Solder Joint Acceptance Criteria: Accept, Reject and Rework

Inspection criteria exist so that two people looking at the same joint reach the same conclusion. Where the criteria are not written down, the conclusion depends on who is looking, and the process drifts.

What a Good Joint Looks Like

A well formed joint shows a concave fillet that wets both the pad and the termination, with a smooth surface and a wetting angle that indicates the solder has flowed rather than sat on top.

The fillet shape is the primary evidence, because it indicates that the solder reached the required temperature and that the surfaces were clean enough to wet.

The fillet size is secondary. A small fillet on a joint that has wetted properly is acceptable, while a large fillet on a joint that has not wetted is a defect.

The Categories of Defect

A process indicator is a condition that suggests something is drifting but that does not by itself reduce the function of the joint. These are handled by adjusting the process rather than by rejecting the product.

A defect is a condition that reduces the function of the joint. These require rework, and the rework must itself meet the criteria.

A critical defect is a condition that creates a hazard or that will certainly fail. These require the unit to be quarantined and the cause established before production continues. Our quality notes describe how each category is recorded.

Common Conditions and Their Treatment

Insufficient solder, where the fillet is smaller than required, indicates that the joint did not receive enough paste or heat. It reduces the mechanical strength and the current capacity.

Excess solder is cosmetically unappealing and it can hide a defect beneath it. It also raises the risk of bridging where pads are close together.

Bridging is a defect because it is a short circuit. It must be removed, and the removal must not damage the mask or the pad.

Conditions That Are Often Misread

A dull surface on a lead free joint is normal for some alloys and indicates nothing on its own. A dull surface on a tin lead joint is a process indicator and may indicate a cold joint.

A visible crack in a fillet is a defect regardless of whether the joint measures electrically good, because the crack will propagate. The measurement that confirms the extent is a section.

A pinhole or a void at the surface is a process indicator, while a void that spans the joint is a defect. The distinction requires the substitution of the void size, and the criteria should state it as a fraction of the joint. Our solder defects notes describe how the voiding is measured.

Inspection Methods and Their Limits

Visual inspection at a defined magnification is the primary method for through-hole and accessible surface mount joints. The magnification and the lighting must be specified, because both change what is visible.

X-ray is used where the joint is hidden, such as under a ball grid array. It shows the solder volume and the presence of voids, and it does not show the wetting at the interface.

Automated optical inspection applies the criteria programmatically and gives consistent results, and it produces false calls on joints it cannot see clearly. The false call rate must be managed so that the operators do not learn to ignore the results. Our inspection notes describe the appropriate technique for each package.

Rework and Its Own Criteria

Rework changes the joint and the surrounding material. A joint that has been reworked twice should be examined more closely than one that has not, because the pad and the mask around it have been subjected to extra heat.

The rework should be performed with the same alloy as the original, and the resulting joint should meet the same criteria as a production joint. A reworked joint that is accepted on the basis that it is only a repair is a latent failure.

A lifted pad from excessive heat is a defect that cannot be reworked without repairing the board. Where the repair requires a wire, the wire becomes part of the assembly and must be documented. Our AOI notes describe how the repaired assembly is re-inspected.

Writing the Criteria

The criteria should be a short document that states, for each joint type, what is acceptable, what is a process indicator and what is a defect. It should reference an industry standard where one exists rather than restating it.

It should include the inspection conditions, including magnification and lighting, so that two inspectors see the same thing.

It should be reviewed when the process changes, because a criterion written for a tin lead process may not suit a lead free one. The document is a living part of the process rather than a formality.

Additional Considerations for This Build

Practical attention to solder joint 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 solder joint 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, wetting is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Process Control and Verification

On a design of this kind, wetting is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

FAQ

Is a dull joint always a defect? No. The alloy, the finish and the cooling rate all affect the appearance, and the criterion should be based on the alloy in use.

How many reworks are allowed? The limit should be stated by the customer or derived from the thermal excursion the pad can take. Rework is not unlimited.

What does gopcb provide for joint inspection? We provide written acceptance criteria per joint type with the inspection conditions, visual, X-ray and automated inspection with managed false call rates, records of rework with the alloy used, and sectioning where a condition cannot be judged from the surface.