How to adjust the temperature of lead-free reflow soldering

The main manifestation of lead-free reflow soldering technology is to see how the reflow soldering operator adjusts the temperature of each temperature zone of the lead-free reflow soldering machine, so as to achieve the most perfect curve of lead-free reflow soldering. The temperature curve of lead-free reflow soldering refers to the curve of the temperature at the test point on the surface assembly device of a PCB changing over time. Therefore, the lead-free reflux temperature curve is an important factor in determining welding defects. Adjusting the temperature of lead-free reflow soldering is of utmost importance in the reflow soldering process. Below, JEENOCE will briefly explain how to adjust the temperature of lead-free reflow soldering.

For lead-free solder paste, the temperature difference between components must be minimized as much as possible. This can also be achieved by adjusting the reflow soldering temperature. Using traditional temperature curves, although the temperature difference between components is inevitable when the board reaches its peak temperature, several methods can be used to reduce it:

1、 Extend the preheating time. This greatly reduces the temperature difference between components before the peak reflux temperature is formed. Most convection reflux furnaces use this method. However, because the flux may evaporate too quickly through this method, it may cause poor wetting due to oxidation of the pins and pads.

2、 Increase the preheating temperature. The traditional preheating temperature is generally between 140~160 ° C, which may need to be increased to 170~190 ° C for lead-free soldering. Increasing the preheating temperature reduces the required peak formation temperature, which in turn reduces the temperature difference between components (pads). However, if the flux cannot withstand higher temperature levels, it will evaporate again, causing poor melting humidity due to oxidation of the solder pad pins.

3、 Trapezoidal temperature curve (extended peak temperature). Extending the peak temperature time of small heat capacity components will allow the components to reach the required reflux temperature with larger heat capacity components, avoiding overheating of smaller components. Using a trapezoidal temperature curve, as shown in Figure 6, a modern combined reflux system can reduce the temperature difference between a 45mm BGA and a small outline package (SOP) body to 8 ° C.