacuum cup removes an area array package without touching the pads at all.

For a single through hole joint, a heated desoldering iron with a hollow tip combines the heat and the vacuum in one tool. The choice depends on the joint type and on the number of joints that have to be cleared. Where the volume of rework is high, having the right tool for each joint type is cheaper than making one tool do everything. Our production flow guide places rework in the wider sequence of the line. Rework should be planned and recorded rather than improvised, because an unrecorded rework is an unmeasured risk.

Protecting the Board During Rework

The main risks are lifted pads, damaged mask, delamination and damaged neighbouring components. Lifted pads come from too much heat and from pulling while the solder is still solid, while delamination comes from moisture in the laminate flashing to steam. Both are preventable, and both are far more common when the rework is done without preheat.

Preheating the board from below reduces the iron temperature needed and the thermal gradient across the joint. Baking a board that has absorbed moisture before any rework also removes the risk of a delamination around the reworked area.

Verifying the Rework

After rework the joint has to be inspected, and where the component is critical it should also be tested electrically. A reworked joint that looks acceptable can still have a weak intermetallic bond if the pad was not properly cleaned. Visual inspection alone is therefore not sufficient evidence that the rework was successful.

The rework record should identify the board, the component, the operation and the operator. Our quality documentation describes how these results are recorded at gopcb, and a record of rework is what allows a recurring fault to be found. Where the same component is reworked on many boards, the record makes the pattern visible and points at the original process step.

Process Control Points

The controls are the braid width and its flux content, the iron tip size and its temperature, the contact time, the preheat, the cleaning after rework and the inspection of the finished joint.

Training matters more in rework than in almost any other process, because the operator is making a judgement on every joint. A written technique note and a periodic skill check are more effective than a temperature specification on its own. Rework is one of the few processes where the operator is the process, and the training should reflect that.

FAQ

Why does the pad lift when solder wick is used? Usually too much heat applied for too long, or pulling the braid while the solder is still solid. Repeating a short operation with fresh braid is safer than one long contact.

Should the iron be hotter when wick is used? No. The braid conducts heat away, and the temptation is to raise the temperature. A larger tip at a moderate setting transfers heat better without stressing the board.

Does a reworked joint need retesting? Where the connection is critical it does, because a joint can look correct and still be weak. The rework should also be recorded so that a recurring fault becomes visible.