Thermal Fatigue: Solder Joint Fatigue Under Thermal Cycling

A solder joint fails under thermal cycling because the board and the component expand by different amounts and the joint has to absorb the difference. The damage accumulates with every cycle until a crack forms and propagates.

Why Expansion Mismatch Drives It

The expansion mismatch between the package and the board sets how much the joint has to deform. The plastic strain that results is concentrated at the point where the mismatch is greatest, which is usually the corner of a package.

The joint geometry decides how that strain is distributed. Our component reliability notes describe the design response.

What the Profile Changes

The temperature range, the ramp rate and the dwell all affect the life. The range sets the strain per cycle, and the dwell decides how much of that strain becomes creep damage rather than elastic deformation.

A profile with a long dwell is usually more damaging than a faster one with the same range. Our thermal cycling notes describe the design of the test.

Where the Crack Starts

The crack starts at the interface between the intermetallic layer and the bulk solder, and it propagates across the joint rather than through it. The failure is ductile at the start and becomes a fast fracture at the end.

The location is why the joint thickness matters. Our joint criteria notes describe the geometry that is preferred.

Acceleration and Extrapolation

A test result is extrapolated to service using an acceleration factor, and the factor depends on the mechanism. Using a factor that belongs to a different mechanism is the most common error in this area.

The extrapolation should state which mechanism it assumes. Our failure investigation notes describe how a field failure is classified.

Comparing Materials and Geometries

A comparison between two options is only valid if both are tested at the same profile and to the same failure criterion. A different criterion can change the ranking.

The criterion should be a measurable increase in resistance rather than a visible crack. Our yield analysis notes describe how the data is recorded.

Design Responses That Help

A thicker joint, a larger pad, an underfill and a lower modulus alloy all change the life. Each has a cost and each acts through a different mechanism.

Our underfill notes describe the option that changes the load path.

Verification

The verification is a profile recorded at the joint rather than at the chamber, a failure criterion based on resistance, and an extrapolation that states the mechanism it assumes.

Our thermal measurement notes describe the recording that goes with it.

Process Control and Verification

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.

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 a larger joint always better? A thicker joint generally lasts longer under cycling, but the joint height is limited by the pad geometry and by the assembly process.

Does underfill always help? It changes the load path and it can transfer stress to the package, which is why the combination has to be tested rather than assumed.

What does gopcb provide for thermal fatigue work? We provide a test profile with the range, ramp and dwell stated, a failure criterion based on resistance rather than appearance, a sample size that supports the comparison being made, an extrapolation that names the mechanism, and a measured joint profile at the interface.