What is SMT reflow soldering and what is the reflow soldering process

2022-07-27 51

Resetter soldering is a process in which solder paste is heated and melted by a large amount of heating so that surface mount components and PCB pads can be reliably combined together by solder paste alloys. Before soldering, the surface mount component relies on a certain amount of solder paste to be fixed on the surface, the alloy solder melts, adheres to the metal surface by wetting, and after cooling and curing, a mechanical and electrical connection is created between the PCB and the component. 

SMT reflow soldering principle specific process: According to the temperature and time changes in reflow soldering, the solder paste at the soldering position will go through the following processes in turn. SMT reflow soldering schematic diagram Standard SMT reflow furnace temperature curve: According to the temperature-time change law, combined with the change stage of solder, the furnace temperature curve can be decomposed into: preheating zone, thermal insulation zone, melting zone, cooling zone standard SMT reflow furnace temperature curve A preheating zone: The initial heating stage needs to pay attention to the fact that the heating rate is not fast. Because the temperature rises too quickly, it may cause damage to the PCB or parts due to thermal stress, and may also cause the rapid volatilization of the diluent and cause four splashes. The limit of the heating rate is generally recommended by the part manufacturer, set below 4 °C/sec, between 1 and 3 °C/sec. Key control point: heating rate preheating area furnace temperature curve B insulation area: The purpose of heat preservation is to ensure that each part of the PCB reaches the temperature before the peak tin melting area. Give the thinner in the solder paste enough time to fully volatilize, rosin and activator to complete its role in cleaning the joint surface. More importantly, it is necessary to ensure that the areas with different temperature rise rates of the PCB can be consistent through heat conduction. However, the time of the insulation area should not be too long, otherwise the flux will also be depleted due to oxidation. The temperature of the insulation area should be determined according to the complexity of the PCB design and the heat conductivity of the recandesing furnace. Key control points: insulation temperature, insulation time insulation area furnace temperature curve C melting zone: the peak temperature of the temperature curve is to make the PCB higher than the temperature of the solder paste melting, and the selection of the peak temperature is the core process in the temperature curve. If the temperature is not high enough, the solder paste cannot melt; If the temperature is too high, it will be damaged by heat. Oxides on the surface of the metal and oxygen in the resoldering furnace can prevent the flux from helping to merge and wet. The higher the temperature, the stronger the effect of the flux, but at the same time the higher the chance of oxidation in the resoldering furnace. The viscosity and surface tension of the solder paste after melting decrease with increasing temperature, which can increase the wetting effect. Key control points: peak temperature melting zone furnace temperature curve D cooling zone: as long as the powder particles in the solder paste melt and can wet the surface to be joined, the faster the cooling rate, the better, the solder joint is bright, the contact shape is good, and there is enough strength. A slow cooling rate will cause more substrate material to melt into the solder paste, creating rough or empty solder joints. In addition, some joint end metals dissolve to make them resistant to wetting or the solder joints are not strong enough. Vibrations that occur before the molten tin is completely solidified can also make the integrity of the solder joints worse. Of course, if some components are extremely sensitive to temperature changes, they also need to limit the cooling rate. Key control point: cooling rate furnace temperature curve in the cooling zone