Highly Accelerated Life Testing (HALT) refers to a reliability testing technique that aims to expose the weaknesses of a product. The main objective is to expose the product to incremental stresses, in order to discover flaws and defects. A HALT test must be conducted during the early stages of the product design process. HALT was originally utilised in the electronics industry, but is now much more widespread. It has also been applied in the medical, military and defence industries.
The stresses that are applied to the product during a standard HALT test are; thermal, vibrational, rapid cycling, shock and combinations of thermal and vibrational.
HALT is not evaluated in terms of passing or failing. Instead, it is desirable for a HALT programme to produce failures.
The failure modes occur typically as a result of the high stress levels, but they are still regarded as legitimate because they are classified as valid failure modes that would be observed in the field.
HALT Testing Standards
There is no overall HALT standard that applies across all products, devices, and industries. HALT is often performed according to specific requests from a client or manufacturer, depending on the product.
For example, the standard often referred to for HALT in power conversion devices for the computer and telecommunications industries, is IPC 9592A. In terms of the HALT test, this standard specifies that the test system is capable of a minimum vibration level of 50 gRMS, a minimum temperature range of -80°C to 170°C, with a thermal change rate of 40 °C per minute.
The details of the basic and ancillary equipment needed to setup a HALT lab, in the context of this particular standard, are also noted. The lab should have calibrated thermocouples, accelerometers, and multimeters for data collection and data monitoring purposes.
Another recommended practice used as a general guideline for HALT testing is IEST-RP-PR003.
How to Plan and Conduct a HALT Testing Programme
A HALT testing programme should be made up of a:
- Separate high temperature step session and a separate low temperature step session
- Vibrational stress session
- Rapid thermal cycles session
- Combination approach session (temperature and shock, temperature and vibration)
The temperature sessions should reveal weaknesses in the product such as warping, melting and design and process issues. The vibrational stress sessions should expose issues such as weak joints, structure and soldering issues and loosening of samples. The rapid thermal cycle sessions should reveal issues such as expansion mismatches, PCB design issues and manufacturing defects. The combination approach should induce different failure modes that unveil problem areas associated with operating limits and destruct limits.
It is essential when planning a HALT test, to first establish the type of stresses that the product will be exposed to, and the parameters associated with each phase. At least four samples should be tested in order to identify as many failure modes as possible.
For each of the applied stresses, a clearly defined starting point must be established, the increments by which the stress is going to be increased, the period of application and the device or equipment limitation for the stress at hand.
HALT Testing Chamber
The HALT testing programme has to be conducted in a HALT chamber. The HALT chamber, is a defined as a chamber that must have the ability to provide the necessary thermal, vibrational, rapid cycle and combination stresses independently and in the same environment at the same time.
The chamber must have features such as vibration at six degrees of freedom, repetitive shock hammers, liquid nitrogen cooling, high temperature heating options and different frequency ranges. Many HALT testing chambers currently on the market have digital temperature recorders and vibration spectrum analysers.
If a company does not have in-house testing facilities then it is advisable to outsource to accredited HALT testing labs. Since a product may have specific testing requirements, it is recommended to contact the accredited lab directly and obtain a quote for the HALT testing costs.
HALT Testing Software
There are vendors that provide HALT testing software for the analysis of results. The industry standard utilised for analysis of HALT results currently is ReliaSoft Weibull++ and ReliaSoft Alta. However, accelerated life testing software such as JMP are also utilised extensively.
The Main Aims of a HALT Testing Programme
The primary aims of a HALT Testing Programme involve identifying as many potential product weaknesses and the root causes of these weaknesses. This allows the system engineers and designers to improve and optimise the design of the product before it reaches the market. Ideally, the product should be overstressed, in an iterative manner beyond its original design specifications.