Most growth does not amount to much below 125☌, but at 150☌ the intermetallic growth is logarithmic to the temperature increase. The growth as a solid is driven by temperature as the solder approaches its melting point, the rate of intermetallic growth increases. This intermetallic will continue to form at a slower rate in the solid phase. During soldering, the intermetallic crystals are precipitated out. As the melting temperatures of these are significantly higher than soldering temperatures, they will not be melted during the reflow process and thus will remain in the joint as occlusions. When soldering to a copper substrate with tin/lead solder, two types of IMCs are formed. The copper-rich Cu 3Sn intermetallic, which melts at 670☌, is found near the substrate, while the tin-rich Cu 6Sn 5, which melts at 415☌, is found throughout the joint. As discussed below, it should be noted that IMCs are a problem of soldering and can lead to long-term failures. However, as most frequently used in soldering, the term “intermetallic” is often used as an abbreviation for intermetallic crystal (IMC), which refers to a specific class of materials. Simply explained, an intermetallic is a combination of the base metals with the solder that provides the mechanical and electrical integrity of the solder joint. Reflow Dynamics- Intermetallics. The goal of the soldering process is to form an intermetallic layer between the solder and the circuit board and the solder and the component. In general, a faster cool down rate will result in a finer grain structure and a stronger and shinier solder joint. However, exceeding 4☌ per second could result in thermal shock to the assembly. The cool down rate of the profile should be controlled within 4☌ per second. It should be noted that this time above liquidus is measured not only during the profile’s rise, but also during its cool-down. The assembly is reflowed above the liquidus point of the solder (183☌ for Sn63, 179☌ for Sn62) for a target time of 60 seconds ± 15 seconds. For the Sn63/Pb37 and Sn62/Pb36/Ag2 alloys, the typical profile length is 3 ½ to 4 minutes from the time the assembly begins to heat up until it reaches its maximum temperature of 215☌ ± 10☌. While varying reflow profiles may be utilized to achieve optimum soldering results, in general these share many of the same details. The guidelines for soldering to various surfaces and with alternative solder alloys also are discussed. This paper shall provide a brief outline of the reflow profile in general, with specific emphasis placed upon the suggested time spent above the melting temperature of the solder. What is clear is that the pains often associated with profiling can be reduced if there is a strong understanding of the variables that can be encountered during the reflow process, as well as the metallurgical dynamics of the soldering process. Despite much research and discussion on the subject of reflow profiling, many questions and a good deal of confusion still exist.
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