Setting of temperature curve for lead-free reflow soldering of SMD LED
At present, the lead-free reflow soldering temperature curves that are widely used in the industry mainly include trapezoidal temperature curves and gradual temperature curves. Combined with the typical temperature curve, the lead-free reflow temperature curve is designed according to the characteristics of 3014LED lamp beads and the performance of lead-free solder 95.5Sn3.8Ag0.7Cu, as shown in the figure.
Lead-free reflow soldering temperature profile
1. Heating zone for lead-free reflow soldering of LED lamp beads
The preheating zone of lead-free reflow soldering can be divided into 3 sub-zones, namely the heating zone, the heat preservation zone and the rapid heating zone. In the heating zone, a heating process with a heating rate of 1.5 to 2.5°C/sec, a maximum heating rate of no more than 3°C/sec, and a time of no more than 90 seconds is used to make the temperature from the working environment temperature reach 130°C. At this stage, the LED circuit board (PCB) reaches the active temperature required for reflow soldering from the ambient temperature, the lower melting point solvent in the solder paste volatilizes, and the thermal shock to the components is reduced. The heating speed should not be too fast, otherwise it may cause the deterioration of the flux composition in the solder paste, the formation of solder balls, bridging and other defects, and at the same time, the components will be subjected to excessive thermal stress and warp. In addition, when welding a PCB carrying high-power and large-size LED lamp beads, in order to make the entire PCB temperature uniform and reduce defects such as warpage caused by thermal stress, you can refer to the relevant heat treatment process and practical experience, and slowly increase the temperature in this interval. And preheating. In the heat preservation zone, use a heating rate of <2°C/sec to make the temperature in the heat preservation zone between 140-160°C and keep it for 60-90 seconds. In this area, the flux starts to become active, and all parts of the PCB are wetted evenly before reaching the reflow area, and the solvent in the solder paste that has not completely evaporated is further volatilized. In the rapid heating zone, which is also called the flux infiltration zone, the temperature quickly rises to the melting point of the solder paste. At this stage, the heating rate is required to be fast, otherwise the flux activity in the solder paste will be reduced, and the solder alloy will be oxidized at high temperature, forming a bad solder joint. At this stage, the flux cleans the oxide layer of the soldering surface and maintains a certain soldering activity, which facilitates the formation of a good intermetallic compound joint in the soldering area.
2. Lead-free reflow soldering area for LED lamp beads
The soldering area (reflow area) solder paste exists in the form of a liquid phase at a temperature higher than the melting point. For the solder 96.5%Sn/3.0%Ag/0.7%Cu, the highest temperature is between 235～245℃, and the melting point above 225℃ is controlled at 40～60 seconds and the time when higher than 230℃ is 10～20 seconds. In this temperature range, the metal particles in the solder paste melt, diffuse, dissolve, chemical metallurgy, and form IMC joints under the action of liquid surface tension. At the same time, if the peak temperature is too high and the reflow time is too long, it may cause the IMC grains to grow too large, and the mechanical and electrical properties will be affected. Damage etc. If the temperature is too low and the reflow time is short, the solder and the PCB may not be completely wetted, forming a spherical solder joint, which affects the electrical conductivity. For components with large heat capacity, the heat is insufficient and the solder joint connection is not firmly formed. Welding. The process of determining the temperature of the soldering zone and the reflow time requires further research for specific LED lamp beads and different PCBs.
3. Lead-free reflux cooling zone for LED lamp beads
After the LED lamp bead circuit board leaves the soldering area, the substrate enters the cooling area. At this stage, the cooling rate is less than or equal to 4°C/s. If the cooling rate is too fast, huge thermal stress may cause cold cracks in the solder joints, LED lamp beads, or even deformation of the PCB, and the PCB light bar will be scrapped. If the cooling rate is too slow, the solder joint crystallization time is long, and the nucleation rate is low. Enough energy will make the IMC grains grow too thick, and it is difficult to form IMC joints with fine grains, which will make the solder joint strength worse, and even LED lamp beads. Shift occurs.
