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I'm pretty used to just blindly specifying high-Tg 170C or greater on my complex multi-layer boards. I do understand what the rating means and have a general idea that higher Tg means they'll survive in the oven at higher temperatures (like for RoHS process).

Anyway I was quoting a tiny 1" x 0.5" 4 layer PCB and the cost difference between Tg 150 and Tg 170 was about 10x for my run. So I'm inclined to go with Tg 150 :) But I'm not sure of myself.

What should I watch out for here? Or maybe better what questions should I be asking myself about my design to ensure I can pick 150Tg with confidence? Max ambient is 55C and the board doesn't generate much heat, maybe an additional 10C rise. I don't have to use RoHS compliant process I just do because it's nicer to the environment.

I did do some reading and googling but didn't really find what I was looking for.

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Tg, Glass transition temperature, is the temperature at which the laminate material losses its properties changing from a glassy state to a rubbery state.

It is known as well as Heat distortion temperature.

As general rule the higher the Tg, the more stable the material is during the PCB manufacturing process and assembly. As well, the higher the Tg, the more expensive the material is.

For FR4 materials this parameter is between 130°C to 190°C. Other types of materials can go up to 260°.

The main aspect to consider here is the PCB complexity. High Tg materials are required for complex multilayer PCBs.

For a small 4 layers PCB, FR4 material with 150°C Tg should work.

If you are not sure about what material you should specify, ask you PCB manufacturer. They know their process and they know what they are capable of.

Anyway, it sounds like a big increase x10 more expensive 170Tg than 150Tg. I suppose that's their preference and they are not really keen on working with 170Tg material.

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