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March 2016 • The PCB Magazine 79 fabricators must select an alternative rated on cost, reliability, shelf life, and customer specifi- cations. In conjunction with the U.S. Environmental Protection Agency (U.S. EPA), IPC conducted an evaluation of alternative solder finishes. Addi- tionally, the EPA, IPC and EIA also conducted a life cycle assessment of lead free solders. • Lead-free Solder (source: IPC) A broad variety of solder alloys have been investigated as potential replacements for tin lead solder. While there is broad agreement that there will be no drop-in replacement for tin lead, alloys based on tin/silver/copper ("SAC") have emerged as the Sn/Pb replacements of choice. • Multilayer Bonders Multilayer bonders are coatings on innerlay- er copper circuitry to enhance adhesion to pre- preg during multilayer lamination. As dielectric resins had to be modified to better hold up to lead-free soldering temperatures, bonders had to be modified. • Thick Film Ceramic Pastes Glass frits containing lead had to be refor- mulated. • Desmear Chemistries Desmear chemistries are typically solvent swelling of dielectric resin followed by perman- ganate etching of the epoxy, to clean the drilled through-holes and/or to micro-roughen the dielectric surface. As base material resins need- ed to be modified for higher Tg to better hold up during lead-free solder reflow, the desmear chemistries had to be modified (made more ag- gressive). • Soldermasks Soldermasks had to be modified to avoid outgassing, discoloration, blistering, and de- lamination at higher processing temperatures. • Other Concerns: Tin Whiskers (Source: IPC) (Note: Lead (Pb), even at low levels, is known to largely suppress tin whisker formation.) A large area of concern with the use of tin- plated or pure tin component finishes is the occurrence of tin whiskers. This condition arises when tin begins to grow tiny filaments known as whiskers. A tin whisker is a sponta- neous columnar or cylindrical filament, which can branch, of mono-crystalline tin emanating from the surface of a plating finish. Tin whis- kers generally have an aspect ratio (length/ width) greater than two; whiskers have been found to be over several mm in length in rare instances. The whiskers can be kinked, bent, or twisted and may be surrounded by striations/ rings. These whiskers are very brittle therefore can lead to intermittent or permanent electrical shorts when broken and re-deposited upon the circuit's surface. Ways of avoiding whiskers in actual field ap- plications include: • Not using pure tin, especially in a 'brightened' format. • Reflow of the tin plating to re-fuse/ re-crystallize and stress-relieve the deposit. • Using barrier materials (over-plating or organic post-coatings) to encapsulate any whiskers which have formed since completion of the plating. C. Examples of minimizing energy consumption, avoiding the use of toxic materials, recycling and re-use of materials • Soldermask developer chemistry: use of polyalcohols. • On-site copper recovery from acidic and alkaline etchants. • Recovery of materials from copper-clad boards/scrap: Example: separation of metals, resins, and glass for re-use. • Replacing electroless copper process with direct metallization processes: Avoids several process steps, reduces water consumption. green legislation and the impaCt on eleCtroniC materials and proCesses