1. The higher melting point sharply reduces the process window. The melting point of lead-tin solder is 183 ° C, the temperature of complete liquefaction is 205 ° C - 215 ° C, and the limit temperature of printed circuit board is 230 ° C - 240 ° C, the existing process The balance is 15 °C-35 °C; the common lead-free solder has a melting point of 217 °C-220 °C, and its complete liquefaction temperature is 225 °C-235 °C. Since the board temperature of the PCB does not change, the process margin is reduced to 5 ° C - 15 ° C, if the narrow process margin requires reflow ovens now have high repeatability, and a tight circuit board indicates temperature difference.
2. The liquefaction time is longer: the liquefaction time of the traditional lead solder paste is 40s-60s; the liquefaction time of the lead-free solder paste is generally
60s-90s.
Below are the differences between the two lead-free reflow profiles and the traditional lead-containing solder paste curves.


Figure 1 is a typical lead-containing graph.
It can be seen from Fig. 2 that if the liquefaction time and the liquefaction temperature are guaranteed, there will be a very high peak temperature of about 260 ° C, which will inevitably cause thermal shock or even damage to the PCB board and components.
Solution:
1 The preheating temperature of the flux is unchanged before the tin melting;
2 Use more than 2 heating temperature zones to make the welding temperature zone;
3 The independent temperature is reduced, and the number of heating zones is increased to facilitate process adjustment;
4 the same production capacity does not minimize the overall size of the heating zone to reduce oxidation;
5 It is recommended to use a nitrogen protection process (not necessary);
6 Designing a new type of intermediate support device to reduce the distribution deviation caused by the intermediate support device