What is the purpose of preconditioning before reliability testing?

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Purpose of Rel Test Preconditioning

Is preconditioning the same as SHRT?

Solder Heat Resistance Test (SHRT) is a reliability testing step performed to determine the ability of a device to withstand the thermal stresses of the board mounting or board soldering process. It is also known by other names, such as RTSH (Resistance to Solder Heat). See SHRT.

'Preconditioning' refers to SHRT that's performed on rel samples as a 'preparatory' step before the samples are subjected to a major reliability test (e.g., HAST, Temp Cycle, etc.). Preconditioning exposes the rel samples to thermally stressful conditions that are equivalent to thermal conditions experienced by units during board soldering.

The purpose of preconditioning is therefore to simulate the board mounting or board soldering process that a device undergoes before they are used in the field. The major rel test that follows preconditioning, on the other hand, simulates the thermo-mechanical stresses that the device experiences while operating in the field.

Thus, subjecting parts directly to reliability testing without preconditioning is like testing the reliability of parts that have not been mounted on the board. In other words, preconditioning must be done on samples prior to a major reliability test to reflect the actual 'use' conditions of the units on the customer side, i.e., they must first be mounted on boards (a thermally stressful process) before they are put into field operation.

Preconditioning ensures that inherently weak parts that are susceptible to popcorn cracking during board mounting will fail and consequently be detected. It will also aggravate marginalities within a package, such that these 'weaknesses' will translate into actual failures during the reliability test succeeding the preconditioning. For example, preconditioning can cause die-to-package delamination in weak parts, which in turn can lead to corrosion during HAST or neck breaks during TCT. Without preconditioning, these corrosion or neck break mechanisms might not occur during the actual HAST or TCT, since no delam was induced in the absence of precon.

Preconditioning generally consists of three distinct steps: 1) baking, which drives away all the internal moisture within the samples; 2) temperature/humidity soak, which drives controlled amounts of moisture into the package; and 3) some form of thermal shock, which simulates the board soldering process itself.

The bake and soak steps must be done according to industry standards since these two steps determine the level of internal moisture when the shock is performed. The thermal shock, which may be provided by a vapor cloud, infrared (IR) heating, convection heating, or a combination of IR and convection heating, must likewise conform to industry-standard requirements.