HALT and HASS are starting to supplant traditional vibration and thermal testing to meet todayï¿½s quality targets.
For decades, product quality has been determined through environmental testing such as vibration, thermal cycling, mechanical shock, and thermal shock. More recently, there has been a significant trend in the marketplace to improve product quality even further.
The Need for Increased Product Quality
One of the most pervasive trends across a wide range of the consumer, industrial, and military markets is the need for increased product quality. In consumer markets, a high rate of product failure can result in the manufacturerï¿½s loss of credibility with an attendant loss of sales, from which it can take years to recover. In industrial markets, a high failure rate can result in expensive field service calls orï¿½potentially worseï¿½significant downtime. In military markets, product failures can translate in the loss of lives.
Although the need for quality is increasing, certain developments are making it more difficult to maintain existing quality levels. The most challenging development has been the increased use of manufacturing subcontractors. The manufacturer whose name goes on a product is likely to be relying on an outside resource, a subcontractor, over which the manufacturer does not have direct control.
This subcontractor is relying on a number of vendors, further weakening the control that the manufacturer has on product quality. Should a product fail, the customer will blame the manufacturerï¿½the one responsible for its quality level.
Another challenge to maintaining quality is a continually decreasing number of engineers with comprehensive QA/QC backgrounds at these manufacturing companies. Many of the highly experienced QA/QC engineers are retiring or being replaced by younger engineers who are far less experienced.
Traditional Vibration and Temperature Testing
Traditional vibration and temperature testing has played an important role in the genesis of todayï¿½s reliable and sophisticated electronic and electromechanical products. The core philosophy of this testing method is to define a set of specifications, usually minimum or maximum temperatures and vibration levels, and conduct the tests by changing only one variable at a time. Vibration testing is performed one axis at a time. If the device still is functional after being tested according to the test specs, it is considered to have passed.
A passing result is a positive outcome. However, a pass result does not help identify the weakest link in the product. In other words, the traditional test cannot help the engineer make the product any more robust.
Furthermore, with the one-at-a-time change in environmental variables and the one-dimension vibration testing, the test specs are not similar to real-world operating environments. As a result, this kind of testing does not provide an accurate indication of how the product might perform in the field.
This critical look at traditional environmental testing is not intended to be a blanket condemnation of that process. After all, this kind of testing has played a key role in the evolution of todayï¿½s highly reliable products. Instead, this examination of certain weaknesses in classical environmental testing can be helpful in understanding how new testing methods, in particular HALT and HASS testing, can lead to even greater levels of product quality and reliability.
The Strengths of HALT and HASS
Highly Accelerated Life Testing
HALT exposes the product to a step-by-step cycling of environmental variables such as temperature, shock, and vibration. It involves simultaneous vibration testing in all three axes using a random mix of frequencies. Finally, HALT can include combinations of multiple environmental variables; for example, temperature cycling plus vibration testing.
Unlike conventional testing, the goal of HALT is to break the product. When the product fails, the weakest link is identified so engineers know exactly what needs to be done to improve product quality.
After a product has failed, weak components are upgraded or reinforced. The revised product then is subjected to another round of HALT, with the range of temperature, vibration, or shock further increased so the product fails again. This identifies the next weakest link.
By going through several iterations like this, the product can be made quite robust. With this informed approach, only the weak spots are identified for improvement. This type of testing provides so much information about the construction and performance of a product that it can be quite helpful for newer engineers assigned to a product with which they are not completely familiar.
HALT must be performed during the design phase of a product to make sure the basic design is reliable. But it is important to note that the units being tested are likely to be handmade engineering prototypes. At Trace, we have found that HALT also should be performed on actual production units to ensure that the transition from engineering design to production has not resulted in a loss of product quality or robustness.
Some engineers may consider this approach as scientifically reasonable but financially unrealistic. However, our customers have repeatedly found that the cost of HALT is much less than the cost of field failures, service calls, blanket recalls, and loss of credibility or business due to poor product quality. One of our clients even includes HALT as a line item on its bill of materials to make sure this testing is included in the product cost right from the beginning.
