Through-Hole Rework: Desoldering Without Damaging the Barrel

Rework on a through-hole joint puts heat, mechanical force and a vacuum into a barrel that is a few tenths of a millimetre of plated copper, and the damage it can cause is not visible from the surface. The method matters as much as the operator, and both are controlled by preheat and by the tool that removes the alloy.

Why Through-Hole Rework Is Riskier

A surface mount joint can be reworked by adding heat and alloy with the pad supported by the board underneath. A through-hole joint surrounds the barrel, so the alloy has to be removed from inside a hole whose wall is the thing that has to survive the operation.

The thermal mass of the barrel also means the joint takes longer to heat than it looks, and an operator who is waiting for alloy to flow will hold the iron in place longer than the pad can support. The result is a lifted pad or a barrel that cracks along its length rather than at the surface. The damage is cumulative as well, so a joint that has been reworked twice has less margin for a third operation than one that has never been touched.

Desoldering Tools and When to Use Them

The vacuum tool removes molten alloy through the tip and is the usual method for a single joint, while braid is used where the vacuum cannot reach or where the alloy must be removed from a row of pins. Hot air is used where the part cannot be touched, such as a connector with a plastic body.

Each method applies a different force to the barrel: the vacuum pulls alloy out of the hole, braid pulls it sideways against the wall, and hot air applies nothing at all but heats a wider area. The choice follows from the joint and from what the board can take.

Alloy, Temperature and the Working Window

The iron is usually set between 300 and 350 degrees Celsius for a lead free joint, and the working time per joint is kept short rather than the temperature low. A low temperature and a long dwell puts more heat into the laminate than a higher temperature and a quick joint does.

Adding a lower melting alloy to the joint is a common technique, because it reduces the temperature needed to remove the original alloy. The residues it leaves have to be cleaned thoroughly afterwards, and the technique is written into the instruction rather than left to the operator. The added alloy has to be removed completely before the joint is remade, because a mixture of two alloys melts over a range and behaves unpredictably during the second operation.

Preheating and Heat Flow

Preheating the board to 80 or 100 degrees Celsius reduces the amount of heat that the iron has to supply and shortens the contact time. The preheat is applied from below, so the board is supported on a preheater plate rather than on a cold bench.

Desoldering a joint on a heavy ground plane is the case that justifies preheat, because the plane draws heat away faster than the iron can supply it. Where a plane is involved, the preheat temperature and the contact time are measured on a sample joint before the operation is released.

Barrel Damage Modes

The barrel can crack along its length, it can be pulled out of the board with the pin, it can lift the pad on one side and it can lose plating where the vacuum tip has scraped it. Each of those has a different cause, and only the first is purely thermal.

A pull-out is a mechanical fault, and it happens when the alloy is not fully molten; a lifted pad follows from overheating or from a sideways force on the iron. Both are prevented by the same discipline, which is to confirm that the alloy is liquid before any force is applied; our hole copper notes show how the barrel is examined afterwards. Plating lost to the vacuum tip is the least obvious of the four, because it appears in a section as a thinned wall rather than as a break, which is why the remaining copper is quoted as a measurement rather than a judgement.

Protecting the Pin and the Component

A pin that is being reused is held so that it cannot move while the alloy is liquid, and a heat shunt is used on a lead that would otherwise conduct heat into a sensitive body. A component that is being replaced rather than saved can be cut free first, which makes the operation much easier on the board.

Where a part is reused, its leads are cleaned and re-tinned before it is replaced, and the number of times a part may be reused is written in the instruction. The limit exists because each cycle consumes some of the plating on the lead, and re-tinning cannot restore it.

Cleaning After the Operation

Rework leaves flux residue inside the barrel and around the pad, and the residue is more concentrated than in a normal assembly because the flux is applied locally. The area is cleaned to the same standard as the rest of the board, and the cleaning method is chosen for the flux that was used.

Where a coating has been removed for access, the area is recoated after the joint is inspected, and the coating is given the same cure and thickness as the original. Where the cleaning step is skipped because the residue is described as no-clean, the reworked area differs from the rest of the board, and the difference becomes visible only when a coating is applied or when a humidity test is run. Our solder joint acceptance criteria describe what the replaced joint should look like before the coating goes back on.

Verification After Rework

The joint is inspected visually for a fillet that wets both the pad and the pin, and the barrel fill is confirmed by X-ray where the joint is not visible. On a reworked joint the fill figure is the one that most often fails, because alloy was removed rather than added during the operation.

Where a board has been reworked several times, a section of one joint gives the evidence that the barrel survived, and it is worth the cost on a high value assembly. The section is taken from a sample rather than from the production board, and it is retained with the rework record.

Records, Limits and Process Control

The rework instruction states the tool, the temperature, the preheat, the alloy used and the maximum number of rework cycles for a given joint. Those limits are what prevent a board from being reworked until it fails rather than until it works.

The record carries the joint identity, the method, the operator and the inspection result, and it is what allows a pattern of failures to be traced to a method rather than to a person. Our board failure notes describe how a reworked joint that fails later is recognised, since the failure usually appears at the barrel rather than at the surface. Where the same joint is reworked on several boards from one lot, the pattern is treated as a process signal rather than as a series of separate repairs.

FAQ

Can a joint be reworked more than once? It can, and every cycle removes some plating and adds thermal stress. The limit is written as a number for a specific joint type rather than as a general rule, and it is counted on the shop floor.

Is braid better than a vacuum tool? Braid gives more control on a large pad and pulls the alloy sideways, which puts force on the barrel. The vacuum tool removes alloy without contact pressure, and it is the usual first choice for a single through-hole joint.

Why does a joint fail after rework rather than during it? Because the barrel may be cracked or thinned in a way that still passes a continuity test. The failure appears after thermal cycling, when the crack opens, which is why the section is worth taking on a high value board.