John, I'm not fluent in SM-785, but can suggest tracking down http://www.amazon.com/Applied-Analysis-Wiley-Probability-Statistics/dp/0471094587
Wayne Nelson's book on Applied Life Data Analysis. The Amazon picture makes it look intimidating, but it is very readable and well written. It may provide some background that will help you answer your question. I do believe 800 as an estimator is overly conservative.
The more failures you find, the narrower will be the confidence interval around the estimated cycles to failure.
Louis Hart
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nieznanski, John A - SSD
Sent: Thursday, December 18, 2008 12:44 PM
To: [log in to unmask]
Subject: [TN] to apply partition correction on individual daisy chains or not?
Hi TechNet List Gurus,
If I have several different serial daisy chains undergoing accelerated reliability testing per IPC-SM-785, can I apply partition correction (IPC-SM-785 Eq. 16) to calculate an "effective" Nf @test (X%) for each daisy chain? Each chain only has a single component type, but there are different numbers of components in each chain.
For example, if the first chain has 10 two-terminal leadless SMT components (assume BETA=4), and the first IPC-SM-785 failure occurs at 800 cycles, then per Eq. 16:
Nf (component) = Nf (m partitions) * [1/m] ^ (1/BETA)
= 800 * (10) ^ 1/4
= 1422
So with 10 components in the first partition or daisy chain, then Nf @test (component) = Nf @test (10%) = 1422.
A more conservative analysis neglecting the partition correction would be to say Nf (10%) = 800.
Which way makes the most sense?
From Nf @test (10%) and BETA, I can calculate Nf @test (50%), then apply the acceleration factors to get Nf @use (50%), then extrapolate to get Nf @use (1%) and Nf @use (0.1%).
Best regards,
John Nieznanski
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