Cold Step Stress

Stress Test Background

Cold Step Stress is a stress test which subjects the unit under test to reducing temperature in steps. It is one of the conventional stresses being used in HALT for defect precipitation for the purpose of design margins improvement. A typical stress profile is shown in Figure 1.

Stress Profile Description

Typically starts at room temperature. The temperature is typically reduced by fixed decrements to a pre-defined minimum temperature providing with dwell time at each temperature step. Functional tests are performed at each temperature step either at specific times or continually. The ability to detect failures will depend on the functional test coverage.

Parameters Determination

Step size

Step size is the temperature difference between two adjacent temperature set points. Step size should be determined by the accuracy required for failure point definition and the time available for testing. 
(Reduced step size as failure point is reached is appropriate, which allows a fine definition without compromising test duration)

Dwell time

Dwell time is the time period for which the temperature remains constant at the temperature set points. Dwell time should be determined by stabilization time of the unit and the functional testing duration. Different thermal stability criteria can be selected for different purposes.

For design improvement purposes (HALT), more stringent thermal stability criteria are more appropriate. For example, an entire unit should reach a target temperature for a unit to be considered thermally stable. This allows defects at unexpected locations be uncovered so design margins can be improved effectively.

For screening purposes (HASS, ESS), less stringent thermal stability criteria may be preferred. For example, locations of interest should reach target temperature for a unit to be considered thermally stable. This allows expected defects can be precipitated over a shorter span of time and without life of other parts/ locations being over-consumed by the long stabilization time.

Target stress level in this case (T_min)

Target stress level is the stress level to be reached for the purpose of the stress test to be achieved. Therefore, it should be determined based on the purpose of defect precipitation.

For design improvement purposes (HALT), stress levels should be high enough to precipitate defects that can cause failures below the target stress limits or before the desired lifetime.

For screening purposes (HASS, ESS), target stress levels should be set in a way that is high enough to precipitate defects, but also allow sufficient remaining lifetime of the good population of products.

Standards with Stress Test Description

For Design Improvement Purpose

- IPC 9592A: Requirements for Power Conversion Devices for the Computer and Telecommunications Industries

- IEST RP PR 003.1: HALT and HASS

- GMW 3172: Highly Accelerated Life Testing (HALT) Highly Accelerated Stress Screening and Auditing 
 

For Testing/ Acceptance Purpose

Potential Defects Precipitated by this Stress