PCB Tech - Factors affecting the welding quality and thickness of pcb assembly

Factors affecting the welding quality and thickness of PCB assembly
Today I will share a topic, this topic is the factors that affect the quality and thickness of PCB assembly soldering. In fact, the quality and thickness of the intermetallic bonding layer in PCB assembly welding are related to the following factors, please see the specific content below.
1. Alloy composition of solder
The alloy composition is a key parameter that determines the melting point of the solder paste and the quality of the solder joint.
Starting from the general wetting theory, the ideal brazing temperature for most metals should be 15.5～71 degree Celsius higher than the melting point (liquid line) temperature. For Sn-based alloys, the recommended temperature is about 30 to 40°C above the liquidus.
Taking Sn-Pb solder alloy as an example, it is analyzed that the alloy composition is a key parameter that determines the melting point and solder joint quality.
(1) The melting point and welding temperature of eutectic alloy are the lowest
The melting point of 63Sn-37Pb eutectic alloy is 183 degree Celsius, and the melting point of 63Sn-37Pb eutectic alloy is 183 degree Celsius.
The PCB soldering temperature is also the lowest, about 210～230 degree Celsius, and the components and PCB circuit board will not be damaged during the soldering process. The liquidus of any other alloy ratio is higher than the eutectic temperature. For example, the liquidus of 40Sn-60Pb (point H) is 232 degree Celsius, and the soldering temperature of SMT patch is about 260～270 degree Celsius, which obviously exceeds the endurance limit temperature of components and PCB printed boards.

(2) The eutectic alloy has the densest structure, which is beneficial to improve the strength of solder joints;
The so-called eutectic solder is the transition from solid phase to liquid phase or from liquid phase to solid phase at the same temperature, and the fine crystals of this composition
The granular mixture is called a eutectic alloy. When the temperature reaches the eutectic point, the solder is all in the liquid phase, when the temperature drops to the eutectic point
id solder suddenly becomes a solid phase, so the crystal particles formed when the solder joint is solidified are the smallest, the structure is the densest, and the solder joint strength is the highest. Other alloys have a longer setting time, and the particles that crystallize first will grow up, which affects the strength of the solder joint.
(3) The eutectic alloy has no plastic range or viscosity range during the solidification process, which is conducive to the control of the welding process;
As long as the eutectic alloy reaches the eutectic point temperature when the temperature is raised, it will immediately change from the solid phase to the liquid phase; on the contrary, as long as the temperature drops to the eutectic point during cooling and solidification, it will immediately change from the liquid phase to the solid phase, so the eutectic There is no plastic range when the alloy melts and solidifies.
The solidification temperature range (plastic range) of the alloy has a great influence on the welding processability and the quality of the solder joints. Wide plastic range
alloy,
It takes a long time to form solder joints when the alloy solidifies. If any vibration (including PCB deformation) occurs in the PCB and components during the alloy solidification process, it will cause "solder joint disturbance", which may cause the solder joint to crack and cause premature damage to the equipment.
From the above analysis, it can be concluded that the alloy composition is the key parameter that determines the melting point and melting point quality of the solder paste.
Therefore, whether it is a traditional Sn-Pb solder or a lead-free solder, the alloy composition of the solder is required to be as eutectic or near eutectic as possible.
Solder alloy
Second, the degree of oxidation of the alloy surface
The oxide content on the surface of the alloy powder also directly affects the solderability of the solder paste. Because diffusion can only enter clean metal surfaces
OK. Although the flux has the effect of cleaning the oxides on the metal surface. But this does not drive away serious oxidation problems. The oxygen content of the alloy powder is required to be less than 0.5%, and it is best to control it below the minus 6th power of 80*10.
In summary, regarding the factors that affect the welding quality and thickness of PCB assembly, the electronic editor will introduce to you here, I hope to help you, thank you.