The winner in today’s electronics market is the company that gets a useful, reliable product out first and grabs the largest slice of the business pie. Accomplishing this means turning a design into a full-scale production run with a minimum number of defects.
To get a design promptly into production, select a process that quickly stresses the
product and points out any possible design and manufacturing weakness. Many tools such as environmental stress screening (ESS), highly accelerated stress screening (HASS), highly accelerated life testing (HALT) or highly accelerated stress testing (HAST) can help you meet your goals. But what are they and what do they achieve?
These terms are not exactly new but they cause confusion because not everyone defines them the same, said Charles Bates of Screening Systems. For example, the term HALT is known as ALT by the IEEE Society. To compound the problem, some manufacturers create their own acronyms to differentiate quality programs that require 100% stress screening from those that only need audit screening.
HALT is used in the product design or development cycle, said Mr. Bates. Many manufacturers use it to step-stress a product to reveal design weaknesses. By fixing these weaknesses, they should have a robust product that can be readily processed through an environmental stress screen using appropriate stress levels of random vibration and temperature cycling.
HALT is also used to define operating and functional limits of a product, according to Wayne Farlow, vice president of sales and marketing at QualMark. You need these limits if you intend to perform HASS because the operating and destruct limits of a product must be determined before you perform HASS.
HASS is part of an overall process that forces failures. It is then followed by failure analysis, corrective action and retested. The HASS process must be continually monitored for effectiveness, and corrected for missed defects.
HASS is based on prior HALT testing agreed Christine Hanse, vice president of Hanse Environmental. It allows you to test to a predetermined limit of a product’s life. If it passes the test, it can be put on the market with confidence that it will behave as designed.
For HALT, the stress levels are greater than those used in the normal product environment, said Mr. Bates. You exceed the operating limits on a sample lot but not the design limits. For example, if you have some defects in a component rated at 0° C, you probably would not find the cause of the defects by testing a small sample at 0° C. You are, however, likely to find them by testing at -20° C.
Some engineers think that HALT is not truly a test because the product is not used in the process strictly for pass/fail purposes, said Ms. Hanse. But whatever name you use, it is an efficient tool to find design limits.
HAST is another stress test typically used on electronic components. It exposes them to accelerated humidity testing by applying high humidity, temperature and pressure. It helps you determine which components might fail while performing the component qualification process.
For HALT or HAST to be effective, you must know what to expect from your product during testing, said Joseph Capitano, Ph.D., president of Advanced Reliability Engineering Technology (ARET). Four basic design parameters will help you understand what to expect:
· The product’s operating design limits.
· The design performance margins at established operating environmental limits.
· The design-margin limits beyond the established design environmental limits where destruction does not occur.
· The destruct limits.
If you do not know your product’s basic design parameters, do not perform any test that may precipitate an anomaly, because high degrees of stimulation will destroy the product, continued Dr. Capitano. Not knowing the life-cycle parameters of a product can cause you to overdesign it, resulting in a waste of time and money. But if a product fails prematurely and you know where it is on the product’s life-cycle curve, you can correct the design.
Determine the amount of product life that is used by exposure to HALT or HAST. You can estimate it by post-failure analysis or by mathematical formula, said Dr. Capitano. For example, a formula developed by ARET derives a correlation between reliability, endurance, field, simulation and stimulation test failures.
But how are HALT and HAST used? The HALT and HAST programs expose products simultaneously or individually to thermal and/or vibration stimulation, continued Dr. Capitano. For thermal exposure, the key variables are thermal extreme limits, the air thermal change rate, the number of thermal cycles, the time held at a temperature and the total test time. The vibration exposure uses sine or random stimulation and varies in g force and time.
When you perform either HAST or HALT, said Dr. Capitano, ask how much stress is too much and how much is too little to obtain the desired results? If aging results can’t be determined, then the only difference between the two is the quantity of tests performed, the limits of extremes and the total test time.
Design Ramifications
As a design engineer, you want to use the right tool to verify a new product and to know if it will save you time. For example, will HALT or HAST get your product to manufacturing sooner and with fewer unforeseen problems?
When HALT is properly used in the early stage of product development, you can find and correct product flaws, allowing products to rapidly pass design-verification testing, said Mr. Farlow. To perform HALT correctly, you must scrutinize any induced- failure mechanisms and examine them with a detailed failure-analysis program. If the analysis shows a failure occurred due to overstress, the failures are ignored. If analysis is not performed and you ignore a failure merely because it occurred due to stresses above the product’s specification, your product could have significant design weaknesses.
Risk you take with HALT occurs when you cause a failure and you do not know the age of a product at the time it fails, said Dr. Capitano. You will not know if you have over- or under-tested the product, and you will not know if it is a true failure or one caused when the product was destroyed.
