Solder Joint Inspection Criteria And Limits
Solder joint inspection criteria describe how a joint is judged against the requirement it was built to. The judgement uses criteria that come from a workmanship standard, from the customer’s specification and from the class of the product, and it is applied to features that are visible from the outside and, for some packages, to features that are only visible in an X-ray image.
This article covers where the criteria come from, what is actually judged, and how to apply them consistently.
Where The Criteria Come From
The primary source is the workmanship standard the product is built to, which defines the classes of product by their reliability requirement and gives an acceptance condition for each feature in each class. The customer’s specification may add or tighten requirements, and the class of the product determines how much of the standard applies. A joint that is acceptable in one class is a defect in another, and the difference is stated in the standard rather than left to interpretation.
The criteria are written for the features that carry the reliability of the joint: the wetting, the fillet, the coverage of the pad and the lead, and the presence of defects such as voids, cracks or a disturbed surface. Where a joint is partly hidden, the standard says what can be judged from the visible portion and what requires a different technique. The way the design supports those judgements is described under the quality characteristics of a board design.
What Is Judged On A Fillet
A fillet is the concave surface of solder that joins the pad to the lead or the termination. It has to be continuous, it has to extend around the joint, and its surface has to be smooth enough to show that the solder was liquid and free to flow. A fillet that is concave and continuous is the normal indication of a good joint on a through hole lead; a fillet that is convex, or that has a visible boundary where the solder stopped, indicates that the solder did not wet the surface.
The coverage requirements are specific. On a through hole lead the solder has to fill the hole and form a fillet on both sides; on a surface mount termination it has to cover the termination and the pad with a defined minimum. Where the pad is not fully covered, the standard states whether the uncovered portion is acceptable, and the answer is different for a leadless part and one with a visible toe. These figures are what an inspector works from, and they are why a photograph is not a substitute for the standard. The metallurgy that the criteria assume is described under lead free versus leaded solder.
Wetting Angle And Its Meaning
The wetting angle is the angle between the solder surface and the surface it is wetting, measured through the solder. A low angle means the solder has spread and is adhering; a high angle means it has beaded. The angle is a direct indication of the surface condition and the thermal history, and it is the feature that distinguishes a cold joint from a properly formed one.
In practice the angle is judged from the shape of the fillet rather than measured. A fillet that is concave and blends smoothly into the pad has a low angle; one that meets the pad at a sharp edge with a convex profile has a high angle and is a reject. The judgement is affected by the surface finish, because a gold or a silver finish wets differently from a bare copper one, and by the flux, whose residue can make a fillet look duller than it is.
Voids And Internal Criteria
A void is a pocket of gas inside the joint, and it is invisible from the outside. It is measured by X-ray, as an area percentage of the joint cross section, and the limit comes from the product specification. The criteria normally combine a total void percentage with a limit on the largest single void and, for a thermal pad, a limit on the voiding at the interface with the pad.
The measurement is only comparable between boards if the equipment settings and the measurement rule are the same, which is why the rule belongs in the process specification. Two numbers produced at different voxel sizes are not the same quantity, and an argument about whether a joint passes often turns out to be an argument about the measurement. The related question of how a partly hidden joint is examined is treated under placement order and pad positioning.
Visual Versus X-Ray Inspection
Visual inspection sees the surface of the joint and the fillets on the exposed sides, and it is the only technique that assesses the wetting angle directly. It cannot see under a package, and it cannot see inside a joint. X-ray sees through the assembly and images the solder by its density, which shows voids and the shape of a hidden joint, but it cannot assess the surface finish or the wetting angle.
Automated optical inspection applies the visual criteria programmatically, using a camera and a set of rules, and it is faster and more consistent than a person. Its limits are the same as a person’s: it cannot see under a package, and a rule that is set too tightly produces false calls while one set too loosely misses defects. The rule set is normally established by correlation with a human inspector on a sample of known boards, and it is reviewed when the product changes.
Applying The Criteria Consistently
Consistency depends on three things: the criteria being written down, the inspectors being trained against the same reference, and the disputed cases being resolved by the same person or group. A shop where each inspector applies a slightly different interpretation produces a varying yield and, worse, a varying escape rate. Reference samples of acceptable and unacceptable joints are the most effective tool, because they turn a description into an object that can be compared.
The criteria should also be reviewed when the process changes, because a change of alloy, of flux or of finish changes the appearance of a good joint. A fillet that looked correct with a leaded alloy may look dull with a lead free one, and an inspector who applies the old appearance will reject good joints. Updating the reference samples with the new process keeps the criteria aligned with what the process actually produces.
FAQ
Are the criteria the same for leaded and lead free joints? The shape requirements are similar, but the appearance differs, because lead free alloys have a duller, more grainy surface. Reference samples should be updated when the alloy changes.
Can a joint be rejected for appearance alone? Only where the appearance indicates a real defect such as poor wetting or an incomplete fillet. Surface texture on its own, without a wetting problem, is not a defect under most standards.
How much voiding is acceptable? The limit comes from the product specification, and it is normally expressed as a total area percentage plus a maximum for a single void. It depends on the measurement rule, so the rule has to be fixed for the number to mean anything.
