Improve Product Design with Accelerated Life Testing Thermal Cycling

Key Takeaways

• Accelerated life testing is a practice where the life characteristics of the system or component are quantified under normal operating conditions by accelerating failure. The accelerated life test includes different subtests, including qualitative and quantitative tests. 

• Accelerated life testing thermal cycling is an established quantitative technique for thermo-mechanical evaluation and the qualification of products. These tests are conducted on the entire population in a batch of products and there is no sampling involved.

• A product under the thermal cycling test undergoes 10 cycles, between -650℃ and +150℃. Components are transferred between two extreme temperatures with a maximum transfer time of 5 minutes. Thermal cycling can take two forms—usage rate acceleration or higher stress acceleration.

Consider using accelerated life testing thermal cycling to ensure reliable components

Replacing old (but still functional) electronics for new versions with updated features is a common practice among the newest generation of electronics consumers—everyone wants the latest, cutting-edge gadget. Fortunately, present-day electronics have a relatively long life and typically work efficiently under normal conditions without much maintenance. However, this fact has made traditional time-to-failure data analysis difficult and this fast design-release process challenges the life data collection. 

Failure modes and life characteristics of any system, component, or product are collected by forcing them to fail faster than they would under normal operating conditions.  An accelerated life test does just that, and includes different subtests, including qualitative and quantitative tests. In this article, we will discuss a particular type of quantitative test—accelerated life testing thermal cycling. 

Accelerated Life Testing Thermal Cycling

In quantitative tests, products are subjected to environmental stimuli to expose, identify, and avoid defects that are not visible through visual inspection of electrical testing. Accelerated life testing thermal cycling is an established quantitative technique for thermo-mechanical evaluation and the qualification of products. Accelerated life testing thermal cycling is conducted on the entire population in a batch of products and there is no sampling involved. It is a method where the life characteristics of the system or component are quantified under normal operating conditions by accelerating failure. 

Let us explore how thermal cycling accelerated life testing works in relation to temperature changes.

Thermal Cycling Reveals Defects

Thermal cycling or temperature cycling is a method of accelerated life testing where the product or component is subjected to temperature variations. These repeated temperature variations result from either self-heating in switching devices or cyclic environmental changes and can cause thermal fatigue or failure of the product after several thermal cycles. 

The objective of thermal cycling is to measure field life and identify possible failures in components by exposing them to harsh temperature conditions. This method of analysis allows for shorter product design times and qualification cycles. Thermal cycling helps to reveal defects in electronic devices and components related to packaging, wiring, and insulation. Additionally, thermal cycling, along with vibratory testing, can uncover various other failures in products. 

The Process of Thermal Cycling

In accelerated life testing thermal cycling, the temperature applied to the product varies from high to low and these values exceed the normal operating temperature of the product. The aim is to impose mechanical stresses on the product due to the thermal contraction and expansion of the materials. For this reason, thermal cycling can also be called thermal shock testing. 

Due to repeated temperature variations, stress concentrates at weak parts of the component while the thermal test accelerates defects and failures. Microcracks, hard precipitates, thin wiring, and insufficient insulation are some stress concentrated areas that can get damaged under thermal cycling induced fatigue. The good components of the product may also experience some fatigue life loss in the thermal cycling process, but this degradation is not enough to make the product faulty. 

The Thermal Cycling Equation

The time taken by the product to fail in thermal cycling is significant in accelerated life testing. The ratio of product life at normal operating conditions to the life at accelerated thermal conditions is called the acceleration factor in thermal cycling. It is calculated by the Coffin-Manson equation and is expressed as:

where ΔT _use is the temperature difference in the field  (°C), ΔT _test is the temperature difference when under thermal cycling test (°C), and m is the fatigue or Coffin-Manson exponent.

Thermal Cycling Stress Acceleration

Accelerated life testing thermal cycling equipment generally consists of a hot and cold chamber to transfer the products between two extreme temperature conditions, with a maximum transfer time of 5 minutes. The product being tested usually undergoes 10 cycles, between -65℃ and +150℃. 

Thermal cycling can take two forms:

1. Usage Rate Acceleration: In this accelerated thermal cycling form, the rate of thermal stresses on the product is accelerated, keeping the applied equivalent stress at a normal value. Either air-air medium or liquid-to-liquid medium is used to increase the thermal rate of change in thermal cycling.  

2. Higher Stress Acceleration: In this approach, the applied stress on the device is increased beyond the value that it would encounter under normal operating conditions. 

Detect Material Defects Before They Occur

In electronic products, accelerated life testing thermal cycling detects defects and/or failures such as:

• Improper packaging 

• Defects in dies or substrates

• Mounting problems

• Bad bonds

• Thermal mismatches of materials  

• Solders, encapsulants, and their interfaces 

Thermal cycling also addresses the aging or evolution of material behavior of packaging materials in complicated electronic assemblies. Stresses and strains on solder joints, die device surfaces, and wire bonds that are embedded inside the electronic product assembly can be measured for reliability testing using thermal cycling.

Accelerated life testing thermal cycling is a cost-effective method of predicting failure in electronic products, components, or systems due to thermal stress. As thermal testing quickly unravels the potential risks and life characteristics of an electronic product, it improves product design, reliability, and shortens the time to market. Consider thermal cycling testing if you want to ensure your electronic products are robust and reliable.