Tin/silver/copper (SnAgCu) is an alloy commonly used as solder. SnAgCu is an environmental friendly soldering alloy which is the reason it is popular in consumer electronics because of new legislations.
Tin/silver/copper (SnAgCu) solder alloys
The Tin/silver/copper (SnAgCu) alloy –also called SAC – is a lead free alloy which is steadily used for electronic solder because it is near eutectic, with good strength, thermal fatigue properties, and wettability. SAC alloys became the main choice for lead-free SMT (surface mount technologies) assembly in the electronics industry. SMT is the process in which components of circuit assemblies are directly mounted on the PCB surface and soldered in place. The “through-hole technology” was replaced largely by SMT where components were fitted using wire leads into holes.
SAC solders are used by nearly two thirds of the Japanese manufacturers for wave and reflow soldering, and nearly three fourths of companies for hand soldering. SAC is a popular lead free solder alloy, based on the reduced melting point of the Tin/silver/copper (SnAgCu) and ternary eutectic behavior.
Tin/silver/copper (SnAgCu) vs SnPb solder alloys
The requirements are quite different for the process of Tin/silver/copper (SnAgCu) and SnPb solders, as they are different in both material and method for electronic assembly. Also, the SnPb solders reliability is well established, while the SAC solders are still being studied. One of the main difference is that the Pb free soldering requires a higher temperatures and more controls on the process to attain same results as that of the SnPb method. Electrolytic capacitors are some of the components vulnerable to SAC assembly temperature.
SAC has shown consistently better results in thermal cycling for Pb-free alloys. Also SAC alloys perform proportionately better in thermal fatigue as the range of thermal cycling decreases. SAC has better performance than SnPb at less extreme cycling conditions.
What is the Soldering process?
Soldering process is using the fusible metal alloy to create a permanent bond between different metal pieces. The solder must be melted first in order to adhere to and connect the pieces together after cooling down, so this requires that the alloy suitable for use as solder should have a lower melting point than the two pieces being joined. The solder should be also resistant to both corrosive and oxidative effects which can degrade the joint over time. Also, the solder used in electrical conductive connections needs to possess proper electrical characteristics.
Lead in Solar industry
The typical 60 cell crystalline silicon solar module that is produced today contains nearly 12 grams of lead. This lead is primarily found within the ribbon coating and the soldering paste used for connecting cells together.
Lead is the usually the ideal material for PV process due to its melting point. The lead presence allows for lower process temperature during stringing, which then reduces the stress placed on the solar cells. Alternative materials for ribbon, includes a pure tin coating or replacing lead with bismuth, but these require a higher process temperature.
Soldering at a temperature of 210 degrees can be done when lead is present in the material. And this is much less stressful than the 260 degrees or higher that you would need otherwise. Increasing the temperature is a possibility for enabling the use of a lead free soldering process. But this increases the risk of the extra stress placed on the cell, which can lead to micro cracking and a higher breakage rate during production.
SnAgCu solder alloys in Solar application
Despite the SnPb alloys still being popular due to their wetting attributes and high reliability, yet with the increase in demand for green technologies and integrated green solutions, SnAgCu provides a good alternative for the tabbing of solar cells.