Tin/silver/lead alloy (SnAgPb) commonly used as solder. Tin/silver/lead alloy (SnAgPb) is a more environmental friendly alloy which is widely used in consumer electronics because of new legislations.
Tin/silver/lead (SnAgPb) solder alloys
The Tin/silver/lead alloy (SnAgPb) –also called SAP- has been steadily used as an alloy system to replace tin-lead because it is near eutectic, with good strength, thermal fatigue properties, and wettability. It is not completely Lead-free solder but offers a trade-off as it has less lead content but still gives good properties with reasonable price. Decreasing Lead content is gaining more demand as awareness of the environmental effects of lead in industrial products is increasing, and as a result of Europe’s RoHS legislation to remove lead and other hazardous materials from electronics. Tin/silver/lead alloy is one of numerous metal alloys is available as bar, ingot, ribbon, sheet, and foil.
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 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 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, include 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.
This directive banning the use of lead, includes a caveat that states that this law “should not prevent the development of renewable energy technologies that have no negative impact on health and the environment and that are sustainable and economically viable.” It specifically includes a permanent exemption for “the photovoltaic panels which are intended to be used in a system that is designed, assembled and installed by professionals for permanent use at a defined location to produce energy from solar light for public, commercial, industrial and residential applications.”
Also the relatively small amounts of lead used in the PV industry, and the fact that the lead is enclosed entirely within the module materials, the PV products by themselves are unlikely to be the cause of a major lead pollution, even in the worst case scenario where a large percentage of panels end up in landfill.
Benefits of Tin/silver/lead (SnAgPb) solder alloys
As mentioned earlier, Tin/silver/lead alloys (SnAgPb) are a friendlier option that is more green than the normal SnPb solders widely used in Solar industries and is cheaper than completely Lead free solders and still providing very good mechanical and electrical properties that decrease degradation of characteristics of the ribbon joint in PV module solder joints.