Using PCB Stress Test Analysis to Ensure Device Reliability

September 30, 2019 Cadence PCB Solutions

Blocking out the source of stress

 

A while back, I was having a pretty hard time sleeping through the night and when I did manage to make it through the night without waking up I would be tired in the morning. One night, my wife woke me complaining that I was having conversations with co-workers and she could not sleep. At this point, I realized that I was wound a bit too tight and needed to find a way to de-stress. After trying a number of methods to relax at night, I came to realize that if I minimized my stress while working then I would not be stressed afterward. Now, I sleep like a baby. 

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 upon 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 the reliability of your boards. 

What is PCB Stress Test Analysis?

PCB stress testing is the performance of evaluations 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 either mechanical, electrical or thermal tests as defined below.

PCB Stress Test Types

Depending upon the environment where your board will be mounted and operating, 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. 

PCB prepared for a vibration test

Mechanical vibration stress test for reliability

 

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

 

PCB Stress Test Types

Board Properties

Stress Issues

Potential Failures

Mechanical

Structural integrity under load

Deformation, cracks, breakage

Electrical

Trace and via ampacity, continuity

Shorts, burns, fire

Thermal

Temperature range, number of heat-cool cycles

Delamination, bad solder joint connections 

 

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 and irrepairable. Therefore, neglecting to ensure that your board’s environment will not place undue stress upon it is a major 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 your PCB development.

What is Device Reliability?

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. 

Structural and Operational Reliability

Meeting the regulatory performance specification standards requires that your boards are both structurally and operationally reliable. These can be defined as follows:

 

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. 

 

Structural and operational reliability; although different attributes, are not exclusive. In fact, a board’s operational reliability is dependent upon its structural reliability. For example, board deformation or cracks will likely alter trace parameters that will impact signal and/or power integrity and 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 in order 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.

Using PCB Stress Test Analysis to Ensure Reliability

The scientific method, which extols the virtues of physical measurement or experimentation is the basis for all good testing processes and this is true for PCB stress testing, as well. And for mechanical testing, this remains the preferred means of evaluating your 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. This fixture-based testing will increase the wait time for your boards and the cost. However, high failure rates in the field will be much more expensive. 

 

Cadence advanced mathematical simulation example

Cadence OrCAD Pspice-Matlab Co-simulation Example

 

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. 

 

PSpice’s SMOKE analysis for design reliability

 

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 OrCAD with PSpice.

PSpice’s unique and advanced SMOKE analysis determines component power dissipation, maximum current limits, secondary breakdown limits, junction temperatures, and breakdown voltage across device terminals. All of which are pivotal understandings when considering device reliability, especially with thermal or electrical testing. 

 

PSpice’s SMOKE reliability analysis covers resistors, capacitors, inductors, BJTs, MOSFETs, Diodes, LEDs, Op-Amps, LVRs, and more components

 

In addition to running simulations within OrCAD, you can also perform co-simulation with Matlab® to perform System-level analysis to make your system reliable and ensure that your boards will be reliable once deployed.

Cadence’s advanced PCB design and analysis capabilities provide you with all the tools needed to fully analyze the thermal and electrical stress ability of your PCBs. With the OrCAD PSpice designer, you can validate your components and optimize your circuit design prior to manufacturing.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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