Grain Structure and Alloy Additives in Solder

A small addition of a fourth element to a solder alloy changes the grain structure that forms when the joint solidifies, and the change is visible in the reliability rather than in the melting point. The additive is a process decision with a measurable result.

What the Additive Does

The additive forms particles in the melt that act as nucleation sites, so the alloy solidifies with more and smaller grains. A finer structure distributes the strain of a thermal cycle over more boundaries and resists fatigue better.

The same additive usually also refines the intermetallic layer at the copper interface, which is where a different class of failure starts. Our thermal cycling notes describe how the benefit is measured.

The Common Additives

Nickel and germanium are added to tin silver copper alloys to control the growth of the intermetallic and the grain structure. Bismuth lowers the melting point and changes the fatigue behaviour in a different way.

Each has a concentration range, and above it the alloy becomes brittle or the melting range widens. Our joint criteria notes describe the appearance that the alloy produces.

The additive and the cooling rate act on the same property from two directions. A fast cooling with no additive can produce a finer structure than a slow cooling with one, which is why the two are specified together.

That interaction is what makes a comparison between alloys difficult without controlling the profile. A trial that changes both the alloy and the profile measures neither. Our profile notes describe how the cooling rate is measured.

The effect is measured by sectioning a joint, etching it and measuring the grain size against a reference, and by cycling a daisy chain to a defined resistance rise. The section explains the mechanism and the cycling gives the number.

The two are reported together because a finer grain that does not improve the cycling result is a change without a benefit. The cycling result alone does not say why.

An alloy with a refined structure still fails where the joint geometry concentrates the strain. A joint that is too thin or too stiff fails at the same number of cycles whatever the grain structure.

That is why the geometry is addressed before the alloy. A change of alloy is the second lever and it is more expensive than a change of pad size.

The practical position is that the additive is part of the alloy specification and not a variable to be adjusted. An alloy is qualified with its additive and the qualification covers the profile that was used.

A change of alloy supplier with the same nominal composition is a change to the qualification, because the additive content and the trace elements differ. Our board quality notes describe how the change is verified.

Acceptance and Its Evidence

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. 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.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

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

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

Verification and Records

A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

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. The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Does an additive raise the melting point? It usually changes the melting range slightly and its main effect is on the structure rather than on the temperature.

Can the benefit be seen without cycling? A section shows the structure and it does not show the life, so both are measured.

What does gopcba provide for alloy selection? We provide an alloy qualified with its additive content, the cooling rate specified with the alloy rather than separately, sections that show the grain and intermetallic structure, cycling that gives the number of cycles, and change control when the supplier or the composition changes.

Checks Before Release

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.

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.