In the process of PCBA processing, there are many production processes, which are easy to produce many quality problems. At this time, it is necessary to constantly improve the PCBA welding method and improve the process to effectively improve the product quality.
I. Improve the temperature and time of welding
The intermetallic bond between copper and tin forms grains, the shape and size of the grains depends on the duration and strength of the temperature when soldering equipment such as reflow oven or wave soldering machine. PCBA SMD processing reaction time is too long, whether due to long welding time or due to high temperature or both, will lead to rough crystal structure, the structure is gravelly and brittle, the shear strength is small.
II. Reduce surface tension
Tin-lead solder cohesion is even greater than water, so that the solder is a sphere to minimize its surface area (the same volume, the sphere has the smallest surface area compared with other geometric shapes, to meet the needs of the lowest energy state). The role of flux is similar to the role of cleaning agents on the metal plate coated with grease, in addition, the surface tension is also highly dependent on the degree of surface cleanliness and temperature, only when the adhesion energy is much greater than the surface energy (cohesion), the ideal dip tin can occur.
III. PCBA board dip tin angle
About 35 ℃ higher than the eutectic point temperature of the solder, when a drop of solder placed on the hot surface coated with flux, a bending moon surface is formed, in a way, the ability of the metal surface to dip tin can be assessed by the shape of the bending moon surface. If the solder bending moon surface has a clear bottom cut edge, shaped like a greased metal plate on the water droplets, or even tend to spherical, the metal is not solderable. Only the curved moon surface stretched into a small angle of less than 30. Only good weldability.
IV. The problem of porosity generated by welding
1. Baking, PCB and components exposed to the air for a long time to bake, to prevent moisture.
2. Solder paste control, solder paste containing moisture is also prone to porosity, tin beads. First of all, use good quality solder paste, solder paste tempering, stirring according to the operation of strict implementation, solder paste exposed to the air for as short a time as possible, after printing solder paste, the need for timely reflow soldering.
3. Workshop humidity control, planned to monitor the humidity of the workshop, control between 40-60%.
4. Set a reasonable furnace temperature curve, twice a day on the furnace temperature test, optimize the furnace temperature curve, the temperature rise rate can not be too fast.
5. Flux spraying, in the over SMD wave soldering machine, the amount of flux spraying can not be too much, spraying reasonable.
6. Optimize the furnace temperature curve, the temperature of the preheating zone needs to meet the requirements, not too low, so that the flux can fully volatilize, and the speed of the furnace can not be too fast.
Post time: Jan-05-2022