Desoldering and Rework Techniques Compared

Every assembly eventually needs a joint remade. Removing solder is a different problem from adding it, because the joint has to be heated through a board that may already be populated, and the surrounding material has seen at least one thermal cycle. The methods differ in how they deliver heat, how much heat they can deliver and how much risk they put on the laminate and the pads.

What Makes Desoldering Harder Than Soldering

Soldering adds alloy and flux to a joint that is at ambient temperature, and the thermal load is limited by the small amount of material being heated. Desoldering has to melt the entire joint including its fillet, and often has to remove the alloy at the same time. The heat sink is the same, but the target temperature is higher and the time available is shorter.

The board has also changed. It has been through reflow, so the laminate has less moisture and the solder mask is fully cured. Neither helps, and in the case of a coated or underfilled assembly the access is worse than it was on the bare board.

Desoldering Wick

Wick is a braid of fine copper strands that draws molten alloy by capillary action. It is the cheapest method and the most controllable, because the operator can watch the joint and stop when the alloy has been taken up. Flux applied to the wick improves the transfer and protects the joint.

The risk is dwell time. Wick has to be held against the joint while it heats, and heat conducts through the braid into the pad and the laminate. Holding it too long, or pressing too hard, lifts pads and burns mask. Wick also leaves a small amount of alloy behind, so a joint may need more than one pass.

Vacuum Desoldering

A vacuum tool heats the joint through a hollow tip and then draws the molten alloy away in one action. It clears a through hole quickly and completely, which makes it the standard method for removing a component from a plated barrel where the alloy must be gone before the lead can move.

The tool concentrates a lot of heat at one point, and the tip can pull the joint rather than the alloy if the vacuum is applied before the alloy is fully molten. The method needs a hot tip, a fast hand and a clean nozzle, and the nozzle has to be matched to the joint size.

Hot Air and Infrared Rework

Hot air heats a whole area at once, which is what makes it suitable for surface mount parts with many joints. A nozzle shaped to the package distributes the heat evenly, and the part can be lifted when all the joints are molten. Infrared works on the same principle but heats by radiation, which is more even on large areas and harder to focus on a small one.

Both apply heat to the component as well as to the joints. The part is usually being removed and can tolerate the heat, but its neighbours cannot, so shielding and a defined maximum temperature are essential. The profile should follow the same logic as a reflow profile, with a controlled ramp rather than an instant application of full power.

<img src="https://www.gopcba.com/wp-content/uploads/2026/05/12温区氮气回流焊-1.jpg" alt="Hot air rework nozzle directed at a small package” />

Choosing by Joint Type

Through hole joints with a small thermal mass are best served by wick or by a vacuum tool. Large through hole joints, particularly those connected to a plane, usually need preheating from below before either method will work. Surface mount parts are handled with hot air, and a fine pitch part may need both hot air and a separate means of lifting the package once the alloy is molten.

Ball grid arrays are a special case. The whole package has to reach liquidus at once, which means a bottom side preheater plus top side hot air, and the package is lifted with a vacuum cup rather than with tweezers. The process and its constraints are described in our article on BGA reballing.

Thermal Capacity and Preheating

Preheating is the difference between a method that works and one that destroys the board. Raising the assembly to around 100 to 150 degrees Celsius from below reduces the heat the tool has to supply, shortens the dwell time and lowers the temperature gradient across the laminate.

Preheating also reduces the risk of thermal shock, which is the mechanism behind many of the pad lifts attributed to operator technique. A board that starts warm can be reworked with a smaller tool and a shorter contact time, which is the opposite of what an operator under time pressure tends to do.

Protecting Pads and Laminate

Pad lift happens when the adhesive strength between the copper and the laminate is exceeded, and it is degraded by every thermal cycle the board has seen. The countermeasures are a lower peak temperature, a shorter dwell, and mechanical support so that no pulling force is applied before the alloy is fully molten.

Flux choice matters too. An aggressive flux left in contact with the surface for a long time attacks the mask and the laminate around the pad, and the damage may not be visible until a later cycle. Cleaning the area after rework is part of the process rather than an optional extra, and the checks described for coating processes apply if the board is subsequently coated.

Verifying the Rework

A reworked joint should be inspected against the same criteria as an original one, and in some cases more strictly, because a reworked joint has a different thermal history and a thicker intermetallic layer. The acceptance shapes are set out in our article on solder joint acceptance criteria.

Where a board has been reworked more than once in the same area, the risk of a latent defect rises sharply. Recording the number of rework operations per unit gives the quality engineer a way to decide when a board should be scrapped rather than repaired again.

FAQ

Is wick or vacuum better? Wick is more controllable on small joints and vacuum is faster on through holes. Most benches keep both, because the choice follows the joint.

Why do pads lift during desoldering? Almost always because force was applied before the alloy was molten, or because the dwell time allowed the laminate to overheat. Preheating reduces both risks.

How many rework cycles are acceptable? Two is a common limit for a given joint, with a full inspection after each. Beyond that the reliability is uncertain and replacement of the assembly is usually cheaper.