Incoming Solder Alloy Verification and Pot Additions

Solder arrives as bars, wire or paste with a certificate that states its composition. The certificate is a claim, and the alloy in the pot is what actually forms the joints. Where a nonconforming bar reaches a wave pot, the effect is spread across every board soldered afterwards and is difficult to reverse. Verification is the control, and it does not have to be expensive to be effective.

What the Certificate Does and Does Not Cover

A certificate of analysis states the composition of the material the supplier tested, which is a sample from a production lot rather than the specific bar in the box. For a supplier with a stable process the claim is usually reliable, but the verification is what protects against a mixed shipment, a mislabelled box or a formulation change that was not communicated.

The certificate also says nothing about the surface condition of the bar. Oxide, contamination and moisture on the surface enter the pot with the metal, and a bar that has been stored badly brings its own problems that the composition data will not show.

Sampling and Spectrometry

Verification is normally a spectrometric analysis of a sample taken from the incoming material. The sample should be representative, which means taking a piece from several bars across the lot rather than from one, and preparing the surface so that the measurement is of the alloy rather than of the oxide on it.

The analytical method has to be capable of measuring the elements that matter at the levels that matter. Trace elements that affect the melting behaviour or the mechanical properties can be present at low concentrations, and a method that reports only the major elements will not detect a problem with them. The choice of method should be agreed with the laboratory and stated in the specification.

Frequency and Acceptance Limits

Full analysis on every lot is not necessary for a stable supplier. A practical scheme is to analyze the first lot from a new supplier or a new part number in full, then to verify periodically and whenever a certificate is missing or inconsistent. The frequency should be increased when a supplier changes a process, a source of raw material or a production site.

The acceptance limits should come from the alloy specification, and they should be stated as ranges with the test method. Where a lot is marginal, the disposition should be defined in advance so that the decision is not made under schedule pressure. A lot that is accepted outside the specification becomes a permanent exception in the pot, and it will be detected later as a defect rather than as a material issue. The pot control that follows is described in solder pot maintenance and dross control.

Bar Identification and Storage

Bars should be identified by lot and kept separate until they are verified. Mixing verified and unverified material destroys the traceability, and the practice of opening a box and using it because it looks the same is exactly how a nonconforming lot reaches the pot. A simple quarantine area with a label is sufficient.

Storage conditions matter for the surface condition. Bars should be kept dry and clean, and they should be handled with gloves. A dusty bar carries the dust into the pot, and a wet bar introduces moisture that produces spatter and porosity. The storage rules are the same ones that apply to the paste and the wire, described in solder paste volume and stencil design.

Pot Additions and Top Up Practice

The pot is maintained by adding alloy as material is consumed, and the addition practice determines the chemistry. Adding a bar of a different composition, or adding a bar intended for a different alloy, changes the bath in a way that a single sample will not reveal immediately. The bars used for top up should be from the same specification as the bath.

The amount added should be recorded by weight, because the record is what makes it possible to compare the consumption with the production volume. A bath that suddenly consumes more alloy than the boards account for is losing material to dross or to a leak, and the record is what makes that visible.

Incoming Inspection of Wire and Paste

The same logic applies to the other solder materials. A wire or a paste that is not what the label says produces the same class of problem as a nonconforming bar, and the incoming inspection has to be appropriate to the form. For wire, the diameter, the flux classification and the surface condition are the checks. For paste, the metal content, the particle size and the viscosity behaviour are the important ones.

Alloy verification for paste usually relies on the supplier certificate plus a periodic check of the metal content by weight loss, which is a simple test that catches a formulation change. The full spectrometric analysis is reserved for the bars that go into a pot, where the volume at risk is much larger and the correction is much harder.

Records and Traceability

The record should link the material to the product. For each lot: the supplier, the certificate, the analysis result and the date it entered the pot. For the production: the boards soldered during the period when that material was in the bath. That chain is what makes a containment possible if a material problem is discovered later.

The gopcb production team keeps that record with the pot history and reviews it when the periodic analysis shows a drift. Where a lot is implicated, the boards produced during its residence in the pot can be identified and assessed, which converts a potential recall into a targeted investigation.

What to Do With a Nonconforming Lot

A rejected lot has to be removed from the flow rather than set aside. The material should be labelled, segregated and returned or scrapped, and the supplier should be notified with the analysis result so that the cause can be investigated. Where the lot has already been used, the boards produced during its residence in the pot have to be identified and assessed.

Assessment is usually electrical and visual rather than destructive, with a sample cross sectioned to confirm that the joints are metallurgically correct. Where the deviation is small and the joints are acceptable, the lot may be accepted with a documented concession. Where the deviation affects the melting behaviour, the joints have to be considered suspect regardless of how they look, because the failure mode appears later.

Verification and Records

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Is a certificate of analysis enough? For a proven supplier it is usually sufficient as a routine control, but it does not cover mislabelling or surface contamination. Periodic analysis verifies the claim.

How often should the alloy be analyzed? First lot in full, then periodically, and whenever a certificate is missing or a process change is reported.

Can top up material change the bath composition? Yes, and that is why the top up bars should match the bath specification and the additions should be recorded by weight.