V. A. 1. Reliability test planning

Determine Success Testing Sample Size

“How many samples do we need?” is a very common question. It is one you will receive when planning nearly any kind of reliability testing. It is a great question.

Having too few samples means the results are likely not useful to make a decision. Too many samples improve the results, yet does add unnecessary costs. Getting the right sample size is an exercise starting in statistics and ending with a balance of constraints.

There are six elements to consider when estimating sample size. We will use the success testing formula, a life test with no planned failures, to outline the necessary considerations. [Read more…]

by Fred Schenkelberg 1 Comment

Waterfall Test Planning

A Waterfall Test Planning Approach to Product Reliability Environmental Testing

I may have the name, waterfall, used incorrectly here.

Years ago I learned from a former Apple reliability group manager how to organize reliability and environmental related testing where samples cascade through a sequence of stress conditions and evaluations. He called it waterfall testing. [Read more…]

by Fred Schenkelberg 1 Comment

ALT Allocation of Test Units

One question that you should consider when planning multiple stress accelerated life test (ALT) is the allocation of test units to the various stresses.

We want to create a model detailing the relationship between stress and time to failure. We also want to project the time to failure estimates to use conditions. Ideally, we test at nominal conditions only and gather time to failure information. We do not have the luxury of time thus explore using ALT.

One method of allocation is to place an equal number of samples with each stress level. Is that the best approach? [Read more…]

by Fred Schenkelberg 2 Comments

Electromigration Accelerated Life Testing

Black’s Equation

Black’s equation for estimating the time to failure due to electromigration is a classic. James Black explored and wrote about electromigration in aluminum metallization within semiconductors since 1969.

He and others have explored other materials used as conductors prone to electromigration. Thus, there are a number of models and constants available to match your particular system.

Let’s take a look at the general equation for a microcircuit conductor after a brief description of the failure mechanisms called electromigration. [Read more…]

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Time Compression Accelerated Life Testing

The Easy One

The easiest ALT is one that you operate an item more often then operated by the customer. Removing spans of time the item is not being bent, moved, heated, etc allows you to use time compression.

For example, a home kitchen toaster may be used for a few cycles during breakfast time in your home. In the lab, we can avoid having to wait the day of idle time and just make toast more often than just at breakfast to accelerate the operation of a toaster.

Time compression ALT is also easy to understand and describe the acceleration factor to cover the ALT results to field use conditions. Let’s explore a simple example, work out the acceleration factor and how to interpret a set of ALT results. [Read more…]

by Fred Schenkelberg 7 Comments

4 Different Types of Reliability Testing

Getting the Right Information from Your Reliability Testing

You cannot test in reliability any more than you can test in quality. Often reliability testing is done though, and knowing the range of testing approaches and their associated results will help you get the most information from each test conducted.

Let’s explore the types of testing that generate information useful as you develop a reliable product. There are 4 different types of reliability testing:

Discovery
Life
Environmental
Regulatory

Within each type there are many variations to the testing details and the specific results generated. Understanding the questions each type of testing has the capability to resolve is a good first step to implementing the right set of tests for your project. [Read more…]

by Fred Schenkelberg 3 Comments

4 Steps to Accomplish HALT

4 Not Always Easy Steps

Highly Accelerated Life Testing, HALT, is a method to discover the weaknesses in a design. Using a step stress approach of single and combined stresses, you can quickly expose the salient weaknesses in your design and/or assembly process.

The value of HALT is it’s quick and often finds problems not previously known. You will destroy one or more prototypes, yet the value of knowing specifically what needs improvement more than justifies the sacrifice of a few photos.

Conducting HALT may be part of your reliability plan. Keeping a few steps in mind will help make sure your HALT does provide value back to your development efforts. [Read more…]

by Fred Schenkelberg 9 Comments

Introduction to Thermal Cycling Life Testing

Materials expand or contract with temperature change. Water expands as it freezes, whereas steel contracts as it cools.

This motion can limit the life of your system.

Materials and mechanical engineers include the expected motion into their designs, well the better engineers do.

Even centuries ago, craftsmen used expansion slots or features when attaching wooden table tops to their frames.

The motion due to temperature change will occur and has the potential to create immense strain within your product. [Read more…]

by Fred Schenkelberg 8 Comments

Introduction to High-Temperature Life Testing

A common reliability test involves ‘baking’ a few units.

Various standards list temperature, duration, and sample size requirements.

When the units survive the test, meaning there are no failures, what does that mean?

How do you interpret a system or component life test using high temperature?

Do the results suggest your product is reliable? Maybe it is maybe it isn’t.

Let’s examine one way to design and interpret high-temperature testing. [Read more…]

by Fred Schenkelberg 2 Comments

Reliability Testing Considerations

Reliability testing to determine what will fail or when will failures occur is expensive.

Organizations invest in the development of a product and attempt through the design process to create a product that is reliable. [Read more…]

by Fred Schenkelberg 22 Comments

Norris-Landzberg Solder Joint Fatigue

Before Norris-Landzberg’s study there was the work of Coffin and Mason.

Coffin-Manson

One way to approach accelerated life testing is to use a model for the expected dominant failure mechanism. One such model is for solder joint low-cycle fatigue originally published by Coffin (1954) and Manson (1953), independently.

Norris-Landzberg

Norris and Landzberg proposed the plastic strain range is proportional to the thermal range of the cyclic loading (ΔT). They also modified the equation to account for effects of thermal cycling frequency (f) and the maximum temperature( T). They and other than empirically fit the parameters for the equation. [Read more…]

by Fred Schenkelberg 13 Comments

Accelerated Life Testing

A form of testing that reduces the time till results are known, ALT provides a means to estimate the failure rate over time of a product without resorting to normal use conditions and the associated duration.

For example, solar photovoltaic cells should operate for 25 years without failure. The product development time is less than a year for a new panel and the team wants to estimate the reliability of the cells over the 25-year duration. [Read more…]

by Fred Schenkelberg 3 Comments

ALT Value

Here is an example of how to determine the future value of a specific reliability task. Many of us face the challenge of how to justify spending product development resources to provide insights and information to the rest of the team. Accelerated life testing (ALT) is particularly difficult: It is time consuming, expensive, and at times statistically complex. Having a clear method to estimate the value serves your career and the organization well, as both benefit from the right investments.

[Read more…]

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Extended Bogey Testing

Reaching for a goal may include taking some risks. In reliability testing, we are often limited by the number of samples available for testing. And, in the case where time is available or the acceleration factor is high we can take advantage of testing longer. [Read more…]