Soldering Contribute to PCB Circuit Board Assembly
A vital component of pcb circuit board assembly is reflow soldering, which creates the metallurgical bond between the components and the PCB. This process uses solder paste, which consists of tiny granules of silver-copper alloy suspended in a medium, and enables a strong and reliable electrical and mechanical connection. To ensure a high-quality and stable solder joint, the PCB must be thoroughly cleaned before solder reflow to remove any contaminants that could negatively impact the performance of the product. This ensures that the solder paste will adhere to and effectively connect the SMD components to the board, creating a strong conductive link that will ensure electrical continuity.
Once the pick and place machine has placed all the SMD components on the PCB, it will be transferred to a conveyor belt that will move it through the reflow oven. The reflow oven is similar to a large commercial pizza oven and consists of a series of heaters that gradually heat the PCB to a temperature of around 250 degrees Celsius. This is hot enough to melt the solder paste, but not so hot that it will damage the components or cause other solder defects.
The first zone of the reflow oven is the preheat zone, which slowly and evenly warms the PCB to a temperature just above the melting point of the solder paste. This helps to minimize temperature variations across the board and prevents thermal shock. It is important to properly design the preheat zone and choose the right temperature profile for the specific assembly, since a poorly optimized preheat phase can lead to poor or non-wetting of the pads and leads of the components and other solder defects.
How Does Reflow Soldering Contribute to PCB Circuit Board Assembly?
Next is the soaking zone, which holds the temperature constant at just above the melting point of the solder to activate the flux and remove oxides from the surfaces of the components and the PCB. The soaking phase is essential for good solder joints as it improves wetting between the components and the PCB, which ultimately reduces defects like cold soldering, voiding, and brittle joints.
During the reflow zone, the temperature rapidly rises to the peak temperature, at which point the melted solder flows and forms a solid metallurgical bond with the component pads and leads. It is important to maintain this temperature within a specified range and duration to avoid damaging the components or causing other solder-related defects, such as oxidation. It is also recommended to use nitrogen during reflow soldering, since oxygen can react with the surface of the solder and form oxides that are not conducive to joining.
Once the reflow soldering has reached its peak, the board is slowly cooled, which allows the solder to cool and solidify into a durable and long-lasting solder joint. A well-designed reflow oven will have multiple cooling zones and a custom profile for each type of PCB, based on the complexity of the assembly, its component density, and the characteristics of the solder paste.