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90 SMT007 MAGAZINE I MAY 2020 (15–20°C above liquidus) to allow wetting of the solder surfaces and the formation of inter- metallic but also not exceed a maximum peak to prevent damage. However, there are compo- nents of different thermal mass (socket, BGA, chip components, etc.) that require different thermal input. A minimum soldering temper- ature is essentially determined by the largest component, such as a BGA, but the maximum is determined by smaller and temperature-sen- sitive components. Even though different products, based on their thermal mass, require different amounts of thermal input, all products must achieve the minimum temperature (temperature above liq- uidus) without exceeding the maximum tem- perature (without damage to any components) within a defined time period (thermal profile). This is the key reason for developing a unique profile for each product. Developing a good profile is a balancing act to ensure intermetal- lic formation in heavier components without causing dewetting in smaller components due to overheating. Future Columns In future columns, I will take on some of the other key points in reflow profile development that I mentioned in the beginning. SMT007 Ray Prasad is the president of Ray Prasad Consultancy Group and author of the textbook Surface Mount Technology: Principles and Practice. Prasad is also an inductee to the IPC Hall of Fame—the highest honor in the electronics industry—and has decades of experience in all areas of SMT, including his leadership roles implementing SMT at Boeing and Intel; helping OEM and EMS clients across the globe set up strong, internal, self-sustaining SMT infrastructure; and teaching on-site, in-depth SMT classes. He can be reached at and has an upcoming SMT class in July (remotely by Zoom). More details at To read past columns or contact Prasad, click here. to cause the component or PWBA damage or discoloration or, in the worst case, delamina- tion or charring of the PWBA. On the other hand, a temperature that is too low may result in cold and grainy solder joints, non-melted solder, or poor intermetallic bonding. As a gen- eral rule of thumb, a higher peak is preferable to a lower peak temperature to prevent opens/ non-wetting. The recommended peak temperature in reflow zone is between 210–220°C (absolute minimum of 205°C) for SnPb and 235–245°C (absolute minimum of 230°C) for Pb-free sol- der alloy. The TAL should be 60–90 seconds but closer to 60 seconds. Extended duration above the solder melting point, or TAL, will damage temperature-sensitive components. It also will result in excessive intermetallic growth, which makes the solder joint brittle and reduces sol- der joint fatigue resistance. 4. Cooling Zone During the cooling zone, various materials will cool at different rates. The BGA pack- age typically will cool faster than the BGA solder joints and much faster than the bare board. This differential cooling can create a mechanical strain on the weakest spot in the interconnection, which is the laminate below the BGA pad, potentially resulting in pad cratering. A faster cooling rate decreases the grain size, improves the joint strength, but increases the warpage and the potential for pad crater- ing. Pad cratering defects have become more common due to the increased stiffness of SAC solders and Pb-free laminates. Pad cratering depends not only on cooling rate but many other factors, such as stiffer lead-free solder and stiffer Pb-free laminate. As a practical mat- ter, in most ovens, turning the cooling fan on and off are the only options for controlling the cooling rate unless the cooling zone has an option for blowing cold air. Challenges in Developing Thermal Profiles There are many challenges in developing a profile. For example, all solder joints must reach the minimum soldering temperature

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