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Troubleshooting Polyimide Browning Issues

Oxygen is the culprit for browning polyimides. It is also responsible for weakening polyimides, so they become brittle. During a normal polyimide cure cycle, a volatile piece of the compound is driven off and the remaining chain reacts and links with other chains. However, if oxygen is present, it reacts with the film and cuts short the polymerization reaction. This causes the chemical structure of the polyimide to deteriorate, any you wind up with small brittle chains instead of one long mighty chain.

Fortunately, the browning of polyimide makes it easy to visually tell if your process has been compromised. I recommend using a vacuum oven such as the YES-450PB Series to thoroughly remove oxygen through a series of nitrogen purges. Polyimide reacts with oxygen above 250 ºC, so most normal processing starts at 150 ºC with lots of air in the oven. First, the vacuum/nitrogen purges start removing oxygen, and then the temperature is ramped up. By the time the oven reaches 250 ºC, the oxygen level is well below 100 parts per million.

There are two other possible causes of polyimide darkening worth checking. Make sure that your oven is properly calibrated--a large temperature overshoot during cure can cause brittle polyimide. I also suggest changing your system’s door seals as part of your regular maintenance to ensure you don’t have any leaks; sometimes fissures in the seals can be undetectable by eye. Rock solid seals paired with a steady nitrogen flow will go a long way towards ensuring the clarity and durability of your polyimide protective coatings. This, in turn, will extend the life and performance of your devices.

If you still encounter issues, we invite you to contact YES.