The reflow soldering process is a surface-mounted board, and its process is more complicated which can be divided into two types: single-sided mounting and double-sided mounting.
A single-sided mounting: pre-coated solder paste → patch (divided into manual placement and machine automatic placement) → reflow soldering → inspection and electrical test.
B double-sided placement: Aside pre-coated solder paste → patch (divided into manual placement and machine automatic placement) → reflow soldering → B side pre-coated solder paste → placement (divided into manual placement and machine automatic placement) → reflow soldering → inspection and electrical test.
The simple process of reflow soldering is "screen printing solder paste-patch-reflow soldering, the core of which is the accuracy of the screen printing. For the patch, the yield rate is determined by the machine's PPM. The reflow soldering is to control the temperature rise and high temperature. And falling temperature curve.
A reflow oven is an indispensable machine in the SMT production line. It is mainly used for soldering circuit boards with mounted components. The solder paste is melted by heating to fuse the chip components and the circuit board pads. Then the solder paste is cooled by a reflow oven to cool the components and the pads which are cured together. A reflow oven is a kind of machine that is completed by soldering the parts on the PCB board through reflow soldering.
A. When the PCB enters the heating zone, the solvent and gas in the solder paste evaporate, and at the same time, the flux in the solder paste wets the pads, component ends, and pins. The solder paste softens, collapses, and covers the solder pad. Isolate the pads and component pins from oxygen.
B. When the PCB enters the heat preservation area, make the PCB and components fully preheated. It can prevent the PCB from suddenly entering the welding high-temperature area and damaging the PCB and components.
C. When the PCB enters the soldering area, the temperature rises rapidly so that the solder paste reaches a molten state. The liquid solder wets diffuse or reflows to the PCB pads, component ends, and pins to form solder joints.
D. The PCB enters the cooling zone and the solder joints are solidified; when the reflow soldering is completed.
PCB soldering technology in the process of electronic industry development in recent years, it can be noted that a very obvious trend is reflow soldering technology. In principle, traditional plug-in parts can also be reflow soldered, which is commonly referred to as through-hole reflow soldering. The advantage is that it is possible to complete all solder joints at the same time, minimizing production costs.
Take the temperature rise-heat preservation-peak temperature curve as an example, the requirements are as follows:
1. The heating rate should be limited to 0.5～1℃/s or below 4℃/s, depending on the solder paste and components.
2. The formula of the flux composition in the solder paste should conform to the curve. Excessive heat preservation temperature will damage the performance of the solder paste.
3. The second temperature rise slope is at the entrance of the peak zone. The typical slope is 3°C/s and the time above the liquidus line requires 50-60s, and the peak temperature is 235-245°C.
4. In the cooling zone, in order to prevent the growth of crystal particles in the solder joints and prevent segregation, the solder joints are required to cool down quickly, but special attention should be paid to reducing the stress. For example, the maximum cooling rate of ceramic chip capacitors is -2 to -4°C/s.