Why have TO3 package have higher Tjmax than plastic?

[Miyuki]

Hi folks,

I just wonder why power transistors in TO3 package have commonly Tjmax 200°C while same device in plastic package have just 175°C or even lower

Is it because some thermal expansion issues with epoxy vs metal as TO3 have free space around chip and bond wires to expand and contract ?

[dzseki]

It is because the glass transition of the epoxy mold of the plastic transistors, it won't melt down, but pressure may build up in the case which may affect the die's electrical properties adversely.