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78 SMT007 MAGAZINE I JANUARY 2019 The Effect of Thermal Profiles on Cleanliness and Electrical Performance This month, my column will cover "The Effect of Thermal Profiles on Cleanliness and Electrical Performance." This paper was orig- inally published in the SMTA International proceedings of 2014 (and is reprinted here with permission), but the topic is timely and worth continuing the conversation here. Introduction The process of thermal profiling for reflow soldering is one of the most important consid- erations when setting assembly parameters. Knowing how to effectively profile includes choosing the proper equipment, understanding the results and being able to adjust as neces- sary. Consideration for some larger multilayer assemblies and assemblies with large thermal mass components should be made to ensure that all areas of the assembly reach the mini- mum recommended temperature for proper solder joint formation as well as rendering no-clean flux residues benign. A review of the assembly drawing is necessary to determine if there are heavy copper layers in select areas. The heavier layers of copper will absorb heat away from the surface of the assembly. This can lead to cold and brittle solder joint defects. Characteristics of Profile There are four different phases or zones to analyze under the reflow curve (Figure 1). The first is the preheating slope (tempera- ture ramp rate), then preheat dwell (soak time), followed by time above liquidus that will include the peak temperature, and lastly, the cooling zone. For the paste flux used in this trial, the preheat slope should be controlled to <2.0°C per second, which allows for gradual evapo- ration of the flux and will yield a higher qual- ity solder joint without increasing the risk of associated solder defects such as solder balls, bridging, etc. The preheat dwell phase is where the flux activators remove oxides and prepare the metal surfaces for joining with the solder paste. This phase brings the entire assembly with compo- nents to a common temperature below the melting point of the solder. This temperature is typically maintained for 60–90 seconds for most paste types. The reflow phase is when the intermetal- lic formation is made. The temperature is commonly anywhere between 20–40°C above the melting point of the solder. Time above liquidus can vary between 30–90 seconds depending on thermal mass and other material choices (Figure 1). Quest for Reliability Feature Column by Eric Camden, FORESITE INC. Figure 1: Example lead-free reflow profile phases.