Alloy: Key Checks Before Release

Alloys marketed for the same purpose differ in their fatigue behaviour, their melting range and their cost. The choice is usually made on melting point and price, and the property that decides the life of the assembly is usually not considered.

What the Alloy Has to Do

The alloy must melt in a window the components can survive, wet the surfaces the process provides, and then survive the movement the assembly will see for the life of the product.

The first two are process requirements and they are verified during development. The third is a reliability requirement and it is verified by thermal cycling, which is slower and more expensive. Our thermal cycling notes describe the test.

Silver Content

Silver strengthens the alloy and improves its fatigue resistance. A lower silver content costs less and is more brittle under thermal cycling, and it is more sensitive to the cooling rate.

The difference appears on assemblies with large components or thick boards where the strain per cycle is high. On a small board with small parts the difference is small. Our reliability notes describe how the strain is assessed.

Additions and Their Purpose

Small additions change the microstructure and therefore the behaviour. Copper raises the melting point and improves the strength; bismuth lowers the melting point and reduces the fatigue resistance.

The additions should be understood rather than accepted, because an alloy chosen for a process benefit can carry a reliability penalty that is not stated on the datasheet.

Cooling Rate and Microstructure

The cooling rate after solidification sets the microstructure, and the microstructure sets the fatigue behaviour. A slow cool produces coarse grains and a different response from a fast one.

The cooling rate depends on the thermal mass of the assembly and on the profile, so the same alloy behaves differently on two products. Our thermal mass notes describe the variation across one board.

Thermal Fatigue Testing

The comparison should be made on the real assembly rather than on a test coupon, because the joint geometry and the thermal mass both change the result.

Where a coupon is used, it should reproduce the joint volume and the pad design of the product, or the result describes the coupon. Our test coupon notes describe how the coupon is designed.

Cost and Availability

An alloy with a better fatigue life can cost more and be available from fewer sources. The supply consideration is a design input rather than a purchasing detail.

Where the alloy is unusual, the second source question applies to the paste as well as to the alloy. Our sourcing notes describe the channel question.

Documentation

The alloy should be stated on the drawing with its composition, not only with a trade name, so that a substitute can be evaluated rather than accepted.

The thermal cycling data that supported the choice should be kept with the design. Our fabrication notes notes list the records that should be kept.

Process Control and Verification

On a design of this kind, thermal fatigue is the item that decides how the rest of the board is arranged. 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. 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.

Process Control and Verification

On a design of this kind, thermal fatigue is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, thermal fatigue is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, thermal fatigue is the item that decides how the rest of the board is arranged. 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.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Is a higher silver content always better? It improves fatigue resistance and it costs more and raises the melting point slightly. The benefit depends on the strain the assembly sees.

Can the alloy be changed without re-qualification? The profile and the thermal cycling result both change, so the change should be qualified rather than substituted.

What does gopcb provide for alloy selection? We provide alloy selection against the process window and the thermal strain, silver content and addition assessment, cooling rate and microstructure review, thermal cycling on the real assembly, supply and second source review, and a stated composition on the drawing with the supporting data retained.