Thermal Shock Versus Thermal Cycling Tests

Thermal shock and thermal cycling both expose a board to a temperature change, and they produce different failures because the rate of change is different. A test that is chosen for the wrong reason qualifies the wrong property.

What Separates the Two

A thermal shock moves the board between two set points as quickly as the chamber allows, so the assembly is never in equilibrium and the temperature difference across it is large. A thermal cycling test moves it at a controlled rate, so the assembly reaches each set point.

The first loads the structure with a gradient and the second loads it with a strain. Our thermal cycling notes describe the test design that follows from that.

A thermal shock finds a weak interface or a void that opens under the gradient, and it does so in a small number of cycles. A thermal cycling test finds fatigue, which appears after many cycles and grows.

The two therefore answer different questions. A shock test qualifies the construction and a cycling test qualifies the life.

A shock test is short and a cycling test is long, often measured in days. That makes the choice partly a schedule decision, and it is the reason a shock test is used as a screen and a cycling test as a qualification.

A screen that is applied to production has to be short, and it has to be shown not to damage good product. A qualification is applied to a sample and it is allowed to take as long as it needs.

The number of cycles is only part of the specification; the dwell at each extreme matters as well. A test with a short dwell never lets the assembly reach the set point, and the cycles that are counted are not the cycles that were intended.

The dwell is set from the thermal mass of the assembly, measured on the board rather than on the chamber. A chamber that reaches the set point quickly can still be waiting several minutes for the board. Our board quality notes describe how the measurement is recorded.

The failure criterion has to be defined before the test starts. A resistance limit on a daisy chain, a visual criterion, or a functional check are all usable and they give different answers.

Continuous monitoring during the test is worth more than an inspection after it, because it gives the cycle at which a failure occurred. A test that is inspected only at the end reports a failure without a time. Our joint criteria notes define the acceptance that the monitoring is comparing against.

The plan normally contains both: a shock test to qualify the construction and a cycling test to qualify the life. Where only one can be run, the choice follows the dominant failure mode of the design.

A design that is dominated by interface quality is qualified by shock and one that is dominated by fatigue is qualified by cycling. The dominant mode is decided from the geometry and the materials rather than from preference.

Process Control and Verification

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.

Is a shock test more severe? It is more severe per cycle for an interface, and it does not accumulate fatigue in the way a cycling test does.

Can a cycling test replace a shock test? For the interface question it usually cannot, because the gradient that opens a weak interface is not reproduced at a controlled ramp rate.

What does gopcb provide for temperature testing? We provide the test selected against the dominant failure mode, dwell set from board measurement rather than chamber readout, failure criteria defined before the test, continuous monitoring with the failure cycle recorded, and traceability from the result to the materials and the process.

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.