Failure Analysis

CALCE has extensive experience in performing failure analysis on a wide range of parts and products (ball grid arrays, printed wiring boards, photodiodes, plastic encapsulated microcircuits, connectors, flip-chips, etc.) and identifying a wide range of failure mechanisms (conductive filament formation, solder joint fatigue, electrostatic discharge, etc.).

It is CALCE's philosophy that failure analysis is a systematic examination of failed devices to determine the root cause of failure and to use such information to eventually improve product reliability. Failure analysis is designed to: identify the failure modes (the way the product failed); identify the failure site (where in the product failure occurred); identify the failure mechanism (the physical phenomena involved in the failure); determine the root cause (the design, defect, or loads which led to failure); and recommend failure prevention methods

The process begins with the most non-destructive techniques and then proceeds to the more destructive techniques, allowing the gathering of unique data from each technique throughout the process. This data when properly analyzed leads to a viable mechanism for the failure. The use of destructive techniques early in the process is discouraged as it can result in the loss of valuable information that might be required later. The recommended sequence of procedures is:

· Visual Inspection

· Electrical Testing

· Non-Destructive Evaluation

· Destructive Evaluation (using relevant techniques)

To increase the reliability, however, this information must be coupled with the results of failure mechanism modeling. The information provided by these physics-of-failure (PoF) models allows designers to select materials and package design attributes, which minimize the susceptibility of future designs to failure by the mechanisms identified in the degradation assessment. In addition, it allows the user to select environmental and operational loads that minimize the susceptibility of the current design to failure during storage or use.

The identification of the important failure mechanisms and failure sites in fielded assemblies also permits the development of a focused accelerated test program. The benefits of a focused accelerated test program are that it allows the proper test stresses (e.g., temperature, relative humidity, temperature cycling) and the levels of those stresses to be selected so as to cause wearout failure in the shortest time without changing the failure mechanism. This is a vast improvement over the old method of choosing a random set of test loads and levels, or subjecting the assemblies to a set of "one size fits all" standard tests prescribed by decades-old military and commercial standards. In addition, the failure distribution in the accelerated tests can be converted to a failure distribution in the intended use environment using the acceleration factors calculated by the PoF models.

This combination of systematic failure analysis and degradation assessment, failure mechanism modeling, and accelerated qualification and life testing can be used to qualify new designs, determine expected lifetimes, or determine the remaining life of an assembly which has been stored or used. This unique process was pioneered at the CALCE Electronic Packaging Research Center. Described as a reliability growth methodology, it consists of three steps:

·conducting an experimental degradation analysis of assemblies which have been stored and/or operated for several years to identify the potential failure mechanisms and to characterize the extent to which degradation has progressed during this period;

·calculating acceleration factors relating the failure distribution under accelerated test conditions to the failure distribution under actual use/storage conditions. These acceleration factors are determined using fundamental stress and damage models (physics- of-failure models) for the failure mechanisms identified in the degradation analysis; and

·determining failure distributions for the elements of the assemblies under accelerated test conditions consisting of appropriate loads and levels to produce maximum test time compression without altering the dominant failure mechanisms.