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Lead-free Alchemy
The Full Story
December 31, 1969 |
Estimated reading time: 3 minutes
By Meredith Courtemanche, SMT
In lead-free manufacturing, R&D dollars are devoted to understanding metallurgy why does lead-free solder work; and what changes will make it wet better, reduce brittle joints, and make any in-the-know person feel safe driving a car with lead-free boards in the control panel? Adjusting reflow profiles, assembling with nitrogen, underfilling and encapsulating, and designing boards with end-use in mind are all tertiary to the laurels for 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 that 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 will form, leaving the assembly vulnerable to cracking. If copper dissolution is too rapid or aggressive, the intermetallic layer will grow disproportionately, creating a thick, weak band.
Underlying these concerns is a solder recipe. One could assume 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 in overcoming the stiffening effects demonstrated when copper is used in solder alloys, especially for assemblies under high strain rates. The more silver found in a particular bulk solder product, the stiffer that solder will be; and brittle failures may result. 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 in reducing the growth of copper/tin (CuSn) intermetallics. Bismuth (Bi) can improve grain structure, making lead-free solder more processable and intermetallic compounds more uniform. These "micro" metals, though they do improve solder properties, require suppliers to vary levels of majority metals, and make other adjustments to maintain workability. Brian Lewis, Ph.D., Cookson Electronics Assembly Materials, notes that his company removed phosphorus from its next-generation SAC lead-free solder to reduce oxidation, accommodating Bi additives.
Other suppliers have developed lead-free systems outside of the SAC formula. Nihon Superior's lead-free solder incorporates tin, copper, and nickel, which is said to control copper dissolution rates more effectively that SAC formulations, and lead to a more robust intermetallic layer. Avoiding silver and phosphorus can prevent aggressive interaction with copper pads and traces. Kester also developed a silver-free, lead-free solder, with two metallic dopants, to regulate intermetallic formation and control defects during reflow.
Simplifying lead-free assembly to omit eutectic alloys in solder, surface finishes, components, and fluxes is a dangerous approach. R&D activities have shown that replicating the role of lead in assembly requires a host of supporting metallics and process changes. EMS providers have discovered that the balance of lead-free assembly is project-dependant, meaning that a 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 noted that prototyping any new lead-free product is essential to manufacturing reliable assemblies. More active fluxes may be required for the tighter lead-free process window. Thicker PCBs may experience hole-fill and joint formation problems not seen in less-complex designs. BGA warpage, a problem facing component manufacturers, can disrupt lead-free joints and is more prevalent under the higher reflow temperatures of lead-free assembly.
REFERENCES:Contact the author for a complete list of references.
Meredith Courtemanche, assistant editor, SMT, may be contacted at (603) 891-9176, mcourtemanche@pennwell.com.