A study of nanoparticles in Sn–Ag based lead free solders

https://doi.org/10.1016/j.microrel.2007.05.004Get rights and content

Abstract

Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb or Au inclusions in Sn–Ag based lead free solders were evaluated to study if these nanoparticles can reduce the growth of intermetallic compounds after four time reflow processes and thermal aging. Also, these nanoparticles were studied if they can reduce the frequency of occurrence of intermetallic compound fractures in high impact pull tests. In addition to intermetallic compound analyses, these nanoparticle effects on solder ball hardness were studied if nanoparticles affects solder hardness and displacement in drop tests. Finally, these nanoparticle effects on drop test performance were studied. This study found that Co, Ni and Pt were very effective for the growth of intermetallic compounds and drop test performance compared to Cu, Ag, Au, Zn, Al, In, P, Ge and Sb.

Introduction

The tin–lead (Sn–Pb) solder alloy has been widely used as interconnection material in electronic packaging due to its low melting temperatures and good wetting behavior on several substrate platings such as Cu, Ag, Pd and Au. Recently, due to environmental and health concerns, a variety of new lead free solders has been developed. Lead free solders lack the toxicity problems associated with lead-contained solders. However, unlike lead solders, the recently employed lead free solders do not have a long history and manufacturing process, and also board level reliability has not been established well. Especially, drop test performance is serious concern for mobile products like cellular phones, cameras, video and so on. Sn–Ag–Cu alloys are leading candidates for lead free solders [1], [2], [3], [4].

However, Sn–Ag–Cu alloys are not enough to meet severe board level reliability requirements. Two lead free solders were introduced in 2003 [5]. One was Sn–Ag–Ni–P. The other was Sn–Ag–Cu–P. Sn–Ag–Ni–P system has a balance of thermal cycling, bend, drop, and internal void test performance. On the other hand, Sn–Ag–Cu–P system has a significant advantage for drop test performance. The combination of Cu and P significantly reduced intermetallic compound thickness (IMC).

But, recent mobile electronic products like cellular phones require a thermal aging process followed by drop and bend tests. Thermal aging affects IMC and Kirkendall voids. Kirkendall voids under IMC reduce the strength of solder joint and degrade drop test performance significantly. It was found that Kirkendall voids in lead free solder joints could be reduced tremendously after adding Ni and In to Sn–Ag–Cu. Sn–Ag–Cu–Ni–In, which were introduced in 2004 [6], could improve drop test performance compared to Sn–Ag–Cu–P.

Previous lead free solders we introduced were four or five element-based lead free solders. Additional element effects on the growth of IMC was not seen, since the number of elements in the solders was many. In this study, we focused on three elements to study the growth of intermetallic compounds. Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb and Au inclusions in Sn–3Ag based lead free solders were evaluated, to study whether these nanoparticles increase IMC thickness and grain size after four solder reflows. Also, IMC element analyses were carried out to study if nanoparticles were dissolved in IMC after one and four solder reflows. Then, Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb or Au inclusions in Sn–3.0Ag based lead free solders were evaluated to study whether these nanoparticles can reduce the frequency of occurrence of IMC fracture in high impact pull tests. In addition to IMC analyses, the thickness of these nanoparticles on solder ball hardness were studied, since large solder displacement under drop impact improves drop test performance. Solder hardness is relative to solder ball displacement, so low solder hardness (soft solder) improves drop test performance [8], [9], [10]. Therefore, the hardness test was performed to study if nanoparticles increase solder hardness after one solder reflow and two solder reflows +100 °C thermal aging (0 h, 100 h, 200 h). Finally, Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb or Au inclusions in Sn–1.0Ag based lead free solders were evaluated to study if these nanoparticles can improve drop test performance. Ni, Co and Pt were very effective for drop test performance. Sn–1.0Ag was used to study drop test performance, since Sn–1.0Ag shows better drop test performance than Sn–3.0Ag [6].

In this study, it was found that Co, Ni or Pt located to the left of Cu in the periodic table, adding to Sn–Ag based solder alloys, did not increase IMC thickness and grain size significantly after the solder reflow process and thermal aging. Hence, these nanoparticles resulted in good drop test performance compared to Cu, Ag, Au, Zn, Al, In, P, Ge, Sb [7].

Section snippets

Nanoparticle effects on solder intermetallic compound grain size and thickness

Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb or Au inclusions in Sn–3Ag based lead free solder balls were evaluated to study if these nanoparticles affect IMC thickness and grain size after four solder reflows [3], [4]. These solder ball samples were attached to OSP Cu solder pads through the reflow process (maximum temperature: 245 °C) [11], [12], [13], [14].

Fig. 1 shows the sample preparation procedure to observe solder IMC grain size and thickness. Following the solder reflow process, solder

Are the nanoparticles dissolved in the IMC?

IMC element analyses were performed to study if nanoparticles were dissolved in IMC after one solder reflow and four solder reflows. FE-SEM was utilized to observe elements in the IMCs [3], [4].

Fig. 14 shows IMC element analysis (wt%) for Sn–3.0Ag–0.03Ni. (a) and (b) show the element analysis after one solder reflow and four solder reflows, respectively. As can be seen in pictures, it is obvious that nickel was dissolved in Cu6Sn5 and subsequently formed (CuNi)6Sn5. The ratio of Ni:Cu:Sn for

Nanoparticle effects on solder ball hardness

Large solder displacement under drop impact improves drop test performance, since large solder displacement can reduce stress in IMCs. Solder hardness is relative to solder ball displacement, so low solder hardness (soft solder) improves drop test performance. Hardness testing was performed to study if nanoparticles increase solder hardness after one solder reflow, and two solder reflows followed by 100 °C thermal aging (0 h, 100 h, 200 h) [15], [16], [17], [18].

Fig. 18 shows solder hardness

Fracture in intermetallic compounds in high impact pull test

Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb or Au inclusions in Sn–3Ag based lead free solders were evaluated to study if these nanoparticles can reduce the frequency of occurrence of IMC fracture in high impact pull tests [15], [16], [17], [18].

Fig. 20 shows the apparatus of high impact ball pull test. Dage bond tester 4000 was used in this test. A pull jaw holds a solder ball and subsequently lifts up at 50 mm/s speed.

Fig. 21 shows fracture mode after high impact pull test. Mode 1 is solder pad

Nanoparticle effects on drop test performance

Co, Ni, Pt, Cu, Zn or Sb inclusions in Sn–1.0Ag based lead free solders were evaluated to study if these nanoparticles can improve drop test performance [19], [20], [21].

Fig. 24 shows the drop test apparatus (Yoshida-seiki HDST-230). The package was a 12 × 12 mm BGA, a 30 × 120 × 0.8 mm PCB with a Cu + OSP (NSMD Type) pad finish, a solder paste (Senju M705-GRN360-K2V). The samples were left for 5 days before drop test was carried out. The daisy chain resistance was monitored. When the resistance exceeds

Conclusion

Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb or Au inclusions in Sn–3Ag based lead free solders were evaluated to study if these nanoparticles increase IMC thickness and grain size after four times solder reflow. Co, Ni or Pt inclusions in Sn–3Ag based lead free solders did not increase IMC thickness and grain size significantly after four solder reflows. Al, P, Cu, Zn, Ge, Ag, In, Sb or Au inclusions in Sn–3Ag based lead free solders increased IMC thickness and grain size after four solder

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