Detailed description of reflow soldering process flow and process requirements
2021-05-31 Classify: Company News Author: 广晟德 Browse: 1786
Reflow soldering is the most widely used method to connect surface mount components to printed circuit board (PCB). The purpose of this process is to form an acceptable solder joint by preheating the component / PCB / solder paste first and then melting the solder without damage due to overheating. The advantage of reflow process is that the temperature is easy to control, the oxidation can be avoided in the welding process, and the manufacturing cost is easier to control. GrandSeed reflow soldering here to share the reflow soldering process and process requirements.
Introduction of reflow soldering process
Reflow soldering is a kind of surface mount board, which has complex process and can be divided into two types: single side mount and double side mount.
A. Single side mounting: pre coating solder paste → mounting (divided into manual mounting and machine automatic mounting) → reflow soldering → inspection and electrical test.
B. Double sided mounting: a side pre coated solder paste → patch (divided into manual mounting and machine automatic mounting) → reflow soldering → B side pre coated solder paste → patch (divided into manual mounting and machine automatic mounting) → reflow soldering → inspection and electrical test.
The simple process of reflow soldering is "screen printing solder paste - chip - reflow soldering". The core of reflow soldering is the accuracy of screen printing. The yield of chip is determined by ppm of the machine. Reflow soldering is to control the curve of temperature rise, high temperature and drop. "
Reflow soldering process requirements
Reflow soldering technology is no stranger in the field of electronic manufacturing. The components on various boards used in our computer are welded to the circuit board through this process. The advantage of this process is that the temperature is easy to control, the oxidation can be avoided in the welding process, and the manufacturing cost is easier to control. There is a heating circuit inside the device, which heats the nitrogen to a high enough temperature and then blows it to the circuit board where the components have been pasted, so that the solders on both sides of the components melt and bond with the motherboard.
1. Reasonable reflow temperature curve should be set and real-time test of temperature curve should be done regularly.
2. According to the welding direction of PCB design.
3. Avoid the vibration of conveyor belt during welding.
4. The welding effect of each printed board must be checked.
5. Whether the welding is sufficient, whether the surface of the solder joint is smooth, whether the shape of the solder joint is half moon, whether the solder ball and residue are present, and whether the continuous welding and false welding are present. Also check the PCB surface color changes. And adjust the temperature curve according to the inspection results. The welding quality should be checked regularly during the whole batch production.
6. Factors affecting reflow soldering process:
a. In general, the specific heat capacity of PLCC and QFP is larger than that of a discrete piece, so it is more difficult to weld large area components than small ones.
b. In reflow soldering furnace, the conveyor belt is a cooling system while reflow soldering the products. In addition, the temperature of the edge of the heating part is different from that of the center, and the temperature of the edge is generally low. In addition to the different temperature requirements of each temperature zone, the temperature of the same carrier surface is also different.
c. The influence of product loading is different. When adjusting the temperature curve of reflow soldering, good repeatability can be obtained under no-load, load and different load factors. The load factor is defined as: LF = L / (L + s); Where l = the length of the assembled substrate and S = the interval between the assembled substrates. The larger the load factor is, the more difficult it is for reflow process to get good repeatability. Generally, the large load factor of reflow soldering furnace ranges from 0.5 to 0.9. This depends on the product situation (component welding density, different substrates) and different models of reflow furnace. To get good welding effect and repeatability, practical experience is very important.