Understanding HALT: A Beginner's Essential Guide

In every field, there are always newcomers. The business environment has shifted due to the economic conditions over the past year or so. Many individuals are choosing early retirement when available. Due to significant layoffs, people are being required to perform tasks they have never handled before. Mergers might result in acquiring equipment that is unfamiliar. Additionally, there will always be individuals entering a field by choice, such as recent college graduates or those transferring from different departments.

Engaging with HALT presents an exhilarating challenge. For more than ten years, I've been thoroughly engaged in this process. I've encountered numerous engineers who possess a strong grasp of the process, but recently, I've noticed many individuals who are not very familiar with it and are eager to learn. Additionally, there's a group that prefers to argue over semantics rather than focus on the task at hand.

To begin, it's important to note that there isn't a single correct method to conduct HALT. Let's take a moment to examine what HALT is and what it is not.

Usefulness of Accelerated Testing

Regardless of the perspective, business is challenging at the moment. Customers, whether they are individual consumers, government agencies, or internal departments, are demanding more for less. They seek high reliability, low cost, cutting-edge technology, and durability.

Admittedly, satisfying everyone is tough. You need to outpace your competitors to market while ensuring your product's longevity. How can you achieve this?

An effective way to address many of these challenges is by expediting the testing process. Waiting 20 years to see if a light bulb will last that long in your neighbor’s kitchen is not feasible. You need to accelerate the process.

While you can make a light bulb fail by dropping it, this only confirms that gravity works. A more logical approach is required, one that "accelerates" time to simulate potential failures. Various computer programs and scientific formulas can help correlate potential lifetimes, allowing you to identify likely failures in the field within days instead of years.

Why is this crucial? One significant company expense is warranty issues. When Ford Motor Company identifies and resolves a weakness before a part reaches a car on the road, their accounting department highlights the savings achieved by Engineering. Detecting issues early can save millions and enhance customer confidence. You benefit by reducing costs and increasing revenue through repeat business.

Starting Temperature

The optimal way to begin the test is at laboratory ambient. Why do I use this term, you might ask? As a member of several IEC (International Electrotechnical Commission) working groups, I've observed that the term "ambient" has evolved to mean different things to different people over the years. Traditionally, ambient referred to the chamber temperature surrounding the product at any given time. However, most people now interpret it as the room temperature where testing occurs. By specifying "laboratory ambient," I aim to ensure that testing starts at approximately room temperature, avoiding confusion with the temperature from the previous chamber test. It's better to be cautious!

Always monitor the product continuously. Ford Electronics, before becoming Visteon, publicly reported that 50% of intermittent failures would not have been detected without constant monitoring. It's insufficient to merely take a reading at the start and another at the end.

A Series of Tests

Considering that there isn't a single perfect method to conduct a HALT, here are some fundamental ideas to remember. HALT consists of a series of tests. Ideally, you should have multiple units available for testing, with one designated for each test in the series.

From a purist perspective, it's best to begin by testing in individual environments, then proceed with combined environments for comparison. We recommend the following six standard tests, although there are other possible approaches:

• Cold only

• Heat only

• Vibration only

• Heat with vibration

• Temperature swings

• Temperature swings with vibration

Additional factors can certainly be included. If you're concerned about lower temperatures, consider adding Cold with vibration. If humidity is a worry, conduct a humidity-only test before combining it with other conditions like thermal and vibration. Power cycling can also be highly advantageous.

Before testing begins, it's crucial to thoroughly understand the product being tested. Familiarize yourself with the end environment where it will be used and test accordingly. Remember: End users are always harder on a product than anticipated, yet they will expect it to function regardless.

Step by Step

After selecting the environment, how do you begin? The key is understanding your product. Is it as small as a PCB or as large as a tank? How long will it take to stabilize at a certain temperature? Are some components more prone to failure?

How do you determine if your product has stabilized at temperature? Consider the example of what I'm testing today: a vehicle console nearly as wide as the van's ceiling. It's impossible to choose one representative spot and assume uniformity across the unit. The rule of thumb is: the larger the product or the more diverse its components, the more thermocouples are needed. Control should still be based on one main thermocouple, placed where it best represents the unit or on the most sensitive component, depending on your primary concern.

A good starting point for all tests is laboratory ambient, typically around 25°C. If necessary, fixture your product and connect any required wiring. Remember, your wiring or cabling will experience the same extremes as your product!

The order of tests is entirely your choice. However, starting with single environments before moving to combinations provides a solid baseline. For instance, if your first test combines heat and vibration and a failure occurs, can you tell which caused it? Or was it the combination? If you've already conducted separate heat and vibration tests, it's easier to identify whether a specific environment or the combination caused the failure.

What is a Failure?

