Solder Ball Attach Defects and Their Causes

A ball attach process places spheres on the package substrate and reflows them, and the defects it produces are mostly hidden until the package is assembled onto a board. The process window is narrow because the sphere has to form a joint and remain round at the same time.

What the Process Has to Achieve

The sphere has to be placed on a fluxed pad, reflowed into a joint with a controlled collapse, and left with a sphere whose diameter is inside a tolerance. A collapse that is too small leaves a weak joint and one that is too large flattens the sphere.

The tolerance on the sphere diameter is what makes the process difficult. Our joint criteria notes describe the acceptance that the figures are written against.

Flux and Its Uniformity

The flux is applied to the pads before the spheres, and its uniformity decides the collapse. A pad with too little flux does not wet and a pad with too much allows the sphere to move during reflow.

The application method is a stencil print or a transfer, and both are judged by the deposit rather than by the setting. A flux that is applied unevenly produces a mixture of collapsed and uncollapsed spheres on the same part. Our paste volume notes describe the same principle in a different process.

Voids Inside the Sphere Joint

A sphere joint can contain voids, and they come from the flux that is trapped as the alloy melts. The void fraction matters most at the perimeter of the package, where the thermal cycling strain is highest.

The profile and the flux quantity are the two controls, and the acceptance is a fraction rather than an absolute. Our <a href="https://www.gopcba.com/bga-inspection-x-ray-vs-aoi-guide/” title=”X-ray”>X-ray notes describe how the fraction is measured.

The substrate and the sphere move differently as they heat, and the difference is largest at the edges of the array. A substrate that warps away from the sphere produces a joint that is thin at the perimeter, or open.

The profile and the support are the controls, and both are developed with the substrate in place rather than on a bare test piece. The measurement is taken on the substrate underside rather than on the carrier.

The characteristic defects are a missing sphere, a sphere that has not collapsed, a flattened sphere, a bridging sphere and a voided joint. Each points at a different variable, and the sequence of checks starts at the sphere placement rather than at the oven.

A single missing sphere on a part is usually a placement or a tape problem, and a whole row of them is usually a nozzle or a flux problem.

Verification is by optical inspection for the sphere positions, by X-ray for the collapse and the voids, and by a shear test on a sample for the joint strength. The three answer different questions and none of them replaces another.

The limit of the optical inspection is that it cannot see the joint, which is why an X-ray check is retained on a sample even when every part is inspected optically. Our thermal cycling notes describe how the result is qualified over life.

Acceptance and Its Evidence

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process.

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure. The sequence of operations is part of the specification, because a different order produces a different result from the same steps.

Verification and Records

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts.

Is a flattened sphere a defect? It is where the tolerance on the diameter matters, because the height of the package depends on it.

Can a ball attach defect be repaired? A missing sphere can be replaced, and a voided joint is a part that is usually scrapped.

What does gopcba provide for ball attach? We provide flux deposit controlled by measurement, a profile developed with the substrate fitted, sphere diameter measured inside a tolerance, void fraction measured by X-ray against a perimeter acceptance, shear tests on samples, and records that tie the result to the sphere lot and the profile.

Checks Before Release

Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.