All, we are working on a thermal analysis and have the following issue.

We are trying to calculate theta-cb (case to board) for arbitrary components. Most datasheets give theta-jc or theta-ja. Theta-jc, unless a part has a thermal pad, is to the top of the part as I understand it. We are trying to figure out how to calculate theta-cb based on the package. The thermal analysis folks would like the thermal impedance to the board for each part, but I am uncertain if there is a way to calculate these values.

 

Question number one, are you sure you need theta-cb? It is usually minuscule compared with theta-jc or theta-ja, principally because solder is designed to be a good thermal conductor for obvious reasons. It also doesn't have a huge amount of relevance to any particular package. See attached for a reference, taken from this white paper - https://www.idt.com/document/apn/thermal-device-analysis-and-design-considerations