SMTA International Conference Proceedings

Inemi Project on Process Development of Bi-Sn-Based Low Temperature Solder Pastes

Authors: Haley Fu, Raiyo Aspandiar, Jimmy Chen, Shunfeng Cheng, Qin Chen, Richard Coyle, Sophia Feng, Mark Krmpotich, Ronald C. Lasky, Scott Mokler, Jagadeesh Radhakrishnan, Morgana Ribas, Brook Sandy-Smith, Kok Kwan Tang, Greg Wu, Anny Zhang, Wilson Zhen
Company: iNEMI, Intel Corporation, Flex, Eunow, Nokia, Celestica, Microsoft Corporation, Indium Corporation, Alpha Assembly Solutions, Wistron Corp., Lenovo
Date Published: 9/17/2017   Conference: SMTA International

Abstract: The 2017 iNEMI Board and Assembly Roadmap forecasts that, due to economic, environmental and technical drivers, use of low temperature solder pastes will increase significantly and reach 10% of all solder paste used for board assembly by 2021.

The Bi-Sn solder system is the primary target for low temperature soldering. To overcome potential drawbacks in the reliability of Bi-Sn solder joints caused by the inherent brittleness of the Bi-Sn eutectic solder composition, solder suppliers have developed new solder pastes adopting due distinct strategies. One is enhancing the ductility of the Bi- Sn alloy using fundamental metallurgical principles; the other is providing a polymeric reinforcement of the solder joint by incorporating resin in the solder paste.

To address the challenge of evaluating these new formulation solder pastes from multiple suppliers by determining their suitability in surface mount processing and resistance to mechanical shock and fatigue stresses, iNEMI initiated the BiSn-Based Low-Temperature Soldering Process and Reliability (LTSPR) Project, with 20+ member participants. This paper reports out the results from the initial phase of this project, which is assessing the processability of these solder pastes through surface mount soldering steps.

Five solder pastes from the ductile Bi-Sn solder paste category and four solder pastes from the Joint Reinforced Paste (JRP) resin containing solder paste category were evaluated. Three Bi-Sn eutectic composition pastes with Ag added in varying amounts and one SnAgCu (SAC) solder paste were also evaluated for comparison as the baseline pastes. The aspects of processability assessed were the printability, reflow profiling, solder joint yield and defects analysis, component removal and site re-dress in the rework operation and the surface insulation resistance (SIR) of the paste flux residue.

The results indicated the following. Ductile Bi-Sn paste and JRP pastes performed equivalent to the baseline Bi-Sn and SAC solder pastes over the range of stencil aperture area ratios studied, except that at the lowest aperture ratio of 0.50 the JRP resin paste have lower transfer efficiencies and coefficient of variation that the others. JRP resin pastes have a trapezoidal reflow profile topography in comparison to the ramp-soak-peak topography for the other category pastes. Initial ramp rate for JRP paste is critical to avoid premature gelling of the resin before the molten solder has full wetted the terminations. Partial wetting and non-wetting will occur if this ramp rate is lower than required. An unusual `hot tearing` solder joint defect was observed for Flip Chip Ball Grid Array (FCBGA) components. This hot tearing phenomena is caused by an interaction of the bismuth stratification at the package substrate interface, the FCBGA dynamic warpage characteristic as it cools down from the peak reflow temperatures and the cooling rate during reflow soldering. Hot Tearing was observed in solder joints formed with the four solder pastes that had the highest bismuth mixing level in the SAC ball.

A quantitative assessment by rework operators for removal of and subsequent site redress for FCBGA and QFN components showed that the lower melting temperature of the solder joints formed with Bi-Sn solder pastes facilitates easier part removal and site redress All solder pastes passed the minimum requirement for the SIR measurement.

Key Words: 

BGA solder joints, low temperature solder, Bi-Sn metallurgy, Mechanical shock reliability, Temperature cycle reliability, Polymeric reinforcement

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