Reflow Soldering Contribute to Circuit Board Assembly
In many cases reflow soldering is used on boards that contain a mixture of surface mount (SMD) components and plated through hole (PTH) parts. In this case a mix of different melting point solders may be utilized and the reflow process allows all the SMD components to be reflow soldered prior to PTH part insertion. In some cases this can allow the wave soldering step to be eliminated completely from the assembly process which can lead to reduced cost.
The reflow soldering process uses a heated oven to melt the solder paste and create the necessary solder joints on the circuit board assembly. During the reflow soldering process the temperature of the PCB and components are controlled to ensure they do not damage the components or the solder paste. This is usually accomplished by using a Ramp-Soak-Spike (RSS) profile.
The RSS profile consists of a preheating zone, a soak or dwell zone and the reflow zone. During the preheating zone the temperature of the PCB and components is slowly ramped up. This ensures that all areas of the board are heated evenly and prevents thermal shock which can damage components or solder paste. During the soak or dwell zone the temperature of the board is held at a stable level to avoid shadowing effects and to reduce the risk of solder bridging.
How Does Reflow Soldering Contribute to Circuit Board Assembly?
During the reflow zone the temperature of the PCB and components rises quickly to the peak reflow temperature. The reflow temperature is reached when the flux reduces the surface tension where the metal joins to accomplish metallurgical bonding and allow the individual solder powder spheres to combine and melt. The reflow temperature for most applications is between 240 and 250 degrees Celsius for a Pb-Free solder.
A successful PCB requires a good quality solder joint. Void formation in the solder joints is an issue that can significantly decrease the strength of the finished product. By understanding the causes of void formation and implementing appropriate processes controls and improvements, manufacturers can achieve high-quality, reliable electronic products.
Various factors can contribute to void formation in the reflow soldering process, including component and PCB preparation, pad design, and misalignment. The most common cause of voiding is tombstoning, which occurs when unintended small solder spheres form during the reflow soldering process. This can lead to poor solder connections and electrical shorts. To prevent tombstoning, the pad size and shape of the component should be compatible with its body size and termination style to help maintain a consistent solder distribution.
Other contributors to void formation include improper handling and storage of the assembled boards, inadequate cleaning and inspection procedures and thermal stress caused by uneven heating of the PCB during the reflow soldering process. These issues can be avoided by ensuring that the manufacturing process follows recommended practices and uses high-quality materials. Keeping the temperature of the soldering process low, and maintaining the correct solder paste viscosity is also key to avoiding voiding.