Different companies evaluate failures in various ways. I visited a company where they set aside products due to pinpoint scratches in the paint job. For them, this was considered a significant "failure" that prevented them from shipping the product to a customer.

Some consider the first intermittent failure as the limit they are willing to accept. Others prefer to reach a complete failure. This is something to consider when planning your test—what will you define as a failure?

The Tests

Having reviewed the key points to consider before testing, let's examine the tests themselves. It's important to understand that more is involved than simply pressing the "Run" button on the computer. You've already seen the need for advance planning. We'll explore the different considerations for each test and the reasons for conducting them.

Cold Step Test

Cold temperatures tend to be the least destructive among single environments, making them a good starting point for testing.

You can determine the size of the steps based on your product knowledge. If you are concerned about cold effects, use smaller steps. If you seek baseline data, start with larger steps and reduce them if a premature failure occurs. Many engineers are comfortable starting with steps of a 10°C per minute change rate in cold conditions.

After deciding on your ramp rate, determine your dwell time. How long should your product remain at a certain temperature before ramping again? The general consensus is to keep the dwell time to the minimum necessary to stabilize the product. For a PCB, this might be only five minutes, while an assembly might require a longer time. Again, rely on your expertise to decide.

Take a look at the following graph:

The chart above shows an example of what could be used.  Starting at laboratory ambient and holding until stabilization, the test lowers 10°C as quickly as possible, and then holds for ten minutes.  These steps are continued until there is a failure.

Heat Step Test

The heat test follows a similar procedure, but in the opposite direction. It relies on the same principles as the cold step test. Start by stabilizing at the laboratory's ambient temperature, then proceed with ramping and dwelling. Since heat is generally more damaging than cold, you might opt to increase the temperature at a rate of only 5°C per minute rather than at a faster pace.

I once had the opportunity to work with a company whose product was intended for use in a hospital setting. Their first significant failure occurred at just 2°C above the typical laboratory temperature. Initially, they weren't worried, thinking that hospitals are air-conditioned and the surrounding air would never become too warm. I shared with them my experience of an extended hospital stay where the temperature was controlled based on the time of year. The heating was activated due to the date, not necessity, resulting in patient rooms reaching nearly 30°C. The company redesigned the board, and their product is now the market leader. Moral: Ensure there is a margin between expected usage conditions and what your product can actually withstand. Consider the worst-case scenario and then add a safety margin. In essence, if something can go wrong, it will.

Vibration Only Test

You've completed the simpler tests, such as heat and cold. You've monitored and recorded the data. You've implemented any changes you deemed necessary. Now, you're prepared for vibration testing.

Begin with the temperature set to laboratory ambient conditions once more. This ensures that temperature will not influence the test. While this may not reflect a real-world scenario, our focus is solely on the vibration for now.

The standard method for measuring a vibration test is by using the g level. But where should we measure the g from?

Measuring from the bottom of the vibration table primarily indicates the table's activity, which may not accurately reflect your product's behavior. The ideal location for the accelerometer is on or near your product. Some opt to attach the accelerometer to the fixture securing the product, which is completely acceptable. If attaching it to the product or fixture is not feasible, mount it near the fixture on the table's surface. This will provide a reasonably close approximation of what the product experiences.

Unlike temperature, vibration needs to be managed in very small steps. It can be challenging to control precisely, so we recommend starting at 2 g’s and increasing by 2 g’s incrementally. The dwell time depends on your understanding of your product. Allow sufficient settling time, often ten minutes. Continue increasing until you observe what you consider a failure, while monitoring and recording data throughout the process.

Combinations

Now comes the exciting part. You've finished the tedious steps. You've gained more insight into your product and the reactions of your team members. ("You're doing what to my design?")

Thermal Swing Test

You've completed tests for cold, heat, and vibration. The next logical step is the thermal swing test, which combines the heat and cold tests.

Adjust the chamber to start at the laboratory's ambient temperature and let the product acclimate. Choose whether you prefer to increase or decrease the temperature initially. Introducing nitrogen into the chamber air will aid in eliminating any residual humidity in the product, so this should be factored into your decision.

Choose the ramp increment, usually 5 or 10°C at a time, and determine your dwell time. Then begin.

As illustrated in the chart, each step expands. Suppose you decide on 5°C increments with a starting temperature of 25°C. You’ve opted for 5-minute dwells and prefer to increase the temperature first. Here is the basic scenario:

Continue testing until a failure occurs.

Note:  It's common for a product to fail at very low temperatures and then start working again once it's warmed up. If you encounter a failure during the cold phase of this test, it's recommended to proceed to the next hot phase to see if it resumes functioning.

