Why 85/85 test is a wrong test choice for PET (polyethylene terephthalate)?

Trelic blog 21/03/2018 – Sanna Lahokallio and Laura Frisk

 

Using a combined high humidity and high temperature test is a very common accelerated reliability test method in electronics. In an 85/85 test, a test temperature of 85°C and relative humidity of 85% are used. This test is often used in R&D phase, as both customers and R&D engineers are accustomed to use this test – it has always been used. For some materials though, 85/85 test is too harsh and causes failures which would never occur in the field.

PET (Polyethylene terephthalate) is a good example of a material which cannot withstand the 85/85 test. As a plastic material, PET is probably best known for its use is water and soda bottles. However, in electronics it is often used as flexible substrate material in low-cost or in printed electronics applications. It is also widely used in various display attachments even in industrial applications.

The popularity of PET is easy to understand: It’s relatively cheap with good electrical properties, making it well-suited for flexible printed circuit boards (PCB) and cables. The main disadvantage of PET is its low glass transition temperate (Tg), preventing its use in applications requiring high use or manufacturing temperature.

In the picture above, you can see a PET flex PCB with cracks penetrating through the flex. The question here is why did the PET film crack, which brings us back to the original question: Why is 85/85-test too harsh for PET?

The answer to the PET cracking is hydrolysis. PET is a polyester material which means that it has ester groups in its polymer chain. When PET is exposed to environments combining high humidity and temperature above its Tg, hydrolysis of the ester groups occurs. This means the polymer chains are broken (often called chain scission) which leads to embrittlement of the material and failures.

You can see another example of PET cracks in the picture on the right. In this one you can also see dark grey traces, which are the silver paste wirings printed on a flex PCB. The black curves in the picture are the cracks in the PET film. In this case the cracks also propagated through the electrical wiring, breaking the electrical path of the wirings and causing a failure.

The PET flex in both picture has been exposed to a rather long 85/85 test. However, it is good to bear in mind that hydrolysis starts to brittle PET material already during a 500h test, and after 2000h the material will lose all its mechanical properties. The same problem may also occur with other ester-based polymer materials, such a PBT and PEN.

Due to hydrolysis, 85/85-test will give too negative results of the reliability of your products, unless they face the 85/85 conditions in their use environment. But how to avoid this? By lowering the test temperature. Hydrolysis only occurs in humid environment exceeding Tg temperature of the polymer. As Tg of PET is typically around 70-80°C, hydrolysis can be avoided by testing the effect of humidity with a lower temperature humidity test, combining for example a temperature of 65°C and a relative humidity of 90%RH.