We hope to obtain finely strengthened eutectic crystal particles and solid solution structure through brazing. We hope that there is a thin and flat bonding layer (0.5 ~ 4um) at the interface to minimize the occurrence of compound layers in the brazing joint. Lead-free soldering hopes to get a solder structure with less segregation.
There are many conditions to obtain the ideal interface organization, for example:
1. The degree of mutual solubility of the brazing filler metal component and the base metal is good;
2. The surface of liquid solder and base metal is clean, free of oxide layer and other contaminants;
3. The role of excellent surface active substances (flux);
4. Environmental atmosphere, such as nitrogen or vacuum protection welding;
5. Appropriate temperature and time (ideal temperature curve);
6.Can maintain a flat reaction layer interface, such as PCB material with small expansion coefficient and stable PCB transmission system.
The lead-free soldering temperature is high. In particular, the PCB material has a small expansion coefficient in the Z-axis direction. It can maintain a flat reaction layer interface, otherwise in the case of segregation, if the PCB is deformed by stress, it is easy to cause the solder joint to warp and even the pad to peel off. In the conditions listed above, under other conditions are constant, the main factors affecting the thickness of the bonding layer (brazing line) and the composition and ratio of intermetallic compounds are temperature and time. If the temperature is too low, the bonding layer cannot be formed or the bonding layer is too thin; if the temperature is too high and the time is too long, the compound layer will thicken, so it is very important to set the temperature curve correctly.
In the previous section where we analyzed the setting of the reflow soldering temperature curve in the SMT patch processing plant, we have done some analysis on the impact of brazing and the formation of excellent solder joints because of the consideration of many PCBA Both are double-sided mounting, which requires a second oven, which results in many solder joints subject to high temperature baking multiple times. How to obtain the ideal interface structure under repeated heating is the SMT patch processing plant One thing that must be treated hard.