Solder Bath Composition and Alloy Contamination Control

The solder in a wave or selective soldering machine is not a fixed material. It picks up copper from the boards, it loses the more reactive elements to oxidation, and it accumulates whatever the process drags into it. The result is an alloy that drifts.

What Changes in the Pot

Copper dissolves into the solder from the pads and the plating. Each board contributes a small amount, and the level rises steadily with the volume processed.

The alloying elements also oxidise preferentially. The dross that forms on the surface is richer in the reactive elements than the bath, so removing dross removes them from the alloy and shifts the composition. Our solderability notes describe the effect on wetting.

Effect of Copper on the Joint

A moderate copper level is normal and it raises the melting point slightly. Beyond the limit the alloy becomes a higher temperature material, and the pot must be run hotter to keep the same flow.

Running hotter accelerates the oxidation, which removes more of the alloying elements, which raises the melting point further. The drift is a feedback loop rather than a straight line. Our joint defects notes describe the appearance of a bath that is out of specification.

Sampling and Analysis

The bath should be sampled on a schedule and analysed by a laboratory that reports the composition, not only a pass or fail. The result should be trended so that the rate of change is known.

The sample should be taken from a defined place and depth, because the composition is not uniform across a large pot. Our fabrication notes notes list the records that should be kept.

Correction

Where the copper is above the limit, the bath is diluted with fresh alloy or replaced. Dilution is cheaper and it can be repeated until the level is inside the window.

Where an element is low, the correction is an addition of a master alloy rather than of pure metal, because the balance between the elements is what defines the alloy.

Pot Maintenance

The dross should be removed with a tool that does not take alloy with it, and the removal should be recorded because it is a composition change as well as a cleaning step.

The pot temperature should be held at the lowest value that gives good flow. A pot run hot to compensate for a drift is a pot that is accelerating its own drift. Our process control notes describe the same discipline applied to a wet process.

Effect on the Process

The flux, the preheat and the contact time are all set for a specific alloy. Where the alloy has moved, the settings that worked no longer describe the process, and the defect that appears is blamed on the flux.

Where the bath is corrected, the process should be re-verified rather than assumed to return to the previous state. The settings and the sample result should be recorded together.

Verification

The verification is the laboratory analysis against the specification, the joint appearance against the reference samples, and the defect rate by joint type.

Where the analysis is inside the specification and the defect rate has risen, the cause is elsewhere and the bath should be excluded before it is changed. Our quality notes describe how the result is recorded.

Process Control and Verification

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. 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.

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. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Verification and Records

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.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

FAQ

How often should the bath be analysed? On a schedule based on the throughput, and immediately after any event that could have added a contaminant, such as processing a batch with a different finish.

Can the copper be removed from the bath? It can be diluted to a lower level, and removing it directly requires equipment that most assembly shops do not have. Replacement is the usual answer above the limit.

What does gopcb provide for solder bath control? We provide a sampling schedule with a defined sampling point, laboratory analysis with trended composition, dilution or master alloy correction to the specification, dross removal with its alloy effect recorded, pot temperature control at the lowest effective value, and process re-verification after every correction.