The reliability of printed circuit boards is influenced by many factors. In the automotive, aerospace or even industrial sectors, the ambient temperatures of printed circuit boards can vary between -40° and +180°Celcius. In order to be able to determine how reliably electronic assemblies perform under these conditions, printed circuit boards can be subjected to a temperature cycling test – also known as a thermal shock test.
On pcb-blog.com we answer five frequently asked questions about our test method and show you how the influence of temperature fluctuations can be reduced.
What is the purpose of the temperature change test?
Electronic devices used in the automotive, aerospace and industrial sectors sometimes have to withstand very strong temperature fluctuations.
For the printed circuit boards installed in them, this means that they must retain their functional properties as component carriers and electrical conductor systems even under thermal stress. This can be simulated by means of a temperature cycling test, which is used to check the aging behavior of printed circuit boards at rapidly changing temperatures.
What characterizes a temperature change test?
The temperature cycling test is often performed as an end-of-life test, i.e. it is terminated as soon as all tested parts have failed. Subsequently, the obtained data are usually subjected to Weibull failure analysis and evaluated based on the characteristic parameters of lifetime, form factor and failure-free time. If a specific failure criterion is to be checked for compliance, this is referred to as a “success run”. This means that all tested parts may only fail after a time limit.
How does a temperature change test work?
In the temperature cycling tests carried out by the KSG Group, the test specimens are installed in a test basket which is cyclically moved into a cold chamber and a hot chamber.
Typical implementation conditions are:
- Cold chamber: -40°C
- Hot chamber: +125°C
- Tempering per chamber: 25 min
- Changeover time between chambers: < 30 s
- Number of cycles: 1,000
The evaluation of the tests can be done visually for real circuits or by means of micrograph inspection. Much more efficient is the use of resistance testing. For this purpose, it is necessary to establish suitable test layouts and define appropriate failure criteria.
What types of faults are tested using the temperature cycling test?
The temperature cycling test focuses on the following types of faults:
- Cracking in the copper sleeve of through holes and large blind holes
- Demolition of inner layer connections of through and blind holes
- Failure of the inner layer connection of LASER vias to the target pad.
- Cracking in solder resist masks
- Delamination of composite superstructures
What is the main cause of PCB failure in temperature cycling test?
A printed circuit board is usually made of copper, epoxy resin and fiberglass mats – materials that differ from each other in their properties such as strength and coefficient of thermal expansion. This means that when the temperature increases, different material expansions occur, resulting in inhomogeneous stress fields. These in turn can cause local stress increases and thus cracking. If these cracks spread, this leads to electrical interruptions and thus to the failure of the printed circuit board.
How can high thermal shock resistance be achieved in printed circuit boards?
To achieve high thermal shock resistance in printed circuit boards, the following main influences should be considered:
- Low pre-aging due to low impact during storage and loading,
- a small extension of the layer composite in the z-direction,
- the use of base materials with the lowest possible coefficients of thermal expansion, especially in the z-direction,
- a high layer thickness of the copper sleeves in holes,
- a drill diameter as large as possible and a uniform distribution,
- a high manufacturing quality of the printed circuit board assembly.
Do you have further questions? For more information on the temperature cycling test, please refer to our XPERTS online seminar “It’s all about layering: Printed circuit boards in the temperature cycling test” with Dr. Swantje Frühauf and Holger Bönitz. To the on-demand version available at any time you can register log in here.