Using Stress Test Analysis to Ensure PCB Reliability

Key Takeaways

• What PCB stress test analysis is
• PCB stress test types
• An overview of structural and operational reliability

Device reliability involves ensuring that a circuit board can withstand shock, environmental factors, and more

A great source of stress when designing circuit boards is how your board will hold up once it is in the field. This anxiety can be relieved by testing your PCB during development to ensure that it will survive the electrical, thermal, and mechanical strains that may be placed on it during operation. Implementing these evaluations and the results obtained is known as PCB stress test analysis. Let’s investigate this testing, the stress test types that are involved, and how they can be utilized to ensure PCB reliability.

What Is PCB Stress Test Analysis?

PCB stress testing is the evaluation of a circuit board’s ability to withstand or endure forces or parameter strains. The objective of this testing is to determine the ranges within which the board will maintain its structural integrity and functionality. The determination of the limits of these ranges is PCB stress analysis. There are various techniques, methods, and processes to perform stress testing; however, most of these can be classified as mechanical, electrical, or thermal tests.

PCB Stress Test Types

Depending on the environment where your board will be mounted and operated, it may be subject to different types of stress. For example, PCBs that are deployed in critical systems, such as aerospace platforms, automotive vehicles, and industrial equipment, are likely to be subjected to forces from continuous or intermittent motion and vibrations. Therefore, performing vibration stress testing, as shown in the figure below, is common for boards developed for these industries.

Vibratory and other mechanical tests are used to determine the structural limits of a board, beyond which physical breakdown will occur. These and other PCB stress test types are listed in the table below.

PCB Stress Test Types

As shown in the table above, your board is susceptible to failures from mechanical, electrical, and thermal stresses, any of which can render it unusable. Therefore, neglecting to ensure that your board’s environment will not place undue stress upon it is a major PCB reliability factor that can be costly if excessive replacements or recalls are required. These undesirable outcomes can be avoided by performing stress test analysis during PCB development.

What Is PCB Reliability and Why Is It Important?

Device reliability and stress testing go hand-in-hand. Stress tests are conducted to ensure that the device is reliable. PCB reliability is the degree of certainty that a circuit board will not experience structural, operational, or electrical failures over its intended lifetime.  Manufacturing reliable devices is a valuable asset in reducing the number of failures and returns a product experiences.

When PCBs are tested in a standardized lab environment, it doesn’t necessarily mean they’ll hold up when deployed. This is why keeping design for reliability (DfR) in mind is crucial; the failure of a circuit board can lead to a loss of time, money, or even life. DfR involves the identification of failure points, as it’s designed so they can be addressed before manufacturing. Conducting stress tests and following DfR protocols will help in reducing costs and highlight issues before they appear on the end product.

PCB Reliability Standards

IPC standards are the industry-adopted set of standards for PCB design, manufacturing, and assembly. Standards set by the IPC ensure that the electronics we use are dependable. Specifically, IPC separates electronics into 3 classes:

• Class 1 defines requirements for general consumer electronics. These electronics often have short life cycles and failure should not cause serious issues.
   
• Class 2 electronics include dedicated service products such as televisions, microwaves, and laptops. Their life cycle is usually longer and continued performance is necessary. 
   
• Class 3 electronics are the highest standard and include high-reliability products targeted toward aerospace, military, and medical applications. These can cause serious issues if they fail, and therefore must deliver uninterrupted service under the strictest guidelines.

The reliability of your board to perform during operation is not only desirable to satisfy clients and end-users, but is mandated by IPC-6012D Qualification and Performance Specification for Rigid Printed Boards and IPC-6013D Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards based upon its performance classification as stipulated in IPC-6011 General Performance Specification for Printed Boards. These standards provide specific dimensions and tolerances to which your manufactured board must adhere.

Absent PCB Reliability and Common Failure Modes

Flying probe tests can be used to ensure device reliability

When creating your PCB, it's critical to keep design for reliability (DFR) in mind, as devices can fail for multiple different reasons without it. Many of the previously discussed stress tests are valuable in detecting failure points before PCBs go to market. Factors from the external operating environment, such as humidity, dust, or other airborne materials, can result in board failure. Some common failure modes include:

• High amounts of electromagnetic interference (EMI) can occur, especially when your board leaves its lab-tested setting. Signals can become corrupt in especially noisy environments. 
   
