What is “HASS”?
A Highly Accelerated Stress Screen (HASS) is a process comprising a set of stresses performed to find potential defects on a product before it is shipped. The stresses are tailored to the product during the HASS Test Development process based on information gathered during the HALT testing process. The key to a successful HASS test is imposing just enough stress to expose defects without aging the product beyond what it is warranted for.
In today’s world of electronics, the reliability of components is getting better and better, and some of the old methods of in-spec screening and burn-in are not as effective as they once were. What is needed in today’s world of electronics and electromechanical devices is a more sophisticated method of screening whereby the screen is tailored and tuned to the product capabilities and weaknesses.
The purpose of Highly Accelerated Stress Screening (HASS) is to develop an appropriate manufacturing screen for the product to identify and eliminate infant mortalities.
VALUE TO YOUR ORGANIZATION
The more effective the manufacturing screen, the fewer field failures due to infant mortalities.
An example of Reliability Integration during HASS is as follows:
Using FMECA results to identify possible wear-out mechanisms that need to be taken into account for HASS
Certain wear-out failure modes are not easily detectable in HALT or even in HASS Development. Therefore, when wear-out failure modes are present, we must rely on the results of a FMECA to help determine appropriate screen parameters.
- HASS Plan: The HASS Plan will outline the process from start to finish, including the equipment trade-off analysis, fixture design, screen development, proof of screen, screen implementation, trend analysis, and the transition from HASS to HASS Auditing (HASA). The plan will serve as the roadmap and can be used as a decision tool during the implementation process.
- Choice of Environmental Equipment/Location of Test: Performance of HASS will probably require specialized environmental equipment. If your facility does not have this equipment, we can help perform the “return on investment (ROI)” analysis to determine if it makes sense to purchase the equipment or to use outside test labs.
If the ROI Analysis shows that it makes sense to purchase, we can help you choose the proper piece of equipment from the possible choices.
If the ROI Analysis shows that it makes sense to use an outside test lab, we can help you decide which lab will best fit your short-term and long-term needs.
- Fixture Design/Qualification: Once the environmental equipment is identified, we can help you determine the number of products that can be tested at one time to maximize throughput and then we can design a thermal fixture and vibration fixture (if vibration is being used) that will accommodate this throughput.
Once the fixture is designed and developed, it must be qualified to assure that it provides for uniform thermal airflow and uniform vibration energy (if vibration is being used) across all products.
- Development of Screen: After the fixture is complete and prior to the implementation of production HASS testing, Safety of HASS testing shall be performed. The purpose of this testing is to demonstrate that the chosen screen leaves products with sufficient life left in them to survive a normal lifetime of field use. Safety of HASS testing shall be accomplished by developing a HASS profile from the HALT results, and then taking one set of products (number of products to be screened at one time during Production HASS) and subjecting it to multiple passes of the proposed HASS. If a failure occurs, then the stress levels shall be reduced and the process repeated. This demonstrates that the selected HASS test leaves sufficient life in the product.
- Proof of HASS Strength: Some customers are also interested to determine if the chosen screen is strong enough to find defects. Because of the nature of HASS and the acceleration factors involved, this is usually assumed to be true. However, if proof is needed, then “No Problem Found (NPF)” samples from the field can be used, if available, to help confirm that the screen can find intermittent design issues.
Another method that can be used is to “seed” a product with defects to assure that the screen is able to detect these types of defects. This is analogous to the process of testing software by inserting bugs and then assuring that the test routines are able to find the bugs. Both of these methods tend to be difficult and may be inconclusive. Therefore, an even better method is introduce the screen as is, to monitor the performance of the product in the field and to then “tune” the screen based on the field results. One method of accelerating this process is to run a higher number of screens at the inception of the screen and to then back off this number based on the screen results and how many screen cycles are needed to find each type of defect.
- Implementation of Screen: Once the screen is developed and the location of the test is known, then we can run this screen at the designated location and deliver the units back for final checkout and shipment.
- Trend Analysis: One key method of this process is to monitor field performance to determine if the screen is missing any infant mortality failures, and to then refine or “tune” the screen. This is a continual process.
- Move from HASS to HASA – The HASA Plan: For a high volume product, as the volume increases and the defect rate decreases, a HASS audit or HASA can then be applied. The key parameters to monitor to determine if a HASA is appropriate is the stability of the process and if the defect rate is at its target. The first step is to write a HASA Plan. This will outline the parameters to decide on such as Detection Shift Level, Alpha and Beta Risk Levels, and Sample Size.
- HASA Implementation: Once the HASA Plan is approved, the process is under control, and the defect rate is at its target, we can implement HASA. We can run this Audit at the designated location and deliver the units back for final checkout and shipment.
- HASA Monitoring: A key step of the HASA process is monitoring the performance of the product and making decisions when to increase/decrease sample sizes. We can perform this on-going statistical analysis to assure that the HASA process is running smoothly.
The following case studies and options provide example approaches. We shall tailor our approach to meet your specific situation.
- Developing a HASS Profile and Proving the Screen Is Safe: A Computer manufacturer needed for us to help them develop a HASS profile that would be safe and to prove that there was sufficient life in the product. For them, we ran a Safety of HASS to assure that the screen was not damaging.
- Developing a HASS Profile and Optimizing the Screen: A Telecommunications manufacturer was having a lot of field issues and needed their screen to be optimized to assure that it was as strong as possible yet still safe. For them, we ran both a Safety of HASS as well as a Proof-of-HASS Strength to optimize their screen.
- Transitioning from HASS-to-HASA: A Medical Device company was a high volume manufacturer and decided early on that they needed to be running HASA. For them, we developed a HASS-to-HASA implementation strategy. First, we developed a screen using the Safety-of-HASS and Proof-of-HASS techniques. Then, during the low volume pilot production, we ran HASS. Then, as the product started approaching the volume ramp-up, we transitioned them from HASS-to-HASA.