Failure Analysis

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:

Electrical Testing is the measurement of all relevant electrical parameters and is a critical part of systematic failure analysis. Correct electrical testing not only provides the failure mode(catastrophic, functional, parametric, programming or timing), but it can also help identify the failure site.

Electrical Testing consists of detecting shorts, opens, parametric shifts, changes in resistance, or other abnormal electrical behavior on the die, between the die and the interconnects, between the interconnects and the circuit card, within the circuit card, and among the various connections between circuit cards. The extensive range of electrical testing equipment available to CALCE personnel allows the CALCE to be a world-leader in the failure analysis of electronic products.

Non-Destructive Evaluation: Non-Destructive Evaluation (NDE) is designed to provide as much information on the failure site, failure mechanism, and root cause of failure without causing any damage to the product or obscuring or removing valuable information. The latter part of this sentence can be very important when the electronic part is still functioning.

Destructive Evaluation (using relevant techniques) having completed the non-destructive analysis, the next step is to use destructive sample preparation techniques to reveal the internal structure of the sample. As much information as non-destructive evaluation (NDE) provides, destructive evaluation is often necessary to verify the failure mechanism and root cause.

Two initial techniques in destructive evaluation or electronic products are decapsulation/delidding and microsectioning. Chemical decapsulation consists of dissolving the plastic encapsulant using fuming nitric or sulfuric acid and delidding involves mechanically removing the lid from a hermetic package. Both decapsulation or delidding allow for internal examination of the die and interconnects by optical, electron, magnetic, or emission microscopy. Additional destructive evaluation can also be performed, using either focused ion beam imaging or transmission electron microscopy. These techniques permit detection of bond pad corrosion, passivation cracking, ball bond lifting, stress driven diffusive voiding, electromigration, metallization corrosion, and other failure mechanisms at the die level.

Microsectioning, also known as cross-sectioning, is performed to reach a surface which reveals an important feature of the sample, such as intermetallic formation in wire bonds or delamination at the fiber/epoxy interface in printed circuit boards. The cross-sectioned surface is often examined using optical microscopy, electron microscopy, and energy dispersive spectroscopy.

Mechanical testing, FTIR, contact resistance, and popcorn assessment.

Destructive evaluation methods:

• Microsectioning
• Decapsulation/Delidding
• Microtesting
• Optical Microscopy*
• Scanning Electron Microscopy (SEM)*
• Energy Dispersive Microscopy (EDS)*
• Focused Ion Beam (FIB) Imaging
• Scanning Magnetic Microscopy (SMM)*
• Transmission Electron Microscopy (TEM)
• Fourier Transform Infrared Spectroscopy (FTIR)*
• Contact Resistance Measurements*
• Assessment of Popcorning in PEMs

*These techniques are also performed during Non-Destructive Evaluation

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.