Forum: Reliability & Maintainability Questions and Answers

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Posted by: S.Arun Kumar (arun_sundararajan@vsnl.net )
Date posted: Wed Feb 9 8:02:39 US/Eastern 2005
Subject: PRISM Model for PCBs
Message:
We are in the process of predicting the FR for an Electronic Assembly consisting of Through Hole Components and SMT Components.
We are using a technique of Mixed model Prediction, in the sense that we use PRISM Models for the Passives and some standard ICs. For Connectors, Crystals & Coils we are using MIL 217FN2. When we want to compute the FR for PCB, we were not able to find a model for it in PRISM. We are Currently using MIL 217FN2 for PCBs. Now the Questions we have are Q1. How do I compute the Failure Rate for the PCBs using PRISM. Q2. In MIL 217FN2 we have Model for PTH and SMT Version of the PCB. My Query is that since we take the solder joint factor, Cycling Rate Factor for most of the components we are using in PRISM, do we need to separately calculate the FR of the PCB (SMT Technology).

Replies:
PRISM Models for PCBs
ddd Thu Feb 17 15:03:05 US/Eastern 2005

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Posted by: Dickie Young (dyoung@herouxdevtek.com )
Organization:Héroux Devtek
Date posted: Mon Jan 31 16:22:25 US/Eastern 2005
Subject: Failure rate
Message:
I am looking for the failure rate for a piston rod scraper use for landing gear application.

Replies:
NPRD Failure Rate
bwd Tue Feb 8 13:56:52 US/Eastern 2005

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Posted by: Grib (kgrib@zodiac.com )
Organization:Aérazur
Date posted: Fri Jan 21 5:06:53 US/Eastern 2005
Subject: Activation Energy
Message:
I am looking for the energy activation for the thermostat component (switchs beteewin 0 and 10°C) in order to perform an accelerated test according to Arrhenius model. My interrogation concerns the existing data on this value of energy of activation or is it necessary to determine it by preliminary tests? Thank you in advance for your support.

Replies:
Activation Energy Thermostat
bwd Thu Feb 3 8:22:47 US/Eastern 2005

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Posted by: Ken poi (kpoi@aapl.com.au )
Organization:Australian Arrow Pty Ltd
Date posted: Mon Jan 10 9:30:55 US/Eastern 2005
Subject: Reliability in the Automotives
Message:
Dear RAC,
I am setting up an ORT SOP for an Body electronics module of a truck. I am advised to use current test matrices used for Design Validation. I understand the failure modes this type of electronic device are usually solder wetting related. I am contemplating sampling 12 units (arbitrarily chosen) for a barrage of tests which are targetted at precipitating the failure modes mentioned above. My goal is 0.97 reliability (meaning p=0.03) with 50% Confidence Level. ( There is no table for p=0.03 in your toolbox, Can it be generated ? ) I propose that some of these 12 units will be run through a series of destructive tests and the others through another series of tests like Power Temp Cycling, Vibration testing, Altitude testing and Temp Storage tests. Test Name 1-3 +1 extra 4-6 7-9 +1 extra 10-12 HIGH TEMPERATURE - STORAGE * * LOW TEMPERATURE - STORAGE * * HIGH TEMPERATURE - OPERATION * * LOW TEMPERATURE - OPERATION * * RAPID CHANGE IN TEMPERATURE * * TEMPERATURE CYCLING * * DAMP HEAT STEADY STATE * * TEMPERATURE STEPS * * THERMAL CYCLING & VIBRATION COMBINATION * * COMPOSITE TEMPERATURE / HUMIDITY CYCLIC * * Objective is to demonstrate reliability of the product. Q: Can I validly use the binomial distribution assumption to define sample sizes and proportion defective in the sample for these samples assigned for their series of tests? Assume no failures will be accepted. Using the tables available from the toolbox on this website with assumptions of proportion defective P=0.10 and expecting no failures, the sample size of 12 ranks in between a confidence levels of 60%-80%. Not satisfactory enough. Q: Does this result suggest that to attain a CL of 90% and expect no failures from the sample, I have to increase the sample size from 12 to 22? Q: Are my assumptions this type of series of destructive testing (or trials) each has an equal probability of success thus a binomial distribution can be validly applied to its sample determination with a fixed number of failures. Regards Ken Poi

Replies:
Confidence Levels and Sample Sizes
bwd Thu Jan 13 9:53:09 US/Eastern 2005

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Posted by: Godder Shao (Godder.shao@sg.com.tw )
Organization:System General Corp.
Date posted: Wed Dec 22 22:47:07 US/Eastern 2004
Subject: How to calculate MTTF for Temperature/Humidity bias life
Message:
I have one product(Power IC) need to perform temperature/humidity bias lift test(THB). I follow JEDEC-22 A110 standard to perform it, and test condition as below,
85 degree C/85% R.H./VDD=20V 1000hrs 30pcs. after this test was finished, the test result as below, 22pcs failed at 768hrs, 6pcs failes at 928hrs, 2pcs failed at 1000hrs. could you teach me how to calculate the MTTF at 30 degree C/60% R.H./VDD=20V ? Thanks!

Replies: (list all replies)
Failure prior to reported failure?
Larry George Sun Dec 26 13:51:49 US/Eastern 2004
85/85 Accelerated Testing
BWD Wed Jan 5 9:15:52 US/Eastern 2005

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