Highly Accelerated Stress Screening (HASS)
HASS screening is performed as an integral part of the production process. The idea is to expose products to increased stress levels to cause those with defects to fail during screening rather than in the infant period of the life cycle. Figure 5 is a product ‘bathtub curve’, which shows how failure rates vary over the lifetime. Machines are like humans: the higher the stress the higher the infant mortality, higher sickness rate and shorter life.
HASS involves a trade-off: while the aim is to reduce failures in actual use, the screening process itself shortens the product’s lifetime slightly because the application of stress contributes to wear-out. Therefore, it is important to carefully consider the balance between benefits and drawbacks when planning HASS. Is it worth using up two months of a ten-year lifetime, for example, if this will eliminate x% of the product’s infant mortality failures? HASS is naturally most beneficial for products with high infant mortality failure rates. Downsides of HASS are that the screening costs are high and production throughput times are increased.
As a practical example of how we use HASS on ABB drives, test cabinets are used to screen main circuit boards and gate driver control boards. The boards are connected to a power supply which is cycled during screening. They are tested several days at temperatures exceeding maximum operational temperatures. This reproduces the stress experienced in a few weeks of normal operation.
Ongoing Reliability Testing (ORT)
The purpose of ORT is to ensure that no changes have occurred in components or production processes that will have a systematic impact on reliability. The methodology is similar to RDT/ALT testing, but the sample units are randomly selected from actual production. In the case of ABB’s drives, testing can be performed on drive modules, IGBT drive packages, PCB boards and even complete drives. Depending on the risk level that has been set, samples are taken from production each week or month. If a failure occurs during testing, a thorough root cause analysis is carried out to check whether the failure was a random event or due to a change in the product or one of the components.