There are valid reasons for conducting the swing test. The difference in thermal coefficients can cause parts to separate from each other, sometimes leading to cracks. By applying these changes rapidly, we are stressing the unit beyond normal conditions. Some argue that you will rarely experience a temperature change of over 30°C per minute, but consider the following:

I live in Michigan. While not the coldest state in winter, it's common to experience a wind chill of -40°C at least once each winter. Imagine my car gets stuck in the snow about a mile from home. I decide to walk (we Michiganders are a resilient bunch).

My cell phone goes from about 25°C to -40°C as I hold it to my ear to call my husband and inform him of my predicament. Since he's on the road, I have to use my reliable palm computing device to find his number.

I arrive home, shivering, turn up the heat, and stand by the heater—still using my trusty phone. The phone and computer warm back up to 25°C as the hot air blows on them. By this time, not only have the electronics been stressed, but so have I! I'm sure you can think of other extreme situations where we subject electronics to stress.

Heat with Vibration Test

Let’s conduct some shake and bake. Not for dinner – although that’s been done! It’s time to combine heat and vibration to test your product.

What vibration level will you select? Suppose you know your product failed at 10 g’s in a previous vibration-only test. Since you already know this is the failure point, there’s no need to reach that level. You want to discover what occurs when thermal factors are combined with vibration.

Most engineers I have worked with opt for a level around 80% of the failure point. In this case, that would be 8 g’s.

What is the optimal method for conducting a combined heat and vibration test? You should rely on your expertise once more. It's advisable to closely follow the original heat test you performed, beginning again at laboratory ambient. This will provide a reference point for anticipating where your product might exhibit a heat-related failure. If the temperature was very high, you might consider omitting some of the lower temperatures. For example, if your product didn't fail until 110°C, you might want to start at around 60°C and proceed from there.

The chart example above illustrates ten-minute dwells, with vibration applied (below failure level) for two minutes every five minutes. Pulsing can lead to failures, but so can continuous operation. It's your decision on the optimal way to conduct the test.

This is where comparison becomes intriguing. Typically, the product will fail at a slightly lower temperature when vibration is introduced compared to using heat alone. However, there are always exceptions. This particular test should provide valuable insights into how your product will withstand a combination, which it will likely encounter in real-world usage.

Thermal Swings with Vibration Test

Utilize the insights gained from your heat and vibration tests to apply similar principles to thermal variations and vibration. If your failure was significantly beyond laboratory conditions, consider skipping several steps.

I discovered something intriguing from a machine design magazine article: some companies use extremely cold temperatures to "de-stress" equipment destined for high-vibration environments. Typically, we learn that any action taken on a product reduces its lifespan. However, after implementing the article's principles, I conducted experiments with customers across various product types. In every instance, combining vibration with cold temperatures allowed the products to endure more vibration than at ambient conditions. You might observe similar results during your swing test with vibration. Additionally, consider including a cold test with vibration if your product needs to withstand low temperatures.

Now What?

With all these numbers and failure times, what should you do next? What are your goals?

There's no need to over-engineer. If your product meets real-world requirements with a reasonable margin, you might not need to make any changes. For example, Microsoft extended their mouse warranty from one year to three after testing, without worrying about warranty issues.

Not breaking something doesn't mean failure on your part. It likely indicates a robust product ready for market. If you do encounter a failure, view it positively. HALT is a learning tool, helping you identify premature product end-of-life quickly, avoiding prolonged learning through warranty replacements.

No Magic Bullet

Will a HALT test predict the exact market lifespan of a product? Probably not. Without field failures on the same or a similar product, establishing a time correlation is challenging. However, if you can replicate field failures in the lab under certain conditions, you can make a good correlation.

Is a rapid thermal cycling chamber with tri-axial vibration the only equipment you'll need? If that were true, I'd be wealthy. Just as a toolbox requires multiple tools, testing equipment should not rely solely on HALT, despite its value.

One positive industry shift is the increased sharing of non-proprietary information among engineers. To learn more about HALT, explore groups like the IEEE AST (Advanced Stress Testing) group, attend seminars, and join user groups. Sharing knowledge helps us avoid starting from scratch. Trustworthy sources are invaluable.

Final Note

When conducting HALT testing, rely on your product knowledge. Preplan with the end-use environment in mind. For plastics, know melting points. Keep an open mind during tests, recognizing product failures as learning opportunities, not personal failures. Collaborate with design, project, production engineers, management, and anyone offering valuable insights.

I enjoy working with HALT and wouldn't choose any other industry. Like BASF's motto, "We don’t make the _____ … we make it better," I help create better products, enhancing reliability, reducing costs, and improving safety. As a child, I wanted to bring world peace, but as an adult, I realize change is gradual. As a teacher and manufacturer, I see that each of us can contribute to a better world. You now have the opportunity to excel at your job, produce better products cost-effectively, and improve others' lives by making them safer, easier, and more reliable.