Rework Practice and Heat Control on Assembled Boards

Correcting a defect on an assembled board is a repair, and the difference between a repair that lasts and one that returns is the control of heat. The techniques differ, and the same technique applied without control produces damage that appears later.

What Rework Does to a Joint

A joint that is reheated grows its intermetallic layer for a second time, and the growth is additional rather than replaced. A joint that has been reworked twice has a thicker and more brittle interface than one that was soldered once.

The consequence is a limit on the number of rework cycles that a joint can tolerate, usually stated as two or three, and a requirement that the rework should be done correctly the first time. Our solder joint notes describe the mechanism.

Heating the Joint, Not the Board

An iron transfers heat through contact, which is fine for a joint with exposed leads and unsuitable for a package whose leads are hidden. A hot air tool transfers heat through the air, which is slower and more even.

The choice of method follows from the package. A gull wing lead can be reworked with an iron, an area array needs hot air with preheat from below, and a package with a thermal pad needs both.

Preheating and Its Purpose

Preheating brings the board to a temperature below the solder melting point, which reduces the heat the tool has to supply and lowers the thermal gradient across the assembly. The gradient is what cracks ceramic components and delaminates the laminate.

The preheat temperature is usually set between a hundred and a hundred and fifty degrees, and it is measured on the board rather than on the tool. A thermocouple taped to the opposite side of the board gives a usable reading. Our reflow profile notes describe the principle.

Rework of Area Array Packages

An area array package has no visible joints, so the rework relies on a controlled profile and on a machine that measures the temperature at the package. Removal is easier than replacement, because the alignment for replacement has to be established optically.

A site that has been used more than once accumulates the effects of the heating, and the pad may lift. The number of times a site can be reworked should be recorded and limited. Our BGA inspection notes describe how the result is verified.

Damage That Rework Causes

The damage appears in three forms: a lifted pad, a delaminated laminate and a cracked component. All three are caused by excessive heat or by mechanical force applied while the joint is molten.

A lifted pad is usually caused by pulling the component before the solder has fully melted, while a delamination is caused by moisture in the laminate turning to steam. Baking before rework removes the second cause.

Tools and Their Control

A rework station should control the tip temperature rather than the power, and its temperature should be verified rather than trusted. A tip that has lost its plating transfers heat poorly, so the operator raises the temperature and the damage follows.

The tip geometry matters as much as the temperature, because a large tip on a small joint transfers too much heat quickly. The rule is the largest tip that fits the joint without touching a neighbour.

Cleaning and Inspection After Rework

The rework leaves flux residue and sometimes small solder particles. The residue should be removed, and the area should be inspected under magnification before the board is returned to service.

A reworked joint should be inspected against the same criteria as an original one, and it should be recorded. A board that has been reworked without a record is a board whose history is unknown.

Deciding Whether to Repair

Not every defect should be repaired. A board with a lifted pad on an inner layer, a delaminated area or a damaged connector footprint may cost more to repair than to replace, and the repair may reduce the reliability of the product.

The decision should be made with a stated criterion rather than case by case. A simple rule, such as no more than two rework cycles per site and no repair of a lifted inner layer, removes the judgement from the operator and the risk from the product. Our design release checklist lists the records to keep.

Process Control and Verification

On a design of this kind, rework is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

Process Control and Verification

On a design of this kind, rework is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Process Control and Verification

On a design of this kind, rework is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

How many times can a joint be reworked? Two or three times for most alloys, and fewer where the joint is thermally cycled in service. The limit comes from the intermetallic growth rather than from the solder itself.

Is a hot air tool always better than an iron? It is better where the joint is hidden or where several joints must be heated together. For a single exposed lead, an iron with a controlled temperature is faster and applies less heat to the surrounding area.

What does gopcb recommend for rework? We recommend preheating, a controlled tip or air temperature, a bake before rework on a board that has been stored, and a record of each rework. Where a site has already been reworked twice we recommend replacing the board rather than the joint.