SRC Forum - Message Replies
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
Posted by: Glen Hanington
Date posted: Mon Mar 5 15:21:34 US/Eastern 2007
Subject: Reliability Testing
We have a problem. We've built up very large number of printed wiring assemblies before we discovered that the board itself has a problem. There are 2 pad areas on the board that form connection points to copper busbars. Each pad is 1/2" x 1/2" in area, solder coated, with a 0.150" hole near the middle. A 6-32 square-cone SEMS screw passes through this hole from the back side of the board and screws into each one of the 2 busbars. The hole is plated through and the back side of the board has an identical pad as on the front of the board. The problem is this: solder mask covers both pads except for a 0.250" diameter circle around the hole on both sides of the board, preventing full surface contact with the busbars. Some of our product models have 83 Amps of current pass from the board through each busbar. We've experimented with removing the solder mask, but that isn't possible due to the other components already on the board. I've been tasked with developi! ng a reliability test to prove that if we were to use these boards we would not have a reliability issue in the field. How would you recommend that I go about performing this test? As I see it, this will involve the combined stresses of current, humidity, and vibration. Is there some way to set up this test, or series of tests, to prove that this will not be an issue within our 5-year warranty period? It is also believed that about 1500 of these boards are in the field, but there have been no reported failures as a result of this issue.
Subject: Test Planning
Reply Posted by: BWD
Date Posted: Mon Mar 5 16:51:37 US/Eastern 2007
All testing requires test planning. One of the steps in the planning is the cost budget. We always set aside a percentage of the test dollars for corrective actions and verification. An estimate for this set aside is usually 10 to 15 % of the test cost. The ideal test scenario starts at the part level and increases to the assembly and unit level,and finishes with the completed system. If the test dollars or the schedule are limited, then one needs to perform and accelerated stress test at the completed system level to maximize the confidence in the test results.
The first step in constructing accelerated tests is to define the failure mechanisms in terms of the materials used in the product to be tested. The next step is to determine the environmental parameters the product will be exposed when operating and when not operating or stored. Based on the failure mechanisms most likely to limit the life of the product, one can choose a test or tests that will accelerate that failure mechanism. Some of the models that can be considered are: Miner's Rule for accumulated fatigue, Arrhenius Temperature test for accelerated temperature and chemical aging effects, Peck's Model for temperature and humidity combined effects, Coffin-Manson non-linear mechanical fatigue damage model and Eyring/Black/Kenney models for temperature and voltage acceleration.