Shear Strength Degradation Modeling of Lead-Free Solder Joints at Different Isothermal Aging Conditions

SAC-based alloys are one of the most common solder materials that are utilized to provide mechanical support and electrical connection between electronic components and the printed circuit board. Enhancing the mechanical properties of solder joints can improve the life of the components. One of the mechanical properties that define the solder joint structure integrity is the shear strength. The main objective of this study is to assess the shear strength behavior of SAC305 solder joints under different aging conditions. Instron 5948 Micromechanical Tester with a customized fixture is used to perform accelerated shear tests on individual solder joints. The shear strength of SAC305 solder joints with organic solderability preservative (OSP) surface finish is investigated at constant strain rate under different aging times (2, 10, 100, and 1,000 h) and different aging temperatures (50, 100, and 150°C). The nonaged solder joints are examined as well for comparison purposes. Analysis of variance (ANOVA) is accomplished to identify the contribution of each parameter on the shear strength. A general empirical model is developed to estimate the shear strength as a function of aging conditions using the Arrhenius term. Microstructure analysis is performed at different aging conditions using scanning electron microscope (SEM). The results revealed a significant reduction in the shear strength when the aging level is increased. An increase in the precipitates coarsening and intermetallic compound (IMC) layer thickness are observed with increased aging time and temperature.

Effect of Bismuth Additions on Wettability, Intermetallic Compound, and Microhardness Properties of Sn-0.7Cu on Different Surface Finish Substrates

The influence of bismuth (Bi) addition on wettability, thickness of interfacial intermetallic compound (IMC), and microhardness properties of Sn-0.7Cu + xBi solder alloy using different types of substrate were examined. The 0.5, 1.0, 1.5, and 2.0 wt. % Bi was added into Sn-0.7Cu and fabricated using the casting process. The result shows that the influence of 1.5 wt. % Bi in the Sn-0.7Cu solder soldered on copper organic solderability preservative (Cu-OSP) and immersion tin (Im-Sn) surface finish has improved the wettability and microhardness. Subsequently, the IMC thickness of Sn-0.7Cu+1.5Bi solder alloy on Im-Sn surface finish gives a better result than reflowed on Cu-OSP. Generally, with the addition of 1.5 wt. % Bi in Sn-0.7Cu solder alloy reflowed on the Im-Sn surface finish had enhanced the performance in terms of wettability, thickness of IMC and microhardness properties compared to on Cu-OSP surface finish.

Effect of Bismuth Additions on Wettability, Intermetallic Compound, and Microhardness Properties of Sn-0.7Cu on Different Surface Finish Substrates

The influence of bismuth (Bi) addition on wettability, thickness of interfacial intermetallic compound (IMC), and microhardness properties of Sn-0.7Cu + xBi solder alloy using different types of substrate were examined. The 0.5, 1.0, 1.5, and 2.0 wt. % Bi was added into Sn-0.7Cu and fabricated using the casting process. The result shows that the influence of 1.5 wt. % Bi in the Sn-0.7Cu solder soldered on copper organic solderability preservative (Cu-OSP) and immersion tin (Im-Sn) surface finish has improved the wettability and microhardness. Subsequently, the IMC thickness of Sn-0.7Cu+1.5Bi solder alloy on Im-Sn surface finish gives a better result than reflowed on Cu-OSP. Generally, with the addition of 1.5 wt. % Bi in Sn-0.7Cu solder alloy reflowed on the Im-Sn surface finish had enhanced the performance in terms of wettability, thickness of IMC and microhardness properties compared to on Cu-OSP surface finish.

Mechanical Reliability of Epoxy Sn–58wt.%Bi Composite Solder Under Temperature-Humidity Treatment with Organic Solderability Preservatives (OSP) Surface Finish

The microstructures and mechanical reliability of Sn–58Bi solder and epoxy Sn–58Bi composite solder joint were investigated with organic solderability preservative surface finishes. The mechanical reliabilities of Sn58Bi and epoxy Sn58Bi solder were evaluated by the board-level drop test and the 3-point bend test after temperature-humidity storage testing. The microstructure and chemical composition of the solder joints were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The addition of epoxy in solder paste did not affect the morphology of the intermetallic compound. The thickness of the scalloped-shaped Cu6Sn5 intermetallic compound of solder/OSP joint increased with aging time. The drop number until fail for the epoxy Sn58Bi/OSP joint was higher than that for the Sn–58Bi/OSP joint; the average numbers of drops withstood by the Sn–58Bi/OSP joint and epoxy Sn–58Bi/OSP joint following the reflow process were fewer than 10 drops and 180 drops, respectively. The drop number of solder/OSP joints decreased with increasing aging time. The result of the 3-point bend tests shows that the number of bend cycles for the epoxy Sn–58Bi/OSP joint was 30 times higher than that for the Sn–58Bi/OSP joint. The number of bend cycles for solder/OSP joints was decreased with increasing aging time.

Assembly and Reliability Studies on Reworked and Non-Reworked QFN Packages

Quad Flat No lead (QFN) packages are being used as alternative components to some of the large form-factor packages due to their smaller footprint, improved thermal and electrical performance. During the past few years, there has been a surge of published papers by end users and component manufacturers on the topic of assembly and reliability of the QFN packages. Both empirical and simulation data has been published, which has helped several manufacturers to develop design guidelines and assembly processes for these packages. There exists a gap in the current published work on the assembly and reliability of high I/O count QFNs on thick boards. In addition, reliability of the rework process on QFNs is not well documented. Therefore, objectives of this research were the following: • Evaluate the second level reliability in thermal cycling for reworked and non-reworked QFNs; • Evaluate the reliability of different sized QFNs on 125 mil thick boards and compare with existing literature; • Failure analysis and recommendations for reliability improvement; • Land pattern comparison; and • Rework process improvement. Studies have been conducted on different QFN packages between 32 pins and 164 pins on 125 mil thick boards with Organic Solder Preservative (OSP) surface finish. Experimental design includes the use of different package sizes, alternate thermal pad patterns, and use of tin-lead versus lead free processes. A subset of some of the package types has also been reworked using variations of the standard BGA rework processes to evaluate its reliability. All packages were daisy chained. Boards were subjected to thermal cycling test between 0 to 100 deg C for 6000 hours. Data is analyzed using Weibull distribution. Failure mechanisms have been analyzed using dye penetration tests and cross sections. The reliability results are discussed in terms of package construction, type of land patterns and rework. Recommendations for process improvement are delineated.