Reliability Engineering

1. Reliability Engineering Richard C. Fries, PE, CRE Corporate Manager, Reliability Engineering Baxter Healthcare Round Lake, Illinois

2. Definition of Reliability The probability, at a desired confidence level, that a device will perform a specified function, without failure, under stated conditions, for a specified period of time

3. Customer’s Definition of Reliability A reliable product: One that does what the customer wants, when the customer wants to do it

4. Reliability Basics Reliability cannot be tested into a product It must be designed and manufactured into it Testing only indicates how much reliability is in the product

5. Purpose of the Reliability Group Determine the weaknesses in a design AND correct them before the device goes to the field

6. Areas Covered by Reliability • Electrical • Mechanical • Software • System

7. Electrical Reliability

8. Mechanical Reliability

9. Theoretical Software Reliability

10. Practical Software Reliability

11. System Reliability

12. Set the Reliability Goal • Based on similar equipment • Used as the basis for a reliability budget • Listed as Mean Time Between Failures (MTBF) in hours or cycles • MTBF = the time at which 63% of the units in the field will have failed • Minimum goal is ten years with a 98% reliability

13. Parts Count Prediction • Uses MIL-HDBK-217 • Indicates whether the design approximates the reliability goal • Indicates those areas of the design with high failure rates

14. Chemical Compatibility • Test plastics with typically used chemical agents (alcohol, anesthetic agents, cleaning agents) • Cleaning agents are the worst

15. Force Puller

16. Component Testing • Cycle/life testing of individual components • Comparison of multiple vendors of components • Determine applicability for the intended use

17. Philosophy of Testing • Test to have the units pass • Test with the addition of stresses to check the margins of functionality

18. Types of Tests • Time terminated, failed parts replaced • Time terminated, no replacement • Failure terminated, failed parts replaced • Failure terminated, no replacement • Test until first failure • Test until all samples fail

19. Determining Sample Size • Uses Chi-Square table • SS = Chi-square Value(MTBF goal)/2 • Chi-square value includes confidence level and degrees of freedom = 2f+2 • Component testing – 90% confidence level • Life testing – 95% confidence level

20. Sample Calculation • Want to test valves to be used for 2,000,000 cycles per year with a 10% failure rate after 10 years • Reliability = e(-t/MTBF) • MTBF = -t/ln Reliability = -20,000,000/ln 0.90 = 389,914,514 cycles

21. Sample Calculation • MTBF = 389,914,514 cycles Number of Samples Number of Cyles 10 89,777,817 50 17,955,563 100 8,977,782

22. Component Test Setup

23. Component Test Setup

24. Component Test Setup

25. Calculating Sample MTBF MTBF = (# of samples)(length of test) # of failures

26. Calculating MTBF Where No Failures Occur • A sample MTBF cannot be calculated • A lower one-sided confidence limit is calculated and the MTBF stated to be greater than that number One-sided limit = 2(#units)(test time) Chi square value for the confidence limit and 2 degrees of freedom

27. Sample Calculation for a No Failure Test • 10 valves are tested for 10,000 cycles with no failures. Calculate using a 90% confidence level. One-sided limit = 2(10)(10,000) 4.605 = 43,431 cycles MTBF > 43,431 cycles

28. HALT • Acronym for Highly Accelerated Life Testing • Used to find the weak links in the design and fabrication process • Usually performed during the design phase

29. HALT Testing • Possible stresses that can be applied: • random vibration • rapid temperature transitions • voltage margining • frequency margining • The product is stressed far beyond its specifications • The test can be set up to find the destruct limits

30. HALT Chamber

31. Goal of HALT Testing • Overstress the product • Quickly induce failures • By applying the stresses in a controlled, stepped fashion, while continuing monitoring for failures, the testing results in the exposure of the weakest points in the design • This test, if successful, will expose weak points in the design

32. Environmental Testing • Operating temperature/humidity • Storage temperature/humidity • EMC • Surges/transients • Brown-outs • Electrocautery • Cell phones • ESD • Altitude

33. Environmental Testing • Autoclave • Shock • Vibration • Shipping • Tip testing • Threshold testing

34. Temperature Chamber

35. Walk-In Temperature Chamber

36. Autoclave Testing

37. Customer Misuse • Excess weight on tabletop • Fluid spillage • Cross connection of wires • Pulling unit by non-pulling parts • Wrong order of pressing keys • “Knowing” how to operate the unit without reading the manual

38. Making a Design Foolproof The biggest mistake engineers make when trying to make a design completely foolproof is underestimating the ingenuity of complete fools

39. Failure Analysis • Failure: device does not operate according to its specification • Determine root cause of the failure • Suggest methods to address the failure

40. Prototype Front Panel

41. Plastic Structure

42. Plastic Structure

43. Autoclave Testing

44. Manifold Port

45. Prototype Port

46. Life Testing • Operate the device in its typical environment and application • Use appropriate on/off cycles • Can be used to verify the reliability goal or a specific period of time, such as the warranty period

47. Tracking Reliability Growth in the Field • Collect manufacturing data on how many units were manufactured by month • Collect field failure data, by month • Develop a reliability growth chart

48. Reliability Growth Example

49. Reliability Growth Example

50. Reliability Growth Example