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Hello,
I was trying to calculate the FIT rate for this component using the ISO26262 standard. I have formulated a calculation where, given a FIT from the supplier using the ISO26262 standard and an standard mission profile (Motor or passenger), I derate the FIT to the mission to the actual mission profile used by us. In this situation, I have been given the following information from TI.
We know the ISO standard formula is:
What I do is, I calculate Lambda_Package and Lambda_Die using this standard formula and motor mission Profile. We know Lambda_Overstress is 0 as specified by TI. Because I have fit as 12 from TI, I subtract Lambda_Package and then divide by the term derived from mission profile in Lambda_Die (second term in Lambda_Die) . This way, I calculate the first term in Lambda_Die, which is all we need to derate as we know everything else from the datasheet/TI safety manual.
The issue I am facing that using this methodology, I calculate a negative first term in Lambda_Die and therefore a negative derated fit. I have rechecked all calculation and the formulas seem to be fine. I choose the following specification -:
"IC Type": "MOS/BiCMOS",
"Package Type": "Two rows (SO; SOP; SOJ; VSOP;)",
"Package Pitch (mm)": 0.65,
"Package Width (mm)": 4.3,
"Package Length (mm)": 4.35,
"Pin Number": 16,
"Given Fit Rate": 12,
"Used Mission Profile": "Motor",
"MP1 - Scenario_2": 0.122183177,
"MP2 - Scenario_2": 5716.846893
"Term_1": -108.8390972,
"MP1": 0.372508359,
"MP2": 4739.707811,
"Alpha_s": 16,
"Alpha_c": 21.5,
"Lambda_3": 1.528878401,
"Power (W)": 1.00E-01,
"Thermal Resistance": 139.6,
"FIT_RATE": -19.86820683
The labels are mostly self-explanatory. You can ignore blue columns. Given FIT rate is the total fit from TI, and so is used mission profile. MP1_Scenario 2 and MP2_Scenario 2 are the mission profile terms in lambda_die and lambda_package respectively calculated using motor mission profile. Then I get term_1 using the motor controller calculations. MP1 and MP2 are the mission profile terms in lambda_die and lambda_package respectively calculated using our board-specific mission profile. Alpha_c and Alpha_s are used to calculate pi_s in the formula using the ISO26262 standard. I am given FR4 as substrate therfore choose 21.5 for alpha_c. I assume 16 for alpha_s as I assume package is plastic in nature. Power and Thermal resistance (in C/W) come from TI datasheet/safety manual. You then see I get a negative derated FIT. When I however change the IC type to be AsGa MMIC, I get a positive fit (still a negative first term) which seems wrong as this is BiCMOS component. Once, I get a term_1, i just forward calculate the derated FIT using board_specific mission profile but all the same values.
I know given a Lambda_Die, I can just do Lambda_Die/(MP1_Scenario 2) to get the first term but I don't always have Lambda_Die provided by the supplier.
Would you know what I am doing wrong in this case? I can understand if this calculation is hard to understand. I would love to hop on a call and have a more detailed discussion. Thanks.
Regards,
Samar
