Lead Free vs. Leaded Solder
The Restriction of Hazardous Substances directive is more commonly know in the electronics industry as RoHS. Adopted by the European Union in 2003, the directive took effect on July 1st, 2006 with the onus being placed on each member state to adopt and fully implement the directive into law. The RoHS directive is aimed at restricting the use of 6 hazardous materials in the manufacture of electrical and electronic devices as follows:
- Hexavalent Chromium
- Polybrominated Biphenyls
- Polybrominated Diphenyl Ether
Closely linked with the RoHS directive is the Waste Electrical and Electronic Equipment directive which is more commonly known as the WEEE directive. The WEEE directive sets collection, recycling and recovery targets for electrical goods.
The effect these two initiatives have had on the electronics industry varies greatly depending on product end use and target sales market. The overall supply chain from individual components to bare printed circuit board manufacturing has shifted from a predominately tin-lead alloy based market to one that caters almost exclusively to lead-free finishes. The result has been limited supply, and in some cases, complete elimination of tin–lead plated components. This has, in effect, forced manufacturers to make design and process changes on products that were traditionally tin-lead based.
The primary difference between tin/lead and lead-free solders, from a rework and repair standpoint, is the temperatures required to form a proper intermetallic bond. For the most widely used tin/lead alloys such as Sn60 Pb40 or more commonly Sn63 Pb37 (eutectic), the melting point is 361°F (183°C). The most commonly used lead-free alloy, Sn96.5 Ag3.0 Cu0.5, commonly referred to as SAC 305, has a melting point of 422°F (217°C) to 428°F (220°C). The resultant increase in melting point will have the effect of reducing the overall process window and can change the traditionally accepted appearance of the finished product.
Prior to the implementation of the RoHS and WEEE directives, the use of tin/lead solder was widely accepted, it’s reliability was exhaustively tested and it’s appearance was easily inspectable. Virtually all electronics assemblies were designed to withstand manufacturing with the use of tin/lead solder and the temperatures they required. Further, virtually all specifications written for the compliance of electronic assemblies in the military, government and consumer markets were written with the same tin/lead alloy in mind.
Today the use of lead-free solder alloys that comply with the RoHS & WEEE directives are in wide use and while various segments of the electronics industry continue to perform reliability and life-cycle testing on complete RoHS & WEEE compliant assemblies and manufacturing practices, the use of individual lead-free components and board finishes is commonplace. Industry specifications have also addressed the differences between the two alloy types to ensure compliance and reliability where applicable.
The Process Guides contained herein will reference the common tin/lead (Sn63 Pb37) and lead-free (Sn96.5 Ag3.0 Cu0.5) alloys and their associated process temperatures for every application.