Six Sigma and Beyond: Design for Six Sigma, Volume VI

AST/PASS

HALT (Highly Accelerated Life Test) and HASS (Highly Accelerated Stress Screens) are two types of accelerated test processes used to simulate aging in manufactured products. The HALT/HASS process was invented by Dr. Gregg Hobbs in the early 1980s. It has since been used with much success in various military and commercial applications. The HALT/HASS methods and tools are still in the development phase and will continue to evolve as more companies embrace the concept of accelerated testing. Many companies use this type of testing, which they call AST (Accelerated Stress Test) and PASS (Production Accelerated Stress Screen).

The goal of accelerated testing is to simulate aging. If the stress-strength relationships are plotted, the design strength and field stress are distributed around means. Let us assume the stress and strength distributions are overlapped (the right tail of the stress curve is overlapped with the left tail of the strength curve). When that happens, there is an opportunity for the product to fail in the field. This area of overlap is called interference.

Many products, including some electronic products, have a tendency to grow weaker with age. This is reflected in a greater overlap of the curves, thus increasing the interference area. Accelerated testing attempts to simulate the aging process so that the limits of design strength are identified quickly and the necessary design modifications can be implemented.

PURPOSE OF AST

AST is a highly accelerated test designed to fail the target component or module. The goal of this process is to cause failure, discover the root cause, fix it, and retest it. This process continues until the "limit of technology" is reached and all the components of one technology (i.e., capacitors, diodes, resistors) fail. Once a design reaches its limit of technology, the tails of the stress-strength distribution should have minimal overlap.

The AST method uses step-stress techniques to discover the operating and destruct limits of the component or module design. This method should be used in the pre-prototype and/or pre- bookshelf phase of the product development cycle or as soon as the first parts are available. Let us look at an example:

We want to discover the operating and destruct limits of a component/module design for minimum temperature. The unit is placed in a test chamber , stabilized at -40 °C, then powered up to verify the operation. The unit is then unpowered, the temperature lowered to -45 °C and the unit allowed to stabilize at that temperature. It is then powered on and verified . This process is repeated as the temperature is lowered by 5 ° increments .

At -70 °C, the unit fails. The unit is warmed to -65 °C to see if it recovers. Normally, it will recover. The temperature of -65 °C is said to be its operational limit. The test continues to determine the destruct limit. The limit is lowered to -75 °C, stabilized, powered to see if it operates, then returned it to -65 °C to see if it recovers. If when this unit is taken down to -95 °C and returned to -65 °C, it does not recover, the minimum temperature destruct limit for this module is determined to be -95 °C. The failed module is then analyzed to determine the root cause of the failure.

The team must then determine if the failure mode is the limit of technology or if it is a design problem that can be fixed. Experience has shown that 80% of the failures are design problems accelerated to failure using the AST or similar accelerated stress test methods.

AST PRE-TEST REQUIREMENTS

Before AST is run on a product, the product development team should verify that:

The product development team will provide the component/module monitoring equipment used during AST and will work with the reliability engineer to define what constitutes a "failure" during the test.

OBJECTIVE AND BENEFITS OF AST

The objective of AST is to discover the operational and destruct limits of a design and to verify how close these limits are to the technological limits of the components and materials used in the design. It also verifies that the component/module is strong enough to meet the requirements of the customer and product application. These requirements must be balanced with reasonable cost considerations. The benefits of AST include:

The failure modes from the AST and PASS are used by the manufacturing team to ensure that they do not see any of these problems in their products.

PURPOSE OF PASS

PASS is incorporated into a process after the design has been first subjected to AST. The purpose of PASS is to take the process flaws created in the component/module from latent (invisible) to patent (visible). This is accomplished by severely stressing a component enough to make the flaws "visible" to the monitoring equipment. These flaws are called outliers, and they result from process variation, process changes, and different supplier sources. The goal of PASS is to find the outliers, which will assist in the determination of the root cause and the correction of the problem before the component reaches the customer. This process offers the opportunity for the organization to eliminate module conditioning and burn-in.

PASS development is an iterative process that starts when the pre-pilot units become available in the pre-pilot phase of the product development cycle. The initial PASS screening test limits are the AST operational limits and will be adjusted accordingly as the components/modules fail and the root cause determinations indicate whether the failures are limits of technology or process problems. The PASS also incorporates findings from process failure mode and effect analysis (PFMEA) regarding possible "significant" process failure modes that must be detected if present.

When PASS development is complete, a strength-of-PASS test is performed to verify that the PASS has not removed too much useful life from the product. A sample of 12 to 24 components is run through 10 to 20 PASS cycles. These samples are then tested using the design verification life test. If the samples fail this test, the screen is too strong. The PASS will be adjusted based on the root cause analysis, and the strength-of-PASS will be rerun.

OBJECTIVE AND BENEFITS OF PASS

The objective of PASS is to precipitate all manufacturing defects in the component/module at the manufacturing facility, while still leaving the product with substantially more fatigue life after screening than is required for survival in the normal customer environment. The benefits of PASS include:

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