Process · Power & Discretes
Thermal cycling
A passive test in which the whole assembly is cycled between temperature extremes in a chamber, stressing the joints between materials of differing expansion coefficients.
It targets substrate-to-baseplate solder rather than the die attach.
- Also known as
- TC test, Passive cycling
- Type
- Process
- Domain
- Power & Discretes, Thermal and reliability
Notes
Ceramic choice, alumina versus silicon nitride, shows up most clearly in thermal cycling rather than power cycling results.
Overview
Thermal cycling, in power electronics, is a passive reliability test in which a complete power module is moved repeatedly between a low and a high temperature in a chamber while no load current flows through it. Because the whole module heats and cools from outside, the strain falls mainly on the large joints between materials that expand by different amounts, above all the solder between the ceramic substrate and the baseplate. It complements power cycling, which heats the chips from within and stresses the die attach and bond wires.
How it works
The unpowered module sits in an air-to-air chamber: either one chamber that heats and cools, or a two or three chamber system that moves parts between hot and cold zones. The automotive qualification guideline AQG 324 from the European Center for Power Electronics (ECPE) calls its version a thermal shock test and bases it on IEC 60749-25. It sets storage temperature limits of minus 40 and plus 125 °C and dwell times of more than 15 minutes at each extreme, so that the whole mass of the module reaches each temperature.
Each swing makes the copper baseplate, the ceramic substrate and the solder between them expand and contract by different amounts. Over many cycles the solder layer fatigues and delaminates, which raises the thermal resistance between the chips and the cooling surface. Engineers therefore measure junction-to-case thermal resistance at set intervals, and AQG 324 recommends scanning acoustic microscopy to record how far delamination has spread at the joint layers.
The test is slow because of the long dwells, and acceleration factors linking it to field life are not well established, so AQG 324 does not require it to run to end of life. Baseplate solder fatigue only arises in modules that have a baseplate, so the test's weight depends on the module's construction.
Examples
- A thermal shock test on an automotive IGBT module between minus 40 and plus 125 °C as part of qualification to AQG 324.
- Acoustic microscope images taken at intervals during cycling to measure how much of the substrate solder has delaminated.
- A rise in junction-to-case thermal resistance after cycling, read as a sign that the substrate-to-baseplate joint is degrading.
Why it matters
Power modules in vehicles and industrial equipment heat and cool many times over their lives, and the large solder joint beneath the substrate has to survive those swings for the module to keep its thermal performance. Power module packaging and design engineers use thermal cycling results to choose solders, substrates and baseplates, and qualification engineers run the test to customer and automotive guidelines.
Related terms
Part of the Power & Discretes glossary, with every term in the topic and the lifecycles, tools and standards that use them.
Where it appears on the Hub
Used on 19 pages of the Hub. In each group, the pages that mention it most come first.
Lifecycle stages and gates
Skills
Suggest an improvement
Suggest an improvement to this entry
A clearer definition, an example, a correction or a source an editor can check. Editors review every suggestion before the entry changes.
Last reviewed . Written by The Semiconductor Hub editorial team. The full glossary