BGA Reballing: Stencils, Reflow Profile and Reliability

Reballing is the process of removing the solder balls from a ball grid array package and attaching a new set. It is used to repair a package whose balls were damaged, to change the ball alloy when a product moves to a lead free process, and to recover components that were removed from a board where the balls were partially consumed.

The process is straightforward in principle and demanding in practice, because the ball pitch is small, the package is thin and the thermal mass of a large device makes the reflow profile difficult to control at the edges.

When Reballing Is Used

The most common case is a package recovered from a board. When a device is removed, the balls that remain on the package are deformed and partly consumed, and they cannot be reflowed a second time onto a new board without losing their shape.

The second case is an alloy change. A product that migrates from a tin lead process to a lead free process needs balls of the new alloy, and a package that is in stock with the old alloy is often cheaper to reball than to replace. The third case is damage: a package with a few collapsed or missing balls can be recovered if the rest of the device is sound.

Preparing the Package

The package is cleaned and inspected before any work begins. Any residue of underfill, adhesive or flux has to be removed, and the pad surfaces under the original balls have to be exposed without damaging the copper or the solder mask.

Moisture is the other preparation issue. A plastic package that has absorbed moisture will expand during reflow and can delaminate or crack, so the device is baked at a controlled temperature for a specified time before the reballing step rather than being processed as received.

Ball Removal

The old balls are removed by heating the package until the solder is liquid and then wiping or vacuuming the surface, or by using a low melting alloy that combines with the existing solder so that it can be removed at a lower temperature.

Either method leaves a small amount of solder on the pad, which is acceptable if it is uniform and clean, and which has to be levelled by a wick or a blade if it is not. The pad surface after removal is inspected under magnification, because a pad that has been torn or a mask that has been lifted will not accept a new ball reliably.

Stencil and Ball Placement

The new balls are placed with a stencil whose apertures match the package footprint. Paste is printed into the apertures, the balls are dropped into place and the stencil is removed, leaving the balls held by the tackiness of the paste.

The stencil thickness and the aperture size are matched to the ball diameter, and the placement tolerance is a fraction of the pitch. For a fine pitch device this is the step that decides the yield, and it is usually performed on a machine rather than by hand.

Reflow Profile

The reflow profile has to bring the balls and the pads to a temperature above the liquidus of the alloy without overheating the package. Because the package is a poor conductor and the balls are small, the operator has to ensure that the measured temperature reflects the joint rather than the air around it.

The ramp rate is kept modest to reduce the thermal shock to the package, and the time above liquidus is kept short to limit the growth of intermetallic compounds. A profile that works for one package size will not necessarily work for another, so each device type is characterised rather than assumed.

Cleaning and Residue

Flux residue left under a reballed package is a reliability risk in the same way as residue under any other component. The cleaning method has to be compatible with the package, since many plastic packages absorb water and cannot be washed aggressively.

Where the device cannot be washed, the flux is chosen to be benign and the cleaning is replaced by a controlled residue specification. The decision should be made from the environment the product operates in rather than from the convenience of the rework station.

Inspection Methods

Visual inspection confirms that every ball is present, that the array is aligned with the package outline and that the ball heights are uniform. X-ray inspection shows the internal quality of the joints and any voids or bridges, and a cross section on a sample confirms the intermetallic structure.

A sample from each batch is often subjected to a shear or a pull test to confirm that the ball attachment meets the specification. That test destroys the sample, so it is performed on a dedicated unit rather than on a product device.

Reliability and Rework Limits

Every reballing operation is a thermal cycle for the package, and the number of cycles that a device can tolerate is finite. A package that has been reballed twice is usually treated with more caution than one that has been reballed once, and the accumulated thermal history should be recorded.

The reliability of a reballed device is dominated by the interface between the new ball and the original pad. Where the original solder has been fully removed, the joint is comparable to a new one. Where a mixed alloy remains, the joint contains intermetallics from the old and new alloys together, and its behaviour under thermal cycling is different from either.

Cost and Decision

Reballing is justified when the device is expensive, when it is unavailable or when its removal was a controlled repair rather than a failure. It is not justified for a low cost package, where replacement is cheaper and more reliable.

The decision also depends on the number of devices. A single repair is a technician task, while a batch of packages that has to change alloy is a process with fixtures, a stencil and a profile that is characterised once and repeated.

Documentation and Process Control

The process is documented with the stencil specification, the alloy, the profile, the cleaning method and the acceptance criteria. Each batch records the package type, the number of units and the inspection results, so that a field failure can be traced to the operation that produced it.

gopcb supports assembly and rework programmes with the stencil, profile and inspection documentation that a reballing process requires, and can advise on the thermal budget of a package before a reball is attempted.

The decision to use BGA reballing should be made with the whole cost in mind rather than the price of a new device. A recovered package carries a thermal history, an inspection cost and a residual risk that a new part does not, and the arithmetic only favours the operation when the device is expensive, when lead times are long or when the board it was removed from is itself valuable. Where a new package is available at a modest price, replacing it is usually the more reliable choice and the cheaper one once the inspection and the risk are counted.

FAQ

Can any BGA be reballed? Most can, provided the pads are intact and the package can tolerate another thermal cycle. A package with lifted pads or a cracked body cannot be recovered.

Does reballing change the ball alloy? It can, and that is often the reason for the operation. The new alloy has to be compatible with the board finish and with the reflow profile used in assembly.

How many times can a package be reballed? There is no fixed number, but each cycle adds thermal history and reduces the margin. Two operations are common, and beyond that the device should be evaluated rather than assumed to be sound.

Related reading: PCBA development process, lead free versus leaded solder, PCB design quality characteristics, and high density interconnect PCB.