Cheryl Tulkoff - Design for Excellence in Electronics Manufacturing

There are no universal best practices. Every company must choose the appropriate set of practices and implement a program that optimizes return on investment in reliability activities. Reliability is all about cost‐benefit trade‐offs. Since reliability activities are not a direct revenue generator, they are strongly driven by cost. By increasing efficiency in reliability activities, companies can achieve a lower risk at the same cost; and addressing reliability during the design phase is the most efficient way to increase the cost‐benefit ratio. Industry rules of thumb indicate the following returns on investment (Ireson and Coombs 1989):

Issue caught during design: 1 cost

Issue caught during engineering: 10 cost

Issues caught during production: 100 cost

Additional Economic Drivers

Use environment and design life

Manufacturing volume

Product complexity

Margin and profit requirements

Schedule and delivery needs

Field performance expectations and warranty budget

Establish a reliability goal and use it to determine reliability budgeting.

Quantify the use environment. Use industry standards and guidelines when aspects of the use environment are common. Use actual measures when aspects of the use environment are unique, or there is a strong relationship with the end customer. Don't mistake test specifications for the actual use environment. Clearly define the median and realistic worst‐case conditions through close cooperation between marketing, sales, and the reliability team.

Perform assessments appropriate for the product and end‐user. These assessments require an understanding of material‐degradation behavior, either by test to failure or by using supplier‐provided data. The recommended assessments include:– Thermal stress– Margin or safety‐factor demonstration (stress analysis that includes step stress tests (e.g. HALT) to define design margins)– Electrical stress (circuit, component derating, electromagnetic interference [EMI])– Mechanical stress (finite element analysis)– Applicable product characterization tests (not necessarily verification and validation tests)– Life‐prediction validation (accelerated life test [ALT])– Mechanical loading (vibration, mechanical shock)– Contaminant testing

Perform design review based on failure mode (DRBFM, Toyota methodology). This readily identifies CTQ (critical to quality) parameters and tolerances and allows for the development of comprehensive control plans.

Perform Design for Manufacturability (DfM) and Design for Reliability (DfR) and involve the actual manufacturers in the DfM process.

Perform root cause analysis (RCA) on test failures and field returns to initiate a full feedback loop.

Best‐in‐class companies have a strong understanding of critical components. Component engineering typically starts the process through the qualification of suppliers and their parts. They only allow bill of materials (BOM) development using an approved vendor list (AVL). Most small to mid‐size (and even large) companies do not have the resources to assess every part and part supplier. Those who are best in class focus resources on those components critical to the design. Component engineering, often in partnership with design engineers, also perform tasks to ensure the success of critical components. This includes design of experiments, test to failure, modeling, supplier assessments, etc. Typical critical component drivers are:

Complexity of the component

Number of components within the circuit

Past experiences with component

Sensitivity of the circuit to component performance

Potential wearout during the desired lifetime

Industry‐wide experiences

Custom design

Single supplier source

From the component perspective, reliability assurance requires the identification of reliability‐critical components and drives the need to develop internal knowledge on margins and wearout behaviors. RCA requires the true identification of drivers for field issues combined with an aggressive feedback loop to reliability and engineering teams and suppliers.

Best in class companies provide strong financial motivation for suppliers to perform well by creating agreements with the supply chain to accept financial incentives and penalties based on field reliability. These practices allow companies to implement aggressive development cycles, proactively respond to change, and optimize field performance.

Establishing a successful, comprehensive reliability program requires planning and commitment. Requisite priorities include:

1 Focus: Reliability must be the goal of the entire organization and must be implemented early in the product development cycle. Separate reliability from regulatory‐required verification and validation activities and mindset.

2 Dedicated staffing: Assignment of responsibilities without assigning resources risks failure.

3 Clearly define reliability goals and the use environment: these drive the rest of the reliability program.

4 Identify critical components, especially those at risk of wearout. Initiate test‐to‐failure and design‐ruggedization activities.

5 Implement step‐stress testing at both sub‐assembly and assembly levels.

6 Perform RCA. Focus on the top three field issues, and repeat; drive to quality assurance as appropriate.

Elements of a comprehensive reliability program include:

Organization: Develop a system that rewards teams for effective reliability engineering focus. Reliability must be the goal of the entire organization and must be implemented early in the process apart from verification and validation (V and V).

Reliability goals: Consider an availability goal as well.

Reliability resources: Resources need to be assigned to reliability characterization and committed in program staffing. Characterization infrastructure needs building.

Software reliability: Frequently overlooked but critical to today's products and systems

Defined use environments: Engage field service to obtain knowledge of relevant stresses (loads, contaminants, electrostatic discharge [ESD], etc.) in use environment, and then exercise these during reliability characterization.

Thermal analyses.

Circuit and component stress analyses.

Derating: Employ systematic derating by computational and/or experimental analysis.

Critical components identification: Identify and prioritize components and subassemblies for extended reliability analysis and test activity during early development. Use supplier life data where possible; create the expectation of minimum supplier reliability competency.

Failure mode and effects analysis (FMEA).

Critical to quality (CTQs) and tolerance identification.

Comprehensive control plans with suppliers.

Design for excellence practices.– Design for Manufacturability (DfM), Design for Testing (DfT), Design for Reliability (DfR), Design for Excellence (DfE), and Design for Sustainability (DfS).– Manufacturing involvement in DfM and DfT.

Step‐stress tests to define design margins (HALT).

Simulation for end‐of‐life prediction

Relevant product qualification tests

Accelerated life test (ALT) to validate the life‐prediction model

RCA on failures and field returns with a feedback loop (design, measure, analyze, improve, control [DMAIC])

These elements will be explored in more detail in the upcoming sections and chapters.

Desired lifetime and product performance metrics must be identified and documented. The desired lifetime may be defined as the warranty period or by the expectations of the customer. Some companies set reliability goals based on survivability, which is often bounded by confidence levels such as 95% reliability with 90% confidence over 15 years. The advantages of using survivability are that it helps set bounds on test time and sample size, and it does not assume a failure rate behavior (decreasing, increasing, steady‐state).