Visual Inspection Criteria for Lead Free Solder Joints
Lead free solder looks different from tin lead. It is more matte, it does not always form a smooth concave fillet and its surface is often grainy. Criteria written for the older alloy reject joints that are perfectly sound.
Why the Appearance Differs
The alloy has a higher melting point and a wider melting range, so it solidifies over a temperature interval rather than at a point. The grain structure that forms is coarser and the surface is rougher as a result.
The flux chemistry also differs, and the residue left on the surface changes the way it reflects light. Our joint criteria notes describe the criteria that apply to the joint rather than to the shine.
What Still Applies
Wetting, fillet shape in the sense of the contact angle, the absence of bridging and the presence of a toe and heel fillet all still apply to a lead free joint.
A joint that has not wetted the lead is a defect whatever the alloy, and it will not be corrected by the profile alone if the surface is contaminated. Our cleanliness notes describe the contamination that causes it.
What Has to Be Judged Differently
The surface finish of the joint and the fillet profile are the two features that differ most. A lead free fillet often has a higher contact angle and a less smooth surface.
The judgement should be made against reference samples from the same process rather than against a photograph from a different one. Our workmanship notes describe the references.
Defects That Look Similar
A cold joint and a correctly formed lead free joint can look similar at low magnification. The distinguishing feature is the wetting at the interface rather than the surface texture.
A joint that is disturbed during solidification has a rippled surface and it is a genuine defect. The two should be separated by the interface rather than by the appearance. Our defect notes describe the difference.
Inspection Practice
The inspection should use the same lighting and the same magnification for every sample, because the appearance of a matte surface changes with the angle.
Where the inspection is automated, the thresholds have to be set on lead free samples rather than transferred from a tin lead program. Our AOI notes describe the programming.
Criteria for Rework
The decision to rework should follow a functional criterion: an insufficient fillet, a bridge, a missing joint or a wetting failure. A surface that is merely dull is not a reason.
Rework adds a thermal cycle and it risks the pad, so the benefit should be clear. Our rework notes describe the exposure it adds.
Verification
The verification is a documented comparison between the reference samples and the production joints, performed by qualified inspectors at a defined interval.
Where the reject rate rises after a paste change, the criteria should be checked before the process is adjusted. Our paste notes describe the change that most often triggers it.
Process Control and Verification
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. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
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.
Checks Before Release
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 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.
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 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.
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.
Verification and Records
A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. 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.
FAQ
Is a shiny joint better? Shine depends on the alloy, the flux and the cooling rate. It is not a criterion for either alloy.
Should the criteria be the same for both alloys? The framework is the same and the appearance references differ, which is why the references are produced per process.
What does gopcb provide for lead free inspection? We provide criteria written against the joint function rather than its appearance, reference samples produced from the same process, lighting and magnification held constant, automated thresholds set on lead free samples, and a rework decision based on a functional criterion.
