Highly Accelerated Stress Screening (HASS) is the ideal tool for both qualifying product changes, and monitoring the reliability throughout a products life. Engineering was fraught with poor quality components in the early days, but this is no longer the case, and therefore traditional production screens that originally identified infant mortalities are not as effective as they once were.
As component reliability has increased, the ability to weed out the ‘good’ from the ‘bad’, has become more of a challenge. No longer is the line between good and bad as clean cut, there is now a requirement to put these newer, more reliable products through a stronger screen.
Where to from HALT?
After conducting HALT on your product, and provided improvements have been made where required, your reliability, and confidence in the products performance should be high. The problem is that the HALT was just a snap shot of the products reliability, it only applies to the particular revision of the product that underwent testing.
Every engineering firm understands the need to constantly maintain and update components and software, while every manufacturer understands the requirement to change their process, and or supplier sourcing from time to time. As soon as changes are made you need to know whether the reliability of the product has been compromised.
HASS is a tailor made screening solution for each of your products. Because you have conducted HALT testing, you know exactly what your product can handle. This knowledge provides you with the ability to create a production screen that won’t damage good hardware, but will find process related issues, along with supplier discrepancies.
The limits attained during HALT are used to create an effective HASS screen. The process of HASS development will ensure the screen is strong enough to flesh out dormant failures, while running a Proof Of Screen will provide confidence that the screen is not going to damage good hardware.
HASS Development Process
HASS is a tailor made reliability test solution for each product; the results achieved during HALT are used to determine an accurate production screen. Because HALT testing has been conducted, the products “limits” are known. This knowledge provides your company with the ability to create a production screen that won’t damage good hardware, but will find process related issues.
Defining the Screen
During HALT testing the products Operating Limits are defined. Using these Operating Limits an appropriate screen is created that can be applied during mass production. There are many different varieties of HASS screen, though the two most common are precipitation and detection, and straight detection.
Precipitation and Detection Screen
Made up of two sections: the precipitation section is designed to ‘break’ any dormant defect in the product by applying higher than normal stress to the product. The detection portion is designed to detect any failure by applying stress at a level the product can be functionally tested at.
A straight detection screen is designed to detect the failure by applying stress at a level the product can be functionally tested at. Typically 2-3 temperature cycles with simultaneous vibration are conducted.
Our engineering consultants will work with your company to establish which type of screen is best suited to your application. There are many points to consider, and the results attained during HALT will be heavily leveraged, in order to provide the best possible base for screen development.
Once the screen type has been decided upon it is time to ensure the products response to the applied stimulus is acceptable. Your product will be secured inside the HALT chamber as it would be during production screening. It is essential that any fixturing material be lightweigh, rigid, and as thermally translucent as possible.
The product is connected to all functional test equipment, as it would be in production. Thermocouples and accelerometers are connected to each unit to monitor the individual unit response to the input stimuli. The HASS begins and functional testing is carried out throughout the screen. In addition, temperature and vibration recordings are attained to ensure each product is receiving uniform response. If screen tuning, or fixture tuning is required then this process is repeated until the desired response is attained.
The defined screen will next be run on production hardware to ensure that no overstress failures occur. This process is called Proof Of Screen (POS).
HASS – Proof Of Screen (POS)
Proof Of Screen is an essential part of the HASS Development process. Proving the HASS screen is not too strong to overstress ‘good’ hardware is the number goal of this stage. Once an appropriate screen is defined it must be run repetitively on known good production hardware a minimum of 15 times to ensure that good hardware will not be damaged during production screening.
Proof Of Screen – The Process
After deciding which screen type to apply during production it becomes necesary to prove that the screen will not damage ‘good’ production samples. The product will be loaded into the HALT chamber and configured as it would be if testing was being conducted during production. All fixturing should, ideally, be identical to that to be used in production.
The product is bolted to the vibration table using an appropriate fixture and all functional test equipment is connected before the Proof Of Screen begins. The proposed HASS screen is repeated a number of times. Each unit is tested throughout each cycle, and all failures must be driven to root cause in order to understand whether the failures are occurring due to overstress – or are actually dormant failures that would occur in the field, over time.
Why conduct Proof Of Screen?
After conducting Proof Of Screen you will have confidence that by running production hardware through the proposed HASS screen, no more than a minimum of 1/nth of the products overall life will have been used up. Determining the products life is not possible using HALT & HASS, and therefore standard reliability tests may be required to ensure this is an acceptable amount of life to lose.
HASS – Production Screening
During production it is essential to monitor build quality, and reliability continuously. Traditional testing does not stress components to a high enough level to ensure that dormant defects are found during the manufacturing process.
Creating a successful production screening process requires information from a range of departments within an organization. It must be supported by Management, and driven by a team able to control, organize, record, and respond to any issues identified during testing.