Kinetics of AuSn4 migration in lead-free solders

Au over Ni on Cu is a widely used printed circuit board (PCB) surface finish, under bump metallization (UBM), and component lead metallization. It is generally accepted that less than 3 wt.% Au in Sn-Pb solder joints inhibits formation of detrimental intermetallic compounds (IMC). However, the critical limit for Au content in Pb-free solder joints is not well established. Three surface-mount package platforms, one with a matte Sn surface finish and the others with Ni/Au finish, were soldered to Ni/Au-finished PCB using Sn-3.0Ag-0.5Cu (SAC305) solder, in a realistic manufacturing setting. The assembled boards were divided into three groups: one without any thermal treatment, one subjected to isothermal aging at 125°C for 30 days, and the third group aged at 125°C for 56 days. Representative solder joints were cross-sectioned and analyzed using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX) to investigate the evolution of the solder joint morphology as a function of Au content and isothermal aging. It was found that, if Cu is available to dissolve in the solder joint, the migration of AuSn 4 from the bulk to the interface as a result of thermal aging is mitigated.

Section: Resultssupporting

confidence: 87%

Section: Resultsmentioning

confidence: 99%

Effect of Gold Content on the Microstructural Evolution of SAC305 Solder Joints Under Isothermal Aging

Powers

Pan

Silk

et al. 2011

“…Additionally, their studies revealed that Au atoms tend to migrate from the matrix to the interface, especially during the first 250 h of aging. 25 Based on our findings and related studies, one can conclude that the Au atoms diffuse back from the solder matrix to the interface during the aging treatment.…”

Section: Microstructure At the Solder/enig/cu Interfacesupporting

confidence: 69%

“…This phenomenon is in line with several previous studies. [25][26][27] Ho and co-authors 25 studied the interfacial reaction when Sn-3.5Ag is soldered over Au/Ni ball grid array substrates, and found that, for the Au (2 lm)/Ni case, many AuSn 4 needles were distributed homogeneously throughout the as-reflowed solder joint. Additionally, their studies revealed that Au atoms tend to migrate from the matrix to the interface, especially during the first 250 h of aging.…”

Section: Microstructure At the Solder/enig/cu Interfacementioning

confidence: 99%

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Zhang

Guo

Liu

et al. 2008

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.

“…Similar preparation techniques have been described in detail elsewhere. 11,12 A schematic drawing of the sample structure is shown in Fig. 2.…”

Section: Methodsmentioning

confidence: 99%

Cross-Interaction between Ni and Cu across Sn Layers with Different Thickness

Chang

Yang

Tu³

et al. 2007

The Ni/solder/Cu material sequence is one of the most common material sequences in the solder joints of electronic packages. In this study, the Ni/Sn/ Cu ternary diffusion couples were used to investigate the solder volume effect on the cross-interaction between Ni and Cu. Experimentally, a pure Sn layer with the thickness of 100-400 lm was electroplated over Cu foils. A pure Ni layer (20 lm) was then deposited over the as-deposited Sn surface. The diffusion couples were aged at 160°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high temperature process, such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that the cross-interaction could occur in as short as 30 min. A detailed atomic flux analysis showed that the Cu flux through the Sn layer was about 25-40 times higher than the Ni flux. Moreover, it was found that (Cu 1-x Ni x ) 6 Sn 5 on the Ni side reduced the consumption rate of the Ni layer, and the cross-interaction also reduced the Cu 3 Sn thickness on the Cu side.