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Lead-free Alchemy
December 31, 1969 |Estimated reading time: 2 minutes
BY Meredith Courtemanche, SMT
In lead-free manufacturing, R&D dollars are devoted to understanding metallurgy - how does lead-free solder work; and what changes will make it wet better, or reduce brittle joints? Adjusting reflow profiles, assembling with nitrogen, underfilling and encapsulating, and designing boards with end-use in mind are all important to global electronics assemblers - the lead-free solder alloy that performs as well as or better than tin/lead.
At the metallurgical level, the crux of lead-free resides in the intermetallic bond formed when a component reflows to a PCB. A homogeneous intermetallic layer is essential to lead-free reliability, states Wolfgang Biben, Centro de Pesquisas Renato Archer (CenPra). The intermetallic compound formed when a component lead or BGA ball reflows to the PCB’s copper pad depends on a solder alloy’s metallic composition. If the solder is too stiff, brittle joints form, leaving the assembly vulnerable to cracking. If copper dissolution is too rapid or aggressive, the intermetallic layer grows disproportionately, creating a weak band.
One could expect that a drop-in lead-free replacement for tin/lead solders - with similar processing parameters, wetting capabilities, and metallic interactions - would allow assemblers to at least shorten time-consuming prototyping, testing, and development work. Solder suppliers are integrating dopants and exotic metals, additives, and new chemistries beyond the standard tin/silver/copper (SAC) formulation in attempts to reach this goal. Additional dopants may be beneficial to overcome stiffening effects seen when copper is used in solder alloys, especially for assemblies under high strain rates. A more ductile solder, with balanced copper-diffusion rates, will be more forgiving to the myriad of stresses an assembly faces over its life.
Nickel (Ni) has shown promise to reduce the growth of copper/tin (CuSn) intermetallics; bismuth (Bi) can improve grain structure. While these “micro” metals improve solder properties, they also require varying levels of majority metals, and other adjustments to maintain workability. Many solder suppliers have developed lead-free systems outside of the SAC formulation.
Omitting eutectic alloys from solder, surface finishes, components, and fluxes is dangerous. R&D activities have shown that replicating the role of lead in assemblies requires a host of supporting metallics and process changes. The balance of lead-free assembly is project-dependant - thicker PCB, BGA components, and other extra-metallurgical factors influence the end product. Solectron’s corporate environment manager Nathalie McKinsey and lead engineer, RoHS process development, Jasbir Bath note that prototyping any new lead-free product is essential to manufacturing reliable assemblies. Thicker PCBs may experience hole-fill and joint formation problems not seen in less-complex designs. BGA warpage is more prevalent under the higher reflow temperatures of lead-free assembly.For the full story, visit www.smtonline.com; ID #304046.