realistic working temperature of FR4

[Simon]

From memory FR4 has a 130 or 140C temperature rating attached to it. What does this translate into in reality? would you use it for power electronics in a 100C ambient or should i use a ore exotic type of PCB ?

[T3sl4co1l]

Tg (glass transition temperature) is where it becomes floppy, and also tends to age rapidly (turn brown -- not necessarily becoming weak or brittle in the process, but getting there).  There's also a lot of expansion associated with that, which would be undesirable for temp cycling.

100C ambient isn't a problem, but how much temp rise is your circuit designed for, on top of that ambient?

You can also try to keep the PCB cool by blowing air over it, clamping it between thermal pads, potting, and so on.

Tim

[Simon]

thank you Tim, it will be potted, this is my 25A motor controller so I'm hoping to make my power tracks as wide as possible to lower any up heat and possibly glue heat-sinks to my mosfets, My other consideration is how thick can i make the copper before i have problems with track widths. Most of my small parts are on a processor board but some will be with the power electronics.

[T3sl4co1l]

Ask fab.  Typically minimum width/space is 2-4 times the thickness, so you can do 7/7 rules at 2-3oz, and more relaxed rules (avoid MSOP, TSSOP parts??) for more.

You may also consider a multilayer board, which gets more copper without needing individual layers being very thick.  4 layers 2oz may even be a standard proto, whereas 2 layers 4oz is usually custom.

Tim

[Simon]

Yes I'm hoping to go with 4 layer 2 oz on the outside, this would get me 4 oz and even 6 oz where required.