I am looking at the thermal resistance numbers in the table below I do not understand how to apply them .
I ran a test where I just let the processor run with a cpu usage number of 25% (number from Top app) for a while in my office environment of 22
Looking at Table 7 it list various thermal items.
Junction to Ambient = 23 C/W, Junction to Case = 6 C/W Junction to Package top = 2 C/W...
So in my test of the internal temp read out of the CPU = 60 and top of BGA = 44 (using a thermal laser device)
1. Is the top of the bga package consider the Case?
2. Since I see a delta for about 15 C between internal and top does that means the processor is doing over 2 watts????
this is based on the Junction to Case = 6 C/W so 15 / 6 = 2.5
This seems unlikely since I am only loading the cpu at 25 % with a simple app that is reading the serial port and scrolling the received HEX data on the screen. So I am guessing, I am misunderstanding how to use these numbers.
What seem more likely is the Junction to Ambient for the 4 layer of 23 C/W is more likely is what is happening so this would be less than a watt 15 / 23 = 0.65 . So if that is true then how do I figure out the numbers for my heat sink.
So if the goal is to keep the processor under 85C for say a 2 watt load then what temperature does the top of BGA need to be? If the 23C/Watt rule is what really applies then I have to keep the top 85 - (23 x2) = 39C which means there is no way to use the processor in a 50C environment.
FYI - another test I did was to put a heat sink on the BGA and ran it in my office again. The heat sink did not make a noticeable difference.