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An important part of any reliability improvement effort is accelerated stress testing, frequently referred to as Highly Accelerated Life Test (HALT). HALT is a stress test regimen that imparts random vibration and temperature stresses to a device under test (DUT). The stresses applied will precipitate failure of the weakest parts of the DUT so that they can be analyzed and redesigned to improve the reliability of the end product. HALT testing is performed in an environmental chamber (HALT chamber) specifically designed to impart temperature and vibration stresses both separately and together. Choosing the right chamber for a product can be complicated. Six issues often overlooked when comparing specifications of a chamber are: internal chamber size, overall chamber external height, input and output ports, chamber operating noise, when to start the ordering process and manufacturers’ warranties.
Internal chamber size. The internal space or volume of the chamber must be large enough to contain the product while allowing room for adjustment, cables and the mechanical fixtures that hold the device to the vibration table. Smaller internal chamber sizes will accommodate a typical microwave oven with room for fixturing. Some drive-in units accommodate a full-size automobile, while still others are tall enough to accommodate large racks of electronic systems.
In selecting the size of the chamber, consider the biggest size DUT that could be placed in the chamber at any time. A small DUT may not translate into a small chamber. Frequently, several DUTs are stress tested at the same time to speed the HALT process, so larger internal volumes should be considered. Additionally, a company may develop larger products in the future, which may not fit in a small chamber.
Working in a chamber that is poorly lit will make setup and troubleshooting more difficult. Lamps affixed to the test chamber’s ceiling are needed to produce enough light from several directions for good lighting coverage. They take up some space so this reduction in total internal chamber height needs to be considered. The best location for these lights is in the ceiling corners, rather than a row of lamps placed in the middle of the ceiling.
Overall chamber external height. Typically, smaller chambers are 5- to 8-feet tall. It is important to consider the chamber’s height relative to the ceiling in the location where it will be used, as well as the ceiling heights of the hallways and elevators the chamber will pass through from the receiving dock to its final destination. If purchasing a large chamber, know the overall crated dimensions and do a practice walkthrough of the path the chamber will take to get to its final destination.
Input and output ports. A major part of the HALT process is dynamic stress testing, meaning that the DUT will be operational during stress testing. The DUT is then instrumented and monitored for correct operation by test personnel to detect when failures occur. Regardless of the instrumentation used to monitor the product, the input/output cables and loads must connect from the instrumentation to the DUT, so a passageway through the chamber wall is needed. Chamber manufacturers provide a variety of I/O ports that range from 6 inches in diameter and up. Some chambers are available with a cutout in the door so that cables can be more easily managed. Through-hole ports have the disadvantage of necessitating the cables be attached to the DUT after the DUT is in the chamber. With the door cut out, the cables can be attached on the test bench where the DUTs operation can be verified and then easily placed into the chamber without having to remove the cables. This avoids passing them through the chamber portholes and reattaching them to the DUT. This can be time-consuming and may well lead to reattachment difficulties later during testing. Specifying a door cutout can be a time saver.
Chamber operating noise. HALT chambers vary in noise levels. High noise levels can cause fatigue to the individuals doing the testing. Selecting a low noise chamber will help. HALT chamber noise levels can vary from 55 dBA to above 85 dBA, depending on the selected level of vibration and manufacturer. When possible, place the chamber in a remote or less traveled area so it will not be a disturbance to others.
When to start the ordering process. Small chambers can be ordered and placed in an existing lab space quickly, but when the chamber is large, additional planning is needed. A year from the date the chamber is to be up and running is typically needed if purchasing a large chamber for the first time. Many items need to be taken into account that can cause both internal and external delays.
Internal delays include gathering several chamber manufacturers’ documents and specifications; chamber specification comparison and selection; upper management’s approval and purchase order signatures; price and warranty negotiations; location preparation; power and other facility installation, such as water or air pressure; and Nitrogen supply determination (concrete slab for a several thousand-gallon tank).
External delays include external tank and air compressor foundation design and preparation; local ordinances and permit approvals—added earthquake installation requirements in some municipalities; and contacting current users for strengths and weaknesses of the chamber.
Manufacturers’ warranties. Chamber manufacturers offer a variety of warranty packages. Typically the warranty covers everything from parts and labor for on-site warranty repair for a 2-year period after date of installation. In addition, some manufacturers provide semiannual preventative maintenance at no cost—including travel and lodging—as part of the warranty. Carefully review the warranty terms and conditions.
To help ease the review process, it is recommended that a chamber specification matrix, such as the example on the previous page, be employed that compares each specification for all of the manufacturers being considered.