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Time of issue:2020-07-15 00:00:00
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Factors affecting PCBA soldering quality and thickness

(Summary description)Generally speaking, the quality and thickness of PCBA soldering (intermetal bonding layer) have a great relationship with the solder, and the alloy composition of the solder is a key parameter that determines the melting point of the solder paste and the quality of the solder joint. From the general wetting theory, the ideal brazing temperature for most metals should be higher than the melting point (liquid line) temperature (15.5~71℃). For Sn-based alloys, it is recommended to be about 30-40°C above the liquidus. Below we take Sn-Pb solder alloy as an example to analyze the influence of alloy composition on melting point and solder joint quality.However, before introducing the...

Factors affecting PCBA soldering quality and thickness

(Summary description)Generally speaking, the quality and thickness of PCBA soldering (intermetal bonding layer) have a great relationship with the solder, and the alloy composition of the solder is a key parameter that determines the melting point of the solder paste and the quality of the solder joint. From the general wetting theory, the ideal brazing temperature for most metals should be higher than the melting point (liquid line) temperature (15.5~71℃). For Sn-based alloys, it is recommended to be about 30-40°C above the liquidus. Below we take Sn-Pb solder alloy as an example to analyze the influence of alloy composition on melting point and solder joint quality.However, before introducing the...

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Generally speaking, the quality and thickness of PCBA soldering (intermetal bonding layer) have a great relationship with the solder, and the alloy composition of the solder is a key parameter that determines the melting point of the solder paste and the quality of the solder joint. From the general wetting theory, the ideal brazing temperature for most metals should be higher than the melting point (liquid line) temperature (15.5~71℃). For Sn-based alloys, it is recommended to be about 30-40°C above the liquidus. Below we take Sn-Pb solder alloy as an example to analyze the influence of alloy composition on melting point and solder joint quality.

 

However, before introducing the alloy composition, let me briefly introduce the eutectic alloy: the so-called eutectic solder is the process of changing from solid phase to liquid phase or from liquid phase to solid phase at the same temperature, and the fine crystals under this composition The mixture of grains is called a eutectic alloy. When the temperature rises to the eutectic point, the solid-phase solder is all in the liquid state; when the temperature drops to the eutectic point, the liquid-phase solder is cooled and solidified and all become a solid state. Because of this, eutectic alloys have no plastic range during melting and solidification. The alloy solidification temperature range (plasticity range) has a great influence on the welding processability and the quality of the solder joints. For example, for alloys with a large plastic range, it takes a long time for the alloy to solidify and form solder joints. If the PCB and components have any vibration (including PCB deformation) during the alloy solidification, it will cause "solder joint disturbance" and may cause solder joints. Cracking, causing premature damage to the equipment.

 

 

Among Sn-Pb alloy ratios, eutectic alloys have the lowest melting point. The melting point of 63Sn-37Pb eutectic alloy (point B) is 183℃, and the PCB soldering temperature is also the lowest, around 210~230℃, which will not damage the components and PCB circuit board during soldering. The liquidus temperature of any other alloy ratio is higher than the eutectic temperature. For example, the liquidus temperature of 40Sn-60Pb (H point) is 232℃, and the soldering temperature of the SMT patch is about 260~270℃. Obviously the soldering temperature Exceeded the temperature tolerance of the components and PCB printed boards. Therefore, when the temperature of the solder joint drops to the eutectic point, the crystal particles formed by solidification are the smallest, the structure is the densest, and the solder joint strength is the highest.

 

In conclusion, in order to ensure the quality of PCBA soldering, when choosing solder, whether it is traditional Sn-Pb solder or lead-free solder, the alloy composition of the solder must meet the requirements of eutectic or near eutectic as much as possible. In addition, the oxide content on the surface of the alloy powder will also affect the solderability of the solder paste. Although the flux has the function of cleaning the oxide on the metal surface, it cannot be completely removed if it is a serious oxidation problem. Therefore, it is required that the oxygen content of the alloy powder should be less than 0.5%, and it is best to control it below the minus 6th power of 80x10.

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