Relying on Surface Mount Technology, the electronics industry is developing rapidly. And reflow soldering is one of the most important technologies in surface mount soldering technology. Let us introduce some technical and temperature setting issues of reflow soldering.
The reflow soldering temperature curve for circuit board assembly includes four major blocks: preheating zone, heat absorption zone, reflow soldering zone and cooling zone.
The preheating zone usually refers to the area where the temperature rises from normal temperature to about 150°C. In this area, the temperature rises slowly (also known as a one-time temperature rise) so that part of the solvent and water vapor in the solder paste can be volatilized in time. Electronic parts (especially BGA, IO connector parts) slowly heat up to prepare for the subsequent high temperature.
The temperature in this near-constant temperature zone is usually maintained at 150±10°C, and the ramp-up temperature usually falls between 150 and 190°C. At this time, the solder paste is just before melting, and the volatilization in the solder paste The substances will be further removed, the activator starts to activate, and effectively removes the oxides on the soldering surface. The surface temperature of the PCB is affected by the convection of the hot air, so that the temperature of the components of different sizes and textures can be maintained at a uniform temperature. If the temperature in this area rises too fast, the rosin (flux) in the solder paste will expand and volatilize rapidly. Under normal circumstances, the rosin should slowly escape from the gap between the solder paste. When the rosin volatilizes too fast , there will be quality problems such as air holes, fried tin, tin beads, etc.
Reflow soldering zone
The reflow area is the area with the highest reflow temperature in the whole section, which is usually called "liquid holding time. It must be noted that the temperature should not exceed the maximum temperature and heating rate tolerance of any temperature sensitive components on the PCB.
The peak temperature of reflow usually depends on the melting point temperature of the solder and the temperature that the assembled parts can withstand. Generally, the peak temperature should be about 25~30°C higher than the normal melting point of the solder paste in order to successfully complete the soldering operation. If it is lower than this temperature, it is very likely to cause the disadvantage of cold welding and poor wetting
After the reflow zone, the product cools and solidifies the solder joints, ready for subsequent assembly processes. Controlling the cooling rate is also critical, cooling too fast may damage the assembly, cooling too slow will increase TAL, which may result in weak solder joints.
The cooling zone should cool down quickly to solidify the solder. Rapid cooling can also obtain a finer crystal structure, improve the strength of the solder joints, make the solder joints bright, and the surface is continuous and meniscus-shaped, but the disadvantage is that it is easier to form holes, because Some gas is too late to dissipate.