The creep of lead-free solders at elevated temperatures

The effects of printed-circuit-board (PCB) surface finish and thermomechanical fatigue (TMF) on the formation and growth of intermetallic compounds (IMCs) between small outline J (SOJ) leads and Sn-3.0Ag-0.5Cu solder were investigated. The thickness of the IMC layer formed initially at the as-soldered SOJ/Sn-Ag-Cu interface over a Ni/Au PCB surface finish was about 1.7 times of that over the organic solderability preservative (OSP) PCB surface finish. The parabolic TMF-cycle dependence clearly suggests that the growth processes are controlled primarily by solid-state diffusion. The diffusion coefficient for the growth of the total IMC layer at the SOJ/Sn-Ag-Cu interface over the Ni/Au PCB surface finish is the same as that over the OSP PCB surface finish, and thus, the total IMC layer at the SOJ/Sn-Ag-Cu interface over the Ni/Au PCB surface finish is thicker than that over the OSP PCB surface finish. Using the Cu-Ni-Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the Cu 3 Sn layer while increasing the initial growth of the Cu 6 Sn 5 layer can be addressed.

Section: Introductionmentioning

confidence: 99%

Effects of different printed-circuit-board surface finishes on the formation and growth of intermetallics at thermomechanically fatigued, small outline J leads/Sn-Ag-Cu interfaces

Huang

Lee

et al. 2004

“…While many studies have demonstrated that Sn-Ag-Cu may substitute for Sn-Pb in general joining tasks, recent results have shown that these solders also provide a marked improvement over Sn-Pb in high temperature, high stress applications, even exceeding Sn-3.5Ag (wt.%) eutectic solder. 1 Although Sn-Ag (T e = 221∞C) is a well-known alternative to Sn-Pb for high temperature applications, alloys in the family of the recentlydiscovered Sn-Ag-Cu ternary eutectic solder (T e = 217∞C) have baseline advantages of a reduced melting temperature and an additional strengthening phase, Cu 6 Sn 5 . 2 When studied in comparison to many other binary, ternary, and quaternary Pb-free alloys, the broad potential of Sn-Ag-Cu solders has been recognized by several industrial groups throughout the world.…”

Section: Introductionmentioning

confidence: 99%

Alloying effects in near-eutectic Sn-Ag-Cu solder alloys for improved microstructural stability

Anderson

Foley

Cook

et al. 2001

205

134

This study included a comparison of the baseline Sn-3.5Ag eutectic to one neareutectic ternary alloy, Sn-3.6Ag-1.0Cu and two quaternary alloys, Sn-3.6Ag-1.0Cu-0.15Co and Sn-3.6Ag-1.0Cu-0.45Co, to increase understanding of the beneficial effects of Co on Sn-Ag-Cu solder joints cooled at 1-3∞C/sec, typical of reflow practice. The results indicated that joint microstructure refinement is due to Co-enhanced nucleation of the Cu 6 Sn 5 phase in the solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and improved the ability of the solder joint samples to maintain their shear strength after aging for 72 hr at 150∞C. The baseline Sn-3.5Ag joints exhibited significantly reduced strength and coarser microstructures.

“…The value of the stress exponent n may vary according to the dependence of the dislocation density on r, and both n and Q are characteristic for the process mechanisms. 13 Studies conducted on Sn-3.5%Ag bulk materials reported stress exponents between 8 and 18, [19][20][21][22] which are significantly higher than stress exponents in pure bulk Sn where n ranges between 4.5 and 9. Some studies on Sn-rich alloys also reported stress exponent ranges varying with the stress values, with n between 4 and 6 at low stresses, [23][24][25][26][27] and between 6 and 16 at higher stresses.…”

Section: Resultsmentioning

confidence: 99%

Constraining Effects of Lead-Free Solder Joints During Stress Relaxation

Zimprich

Saeed

Weiß

et al. 2008

Reliability and quality control of microelectronics depend on a detailed understanding of the complex thermomechanical properties of miniaturized lead-free solder joints. With the continuous reduction in size of modern electronic devices, including also the size of the solder joints themselves, mechanical constraint effects may become of importance for the reliability of the joints. In the present study stress relaxation tests in tensile mode were performed on model solder joints consisting of eutectic Sn-3.5Ag solder between Cu substrates. The gap size of the joints was varied between 750 lm and 150 lm in order to investigate the variation of the mechanical properties as a function of the gap size. As it turned out, stress relaxation was dramatically reduced when the solder gaps became smaller due to constraint effects already well known from earlier measurements of the tensile strength. By employing a traditional creep model, the stress exponents and the activation energies were derived and compared with available data in the literature. The consequences of these constraint effects for the case of thermomechanical fatigue are discussed.