Manufacturability and Reliability Screening of Lower Melting Point, Pb-Free Alloys Containing Bismuth

Abstract

This paper explores the manufacturability and reliability of three Pb-free Bi-containing alloys in comparison with conventional SAC305 and SnPb assemblies. The first alloy included in the study is a Sn-based alloy with 3.4%Ag and 4.8%Bi, which showed promising results in the National Center for Manufacturing Sciences and German Joint projects. The other two alloy variations have reduced Ag content, with and without Cu. BGA and leaded components were assembled on medium-complexity test vehicles using these alloys, as well as SAC305 and SnPb as baseline alloys, for comparison. Test vehicles were manufactured using two board materials, 170°C glass transition temperature (Tg) and 155°C Tg, with three surface finishes: ENIG, ENEPIG, and OSP. The accelerated temperature cycling (ATC) testing was done at −55°C to 125°C with 30-min dwells and 10°C/min ramps, for 3,000 cycles. Detailed microstructure examination before and after ATC testing is described, as is failure analysis. All three experimental alloys showed excellent performance in harsh-environment thermal cycling. Vibration testing at two G-force test conditions with resistance failure monitoring was performed on the daisy-chained components. A detailed description of the technique for the vibration testing using 2 G and 5 G harmonic dwells is provided. The lowest failure rate found at both the 2 G and 5 G levels was for the Cu-containing alloy known as Violet. These results provide data for further statistical analysis leading to the choice of proper combinations of the solder alloys, board materials, and surface finishes for high-reliability applications.

Precipitation and coarsening of bismuth plates in Sn–Ag–Cu–Bi and Sn–Cu–Ni–Bi solder joints

“…Examples of aligned Bi plates are given in Figs. 3, 4, 6b and 7, and similar examples can be found in the literature [7,31,48,49]. The shape of (Bi) precipitates suggests the existence of preferred interfaces between (Bi) and βSn.…”

“…and the combination of Pb and Bi produced a low temperature eutectic that could lead to early failure [1,4,5]. Now that the industry is truly Pb-free, Bi can be used without this risk and, recently, there has been a resurgence of interest in Bi additions mainly for designing the third-generation, high-reliability electronic solders targeting applications in harsher environments [6][7][8][9][10][11][12][13]. Broadly, there are three categories of Bi-containing solders under investigation and entering service: (i) low Bi solders (typically < 2 wt% Bi) strengthened by Bi solid solution strengthening of βSn such as Sn-0.7Cu-0.05Ni-1.5Bi (wt%) [9,10]; (ii) medium Bi solders (typically 3-7 wt% Bi) strengthened by both Bi solid solution strengthening and (Bi) precipitates such as Sn-3.8Ag-0.7Cu-3Bi-1.5Sb-0.15Ni [14] and Sn-2.25Ag-0.5Cu-6Bi [7]; and (iii) solders based around the Sn-57Bi low melting eutectic for low process temperature electronics such as Sn-57Bi-1Ag, Sn-57.5Bi-0.5Sb and Sn-58Bi [15,16].…”

“…Bi additions have been demonstrated to also improve the wetting and spreading performance of lead-free solders [2,19,21], to decrease their liquidus temperature [26], to reduce the excessive growth of the Cu 3 Sn IMC layer [21,27,28] and to prevent Sn-whisker growth [29,30]. Bi additions have been further demonstrated to retard the rapid degradation of solder joint mechanical response/reliability during ageing, which is typical behaviour for SAC compositions subjected to elevated temperatures [7,8,11,31].…”

“…This work explores the precipitation and coarsening of (Bi) plates at room temperature in four widely used Pb-free solders each with an addition of 4 wt% Bi: Sn-3Ag-0.5Cu-4Bi (SAC305 + Bi), Sn-1Ag-0.5Cu-4Bi (SAC105 + Bi), Sn-0.7Cu-4Bi, and Sn-0.7Cu-0.05Ni-4Bi (SN100C + Bi). 4 wt% Bi was chosen as it is within the range of typical Bi additions under investigation for 'medium Bi' solders [7,8,28] and is known to be sufficient to cause the formation of both non-equilibrium eutectic (Bi) particles and the solid-state precipitation of (Bi) plates [10,32]. Additionally, we study (Bi) precipitation in near-eutectic Sn-57Bi to compare (Bi) precipitation in high-Bi versus medium-Bi solders.…”

Properties of Mixing SAC Solder Alloys with Bismuth-Containing Solder Alloys for a Low Reflow Temperature Process

“…[29][30][31] Also the goal remains for Electronic Manufacturing Service (EMS) providers to once again utilize low Tg substrates and other component materials previously reliable for SnPb to further reduce costs. [32] The large efforts to identify a Low Temperature solder alloy drives the need for understanding the metallurgical condition through refl ow and correlate that to reliability. Areas of knowledge gaps regarding mixed alloy assemblies are associated with the challenges in manufacturing processes and reliability.…”