Pb-free solder alloys are lead free alloys which are becoming widely used in consumer electronics because of new legislations.
What is the soldering process?
Soldering 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. 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 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.
Pb-free solder alloys in Solar industry
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.
Pb-free solder alloys: application
The adoption of the Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1)in 2003 was one of the main driving forces that mandated the electronic industries to look for alternatives to replace the SnPb eutectic alloy used for decades.
The RoHS was adopted in early 2003 and came into effect on July 1, 2006, restricting the inclusion of any lead in most of consumer electronics sold inside the EU region, and having a broad effect on all consumer electronics sold worldwide. In the US for example, manufacturers can receive tax benefits by the reduction of using lead based solder. The Lead free solders commercially used, may contain tin, bismuth, zinc, indium, antimony, copper, silver and traces of other metals. The lead-free replacements for the conventional 63/37 and 60/40 Sn-Pb solder have melting points ranging between 50 and 200 °C higher. Yet there are also solders which have much lower melting points. Lead free solder typically needs around 2% flux by mass for adequate wetting ability.
The Lead free solder may be less desirable for some critical applications, like aerospace and medical projects, as its properties are not yet comprehensively known.
Lead free solders have been rapidly increasing and are having a high demand because of regulatory requirements in addition to their health and environmental benefits of avoiding any lead based electronic components. Also used in applications directly relating to health which can be harmful due to Lead poisoning.