Lead Solder vs Lead Free Solder: Conductivity

The industry trend over the past two decades has been a move to lead-free solder, as most of the world is switching to these solders for various reasons. Some question this trend, objecting to the change in industry-standard practices. 

There are a number of good reasons to consider lead-free solders. Reduction of lead pollution is one such reason. Ground, air and water pollution from lead is hazardous to the ecosystem but also to people. Lead concentrations in air have reduced over time, thanks to lead reduction efforts in the paint industry and others such as the electronics manufacturing industry. Additionally, lead exposure can be harmful – fatal, in high enough doses – to employees. This is not only a physical hazard but a moral one as well. 

However, is there a benefit to lead-free solders? Are they more conductive than leaden solder alloys? Which solder should you decide to use?

Conductivity Of Leaden and Lead-Free Solders

Electrical conductivity of solder is determined by its composition as some substances have more resistance than others. Copper is the most conductive material known, which is why the standard measure of conductivity is the International Annealed Copper Standard – or IACS –  expressed as a percentage of the conductivity of annealed copper at 20 degrees C. 

A data sheet from Farnell.com shows testing of various solders for various attributes, including conductivity. The higher the score, generally the better though solder is best matched to the application for which it will be used. A solder for electronic components should score at least a 7 percent. 

Solder Conductivity Scores

Pure tin, in Multicore’s data sheet, had an IACS score of 13.9 percent, so clearly lead-free solders can achieve good conductivity. However, a 15/85 lead-tin alloy scored 8.5 percent. Slightly bumping the alloy to 20/80 had only an incremental improvement, scoring an 8.7 percent. However, solders with higher concentrations of lead – such as Multicore’s Sn62 and Sn63, which are 62 and 63 percent tin, 36 and 35 percent lead and 2 and 1 percent silver, respectively, scored an 11.9 percent for Sn62 and 11.5 percent for Sn63. 

The solder chart of the Indium Corporation – which can be found online reveals that many lead free solder alloys are capable of similar conductivity.

For instance, an 80/20 alloy of gold and tin (Indalloy #182) scored a 14.51 percent. A solder composed of 88 percent gold to 12 percent Germanium, which is Indalloy #183, scored a 14.67 percent, and a solder composed of 82 percent gold to 18 percent Indium – Indalloy #178 – scored a 14.9 percent. 

These are just a few examples; hundreds of different solder alloys are available on the market today. A more in-depth study of technical data on lead-free solders is available from the National Institute of Standards And Technology in PDF format. 

A Specific Lead Free Solder Is Perfect In The Right Application

Going over technical data comparing lead free solder, lead-tin and other lead alloys gives you an idea of certain properties, but they don’t give you the whole story. 

For instance, the above mentioned lead-free solders by Indium are eutectic, so they have more specific applications requiring only brief changes in temperature. That does not make them well-suited to boards that require longer soldering times under heat. 

Pure tin is very conductive, but is also very weak. Multicore found it had around 60 percent of the tensile strength of Sn63 and less than 50 percent of that of Sn62. Clearly, pure tin can only be employed if not subjected to much, if any, mechanical stresses.

By contrast, the Indalloy #204 family – lead/indium alloy solders – are some of the best general purpose solders on the market with scores ranging from 8 percent to 9 percent of IASC. Sn62 is also known for being a fantastic all-purpose solder. 

So, is it so much that lead free solder is a better conductor than lead alloy solders? The right alloy can be. However, that doesn’t matter as much as matching the solder to the components you’ll be soldering them with. 

Solder, just like a selective soldering machine, is only a tool, which must be used for the task for which it is best suited.