DFR : Design for Reliability

Introduction

Design For Reliability (DFR) is a method belonging to a broader set called Design For Excellence (DFX). DFR focuses on recommendations related to the product design phase. The goal is to ensure the required level of product reliability.

What is reliability?

"Reliability is defined as the probability that a product, system, or service will perform its intended function adequately for a specified period of time, or will operate in a defined environment without failure"[1]

Key Aspects of Reliability

• Consistency. The product performs its functions consistently across multiple uses and under varying conditions.
• Dependability. The product will function when required.
• Durability. The product withstands wear, stress, and environmental factors.
• Maintainability. The product can be easily maintained and repaired to continue functioning as expected.
• Low Failure Rate. The likelihood of the product failing is minimized and falls within acceptable limits.

Steps of Design For Reliability

Design For Reliability (DfR) involves several steps and methods to achieve its goals. DFR starts early in the product design process, preferably during the initial concept phase. Consider the following steps:

Set Reliability Goals

Define clear and measurable reliability goals based on customer needs and industry standards. Define the expected lifetime bound by confidence intervals.

Concept phase

Define the top-level system architecture with reliability goals in mind. Use block diagrams to represent the main subsystems of the product. Define primary functions and interactions. Use S-FMEA to identify risk. Define mitigation strategies for high-priority failure modes (e.g., adding a redundancy block). Cast down the specific requirements to the subsystems.

Design phase

Design the subsystems according to previously defined requirements. Use tools and methods like:

• Design rules for high-reliability products.
• Physics of Failure (PoF).
• Monte-Carlo analysis.
• Derating.
• Redundancy, and fail-safe mechanisms.
• D-FMEA analysis.
• FMEDA (for some applications only).
• Select components with proven reliability records.

Prototyping and Testing

Subject the prototypes to various tests, such as functional, test-to-failure (ALT, HALT), and deterioration tests. Gather failure data. Analyse failed prototypes to understand the causes. Improve the design.

Reliability Analysis

Use the test results and field history to predict a product's reliability over time. Apply adequate statistical methods, such as Weibull Analysis.

Design Verification and Validation

Verify and then validate that the current design meets all specified requirements.

Production Process Control

Transfer reliability requirements to the manufacturing process. For example, provide a list of special characteristics and required control methods. The manufacturer shall implement the required controls. For special characteristics use Statistical Process Control (SPC) or 100% test. Document the control methods in the relevant P-FMEA and Control Plan.

Continuous Improvement

Monitor production and field failures. Analyse returned units to understand the failure mechanisms and underlying causes. Review and update reliability analysis to fine-tune the prediction model. Continuously improve reliability through lessons learned and feedback.

Summary

Design For Reliability (DFR) helps to ensure that a product performs its intended functions over time, under specified conditions, and with minimal failure rates. DFR is crucial for ensuring customer satisfaction, product success, and the company's overall reputation.