Reflow soldering of solder paste is the main board-level interconnection method used in the SMT assembly process. This soldering method combines the required soldering characteristics excellently. These characteristics include ease of processing, wide range of SMT designs Compatibility, high soldering reliability, and low cost; however, when reflow soldering is used as the most important SMT component-level and board-level interconnection method, it is also challenged to further improve soldering performance. In fact, whether reflow soldering technology can withstand this challenge will determine whether solder paste can continue to be the primary SMT soldering material, especially in the case of continuous progress in ultra-fine fine-pitch technology. Below we will discuss several major issues that affect the improvement of reflow soldering performance. In order to stimulate the industry to develop new methods to solve this problem, we briefly introduce each issue as follows:

Fixing of bottom elements

Double-sided reflow soldering has been used for many years. Here, the first side is printed and wired first, components are installed and reflowed, and then turned over to process the other side of the circuit board. In order to save more, some processes are omitted. For the reflow of the first side, but reflow the top and bottom sides at the same time, a typical example is that only small components such as chip capacitors and chip resistors are installed on the bottom side of the circuit board, due to the design of the printed circuit board (PCB) It is getting more and more complex, and the components mounted on the bottom surface are getting bigger and bigger. As a result, component drop-off during reflow becomes an important problem. Obviously, the phenomenon of component falling off is due to the insufficient vertical fixing force of the molten solder on the component during reflow, and the insufficient vertical fixing force can be attributed to the increase in component weight, poor solderability of the component, wettability of the flux, or insufficient solder volume Wait. If there is still component shedding after improving the latter three factors, SMT adhesive must be used. Obviously, the use of adhesives will make the self-alignment of the components worse during reflow.