According to the temperature profile for the Sn63Pb37 solder paste, a completed reflow soldering process contains the following stages:
- Preheat Zone: The heating rate in the preheat zone should be 2°C to 4°C/second and the peak temperature in this zone should be 100-125°C. During preheat if the temperature ramp is too fast, the solder paste may splatter and cause solder balls. Also, to avoid thermal shock to sensitive components such as ceramic chip resistors, the maximum heating rate should be controlled. (<4.5F/second)
- Soak Zone: The soak zone is intended to bring the temperature of the entire board up to a uniform temperature to minimizing temperature gradients. The soak zone also acts as the flux activation zone for solder paste. The ramp rate in this zone is very low, almost flat and the temperature is raised near the melting point of solder. The consequences of being too high in the soak zone are solder balls due to insufficient fluxing and solder splatter due to excessive oxidation of paste. Soak times are usually around the range of 130 -170°C for 60 to 90 seconds. (0.9F-1.08F/second)
- Reflow Zone: In this zone the temperature is kept above the melting point of the solder for about 30 to 60 seconds. The peak temperature in this zone should be high enough for adequate flux action and to obtain good wetting. The standard primary fluid used for surface mount assemblies has a boiling point of 215°C at sea level. A peak temperature range of 215 – 220°C is generally considered acceptable. (2.34F-2.88F/second)
- Cooling Zone: The cooling rate of the solder joint after reflow is also important. The faster the cooling rate, the smaller the grain size of the solder, and hence the higher the fatigue resistance of the solder joint. Unfortunately, cooling too fast will result in residual stresses between TCE (Thermal Coefficient of Expansion) mismatched components. So the cooling rate needs to be optimized. For a given system, cooling rate is directly associated with the resulting microstructure which, in turn, affects the mechanical behavior of solder joints.
Resource: http://www.anadigics.com/sites/default/files/application_notes/SolderReflowReport.pdf
Also, today we worked on making a more detailed schematic for our project. This time we want to show how devices are connected together, using which ports and what kind of signals are sent from each separated device. Below is our initial graph:
