Issue link: https://iconnect007.uberflip.com/i/1496178
14 SMT007 MAGAZINE I APRIL 2023 In material: • To choose alloy composition properly (e.g., possessing adequate plastic strain) • To choose alloy composition with narrow pasty range (less than 10 degrees) • To choose alloy composition having good intrinsic metallurgical wetting • To assure adequate wetting condition on PCB land • To assure the compatible soldering flux What Was Learned Achieving one performance property that could jeopardize another performance parameter should be avoided at the outset (i.e., design stage). This may sound as though it's stating the obvious, but the mishap resulting from the lack of holistic understanding of process, materials, and the compatibility between the process and materials has happened, and happened too frequently. Bi is a unique element which can deliver significant utilities that are beneficial to electronic products, but it has to be used properly and scientifically. The interplay of the four elements—Sn, Ag, Cu, Bi—in Sn-based lead-free systems is intricate in the underlying metallurgical interactions and reactions. For a Sn-based lead-free alloy system, adding or removing Bi is a unique element which can deliver significant utilities that are beneficial to electronic products, but it has to be used properly and scientifically. an element, solely reducing or increasing the dosage of an element to target one property or performance parameter, is not a robust approach from the reliability perspective. Manufacturing process, materials, and compatibility are closely intertwined in relation to reliability; setting, selecting, and designing an electronic system must consider process, material, and compatibility as an inter-dependent triad. Caution (Awareness) The discussion in this writing is on a Sn-based system that contains Bi element— in practice, the "base" is practically defined as the element that constitutes 50 vol% or greater of an alloy (i.e., serving as the metallurgical matrix). Distinctions should be drawn between Bi-containing Sn-based alloys and Bi-based alloys. This distinction is profoundly important when we design, select, and use lead-free solder alloys for microelectronic/ electronic packaging and assemblies. • Bi-containing Sn-based alloys e.g., 93Sn3.0Ag1.0Cu3.0Bi e.g., 93Sn3.0Ag1.0Cu1.0Bi2.0XYZ • Bi-based alloys e.g., 57Bi41Sn2Ag e.g., 57Bi41Sn1.5Ag0.5XYZ (dopants) The distinction in properties, performance, and most importantly to the reliability of electronic products, will be discussed in future columns. SMT007 References 1. U.S. Patent 5,520,752: "Composite Solders," May 28, 1996. 2. U.S. Patent 5,985,212: "High Strength Lead- Free Solder Materials," Nov. 16, 1999. 3. U.S. Patent 6,176,947 B1: "Lead-Free Solders," Jan. 23, 2001. 4. Internal Research Reports, Z. Guo and J. S. Hwang, H-Technologies Group, 1991–2002. 5. Internal Research Reports, J. S. Hwang and H. Koenigsmann, H-Technologies Group, 2000– 2007.