A catastrophic failure leaves an assembly inoperable. Such failures, as shown in Figure 2-1, cause significant damage and are difficult to troubleshoot because evidence is often destroyed during failure.

Failures can occur at various stages of a product’s life cycle and are represented by the familiar bathtub curve shown in Figure 2-2. Defects will usually show themselves in the beginning and represent Infant Mortality. The Normal Life span of the curve represents random failures not product related (lightening, earthquakes, etc.). The End of Life part of the curve is the result of subtle, internal defects which develop over time.

Failures can happen at the board level or at the component level. A common board level failure is Conductive Anodic Filament (CAF). The result of this failure mechanism is a short occurring within the board causing varying damage depending upon the currents involved.

CAF is a form of electrochemical migration (ECM) occurring in PCB layers. The difference is that CAF happens within the PCB and the direction of the growth is from the anode to the cathode and ECM is just the opposite and occurs on the surface. (1) Because CAF occurs in the layers it is harder to observe. Tools like Ion Chromatography (IC) and Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) allow for indirect measurement of the potential causes of CAF while Surface Insulation Resistance (SIR) testing brings forth CAF by providing the right environment (temperature and humidity with a bias voltage) in the presence of resin ionic impurities. Table 2-1 gives the results of a study of board and laminate materials in which current leakages occurred with the PCBs. In the case of the PCBs, there were levels of bromide above recommendations. Bromide in freely ionized form (e.g., flux residues) can be detrimental, whereas organo-bromide used in laminate flame retardant is somewhat benign. The specific source for the bromide levels observed here was not identified; however, for a bare board, these levels appeared atypical.

The traditional electrochemical migration occurring on the surface is shown in Figure 2-3. Based upon the SEM/EDS analysis of this example, it was composed of tin (Sn), lead (Pb) and calcium (Ca)-chloride (Cl) salts.

At the board surface, ordinary corrosion is a problem (Figure 2-4). In this instance, a mixed process of water soluble and RMA flux residues with a compromised conformal coating resulted in corrosion of pad and lead areas on an assembly.
At the component level, you can have gross defects (Level I failure) where there is an obvious issue.

The failure in Figure 2-5 is a magnification of the tantalum capacitor which was the origin for the damage displayed in Figure 2-1. The root cause of this failure was not clear. Attempts to repeat the failure in a controlled environment indicated that such damage was due to a significant amount of current with a sustained burn.

Tantalum capacitors can fail from a number of mechanisms, including but not limited to:

1. Reverse biasing - Solid tantalum capacitors are polarized devices designed to operate only under forward voltage bias conditions. Such devices have been known to operate initially in reverse mode if a threshold voltage is not exceeded. According to A. Teverovsky (3), depending upon the type of capacitor, this threshold limit is between 15-25% of the rated voltages. Joule heating occurs from this mode of operation and is the ultimate reason for the mortality. As a result, operation of a tantalum capacitor in an AC-only circuit is not recommended (4).
2. Thermal stress. Operating temperatures will shorten life times and operating voltage ratings are temperature-dependent.
3. Ripple current. Such variations in current induce thermal stress and failure. Careful review of the application notes for recommended limits for ripple current is important.
4. Moisture. Despite best efforts, hermetic sealing of tantalum capacitors is not possible. As a result, moisture will increase capacitance and induce failures (5).

Another failure mode is mechanical stress. The most common example is mechanical stress through mismatched coefficient of thermal expansion (CTE) which will often lead to stressed solder joints. Out-gassing of components, particularly BGAs, is a good example of another type of mechanical failure. In this instance, absorbed moisture will out-gas during reflow, causing the often recognized popcorning effect, which results in stressed solder joints. The solution to this problem is baking the parts and board materials prior to assembly.

Preventing catastrophic failure
The first step is proper Design for Manufacturability (DFM). Ensure that raw materials and components meet the necessary criteria of the design and the end use requirements. Bread boarding a design will identify when components are operating at or above recommended limits or where current limiting is necessary.

Effective DFM also involves proper processing steps such as:

1. Cleaning of bare boards immediately after solderingsteps (do not allow flux residues to dry), and prior to storage. Use of high quality deionized water (Resistivity of 12MW or greater) with the proper chemistry is important.
2. Drying components prior to assembly.
3. Conformal coating of assemblies. As assemblies become more densely populated and line spacings become smaller, failures are more likely. Those closely packed components and traces are now protected by less and less dielectric board material thus increasing the likelihood of corrosion, shorts or electromigration.

Summary
Many of the problems mentioned so far do not show themselves until in the field and are often not noticed unless in use. As a result, a number of screening tools from recognized standards (JEDEC, IPC, ASTM, etc.) are available which address reliability concerns. These include, but are not limited to: temperature and humidity exposure, thermal cycling, highly accelerated stress test (HAST), high accelerated life test (HALT), vibration salt fog exposure, high temperature operation life (HTOL), moisture and insulation resistance, dielectric withstanding voltage, flexibility, chemical resistance, humid sulfur environment, and mixed flowing gas.

In the examples mentioned, a number of analytical tools can be used to troubleshoot catastrophic failures. The EMPF has a diverse range of on-site analytical tools and established partnerships to solve most problems.

References
1. Bob Neves, “Setup, Procedures, and Patterns for CAF and ECM Testing,” PC Fab, April 2002.
2. http://www.circuitree.com/CDA/ArticleInformation/features/ BNP__Features__Item/0,2133,79336,00.html
3. Alexander Teverovsky, Ph.D. “Reverse Bias Behavior of Surface Mount Solid Tantalum Capacitors,” CARTS 2002: 22nd Capacitor and Resistor Technology Symposium, 25-29 March 2002.
4. HITACHI application note: “Precautions in using Tantalum Capacitors.”
5. Alexander Teverovsky, “Effect of Moisture on Characteristics of Surface Mount Solid Tantalum Capacitors,” QSS Group, Inc./Goddard Operations NASA/GSFC, Greenbelt, MD 20771, Code 562.
6. IPC-HDBK-830, “Guidelines for Design, Selection and Application of Conformal Coatings.