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termination thickness is dissolved, the solder will dewet from the ceramic part, leaving an open joint if the entire termination is dissolved, or a substantially weakened one if it is dissolved only in some areas. Use of 63Sn-36Pb-2Ag substantially reduces this problem, since the presence of Ag in the solder reduces the dissolution rate of the Ag forming the termination. Finally, eutectic and near-eutectic Sn-Pb solders form intermetallics with the termination metals that influence the properties of the finished joints. On the most common termination metals, Cu and Ni, continuous intermetallic layers form: Cu3Sn and Cu6Sn5 on copper, and Ni3Sn4, Ni3Sn2, and Ni3Sn on nickel. These intermetallic layers are composed of compounds that are hard and brittle in comparison to both the solder and the termination metals. Although there have been few systematic comparisons made, it is general wisdom that when the intermetallic layers are very thick, solder joint reliability is reduced. While solder joint thermal fatigue mostly involves cracking in the solder, the intermetallics can affect their ability to withstand mechanical stresses, particularly in tension. The Ni-Sn intermetallics are particularly brittle. In every case, an effort should be made to minimize the total time the solder is molten during processing and to minimize the time above about 150 C once the solder joint is formed. 57.5.2.2 Other Solders. A number of other solders are used in specialized applications, including 80Sn-20Pb for lead finishes, 50In-50Pb for solder on thick Au, high-Pb solders such as 95Pb-5Sn and 97Pb-3Sn for flip-chip assemblies (usually on ceramic substrates), and lowtemperature solders such as 58Bi-42Sn and 52In-48In, where hierarchical soldering is desirable. Further information can be found in Refs. 23, 24, and 38. 57.5.2.3 Conductive Adhesives. Electrically conductive adhesives are used today for specialized applications such as connections to LCD displays and attachment of small resistors and capacitors. These materials consist of conductive particles, usually silver flakes or carbon, suspended in a polymer matrix, most commonly epoxy. The electrical resistance of the contact to the PCB tends to be unstable over time, so these materials are not suitable for applications requiring a constant, low-resistance contact. The primary failure mechanism is moisture migration through the epoxy to the interface, resulting in oxidation of the contact metal. Adhesion strength is also a reliability concern. New materials suitable for a broader range of applications are under development. Further information can be found in Ref. 39.
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Components Components and their packages influence many of the field failures of electronic assemblies. Packages are primarily designed and selected for their ability to protect the electronic components inside; for example, ceramic packages may be selected over plastic ones for their greater hermeticity.This section will discuss the ways in which package selection can influence solder joint and cleanliness-related failures. 57.5.3.1 Package Selection to Minimize Solder Joint Thermal and Mechanical Failures. Minimizing solder joint thermal and mechanical fatigue failures means minimizing the global and local mismatches in the system and introducing compliance that minimizes the stresses and strains transmitted to the solder joints. Figure 57.21 illustrates the important features of these systems. The following describes how component parameters can influence the incidence of solder joint failures. Surface-Mount Technology vs. Through-Hole Components. Although there is little compliance in the system, the reliability of through-hole solder joints generally exceeds that of surface-mount joints in thermal fatigue (assuming good solder fillets are present in both cases), because the loading geometry makes it difficult for a crack to propagate far enough to cause an electrical failure. However, the PTHs themselves may be susceptible to failure if they are exposed to even a few thermal cycles well above Tg.
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