Highly Accelerated Stress Screening
HASS, an abbreviated form of HALT, is an ongoing screening test performed on regular production units. Here, the idea is not to damage the product but rather to verify that actual production units continue to operate properly when subjected to the cycling of environmental variables used during the HASS test. The limits used in HASS testing are based on a skilled interpretation of the HALT parameters but do not exceed a productï¿½s operating limits.
The importance of HASS testing can be appreciated when you consider todayï¿½s typical manufacturing scenario. Circuit boards are purchased from a vendor who uses materials purchased from other vendors. Components and subassemblies are obtained from manufacturers all over the world.
Often, the final assembly of the product is performed by a subcontractor. This means that the quality of the final product is a function of the quality or lack thereof of all the components, materials, and processes that are a part of that final product. These components, materials, and processes can and do change over time, affecting the quality and reliability of the final product. The best way to ensure that production units continue to meet reliability objectives is through HASS testing.
The benefits of HALT/HASS testing can be seen in two case histories.
Automotive Lamp Assembly
A manufacturer of automotive lamp assemblies (headlight, brake light, and third brake light units) provides an example of the benefits of using HALT/HASS throughout the development of a new product.
An engineer at this company decided to submit a production sample for an abbreviated suite of HALT. The unit failed, and it was redesigned. When submitted for a retest, a full HALT was performed, with the power to the bulbs in the assemblies cycled on and off during the testing process. During HALT, temperatures were varied over the range of -100ï¿½C to +85ï¿½C, with vibration parameters of 0 to 50g rms (Figure 1).
Special fixtures were made to hold the assemblies at the exact same angle and under the exact conditions they would experience when installed in a car. The manufacturer was careful to test actual production units to ensure that the test results were an accurate reflection of product quality.
Automakers have been champions of sophisticated quality testing for years. When they saw the test setup and the test results from this lamp assembly manufacturer, the automakers were so impressed that they made the manufacturer a prime vendor for these assemblies and started requiring HALT from all their vendors.
A manufacturer of custom power supplies used in telecom switching systems wanted to ensure reliability in the field, so the company contacted Trace Labs for HALT to verify and refine the basic design. After several iterations, the basic design was made reliable. The power supplies were HALT tested over the temperature range of -50ï¿½C to +130ï¿½C, with vibration levels ranging from 0 to 10g rms.
Next, the manufacturer developed the handmade units into production designs. We recommended the production units be HALT tested, but this recommendation was declined.
It turned out that in developing the design for production, the power supply manufacturer reacted to price pressure from its customer, reduced the cost of various aspects of the production design, and had inadvertently compromised the high reliability of the original design.
Now facing a serious field-failure problem, the manufacturer submitted actual production units for HALT. After five iterations, the design of the production units had been refined to provide good field reliability. Ironically, the cost of the redesigned production units was only 2% more than the amount specified in the original contractï¿½a cost the customer was willing to pay.
However, damage had been done to the power supply vendorï¿½s relationship with the customer. The customer next required 100% HASS testing of all power supplies from this manufacturer, and the manufacturer was not invited to submit quotes on subsequent RFQs. The entire problem could have been avoided if the manufacturer had been willing to spend the upfront costs for HALT on the original production units.
Fortunately, this story does have a happy ending. After three years of producing reliable power supplies, proven through HASS testing as well as successful field experience, the manufacturer once again is regarded as a primary vendor.
Classic vibration and temperature testing definitely have helped improve product quality over the years, but todayï¿½s very high standards for product quality are requiring tests better able to reduce, or even eliminate, field failures.
HALT provides a controlled, repeatable method of determining product quality under conditions comparable to field operating conditions and is critical for proving the basic design of a product. HASS testing is a quick, effective screening process that can be used to ensure production units continue to meet quality standards.
While it is true that HALT and HASS testing can add to the short-term manufacturing cost of a product, the increment is surprisingly small in most cases. In the long run, the cost of the testing is much less than the cost of field failures or the loss of business due to reliability problems.
About the Author
William Lagattolla is manager of vibration and shock testing and oversees the HALT/HASS testing department at Trace Laboratories-Central. Previously, he worked as a mechanical engineer in the telecommunications industry, designing and conducting shock/vibration and HALT testing procedures. Trace Laboratories-Central, 1150 W. Euclid Ave., Palatine, IL 60067-7368, 847-934-5300, e-mail: [email protected]
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