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90 SMT007 MAGAZINE I JULY 2019 The considerations necessary to achieve good solder joints are (Figure 1): • The formation of an intermetallic layer • Solder joint structure • Joint temperature (military standard) • Tip temperature vs. joint temperature • Maintenance of the soldering profile (similar to that found in a reflow oven) When copper comes in contact with molten solder, it forms two distinct intermetallics between the copper and the tin contained in the solder (Figure 2): • Layer of "e-phase" (Cu3Sn) next to copper • Layer of "h-phase" (Cu6Sn5) a thicker layer above Tin is depleted by the formation of interme- tallics, so in tin-lead solders, there will be a resultant lead-rich region. A thin, intermetallic layer is necessary to produce wetting; however, thicker intermetal- lic layers may alter the appearance of the joint and have an adverse effect on its integrity. Some of the reasons are: • Intermetallics are frequently brittle, and differences in CTE (coefficient of thermal expansion) between intermetallic and solder can contribute to internal stress • Depletion of one element of the surface may impair solderability Nevertheless, without an intermetallic layer, there is no valid soldering joint, and once created, the layer grows at any temperature and accelerates exponentially as temperatures increase. This growth continues until the inter- metallic compounds (the base metal) occupy the entire joint, and/or the solder is exhausted. The rules for good solder joints are: • Solder as quickly as possible • Use the lowest possible soldering temperature that yields acceptable joints • Avoid repeated soldering to improve the appearance of the joint since added exposure to high temperatures only increase the intermetallic layer. The joint may look pretty but is weaker or stressed • Remember that the intermetallic layer grows at any temperature but accelerates exponentially at elevated temperature levels. Rates of dissolution of various metals also will rise with increases in temperature (Figures 3 and 4) Reliable Solder Technique Having established what considerations are necessary to form a reliable solder joint, the next step is to understand how to achieve this and what processes should be followed. While much of the focus in soldering is placed on the Figure 1: Components of a good solder joint and their relative placement during the creation of a solder joint. Figure 2: Cross-section showing the two distinct intermetallics between the copper and tin.

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