Thermal Cycling Test Design and Interpretation

A thermal cycling test is not a single test. The ramp rate, the dwell time, the temperature extremes and the number of cycles each change the mechanism that the test exercises, and a test that does not match the application can pass a design that will fail.

What Thermal Cycling Tests

The test drives the difference in expansion between the materials and loads the joints and the interfaces with shear. The damage accumulates with each cycle and it appears as a fatigue crack.

The mechanism is not the same as a thermal shock, which is limited by the rate of change. The two tests answer different questions, and a design may pass one and fail the other. Our reliability notes describe how the mechanisms are distinguished.

Extremes and Dwell

The extremes should bracket the condition the product will see with margin. Where the extremes are wider than the application, the test may introduce failures that cannot happen in use, and the resulting redesign has no benefit.

The dwell must be long enough for the entire assembly to reach the extreme. A dwell that is too short leaves the centre of a large part at an intermediate temperature and the test underestimates the stress.

Ramp Rate

The ramp rate sets how much of the strain is applied while the materials are still creeping. A slow ramp allows the solder to relax and produces less damage per cycle than a fast one.

The rate should be stated, because two laboratories using the same extremes and the same dwell can produce different results with different ramps.

Sample Size and Monitoring

The number of samples decides the confidence in the result. A test that reports “no failures after a thousand cycles” on three samples describes three samples, not a failure rate.

The assemblies should be monitored in situ where the failure mode is an intermittent open, because the crack closes when the assembly returns to room temperature. Our probe notes describe the measurement arrangement.

Failure Analysis After the Test

A failed sample should be cross sectioned at the failure site, and the crack path should be compared with the expected mechanism. A crack through the intermetallic layer is a different finding from one through the bulk solder.

The analysis should include samples that have not failed, because the extent of the damage in them shows how close the design was to the limit.

Interpreting the Result

The result should be expressed as the cycles to a defined failure criterion at a stated condition, not as a pass. That is what allows the number to be compared with the application.

Where the application requires more cycles than the test showed, the design change should be evaluated against the mechanism rather than against the number. Our quality notes describe how the result is recorded.

Records

The profile, the chamber identity, the sample identification and the monitoring arrangement should be recorded with the result, because a second test has to be comparable with the first.

Where the result is used to qualify a product, the record is the evidence, and it should be kept with the design rather than with the laboratory. Our fabrication notes notes list the records that should be kept.

Process Control and Verification

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

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, sample size is the item that decides how the rest of the board is arranged. 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, sample size is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, sample size is the item that decides how the rest of the board is arranged. 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 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 more cycles always better? Only if the cycles match the application. Additional cycles beyond the requirement consume time and samples without adding information about the product.

Should the test be run on the bare board or the assembly? On the assembly, because most of the failures that matter are in the joints, the interfaces and the plated features that only exist after assembly.

What does gopcb provide for thermal cycling? We provide a profile matched to the application with the ramp and dwell stated, extremes bracketing the use condition with margin, sample sizes matched to the confidence required, in situ monitoring for intermittent faults, cross section analysis of failed and surviving samples, and records that make the result comparable and traceable.