Reflow Profiling Optimize Soldering in SMT Assembly
In surface mount technology (SMT) assembly, a thin circuit board of non-conductive material like fiberglass or plastic is coated with copper traces that serve as pathways for electrical current. The board is then loaded with small electronic components using an automated pick-and-place machine. A mix of solder paste and flux, a liquid mixture of tin and copper powder, is then applied to the component pads. The printed circuit board then goes through a heating process known as reflow soldering to bond the components to their corresponding traces and PCB pads. The reflow soldering process must be carefully controlled to ensure proper melting and bonding of the SMT components. A critical step in reflow soldering is thermal profiling, which optimizes the temperature profile of the reflow oven.
A reflow profile is a set of temperatures mapped out on a time-versus-temperature graph that describes the exact steps in the reflow soldering process, from room temperature to peak temperature and then cooling to solidify the solder. A well-defined reflow profile is the foundation for consistent and successful SMT production.
An important first step is the determination of the preheat zone temperature, or T1, which establishes the minimum acceptable temperature for the smt assembly and its components during the reflow process. To determine T1, simply identify the highest temperature at which any component in the assembly gets into trouble and subtract a buffer of 5K from that value. The resulting value is the maximum temperature the components in the assembly should reach during reflow. This is the MVC temperature and staying below it will prevent thermal pain to sensitive components.
How Does Reflow Profiling Optimize Soldering in SMT Assembly?
Next, the reflow zone temperature, or T2, must be established to ensure that all of the tin and copper in the SMT components will be melted to form the required intermetallic compound. This requires a temperature that is below the MVC but above the reflow melting point of the solder paste used to apply the tin-copper powder. Finally, the peak temperature and time above liquidus, or TAL, must be defined to ensure that the individual powder tin spheres are sufficiently melted to fuse together into a solid mass with minimal defects.
A well-defined reflow profile also includes a soak zone, or soaking phase, which is intended to reduce the delta T between adjacent components by allowing the tin and copper to reach a lower temperature. The duration of the soak phase can vary depending on the application and the PCB design, but a soaking period of 30 seconds or less is typical. Finally, the reflow profile must include a reasonable cooling rate to minimize stress on the joints and the assembly substrates due to differences in their coefficients of thermal expansion.
Typically, a rate of 4K/s over 20s is recommended. The reflow profile is one of the most crucial and complex processes in SMT assembly and should be analyzed on a regular basis to ensure quality product manufacturing. Without a high-quality reflow profile, voiding, poor solder joint strength and other defects will result.
