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FIGURE 57.9 Migration of corrosion products across the surface of FR-4 bridging two conductors. From Ref. 11. (From IPC-TR-476. Courtesy of Department of Defense.)
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FIGURE 57.10 Transmitted light micrograph through a PCB that failed in the field. Dendritic growth has formed at the interface of a UV-cured screened solder mask and the FR-4 surface.
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cathode. The dendrite resembles a tree, since it consists of a stalk with branches. When the growth touches the other conductor there is an abrupt rise in current, which sometimes destroys the dendrite but may also cause an electrical circuit to temporarily malfunction or damage a device. It has been proposed that the absorption of moisture produces an electrochemical cell.The following electrode reactions for Cu are an example: At the anode: Cu Cun+ + ne H2O 1 2O2 + 2H+ + 2e At the cathode: H2O + e 1 2H2 + OH
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where the majority of the leakage is due to the electrolysis of water. Copper metal is dissolved at the anode and migrates to the cathode, where it is no longer soluble. The dendrite that forms follows the resulting pH gradient.10 The voltage difference between the cathode and anode also affects the rate of dendrite growth. When the cathode and anode are the same metal (e.g., Cu), the voltage difference is determined primarily by the applied bias, although the access of moisture and air also has an effect. Corrosion can be accelerated in a crevice because an oxygen concentration differential between the anode and the cathode develops. When the metals are dissimilar, galvanic corrosion may occur without a bias voltage. If there is an applied bias, dendritic growth will occur almost instantly if the cathode and anode are under water.A simple laboratory experiment can prove the point.A 6-V bias across two conductors is sufficient to induce rapid growth (readily observable with a low-power microscope) even when distilled or deionized water bridges the conductors, although growth will occur faster with tap water.11 Galvanic Corrosion. Galvanic corrosion occurs between dissimilar metals because they have differing affinities for electrons (i.e., they are more or less electronegative). Galvanic series have been compiled for many common metals and alloys (see Table 57.1). Metals near the top of the series (noble metals) do not corrode; those near the bottom corrode easily. When these metals are near each other, the more noble metal becomes the cathode, the less noble the anode. Moisture is required to couple the two metals electrically. Applied bias is generally not required, but may accelerate the reaction if the polarity is correct. When the anode is very small compared to the cathode, its corrosion can be very rapid. Conversely, if the anode is much larger than the cathode, corrosion is unlikely to be serious, particularly if the difference in electronegativity is small. Conductive Anodic Filament Growth. Conductive anodic filament growth (CAF) causes electrical shorts when a metal that dissolves anodically is redeposited at the interface between the glass (or other) fibers and the resin matrix of a printed circuit board. Conductive anodic filament growth is promoted by delamination at the glass-polymer interface, which may in turn be promoted by various environmental stresses including high temperatures (greater than about 260 C for FR-4) and thermal cycling. Shorts seem to occur most rapidly when a single fiber bundle connects two pads. Once delamination has occurred, the metal migration that causes shorts to occur is promoted by increasing temperature, relative humidity, and applied voltage. Small conductor spacings also significantly decrease times to failure.12 In multilayer boards, failures occur faster on outerlayers than innerlayers because the surface layer absorbs moisture more readily. By the same reasoning, solder mask and conformal coating both increase the time-to-failure because they slow the absorption of moisture from the atmosphere into the board. Whiskers. Whiskers are faceted filament-like structures that grow spontaneously on the surface of a plated metal and can cause shorts between closely spaced conductors (see Fig. 57.11). Whiskering can be differentiated from other causes of shorts such as dendritic growth, because neither an electrical field nor moisture is required for whiskers to form. Whiskering is a particular problem with pure tin. The whiskers grow in response to internal stresses in the plating or external loads. Sn whiskers are commonly 50 mm long and 1 to 2 mm in diameter.
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