It is easy to understand why the term HALT (Highly Accelerated Life Test) is so tightly couple to the equipment called “HALT chambers” systems. Many do not think they can do HALT processes without a “HALT Chamber”. Many know that Dr. Gregg Hobbs, who coined the term HALT and also HASS (Highly Accelerated Stress Screens), spent much of his life promoting the techniques and was also the founder of two “HALT/HASS” environmental chamber companies. I was very fortunate in working with and learning from Dr. Hobbs beginning 23 years ago at Storage Technology, Inc., before he founded his first chamber company Qualmark, Inc.. Later he asked me to join him at Qualmark in promoting the methods and equipment for HALT and HASS and we worked together at Qualmark for a year.
Back in the 1980’s Gregg taught that the best and basic approach for HALT was to use a single stress in a stepwise fashion to a point of empirical operational discontinuity, that is an operation limit or destruct limit. Some stresses were more useful and universal than others such as rapid thermal change and multi-axis vibration (pneumatic hammer driven) which required a combined environmental chamber and became referred to as a “HALT chamber”.
Even though Gregg made HALT chambers, still the HALT processes he taught were a fundamental paradigm shift in in that it is empirical test process over theoretical, probabilistic failure prediction methods used in traditional reliability engineering. Increasing voltage stress, clock frequency stress, software application stresses, drop height shock stress, and increasing pressure in hydraulic systems are all stresses that could be used for HALT processes and no HALT chamber is required.
To illustrate I recall almost twenty years ago at Qualmark I was in Gregg’s office and he was very excited to tell me about a call he just had with some test engineer who had a big success with a vibration HALT procedure, and the reason I remember this so well is that just as I was leaving Gregg’s office he told me, “…oh by the way, he used a classical [Electrodynamic] shaker!”. Many do not realize that vibration HALT could be performed on a classical shaker by taking the product to a empirically determined vibration stress limit, as Gregg knew it could. Thermal HALT methods could be done in a traditional mechanical refrigeration chamber, although maybe not as efficiently.
HALT is not a chamber, or type of stress or combinations of stress; it is a basic method of using an applied stress to for empirical design or manufacturing weakness discovery. A much better term to describe HALT would be “Highly Accelerated Limit Test”.
“HALT/HASS” chambers are typically defined by the fact that they are capable of producing multi-axis vibration from pneumatic hammers and rapid thermal stresses from liquid nitrogen supplied cooling and large banks of electric resistive heating coils. In these systems the multi-axis vibration intensity can be controlled but the frequency spectrum is uncontrolled. It is like the real world in that vibration for most products life cycle environmental stress is uncontrolled intensity, axis, and frequency. Temperature ramp rates can force rapid product thermal changes of 60°C per minute or greater depending on the product mass and chamber configuration. Even though they are typically referred to as HALT/HASS chambers and are excellent for that purpose, their capabilities are really shine in HASS processes when combined environments should be used to induce the fastest stress and fatigue damage in the shortest period of time. The quicker a latent defect is detected the shorter and less expensive a HASS process is for those companies doing HASS. If HALT methods are used to make a robust design as it should be, then most of the failure mechanisms found in HASS occurs in the “infant mortality” region of the life cycle bathtub curve and vast majority of these early life failures arise from assignable errors or from wide variations in the manufacturing processes. HASS is an ongoing insurance process. It can be a very useful yet expensive ongoing test process if not coupled with RCA to eliminate the cause of manufacturing defects.
To be sure, “HALT” chambers [multi-axis pneumatic, rapid thermal] are the fastest most efficient chambers capable of applying some of the most useful universal environmental stresses to find latent design errors or manufacturing errors. Yet at the core of the new methodology and philosophy of HALT is a conceptually simple empirical process. The HALT methodology is performed by increasing stress in a controlled application to an operational and sometimes destruct limit. Many useful stresses to find operational and destruct limits do not require a chamber to find limits, improving operating margins and comparing limits between samples and products. That is what HALT is.