Anodic Filament Formation

Heat transfer Test conditions Wave solder, face-up Float Reflow, float  

A model developed by Bell Labs researchers (Ref 22) in the late 1970s (Ref 23) details the mechanism by which CAR formation and growth occurs. The first step is a physical degradation of the glass/epoxy bond. Moisture absorption then creates an aqueous medium along the separated glass/epoxy interface that provides an electrochemical pathway and facilitates the transport of corrosion products. 

CAR can be observed microscopically on double-sided PWBs using back lighting (Rig. 12). Subsequent scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM/EDS) analysis shows that it contains copper and chloride (Fig. 13). A CAR sample was extracted and transmission electron microscopy (TEM) analysis was used to obtain electron diffraction data. Analysis of these results identify CAR as atacamite, Cu2(OH)3Cl (Ref 24). The Pourbaix diagram (Ref 25) (Fig. 14) of the cop- 

Substrate Material Choice. FR4 was compared with several substrates G-10 (a non-fire retardant epoxy/woven glass material), polyim-ide/woven glass (PI), triazine/woven glass. 

To ensure immunity to CAF, the laminate of preference is BT. However, there is a cost penalty to consider. Newer CAF resistant materials have appeared in the past few years. A number of these materials have been examined, as well 

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Lowe N. Removing paste and adhesive. Electronic Packaging and Production. Aug. 1999. Bent WR, Turbini LJ. Evaluating the effect of conformal coatings in reducing the rate of conducive anodic filament formation, Proc. APEX. San Diego, CA Jan. 2002. 

Bent, W. R., and Turbini, L. J., Evaluating the Effect of Conformal Coatings in Reducing the Rate of Conducive Anodic Filament Formation, Proc. Apex, San Diego, CA (Jan. 2002)... 

Ready, W. I, and Turbini, L. I, The Effect of Flux Chemistry, Applied Voltage, Conductor Spacing, and Temperature on Conductive Anodic Filament Formation, Journal of Electronic Materiab, Vol. 31, No. 11,2002, pp. 1208-1224. 

W.J. Ready and L.J. Turbini, The Effect of Rux Chemistry, Apphed Voltage, Conductor Spacing, and Temperature on Conduc-hve Anodic Filament Formation, J. Electron. Mater., Vol 31 (No. 11), 2002, p 1208-1224... 

The ability of a material to resist water absorption, either from the air or when immersed in water, is important for printed circuit rehabUity. Besides the obvious concerns of moisture causing defects when a material is subjected to thermal excursions, absorbed moisture also affects the ability of a material to resist conductive anodic filament (CAF) formation when a... 

Conductive anodic filament (CAF) formation is a term used to describe an electrochemical reaction in which conductive paths are formed within a dielectric material due to transport of metal or metal salts through the dielectric. These paths may form between two circuit traces, between two vias, or between a trace and a via, as illustrated in Fig. 9.12. CAF formation between a hole and a plane inside the PCB is also possible, and is similar in concept to hole-to-trace CAF formation. By definition, as circuit density increases, the space between these features decreases. With shorter paths between features, CAF growth becomes a more critical reliability consideration. 

Conductive anodic filament (CAF) As the base material system degrades, the resistance to CAF formation decreases. 

D. J. Lando et al. first used the term conductive anodic filament (CAF) to describe this faUtrre in 1979. They defined the mechanism of CAF formation as a two-step process degradation of the epoxy/glass interface, followed by the electrochemical reaction (see Eq. 56.2). 

Lando,D.X, Mitchell, IP., andWelsher,T. L., Conductive Anodic Filaments in Reinforced Polymeric Dielectrics Formation and Prevention, IEEE Reliability Physics Symposium Proceedings, Vol. 17, 1979, p. 51-63. 

Ready, W. 1.,Factors Which Enhance Conductive Anodic Filament (CAF) Formation, Master Thesis in Materials Science and Engineering, Georgia Institute of Technology, 1997. 

D. J. Lando, J. P. Mitchell, and T. L. Welsher, Conductive Anodic Filaments in Reinforced Polymeric Dieletrics Formation and Prevention, 17th Annual Proceedings of IEEE Reliability Physics Symposium, San Irandsco, April 24—26,1979, pp. 51-63. 

In electrochemical migration, failure can occur due to dendrite growth, open circuit short, or conductive anodic filament (CAP) formation. Dendrites can form on the surface due to con-taminahon left by the solder flux (paste) or other residues. Under a bias voltage, the metal at the anode goes into solution, migrates toward, and plates-out at the cathode. The nature of the dendrite will depend upon the surface metallization. The following oxidation reactions can occur at the anode ... 

S. R. Stock, Analysis of Catastrophic Failures Due to Conductive Anodic Filament (CAF) Formation, Electronic Packaging Materials Science X, Vol 515, D.J. Belton, M. Gaynes, E.G. Jacobs, R. Pearson, and... 

Filiform corrosion is characterized by formation of interconnecting filaments of corrosion under a paint film upon exposure to a humid environment. Filiform corrosion typically occurs only when the relative humidity exceeds about 65%. The mechanism is complicated and has been the subject of considerable discussion in the literature (, 1A, 2 1, 2, 24). Basically, a localized corrosion mechanism is responsible (Figure 8). The head of the growing filament is anodic, and as a result the filiform corrosion process has been termed a specialized form of anodic undermining... 

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