RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
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TABLE 57.1 Standard Electromotive Force Potential (Reductions Potentials) for Elements Commonly Found in Electronic Assemblies Standard potential (Volts vs. standard hydrogen electrode) +1.498 +1.358 +1.229 +1.2 +0.799 +0.771 +0.401 +0.337 +0.15 0.000 0.126 0.136 0.250 0.440 0.744 0.828 2.714 2.925
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Reaction Au3+ + 3e = Au Cl2 + 2e = 2Cl O2 + 4H+ + 4e = 2H2O (pH 0) Pt3+ + 3e = Pt Ag+ + e = Ag Fe3+ + e = Fe2+ O2 + 2H2O + 4e = 4OH (pH 14) Cu2+ + 2e = Cu Sn4+ + 2e = Sn2+ 2H+ + 2e = H2 Pb2+ + 2e = Pb Sn2+ + 2e = Sn Ni2+ + 2e = Ni Fe2+ + 2e = Fe Cr3+ + 3e = Cr 2H2O + 2e = H2 + 2OH Na+ + e = Na Active K+ + e = K Noble
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Source: A. J. deBethune and N. S. Loud, Standard Electrode Potentials and Temperature Coefficients at 25C, Clifford A. Hampel, Skokie, Ill., 1964.
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Once started, they may grow as fast as 1 mm per month. The tendency toward whisker growth is influenced by a variety of factors including plating conditions and the characteristics of the substrate. Growth may be inhibited by a Cu or Ni barrier layer beneath the tin coating. Pb seems to suppress whisker growth; eutectic Sn-Pb solder is considered almost immune. Whiskers do not cause the corrosion resistance or solderability of the tin coating to deteriorate, so tin may be used as a temporary finish. To avoid whiskering, plated pure Sn should not be used on closely spaced conductors that could short during service, such as connector terminations or component leads.13,14 57.2.2 Interconnect Failure Mechanisms 184.108.40.206 Thermally Driven Failure Mechanisms.
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Thermal Fatigue of Solder Joints. Thermal fatigue of solder joints has been extensively researched in the last decade. The mechanism of fatigue, accelerated testing methods, and methods for predicting life have all been described at length, although there is still much controversy about many of the details.15 17 These references also illustrate how modern finite element methods can be used to model the strains in the solder under both operating and accelerated test conditions. This section briefly reviews some of the important principles underlying solder joint thermal fatigue. The focus of the discussion is on surface-mount solder joints, which have been extensively researched; however, many of the same principles also apply to through-hole solder joints. Through-hole joints are generally less prone to solder joint fatigue failures, so long as the through-hole barrel is full of solder. Ideally, complete fillets should be observed on both surfaces of the board. Reviews of this topic can be found in Refs. 18 and 19. Thermal fatigue in solder joints occurs because of the thermal expansion (CTE) mismatch between the PCB and the component interconnected by the solder joint (see Fig. 57.12).
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FIGURE 57.11 Tin whisker growth on a tin-plated surface. From Ref. 11. (From IPC-TR-476. Courtesy of Burndy Corporation.)
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The imposed thermal cycle T results in an imposed cyclic strain e of the solder joint, which is generally the weakest part of the system. The relationship is simple under the assumptions that the part and substrate are rigid, the solder joints are relatively small, and that homogeneous shear deformation caused by the global CTE mismatch predominates: e = ( T)( a)l h (57.1)
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2l Component ( 1) h Substrate ( 2)
Low temperature High temperature = 2 1; 2 > 1
FIGURE 57.12 Schematic illustration of strains imposed on solder joints to leadless and leaded surface-mount components during a thermal cycle. Although the relative displacement of the substrate and component body is the same, the strain on the joint is reduced in the leaded case by the deflection of the lead.