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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Posted by: Matt Wiltshire ( )
Date posted: Thu May 27 11:51:59 US/Eastern 1999
Subject: MIL-STD-810E Salt Fog Test
How many hours would the MIL-STD-810E Salt Fog Test (48 hours, 5% salt solution, 95 degrees F) be equivalent to in a worst case land environment such as the coast of Florida in the summer and a bast case in Nebraska (if there is a better one let me know)? If numbers can be directly applied to the 48 hour test, can it be applied linearly to an extended test (much > 48 hours)? Can you recommend any studies that address this? Thanks, Matt Wiltshire


Subject: Test Acceleration
Reply Posted by: Dave White (white@spectralsolutions )
Organization: Spectral Solutions
Date Posted: Tue Jun 1 14:29:49 US/Eastern 1999
That is the $64,000 question! The trouble is, there just isn't enough information to answer it. Things like product geometry, activation energy and material selection play too big a part, so without these any estimate of test acceleration comes out as little better than a guess. Your best bet is to compare test results to actual field results for your product (if these are available). If it is a new product, and field data doesn't exist yet, you may get some insight from "similar" products that do have both field and lab data available. In this case "similar" doesn't have to be functionally similar, but similar materials and geometries. I once had a manager who just wouldn't accept this. In the end he told his boss that one day of testing equalled one year of field use (he didn't say where in the field, as you point out - all fields are not the same!). I don't know where he pulled this number from, and I would not like to try and defend it. If you do find any referance material that addresses this I'd like to know about it too. Good luck, and please post any info you find. Thanks.


Subject: mwiltshire@spectraprecisionday.c
Reply Posted by: Joel Mandel ( )
Organization: teledata
Date Posted: Sun Jun 6 4:34:53 US/Eastern 1999
Point information for telecomminication outdoor cabinets what you find outside your house the requirements for Bellcore equipmnet is 14 days!!!! .See GR-487. I dont't know where the received their information but its interesting.


Subject: Salt Fog Acceleration
Reply Posted by: Bruce Dudley ( )
Organization: Reliability Analysis Center
Date Posted: Mon Jun 7 15:35:37 US/Eastern 1999
Determining the equivalent use time based on an accelerated test condition is not an easy operation. If you can determine the specific accelerated test paramenter(s) that are under test, one can approximate the equivalent use times using single or multiple variable relationships. Assuming that the item you are considering for the test is an electronic or electromechanical component, this salt fog test could represent a temperature and humidity acceleration test. Temperature/humidity relationship tests are very useful in the electronic accelerated test arena. Mr. D.S. Peck in several reports developed a relationship which is the combination of the humidity acceleration and the arrhenius relationship for temperature acceleration. This model is called the "Peck Model". Since the temperature of this test is only 95 degrees F, it's acceleration factor is close to zero as the normal operating conditions at both Florida and Nebraska have temperatures this high or higher. Therefore, this part of the Peck Model is not considered relevant. For the humidity part, the use humidity is expected to be far less on average than the 85% or greater test condition. The accelerating equation for the humidity is: (RHuse / RHaccel)^n. Where RH is the relative humidity for use and test and n is the accelerating factor usually established by experimental testing. The book by Dr. Wayne Nelson, "Accelerated Testing", John Wiley and Sons, 1990, indicates that a generalized "n" factor for electronic components using 85 degrees C and 85% RH to be 2.7, which was determined by experimental testing. Thus, for the Florida location if the relative use humidity is 75% and the Nebraska location is 50%, the humidity related acceleration factors for each site would be about 1.4 for Florida and 4.2 for Nebraska. This translates to approximately 70 hours and 200 hours of equivalent use time, respectively for the 48 hour stress test. This is an estimation that really needs to be tested in order to determine the actual acceleration factor "n".

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