barcode scanner input asp.net Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use. in Software

Encoding QR in Software Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.

Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
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FIGURE 56.1 AT&T test vehicle compared: (a) double-layer glass reinforcement; (b) single-layer glass reinforcement; (c) single-layer glass reinforcement with extra buttercoat. 2 1976 IEEE
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The second mode involved shorts between conductors on the same side of the board in which conductive material accumulates between the glass bundles and the epoxy (see Fig. 56.3). R. H. Delaney and J. N. Lahti2 noted that the thicker the buttercoat, the less this failure was observed. They also detected a failure, which they termed anodic eruption failure mode, in which corrosion
Subsurface substrate failure site
Through-substrate short
FIGURE 56.2 IEEE Through-substrate short.2 1976 FIGURE 56.3 Subsurface substrate failure.2 1976 IEEE
CONDUCTIVE ANODIC FILAMENT FORMATION
Eruption products emerged from the anode to the covercoat surface, charring the surface, and then growPlume ing back through the covercoat to the cathode, where it shorted (see Fig. 56.4). In 1976,Aaron Der Marderosian at Raytheon, while studying measling, crazing, and delamination, examined the reliability of multilayer PWBs by applying a DC voltage between ground planes and conductor traces (see Fig. 56.5).3 Test coupons were biased at 100 V and aged at 65 C and 95 percent RH for 10 days. Based on the results, Der Marderosian reported a failure that Early stage he termed the punch thru phenomenon. This is similar to the thru substrate shorts reported by Delaney.2 To study this failure further, Der Charring of Marderosian obtained test coupons from three covercoat & different vendors biasing some at 100 V DC, failure others at 100 V AC, and a third group was unbiased. Since punch thru was observed only when DC bias was applied, Der Marderosian concluded that this failure was due to electrochemically initiated metal migration. He reported that the number of incidents of punch thru decreased as aging voltage was decreased. The addition of a urethane conformal coating appeared to accelerate, not suppress, the problem. Late stage Punch-thru is an electrical failure that FIGURE 56.4 Anodic eruption failure mode.2 eventually manifests itself in a rupture of the 1976 IEEE insulation between two layers of copper metallization. In the early stages, Der Marderosian observed conductive copper-containing deposits that he assumed were CuO along the glass fibers. Eventually these deposits shorted to the cathode, carbonizing the epoxy, which caused
(+) biased copper track CuO
Copper ground plane ( )
FIGURE 56.5 Raytheon test coupon with punch thru from anode trace to copper ground plane.3
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the epoxy to be more conductive. Epoxy blowout then ruptured the glass fibers. In the later stages, Der Marderosian observed melting of the metal traces. Equation 56.1 represents Der Marderosian s concept of the production of CuO and Cu(OH)2 at the anode. At the cathode, reduction takes place yielding copper and hydrogen gas. He noted that copper hydroxide decomposes to copper oxide above 60 C. 3Cu + 3H2O = CuO + Cu(OH)2 + 2H2 + Cu anode cathode (56.1)
D. J. Lando et al.4 first used the term conductive anodic filament (CAF) to describe this failure 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). Anode: Cathode: Cu = Cun+ + ne H2O = 1/2 O2 + 2H+ + 2e 2H2O + 2e = H2 + 2OH H2O + 1/2 O2 + 2e = 2OH n+ Cu + ne = Cu
(56.2)
In 1980, T. L. Welsher et al.5 reported that CAF was potentially a serious reliability problem for closely spaced conductors and proposed a mean time to failure (MTTF) based on Eq. 56.3: E L2 MTTF = a( H )b exp a + d V RT where H = humidity Ea = activation energy R = gas constant T = temperature in Kelvin L = conductor spacing V = voltage a, b, Ea, d = material-dependent (56.3)
Welsher et al. noted that additional work was needed to determine the exact dependence of CAF on conductor spacing and humidity. They also reported that glass-reinforced triazine is CAF-resistant. In 1981, they expanded their view of the MTTF and reported it as shown in Eq. 56.4: Ln E 1 + H exp a V kT where a, b = material-dependent constants g = humidity-dependent constant n = related to the orientation of the conductors L = spacing V = voltage H = humidity Ea = activation energy k = Boltzman s constant T = temperature in Kelvin (56.4)
W. J. Ready and L. J. Turbini6 studied the effect of voltage and spacing on CAF failures for a hole-to-hole test pattern (see Fig. 56.6). Using two different spacings (0.50 mm and 0.75 mm)
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