Ensure Durable Boards With Reliability Assurance

Key Takeaways

• Motivations for design for reliability and some useful metrics therein.

• The interplay between yield and reliability.

• A non-exhaustive list of reliability tests available.

Reliability assurance, like visual inspection using an electron microscope, is a subset of quality assurance

After months of product development and manufacturing, the million-dollar question is no longer about how a product was designed. Instead, the question is whether it was built correctly. PCBs are a manufacturing process of refinements, and that doesn’t end once design for manufacturing sets in. The hope at that stage is that changes are gradual evolutions and improvements rather than full-scale revisions. To support this endeavor, all teams involved need feedback on the actual logistics of board fabrication and assembly to check processes in real-time, especially before entering into large production lots where an appreciable defect percentage will result in significant losses. Reliability assurance enacts checks and benchmarks at multiple steps throughout manufacturing to ensure high-quality boards that meet stated longevity goals, but the seeds of a successful design begin far in advance of any production.

Why Designers and Manufacturers Must Predict End Usage

Design for reliability is a concentration of PCB design that concerns choices and best practices to promote the long-term success of a board. One of the difficulties with electronic design in any era is accounting for the environment and conditions the board may undergo at any point during its service life; increased portability has only compounded this consideration. In general, electronics must be well-suited for a variety of temperatures, humidity, and other factors related to their surroundings. Fortunately from the component end, most devices are rated for common usage, and engineers and designers only need to take additional caution for procurement when operating settings require more specified attention. 

Quantifying Reliability 

Quantifying reliability can be done in a number of ways, but two of the most common methods are the mean time between failures (MTBF) and failure in time (FIT). Though both measure failure, they do so in different ways, with the former tracking the time between failures (both critical and non-critical) and the latter tallying the total number of failures that occur within one billion hours of operation. Since FIT tracks device hours, it can be spread across multiple devices, such as tracking the number of failures within one million devices during the first thousand hours of operation. Unlike MTBF, which assumes a constant rate of failure, this aids in modeling the infant mortality and wear-out regions of the failure curve. 

The Evolving Nature of Manufacturing Feedback in Reliability

A notable fork in design for reliability comes in the size of the lot. Quality, yield, and other manufacturing metrics are far more forgiving in prototyping and other small runs than they are in mass production. The prototyping stage also provides invaluable data and refinement opportunities for system processes, which have further downstream benefits, such as reducing the burn-in time by removing high-likelihood defects prior to testing. Although there is no standardized value, 10 FIT can generally be regarded as a mark of high reliability from a manufacturing standpoint and is usually associated with Class 2 or 3 industries.

For the best possible outcome, reliability must be approached predictively by the designer, actively by the manufacturer, and retrospectively by the tester. All three have important roles to play in the design workflow, but it is arguably the latter who is in the driver’s seat. Up until the final production, the board design is a heavily iterative process where even small changes from the engineer or design team can realize extraordinary savings due to increases in yield or reduction of complexity/processing. The feedback for these revisions comes from reliability testing performed at the bare board or PCBA level, providing guidance on how to maintain design intent while incorporating the limitations of manufacturing technology.

Reliability Assurance Testing By Type

With the inherent complexity of modern PCB manufacturing, it should come as no surprise that testing is equally expansive. Though this list is incomplete, and not every test will be applicable for every board, it should give an idea of what parameters of common failure modes are under examination:

• Electrical

  • Power cycling - Estimates power device service life while monitoring temperature, voltage, current, and other essential parameters.

  • Insulation/conductor resistance - Humidity and temperature can affect the long-term viability of conductive elements (via barrels, traces, etc.) as well as insulating layers. By measuring the resistance, test equipment can deduce the point of material breakdown.

• Battery

  • Charge/discharge testing - Determine the number of cycles to failure for the battery alongside the number of thermal cycles to failure.

  • Short test - Performed externally (connecting positive and negative terminals with a resistor) or internally via nail puncture. Confirms the safe operation of the battery in common worst-case scenarios.

• Mechanical

  • Drop test - Studies the effects of mechanical shock on a board when dropped from a fixed height. With the increasing prevalence of portable electronics, this test is becoming more commonplace.

• Corrosion

  • Salt mist - Examines effects of corrosion on PCB or box build, from a spray to full saltwater immersion. Can be coupled with temperature cycles.

• Environmental

  • Low air pressure - Determines the reliability of electronics at high altitudes, such as in the mountains or inside an aircraft.

  • Vibrational test - Can be randomized, performed at a set frequency, or used to discover a device’s resonance. Temperature/humidity factors can be combined to build a more accurate model of the phonon response.

  • Sand test - Checks the effects of abrasive particulates on a board.

• Humidity

  • Dew cycle - Alternates temperature and humidity control to promote insulation degradation via a layer of condensate and evaporate at the surface of the board.

  • Highly Accelerated Stress Test (HAST) - Saturates the environment to a point above the internal water vapor pressure of the device for moisture penetration. 

• Temperature

  • Damp test - Combines individual humidity and temperature cycling tests to provide test conditions conducive towards electromigration, whiskering, and moisture absorption.

  • Dry test - An elevated temperature environment to evaluate the aging of materials.

  • Temperature cycling - A rapid temperature change test to measure susceptibility to thermal shock. Can utilize air-to-air or liquid-to-liquid media, with the latter phase allowing for greater temperature modulation.

Stay Ahead of Design for Reliability Testing With Software Solutions

Reliability assurance is a cornerstone of successful PCB manufacturing. Providing end users with boards that have passed thorough testing instills a high level of brand confidence. Though reliability is engaged by testers and operators on the floor, the best results are achieved by designers taking an active role in planning the board for reliability. To achieve this goal, Cadence offers a powerful suite of PCB Design and Analysis Software tools that are able to prevent some of the most common failure modes. Paired with the rule governance of the Constraint Manager found within the Allegro X Design Platform, layout teams can design in full confidence knowing that they’re meeting manufacturing guidelines to promote longevity in the final product.

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