BGA Rework: Removal, Site Preparation and Reballing

A ball grid array cannot be soldered twice the way a leaded part can. The joints are underneath the package, the whole array has to reach the melting point at the same time, and the surrounding board has to stay cool enough that the joints around it are not disturbed. BGA rework is therefore a profile problem before it is a handling problem.

When it is done properly, the replacement is indistinguishable from the original joint in every way that can be measured. When it is done badly, the board passes the next test and fails months later, which is why the operation deserves a written procedure rather than the judgement of whoever happens to be at the station.

Why Area Array Rework Is Different

The package hides its own joints, so the operator cannot see when the alloy has melted. Everything is inferred from the profile and from the behaviour of the component when it is lifted, which means the process has to be predictable in a way that hand soldering of a visible joint never has to be.

The whole array melts together, which concentrates the thermal load in one place. The board around it is heated by conduction and by the air, and the parts nearest to the site are the ones most at risk of being disturbed or damaged during the operation.

Preheat and Board Support

Preheating from below raises the assembly to a temperature below the solidus, usually a little over a hundred degrees, so that the top side heater only has to supply the difference. That reduces the gradient across the board and the stress that the gradient produces.

The board has to be supported while it is hot, because the laminate is softer above its transition temperature and will sag if it is held only at the edges. A support plate or a set of pins under the site keeps the board flat, which matters because the placement of the replacement depends on the site being in plane.

Nozzle Selection and Heating

The nozzle controls the area that the top side heater warms. A nozzle matched to the package outline heats the whole array without cooking the neighbouring parts, and a nozzle that is too large spreads the heat into the surrounding components for no benefit.

Airflow matters as much as temperature, because it sets the rate at which heat is delivered. A high flow brings the site to temperature quickly and can move small components nearby; a low flow is gentler and takes longer, which exposes the whole area to heat for an extended period. The balance is found by measurement on a scrap board.

Removal Without Damaging the Site

The package is lifted with a vacuum nozzle when the alloy at every ball is molten. Lifting too early leaves half of the joints solid, and the pads they were attached to come away with the component. That single error converts a repairable board into scrap.

The signal that the array has melted is the movement of the package, which is why the thermal profile and the moment of lift are tied together as the surface tension releases, and it can be seen through the alignment prism of the machine. Waiting for that movement, rather than lifting at the end of the profile, is the practice that separates a controlled removal from a destructive one.

Site Preparation and Pad Levelling

The site left behind carries residual solder of varying height, flux residue and sometimes a lifted pad. The residual solder has to be removed with a wick and the pads levelled so that the replacement sits flat with a uniform gap.

Levelling is done with a wide blade tip and a small amount of flux, working across the site rather than on individual pads. Pads that are visibly lifted, or that have lost their solder mask collar, indicate that the removal was too aggressive against the process tolerances, and the board’s suitability for further work should be reassessed.

Reballing and Stencil Methods

Reballing puts new solder balls on the component or the site so that the replacement has a defined volume of alloy. The common method uses a stencil to place paste or preformed balls onto the pads of the device, followed by a reflow cycle that attaches them.

The stencil method is fast but requires a tool for each package, and recovering a package with its original ball array is often more economical than reballing a component that has already been through several thermal cycles. Where the component is expensive, reballing is justified, and the result is inspected before it is used.

Placement and Alignment

The replacement is placed with the same vision system that the placement machine uses, or with the alignment prism of the rework station. The alignment is checked in both axes and in rotation before the reflow cycle begins.

A small misalignment is corrected by the surface tension of the molten solder, which pulls the package towards the centre of the pads, but that self centring only works if every ball is on its pad. Where the offset is larger than the pad can tolerate, the part has to be lifted and repositioned before the reflow, not after it.

Reflow of the Replacement

The reflow profile for the replacement is the same one that was developed for the removal, with the addition of a soak long enough for the whole array to reach temperature evenly. The peak temperature has to be above the liquidus of the alloy and below the limit of the component and the board.

The thermal profile should be verified with a thermocouple on the site, and the measurement should be repeated for each package type, because the thermal mass of the assembly changes the result. A profile that works for a small package at the edge of a board will not work for the largest device in the middle of a dense assembly.

Inspection After Rework

X-ray inspection is the standard check, and it is the only way to see the balls after the operation. The image is compared with an untouched joint of the same type on the same board, which is a more reliable reference than a library image.

The features that matter are the size and roundness of the balls, the presence of bridging and the voiding inside the joints. A reworked array that looks different from its neighbours has to be examined further, and the criteria should be the same as those recorded for the quality control of the original assembly.

Reliability and Records

A reworked package has a different thermal history from the rest of the board, and the joints have seen at least two reflow cycles. That is not automatically a reliability problem, but it does mean the assembly should be treated as a variant when a field failure is investigated.

Recording the operation, the profile, the operator and the board serial number makes that investigation possible. The records also allow the rework rate to be tracked, which is one of the more useful indicators of the health of the assembly process, and they are kept alongside the material handling requirements for moisture sensitive components that have to be respected before the part is even placed.

FAQ

How many times can a BGA be reworked? Once or twice in most cases. Each operation adds thermal cycles to the board and to the site, and the pads weaken with every removal.

Can a BGA be reworked without a dedicated station? It can be attempted, but the control of the profile and the support of the board are what make the operation reliable, and both are difficult without the equipment.

Does a reworked joint need to be recorded? Yes. Knowing which units were reworked is the only way to correlate a later failure with the operation, and it costs nothing to record.