• Thermal reliability issues can occur when cycling between two temperature extremes repeatedly, thermal shock, or without good general thermal management to eliminate hotspots. For this reason, it’s imperative that you design your layout for even heat distribution, avoid stacking too many buried vias, and use the right substrate and solder materials.
   
• Component selection mismatch–if you expect your board to operate with tight tolerances, whether that is temperature-wise or in designing for specific capacitance and inductance, using components such as resistors and capacitors with a tighter 1% tolerance rather than the standard 5 or 10% will improve reliability.
   
• Power delivery network (PDN) issues– power supply issues can happen with virtually any device. Fluctuations in the supply line can cause brownouts in microcontrollers, ICs to fail, and even worse.

Other Reliability Tests

DfR begins at the schematic and goes all the way to verification of the end product. Beyond the discussed reliability tests, other analyses that you might consider conducting are:

• Cleanliness tests that measure how well the PCB resists environmental hazards
• Environmental tests that quantify how well the PCB resists water absorption
• Solderability testing that ensures all components are securely attached to the board 
• Design for manufacturing (DfM) testing
• Design for assembly (DfA) testing
• Design for supply chain (DfSC) testing
• Flying probe and fixtureless in-circuit (FICT) testing

Keep in mind that no single test will ensure the reliability of your board. Consider a combination of testing methods that adequately reflect your board's intended use case (in addition to its extremes!).

Structural and Operational Reliability

Meeting regulatory performance specification standards requires that your boards are both structurally and operationally reliable.

PCB structural reliability is the ability of a circuit board to maintain its physical form throughout its operational lifecycle, provided it is not subjected to forces that exceed the boundaries of its range of structural endurance.

PCB operational reliability is the ability of a circuit board’s electrical circuitry to consistently function as developed over its operational lifecycle.

Although they are different attributes, structural and operational reliability are not exclusive. In fact, a board’s operational reliability is dependent on its structural reliability. For example, board deformation or cracks will likely alter trace parameters, which will impact signal and/or power integrity. Breakage could sever surface routes, signal layers, or ground planes, resulting in erratic behavior or no functionality at all. Thermal and electrical stresses can be similarly debilitating for board operation. Therefore, it is imperative to know the limitations of your board during development to ensure that it will not be subjected to stresses that will likely cause it to fail in the field. The best way to achieve this is by using PCB stress test analysis.

Ensure PCB Reliability With Stress Test Analysis

The scientific method, which extols the virtues of physical measurement or experimentation, is the basis for all good testing processes. This is true for PCB stress testing as well. For mechanical testing, this remains the preferred means of evaluating a board’s propensity to endure physical force stresses. Many contract manufacturers (CMs) can perform these types of tests on individual boards or through in-circuit testing such as flying probe testing. This fixture-based testing increases the wait time and cost for boards; however, high failure rates in the field will be much more expensive.

In contrast to mechanical stress testing, it is advantageous to know your board’s thermal characteristics prior to manufacturing. This enables you to make the best material selections and layout your board for the best thermal distribution. Both of these will aid your board build.

During assembly, your board is subjected to high temperatures for significant periods. These temperatures can be as high as 250°C for lead-free soldering. Additionally, it is necessary for the solder to distribute throughout the area for good solder joint connection quality.

Electrical stress testing, which is the determination of the range of electrical parameter variation that your components and traces can endure, should be done during design as well. In fact, these determinations should be major factors in component selection and routing your layout. For operational reliability, thermal and electrical stress are interdependent. Excessive heat can impact the operation of components, and some components themselves generate heat.

Stress Testing With PCB Design Software

Performing thermal and electrical testing can and should be done during design; however, it does require that your PCB design software is capable of performing these functions. Simulations to evaluate your board’s thermal and electrical stress capabilities are easily performed with Cadence’s suite of PCB design and analysis tools.

PSpice’s advanced SMOKE analysis determines component power dissipation, maximum current limits, secondary breakdown limits, junction temperatures, and breakdown voltage across device terminals. This information is pivotal when considering device reliability, especially with thermal or electrical testing.