Solder Ball Design Guide

A solder ball is a sphere of alloy attached to the underside of a ball grid array package, and it becomes the joint when the package is reflowed onto the board. The ball is supplied with the package, so the designer usually chooses the board and the paste rather than the ball. The diameter, the pitch and the collapse behaviour are still part of the design, because they set the pad, the paste volume and the inspection method.

What a Solder Ball Is

A solder ball is a sphere of an alloy that is reflowed to the package substrate during its manufacture. It sits on the pad of the package and it is what the assembler sees.

The ball is the largest part of the joint volume, and it is what determines the standoff between the package and the board. The paste that is printed on the board adds a smaller volume.

The sphere is made by a casting or a dropping process, and the diameter tolerance is a few per cent. The tolerance contributes to the coplanarity of the package, which is one of the assembly parameters.

The alloy is usually the same tin silver copper that is used for the paste, so that the joint melts as one during reflow.

Sphere Diameter and Package Pitch

The sphere diameter is chosen by the package manufacturer from the pitch. A typical relation is that the ball diameter is about half the pitch, which leaves room for the pad and the clearance.

A pitch of 0.8 mm uses a ball of about 0.4 to 0.5 mm, while a pitch of 0.4 mm uses a ball of about 0.25 mm. Below a certain size the ball becomes difficult to handle and the package becomes a chip scale device.

The diameter also sets the volume of alloy in the joint, which sets the standoff. A larger ball gives a taller joint and a better resistance to thermal cycling, at the cost of a taller package.

The ball diameter and the pad diameter on the board are related, and the pad is usually smaller than the ball so that the ball self centres during reflow.

Paste, Flux and Placement

The paste on the board provides the flux that cleans the surfaces and a small part of the alloy. The volume is much smaller than the ball, so the paste is a process aid rather than the main source of solder.

The paste also holds the package in place before reflow, which is why the tackiness matters. A package on a board with too little paste will shift on the conveyor.

Some assemblies use a flux only process, where the board is coated with a tacky flux and no paste is printed. The method gives a lower void content and is used for a fine pitch device.

The placement pressure is set by the machine and it should not crush the balls. On a fine pitch package the placement force is small and the machine has to be calibrated for it.

Collapse and Standoff

During reflow the ball melts and the surface tension pulls the package down until the joint reaches equilibrium. The final height is the standoff and it is set by the ball volume, the paste volume and the pad area.

A collapsible ball, which is the usual type, changes shape and forms a barrel between the two pads. A non collapsible ball keeps its shape and is used where a defined standoff is needed for thermal or mechanical reasons.

A larger standoff is better for thermal cycling, because the joint can absorb more of the movement between the package and the board. It is also better for cleaning under the package.

The standoff is measured on a section, and the value should be compared with the specification rather than assumed from the ball diameter alone.

Defects

Bridging between two balls happens when the paste volume is too high or the placement is off centre. On a fine pitch array the gap between the pads is small and the process window is narrow.

A missing ball is a package defect that is found by inspection before assembly, and a missing joint is found by X ray afterwards. The two look different and are fixed differently.

A head in pillow defect occurs when the ball and the paste do not fully merge, leaving a joint with a visible seam. It is caused by a warped package or by an insufficient peak temperature, and it is a common cause of an intermittent open.

The joint appearance and the causes are listed in the solder defect guide, and the inspection is covered by the X ray and AOI material.

Rework and Reballing

A ball grid array can be removed and replaced, and the board can be reused if the pads are undamaged. The process needs a controlled heat profile and a fixture that holds the package flat.

A package that has to be reused needs new balls, which is a reballing operation with its own fixture and its own stencil. The process is described in the reballing material.

The board side of the rework is the critical part, because a pad that is lifted or a solder mask that is damaged makes the site unusable. The rework site should be inspected before the new package is placed.

The cost of rework is high, so the design should avoid placing a ball grid array in a position where it cannot be reworked, and the cost reduction review should include the rework strategy.

Practical Rules

Choose the ball diameter and the pad from the package drawing, and do not scale one without the other.

Set the paste volume for the flux and the tack, not for the alloy, and reduce it rather than increase it when the bridging risk is high.

Check the standoff on a section of the first article, and compare it with the value that the thermal calculation assumed.

Plan the rework before the layout is fixed, and keep the rework site accessible for the fixture and the nozzle.

Additional Considerations for This Build

Practical attention to paste flux 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 paste flux 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, solder ball 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

Is the ball or the paste the main source of solder? The ball. The paste provides the flux and a small volume of alloy, and it also holds the package in place before reflow.

Why does a ball grid array need a defined standoff? Because the standoff sets the clearance under the package and the ability of the joint to absorb thermal movement. It is measured on a section.

What causes a head in pillow defect? A warp in the package or a peak temperature that is too low, so the ball and the paste do not merge. The result is an intermittent open joint.