The test profiles used in HALT and ESS provide an effective quality screen, said Christopher Williams of Ling Dynamic Systems. However, it is challenging to develop a good screen because most products tested are assemblies or subassemblies that exhibit complex local characteristics, especially during vibration.
For example, a high-mass component such as a transformer mounted on a board will have a significantly lower resonance than smaller components assembled on the same board. A closed-loop system can subject the whole board to an effective screen over the entire frequency range without premature failure in the transformer area, said Mr. Williams.
The HALT method may not shorten the design and development time, but it does eliminate redesign and redevelopment time, said Mr. Bates. It helps release designs to production, lessen the number of engineering change orders and meet the time-to-market demands of the company.
The proper use of accelerated aging can result in a life test taking about one to two weeks to age a product 10 years, said Dr. Capitano. Some special test programs may take three or four weeks if the product is large.
HALT and HAST programs normally take months to complete due to the sorting of the failures. They also need robust equipment and extensive test programs. But accelerated aging can be accomplished economically if you are careful when choosing the equipment.
The backbone of any HALT or ESS program is a sound root-cause-failure-analysis program, added Mr. Bates. All decisions should be based on the results of this analysis.
ESS Products
Chambers Accelerate
Propagation of Failures
The LPB Series Environmental Test Chambers for highly accelerated stress testing (HAST) speed up time to propagation of failures by increasing test temperatures above 100° C. The system eliminates condensation on parts when controlling relative humidity at 95% or less during ramp-up, hold and ramp-down cycles. The chambers use a pressurized environment to elevate temperatures to 160° C from 50% to 100% relative humidity. Interior chamber volume ranges from 1.0 to 6.7 cubic feet. Call company for price. Despatch Industries, (612) 469-5424.
Thermal Shock Module
Available for Thermal Chamber
The Thermal Shock Module converts a conventional AGREE chamber to a thermal shock chamber. The typical recovery rate from hot to cold or cold to hot ranges from 2 min to 3.5 min for product payloads of 75 lb to 300 lb with a temperature range from -54° C to +71° C. The rate of temperature change is 35° C/min to 60° C/min. The module connects directly to AGREE, Thermotron, Tenney, Envirotronics and Ransco chambers. $17,000 to $25,000. Environmental Screening Technology, (616) 772-5485.
Electrically-Heated Oven
Provides Temperatures to 500° F
The Grieve No. 664 is an electrically heated walk-in oven with humidity control. It has a heat-input capacity of 30 kW and provides operating temperatures to 500° F. The oven is equipped with 4″ insulated walls and an insulated floor. Work-space dimensions measure 54″ W ´ 84″ D ´ 74″ H. A stainless steel insulated door has a silicone gasket and an adjustable latch. The temperature is microprocessor controlled and maintained. Another controller regulates the humidity from 0% to 100% RH. From $24,000. Grieve, (847) 546-8225.
Combination System Suitable
For ESS Applications
The combination of the Series 800 Shaker, the SPA-K Amplifier and the DVC 4000 or DSC4 Controllers is suited for many ESS applications. Shakers cover a frequency range from 0 to 3,000 Hz. Force ratings extend from 2,200 lbf to 8,000 lbf, and the amplifier series has a power range from 5 kVA to 50 kVA. The DVC 4000 Controller includes sine, random and transient control. The DSC4 Controller provides 16 user-defined sine-vibration programs. Call company for price. Ling Dynamic Systems, (800) GO TO LDS.
System Accommodates Memory
Or Logic Device Burn-In
The Multiple Temperature Zone System can be configured to burn-in and test either memory or logic devices. It is a member of the Automatic Burn-In/Environmental System family. The oven features up to four individual heat-only or hot-cold temperature zones. Memory burn-in configurations provide 14-MHz operation and accommodate 128 data I/O lines. Logic burn-in offers on-the-fly timing, five programmable voltages and 256 I/O drivers per board. $50,000. Micro Control, (612) 786-8750.
System Has Thermal Chamber
And Tri-Axial Vibration Table
The QRS™-600V Power Screen System consists of a thermal chamber that features separate heating and cooling compartments and a triaxial quasirandom vibration system. The Auto-Height™ chamber ceiling optimizes performance and saves energy. The SSC-2000 Controller handles the programming, operation and control
of the system. The 44″ ´ 44″ shaker table uses 16 precision pneumatic actuators. Vibration levels of 30 grms are attainable and loads up to 600 lb are accommodated. The temperature chamber provides transition rates to 60° C/min. Call company for price. Screening Systems, (714) 855-1751.
Copyright 1996 Nelson Publishing Inc.
December 1996