Forum: Reliability & Maintainability Questions and Answers
Posted by: Glen Hanington
Date posted: Thu Apr 25 20:18:06 US/Eastern 2002
Subject: Heatsink mounted TO-247's, no strain relief.
My company is using a technique for mounting TO-247 power FET's which I believe to be unreliable. With the heatsink screwed to the top of the PCB the FET's are held to the aluminum heatsink with a metal clip that presses on the center of the device, providing good thermal conductivity to the heatsink through a Silpad. However, the leads from the FET's go straight down into the PCB and are soldered in place without any strain relief. Depending on the model, this distance can vary from 0.25" to 0.50". Do you know of any studies modeling this arrangement vs. others with strain relief, and whether it can promote internal fracturing of the die? Our units typically function in a laboratory or industrial type environment, but the heatsink temperature can get up to 85degC. I believe that the aluminum has a slightly higher coefficient of thermal expansion than the copper leads of the devices, but I don't know what forces would be generated and if it would be enough to crack anything. Since I just started working here I've not had the opportunity to review field failure data.
Posted by: Raghavendra madgulkar (firstname.lastname@example.org, email@example.com )
Date posted: Wed Apr 24 4:51:26 US/Eastern 2002
Subject: Reliability prediction
I am looking for a method to predict reliability if I know the failure rate for last quarters. Suppose the failure rate for last eight quarters is 7,8,13,25,23,25,27,24 and the products I shipped during this months is 1975,1675,1678,1196,1851,1522,1668,1223. How can I predict the failure rrate for next four quarters.I am not using any software for this.. Can anybody help me on this,,,,,,,,,,,,,,
Posted by: Steven Lu (firstname.lastname@example.org )
Date posted: Tue Apr 23 10:21:25 US/Eastern 2002
Subject: Service life for brushes in brush motors
How to determine the service life for brushes in brush motors? What kind of data do I need to calculate the service life for brushes if there's a way to calculate it?
Posted by: IC
Date posted: Thu Apr 18 9:43:06 US/Eastern 2002
Subject: Is 217 Obsolete
I have came across several articles stating that 217F is longer being used to the same extent for theoretical analysis purposes. If this the case what will replace it?
How are engineers now calculating reliability figures? If there is no in-service/field data available how do we now calculate a realistic failure rate? I would be grateful for any pointers in this matter. Thanks in Advance.
Posted by: tom c
Date posted: Tue Apr 16 12:28:16 US/Eastern 2002
Subject: Test Failure: Continue Test or Restart Test?
I am interested in setting up a fixed duration time terminated reliability test in which I will allow zero failures in the test time interval (test time is based on 90% conf limit). The test article happens to be a complex electronic system and I cannot afford to test more than one unit. If I do get a failure during my test interval, is it preferred to: a) replace the failed LRU and continue testing or b) repair the failed LRU and repeat testing in its entirety or c) do something else (must be specific here).