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Q: We have tried to switch several jobs from polyimide to high temperature epoxies without too much success. Why won't the high Tg epoxy products perform satisfactorily in high layer count multilayer boards?

A: Polyimides have been used for years in high temperature applications such as bum-in boards and downhole drilling. For many MLBs with higher layer counts and for applications where the process itself may be severe, such as in sequential lamination jobs or where there are lead-free soldering requirements you may want to consider going back to polyimide to "beat the heat."

Polyimides have been in service for many years in tough service and where frequent rework may be necessary. Much of the US domestic military electronics produced over the last 20 years has been made on polyimide, both to protect against latent PTH defects caused by processing and where the applications themselves reqmre periodic rework such as in the upgrading of chip sets.

Polyimides have glass transition temperatures (Tg) of 250[degrees]C+ and thermal decomposition temperatures (Td) in the range of 400[degrees]C. T300 values range from 10 minutes for V0 flame retardant versions to >60 minutes for pure polyimides. With total Z-direction expansion from room temperature to 2500(2 typically around 1.5% it is easy to see why plated through hole reliability of polyimide designs has always been without parallel.

Arlon offers three standard grades of polyimide that vary primarily in flame retardance. 35N (UL-94 V1) is our most popular grade, which also offers a 90 minute accelerated cure at epoxy cure temperatures. 85N (UL 94 HB) with no brominated flame retardant will provide maximum long-term thermal stability. In critical applications requiting better flame retardance, 33N (UL 94 V0) is also available.

For additional information on Arlon's polyimide offerings, contact your Arlon sales or technical service representative or call Arlon directly at [phone].

Dave Nelson,

Technical Sales &

Service Engineer