Double sided reflow soldering method

The double-sided circuit board process has become increasingly adopted and more complex. This is because it can provide designers with larger and more flexible design space. The method of double-sided reflow soldering is more complex than single-sided reflow soldering. Below, JEENOCE will explain the method of double-sided reflow soldering.

The presence of components on both sides of the circuit board greatly enhances the actual utilization of the PCB, thereby reducing manufacturing costs. At present, the commonly used process for double-sided circuit boards is to pass through a reflow furnace on top and a wave soldering furnace on the bottom.

Today, most people are gradually inclined towards double-sided reflow soldering as a better method. But there are still some issues with the process, such as: when refluxing again, the larger components at the bottom may fall off, or the solder joints at the bottom may partially re melt, which affects the reliability of the solder joints. There are several methods that have been developed to use double-sided circuit boards for secondary backflow connection Cure the adhesive on the first side of the component so that it does not fall off the board during secondary reflow and remains in the correct position; 2、 It uses solder paste with different melting points, and the solder paste used during the second reflow has a lower melting point than the one used during the second reflow.

However, there are some serious issues that need to be noted in the usage method: the first is that it causes the finished product to have a "too low" melting temperature during maintenance; The second issue is that using a higher reflux temperature can cause thermal shock to the components and substrate.

For most components, during secondary reflow soldering, the surface tension of the solder joint is sufficient to maintain the adhesive force of the component at the bottom, ensuring that the component is firmly fixed on the substrate. Here is a proportional relationship between component weight and pin (pad) tension, which can calculate whether the component can stick to the bottom of the substrate during secondary reflow without falling off, thus eliminating the need for actual testing of each component. 30g/in ² It is a conservative value that can be used as a design standard.

Another method is to adopt the concept of blowing cold gas over the bottom of the substrate, so that the temperature at the bottom cannot reach the melting point during secondary reflux. However, the temperature difference between the top and bottom of the substrate may lead to potential stress generation.