I need help in setting up life profile of a circuit borad (having hundreds of components of different types viz. passive, active, analog, digital, mixed signal etc.) in a housing (also called sub-assembly) for following condition;
1. Ambient temp. of housing is 55 degC controllable with in +/- 2 degC (delta-T as 4 degC) range duirng operating as well as non-operating phases.
2. While in operation, Case Temp. (or solder joint temp.) of the components used on the circuit board inside this housing varies between 60 degC to 90 degC at 55 degC ambient (housing) temp.
3. While not in operation, case temp. of the components is same as the ambient temp. i.e. 55 degC +/- 2 degC.
Possible solutions:
Sol-1: Consider average temp. rise of the components used on the circuit board i.e around 72 degC which is 17 degC above ambient temp. and add ambient temp. variations to this and thus decide the delta-T i.e. 19 degC in this case.
Prob: in this case, high dissipating component having higher case temp. (and higher delta-T in turn) will have lower Failure Rate as compare to actual opertaion condition and components with lower case temp. will have higher FR. The results will not be the true representative of the actual condition.
Sol-2: Define life profile for each individual component and assign the delta-T according to the actual case temp. observed by the component.
Prob: This will require extensive efforts in defining the life profile and thus arriving at the FR of the entire sub-assembly. for a system with multiple sub-assemblies, this will become practically prohibitive.
Hope I was able to express my doubt effectively.
Thanks in advance.