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Tuesday, 29 January 2013 17:48

PCB Design

“Bottom Termination Component Land Pattern Design and Assembly for High Reliability Electronic Systems”

Scott Nelson; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Abstract: With the rapid implementation of BTCs in high-reliability applications, two key issues have evolved. The first is improper land pattern design, and the second is improper solder stencil design. These two issues are at opposite extremes of the product cycle. When issues arise, this can make it difficult to answer the question, “Is it a design problem or is it a manufacturing problem?” In many cases the answer has been found to be a combination of both. (SMTA International, October 2012)


“Study of Sulfide Films Grown on Printed Circuit Boards”

Authors: Anil Kurella, Ph.D., and Balu Pathangey, Ph.D.; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Abstract: Sulfide films were grown on PCBs with immersion silver (ImAg) and organic solderability preservative (OSP) surface finish. Time of flight secondary ion mass spectroscopic (TOF SIMS) depth profiling showed significant moisture accumulation in the region below the corrosion product. In both cases (ImAg and OSP) copper was seen to diffuse through the surface finish and react with the sulfide-rich environment. (SMTA International, October 2012)

“The Role of Pd in Sn-Ag-Cu Solder Interconnect Mechanical Shock Performance”

Authors: Tae-Kyu Lee, Bite Zhou, Thomas R. Bieler,  Chien-Fu Tseng and Jeng-Gong Duh
Abstract: The mechanical stability of solder joints with palladium added to SAC alloy with different aging conditions was investigated in a high-G level shock environment. A test vehicle with three different strain and shock level conditions in one board was used to identify the joint stability and failure modes. The results revealed that palladium provided stability at the package-side interface with an overall shock performance improvement of over 65% compared with the SAC alloy without palladium. A dependency on the pad structure was also identified. However, the strengthening mechanism was only observed in the non-solder mask defined (NSMD) pad design, whereas the solder mask-defined pad design boards showed no improvement in shock performance with Pd-added solders. The effects of tin grain orientation on shock performance, interconnect stability, and crack propagation path with and without palladium are discussed. The SAC 305 + Pd solder joints showed more grain refinements, recrystallization, and especially mechanical twin deformation during the shock test, which provides a partial explanation for the ability of SAC 305 + Pd to absorb more shock-induced energy through active deformation compared with SAC 305. (Journal of Electronic Materials, February 2013)

Solder Materials

“Study of Intermetallic Growth and Kinetics in Fine-Pitch Lead-Free Solder Bumps for Next-Generation Flip-Chip Assemblies”

Ye Tian, Justin Chow, Xi Liu, Yi Ping Wu, and Suresh K. Sitaraman
Abstract: With continued advances in microelectronics, it is anticipated that next-generation microelectronic assemblies will require a reduction of the flip-chip solder bump pitch to 100µm or less from the current industrial practice of 130µm to 150µm. With this reduction in pitch size, and thus in bump height and diameter, the interaction between die pad metallurgy and substrate pad metallurgy becomes more critical due to the shorter diffusion path and greater stress. Existing literature has not addressed such metallurgical interaction in actual fine-pitch flip-chip assemblies. This work studies intermetallic growth and kinetics in fine-pitch Pb-free solder bumps through thermal aging of flip-chip assemblies. Based on this study, it is seen that nickel from the die pad diffuses to the substrate pad region and copper from the substrate pad diffuses to the die pad region, thus the resulting intermetallic compounds at the die and substrate pad regions are influenced by the other pad as well. Such cross-pad interaction is much stronger in fine-pitch solder bumps with smaller standoff height. It is seen that the die pad region contains Ni3P and (Cu,Ni)6Sn5 after thermal aging, while the substrate pad region contains Cu3Sn and (Cu,Ni)6Sn5. By digitally measuring the thickness of the interfacial phases, the kinetics parameters and the activation energy were calculated for the growth of (Cu,Ni)6Sn5 on the substrate side. The copper diffusion coefficient through the intermetallic compound (IMC) layer was found to be 0.03370µm2/h, 0.1423µm2/h, and 0.4463µm2/h at 100°C, 125°C, and 150°C, respectively, and the apparent activation energy for the growth of compound layers was 67.89 kJ/mol. (Journal of Electronic Materials, February 2013)

This column provides abstracts from recent industry conferences and company white papers. Our goal is to provide an added opportunity for readers to keep abreast of technology and business trends.

Last Updated on Wednesday, 30 January 2013 17:29


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