Solder Ball Removal and Acceptance Criteria

A solder ball is a small sphere of alloy on the board surface that is not part of a joint, and the question it raises is whether it can move. A ball that is held by the mask is a cosmetic defect and one that is free is a reliability defect.

Where the Balls Come From

The paste sputters during reflow, the paste that escaped under the stencil leaves residue, and a deposit placed on the mask rather than on the pad forms a sphere. Each source produces balls in a different place on the board.

Controlling the source is cheaper than removing the result, which is why the investigation starts with the print rather than with the cleaning. Our board quality notes describe how the surface is judged.

Acceptance and the Movement Question

The acceptance is written around whether the ball can move. A ball trapped under a coating, contained by a mask dam or held in a via is not free to travel, and one on a flat area beside a connector is.

That distinction is what makes the criterion workable rather than absolute. A criterion that requires no balls at all is a criterion that is not met by most processes. Our joint criteria notes describe how the acceptance is written.

A brush removes the loose balls and moves the adhered ones. Compressed air removes the loose ones and moves them elsewhere. A wash removes most of them and leaves the ones that are held by flux residue.

The method is chosen from where the balls are and what is underneath them. Brushing a board with a fine pitch part moves the balls into the part. Our cleanliness measurement notes describe how the result is verified.

The print is the main source, and the controls are the stencil underwipe, the paste volume and the aperture design. A print with a clean board surface produces few balls.

The profile is the second source, because a fast ramp causes spatter. A profile that is developed with the paste produces fewer balls than one that is copied.

Inspection for balls is visual, and the field of view has to include the areas where they can travel. An inspector who looks at the joints only does not see the balls on the mask.

Where the assembly is critical, the check is done under magnification after cleaning and before the unit is packed. Our inspection notes describe how the machine side is programmed.

A free ball that reaches a customer produces an intermittent fault that is difficult to diagnose and expensive to contain. That is the reason the criterion is written around movement rather than around the count.

A board that is cleaned and verified is cheaper than a containment, and a print that is controlled removes the need for both.

Acceptance and Its Evidence

The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review. 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.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts.

Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made. Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

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. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Checks Before Release

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. 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.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

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.

Verification and Records

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.

Is a solder ball always a defect? It is where it can move. A ball held by the mask or by a coating is usually accepted by a defined criterion.

Does cleaning remove every ball? It removes the loose ones and it leaves the ones that are held, which is the reason the criterion is written around movement.

What does gopcba provide for solder balls? We provide the print controlled to reduce the source, a profile developed with the paste, acceptance written around whether the ball can move, removal chosen for the area beneath the balls, and verification under magnification after cleaning.

Points to Confirm at First Article

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. 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.

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.