Reliability of Solder Joints
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Reliability of Solder Joints D.R. Frear and F.G. Yost Introduction In early electronic technologies, circuit components were attached to circuit boards by mechanical means. The electrical leads were either twisted together or mechanically interlocked to a board prior to soldering. The possibility of an unreliable solder joint causing any kind of circuit failure was remote. Interconnections were made intrinsic to the board by applying solder to increase electrical and thermal conductance. Technological advances and the need for high-density electronics have since eliminated the luxury of mechanical interlocks. Soldering in advanced applications, like surface mount technology (SMT), provides electrical, thermal, and mechanical interconnections between the board and its electrical components. In SMT, solder joints are the only mechanical features on the board and must hold components in place in a wide range of environments. The solder joints themselves are decreasing in size as increased chip functionality and clock frequencies become available. The failure of a single solder joint can render a device, or an entire electrical system, inoperable.1 Therefore, as insignificant and innocuous as they may seem, solder joints have become a critical aspect of electronic circuit reliability. To highlight the critical nature of this reliability problem, five library databases were searched for technical papers containing the words solder, failure, and relia-
bility in any order of appearance and separated by no more than five words. Duplicate entries in the various databases were removed. The number of such citations in a given year were divided by the total number of papers recorded in the five databases that year. The results plotted versus citation year are shown in Figure 1. Over the past 25 years the normalized data trend is one of exponential growth. Since most profit-oriented corporations are reluctant to publish information on product failures, the data in Figure 1 is an underestimate of the real trend. If the number of such articles reflects the level of concern, solder joint reliability is generating
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Figure 1. Plot of the normalized number of citations in the literature containing the words solder, failure, and reliability in any order of appearance and separated by no more than five words over the total number of citations of technical articles published per year (in citations per million).
Aging Failures When molten tin or solder contacts a clean metal surface, the two materials react to form one or more IMCs.2 For example, when Sn-Pb solder is joined to Cu, the Sn and Cu react to form Cu3Sn and Cu6Sn5 at the interface as shown in Figure 2. The failure of solder joints under aging conditions depends on the growth kinetics of the intermetallic layer. The presence of an IMC indicates that a good metallurgical bond has formed, but the reaction layer can grow throughout product life and cause abrupt failure. The mechanical properties of a solder joint can be deg
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