Solder Alloy Selection for High Temperature Service

An assembly that operates hot needs a solder whose melting range leaves a margin above the service temperature. The margin is what keeps the joint from creeping and from coarsening while it is in service.

Why the Melting Range Matters

A solder with a wide melting range has a pasty zone, and a joint held inside that zone is partly liquid and partly solid. The structure coarsens, the strength falls and the joint creeps under load.

The service temperature should sit well below the solidus. Our joint criteria notes describe the acceptance that follows.

Alloys That Are Used

Tin antimony, tin silver and gold tin alloys are the common high temperature choices, each with a different melting range and a different cost. The choice follows the temperature and the assembly rather than the data sheet alone.

High lead alloys remain in use where legislation permits and the thermal requirement is extreme. Our component selection notes describe the wider trade.

Intermetallic Growth

The intermetallic layer between the solder and the copper grows with time and temperature, and it is brittle. A joint that spends its life hot develops a layer thick enough to crack under thermal cycling.

The growth is the mechanism that limits the life. Our thermal cycling notes describe the test.

Thermal Fatigue in a Hot Application

A hot application often has a small temperature swing, which is easier on the joint, or a large one during power cycling, which is harder. The duty cycle matters as much as the absolute temperature.

The profile should represent the real cycle rather than a standard one. Our fatigue notes describe the extrapolation.

Flux and Process Compatibility

A higher melting alloy needs a higher process temperature, which the components and the laminate have to tolerate. The flux has to remain active at that temperature without decomposing.

The process window narrows as the alloy temperature rises. Our profile notes describe the verification.

Compatibility Between Alloys

A board that uses two alloys, one for the first pass and one for the second, has to have a sufficient gap between their melting ranges so that the first joint is not disturbed.

The gap should be checked against the profile rather than assumed. Our reflow notes describe the arrangement.

Verification

The verification is a service temperature stated with its duty cycle, a chosen alloy with a solidus above it, a process window confirmed on the assembly, and an accelerated test whose profile represents the real cycle.

Our quality notes describe how the records are kept.

Process Control and Verification

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. 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.

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.

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.

FAQ

Is the highest melting alloy always the safest choice? No. A higher melting point raises the process temperature, which can damage components and laminate more than the alloy gains.

Does a small temperature swing remove the concern? It reduces the fatigue but not the intermetallic growth, which continues at temperature regardless of cycling.

What does gopcb provide for high temperature assemblies? We provide an alloy chosen with its solidus above the stated service temperature, a process window confirmed on the assembly rather than on a coupon, a check on the gap between two alloys used on the same board, and an accelerated test whose profile represents the real duty cycle.