BGA Rework Process Control

Reworking a ball grid array is the most demanding repair operation in electronics assembly. The joints are underneath the package and cannot be seen, the package is thermally fragile, and the board around it carries components that must not be disturbed. A successful rework is the result of a controlled process rather than of an operator skill alone.

This article covers the steps of that process, the parameters that matter at each one, and the checks that confirm the result.

Why Rework Of A BGA Is Different

The difference is that the joints cannot be inspected or touched during the operation. In a leaded component a joint can be examined and repaired individually, while in a ball grid array the whole array is made or lost in a single thermal cycle. The process therefore has to be right the first time, and the parameters have to be recorded so that a second attempt uses the same conditions.

The second difference is the thermal mass. The package, the board and the surrounding copper all have to reach temperature together, and the risk is that the package reaches the melting point while the board is still cool, or the reverse. The temperature has to be uniform through the joint, which is what the preheat is for.

Preheating And The Board Fixture

Preheating raises the whole area to a temperature below the melting point of the solder, typically in the range of a hundred to a hundred and fifty degrees Celsius depending on the alloy. The purpose is to reduce the temperature difference that the top side heater has to produce, so that the joint reaches the melting point without the package being exposed to an excessive peak.

The fixture has to hold the board flat and support it directly beneath the package, because the board softens at preheat temperature and would otherwise sag. Support pins placed under the package area prevent the board from moving when the vacuum nozzle lifts the component, and they also help to keep the thermal path consistent between operations. The preheat should be applied from below over a large area rather than concentrated on the package.

The Reflow Profile For Removal And Replacement

The profile has a ramp, a soak and a short period above liquidus, and the peak has to be high enough to melt every ball in the array with margin but low enough to stay within the package rating. The temperature is measured at the package and at the board beside it, and the difference between the two should be small.

The duration above liquidus is the parameter that limits the damage. A short time reduces the growth of the intermetallic layer and the stress on the package, while a time that is too short leaves part of the array unmelted. The profile that was used to assemble the board originally is the best starting point, because the package survived that cycle once and the array is known to melt within it.

Site Dressing And Pad Condition

After the package is removed the site carries the remains of the old balls, and those have to be removed without damaging the pads. The usual method is a controlled wick with a flat iron, followed by a clean with a suitable solvent. The pad has to end up flat and clean, with the original plating intact and no copper exposed.

A pad that has been overheated during the removal shows a dull or discoloured surface, and one that has been over wicked loses plating. Both reduce the wettability, and a pad that does not wet properly will produce an open joint that is invisible after the new package is placed. Inspecting the site under magnification before the new package is placed is the check that catches this, and it is the last opportunity to reject the site.

Placement And Self Alignment

The new package is placed with the balls in contact with the paste or with the residual flux, and the surface tension of the molten solder pulls the package into alignment with the pads as long as the initial misalignment is small. The self alignment is effective but it has a limit, and a placement error larger than a fraction of a ball pitch will not be corrected.

The placement force matters. Pressing the package down to seat it can flatten the balls and produce bridges, while placing it without contact leaves a gap that the solder cannot bridge. The nozzle should lower the package until it touches and then release, and the position should be confirmed through the optical split prism if the station provides one. The alignment mechanisms are the same as those that operate in a normal reflow, which are described under component shift.

Verification Of The Result

The reworked joint is verified by X-ray inspection, which shows the ball array, the presence of bridges and the size of any voids. A comparison with an unreworked area of the same board is more informative than an absolute judgement, because the reworked joints may legitimately differ in appearance from the original ones.

Where the equipment is available, a cross section of a sample confirms the intermetallic structure and the presence of any voiding at the interface. The electrical test then confirms the function, and the thermal cycle test on a sample confirms that the reworked joints survive the same cycling as the original assembly. The acceptance criteria for the joints themselves belong to the general quality characteristics of the product, and the overall sequence that a repair sits within is described in the assembly development process.

Additional Considerations for This Build

Practical attention to BGA rework pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating BGA rework explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Deliberate attention to thermal profile pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating thermal profile explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, thermal profile is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. 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 BGA be reworked? The practical limit is set by the number of thermal cycles the board and the package can take, often two or three. Each cycle grows the intermetallic layer and stresses the plating.

Is preheating optional? No. Without it the package has to absorb the whole thermal load and the board does not reach temperature, which produces joints that look right and are not.

Can a reworked joint be distinguished from an original one by X-ray? It can differ in appearance, because the ball shape and the void content change. The comparison that matters is with an unreworked reference on the same board rather than with an ideal image.