![]() ![]() The cooling rate should therefore be 3–4 ☌/s down to around 130 ☌. On the other hand, the components can crack if the temperature drops too rapidly. A lower rate may increase the grain size of the intermetallic compounds making the solder joint brittle and weak. To obtain a strong joint between the solder pad and the components' terminal, the cooling should be fast. In the cool down phase, a reliable solder joint is formed. It is important to monitor the time at peak reflow temperature in several areas of the PCB. The typical peak temperature is 30–40 ☌ higher than the solder alloy melting point. This soak time should be long enough to allow the flux to clean the surfaces, but not so long that the flux is evaporated prematurely.ĭuring the reflow phase, the temperature increases further to melt the solder alloy and subsequently, intermetallic layer will form at the boundary surfaces. The pre-flow or soak and the ramp-up section, also called the flux activation stage, brings the entire assembly up to the temperature at which the flux activating and the alloy in the paste changes its phase from solid to liquid. A typical preheat phase has a slope of 0.5–1.0 ☌/s. Second, the solder paste can slump, because of a too slow temperature rise. First, solder particles can be spread when the solvents burst through the flux surface membrane. If the temperature rises during the pre-heat too rapidly or two slowly, two failures can occur. During the pre-heat phase, the solvents evaporate from the solder paste. The soak profile can be divided into five zones: pre-heat, pre-flow or soak, ramp-up, reflow, and cool down. ![]()
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