Interfacial bonding behavior with introduction of Sn Zn Bi paste to Sn Ag Cu ball grid array package during multiple reflows

Transcription

1 Interfacial bonding behavior with introduction of Sn Zn Bi paste to Sn Ag Cu ball grid array package during multiple reflows Po-Cheng Shih and Kwang-Lung Lin a) Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan 701, Republic of China (Received 20 July 2004; accepted 18 October 2004) Sn 8Zn 3Bi solder paste and Sn 3.2Ag 0.5Cu solder balls were reflowed simultaneously on Cu/Ni/Au metallized ball grid array (BGA) substrates to investigate the interfacial bonding behaviors for multiple reflow cycles at two different soldering temperature of 210 and 240 C. The different intermetallic compounds that formed at the interface after one reflow cycle were respectively the island-shaped Ag Au-Cu-Zn (near the solder) compounds and the Ni Sn Cu-Zn (near the metallized pad) compounds in 210 or 240 C soldering systems. Layered Ag Au Cu Zn, Ag 5 Zn 8, and Ag Zn Sn compounds were also observed within the solder near the interface after single reflow cycle. After ten reflow cycles, the Ag Au Cu Zn compounds significantly decomposed, while the Ag 3 Sn and Ni Sn Cu Zn compounds coarsened obviously. I. INTRODUCTION Sn Pb solder alloys have been widely used in electronic packaging industries due to their low eutectic melting temperatures (around 183 C) and good wetting behavior on several substrates such as Cu, Ag, Pd, and Au. 1 3 However, the Pb contained in these solders is toxic and detrimental to the environment and to human health. As a result, the use of Pb-free solders will soon be required for the manufacture of electronic products, driving the need for a Sn Pb solder substitutent. Among the potential candidates for lead-free solders, Sn Ag Cu holds promise because of its good resistance to thermal fatigue, high ductility, 4 and better solderability on copper than Sn Pb solder. 5 However, the ternary eutectic composition of Sn Ag Cu solder takes place within the composition range of wt% Ag and wt% Cu at 217 C, 5 which is about 40 C higher than that of Sn Pb eutectic solder. This is expected to result in the excessive growth of intermetallic compounds (IMCs) during the soldering process. 6 The overgrown IMCs are known to be detrimental to the bonding strength between the solder and the substrate due to increased brittleness. To overcome this problem, a nickel layer is often used as a diffusion barrier to prevent copper from diffusing into the solder to form the IMCs. A gold layer, plated on the Ni layer of the metallized pads, not only improves wettability but also increases oxidation resistance. 7 Sn Zn solder (Sn 9mass% Zn for the eutectic composition), with a eutectic melting temperature of 199 C, a) Address all correspondence to this author. matkllin@mail.ncku.edu.tw DOI: /JMR FIG. 1. The cross-sectional schematic of the specimen before reflow process. J. Mater. Res., Vol. 20, No. 1, Jan Materials Research Society 219

2 FIG. 2. The elemental analysis at the interfacial area of the single reflow specimen soldered at 210 C (a) backscattered electron image, (b) Zn, (c) Au, (d) Ag, (e) Sn, (f) Cu, (g) Ni, and (h) Bi. 220 J. Mater. Res., Vol. 20, No. 1, Jan 2005

3 FIG. 3. The SEM images of the IMCs formed after single reflow at 210 C: (a) the layer compound [A: the upper part, B: the lower part], (b) the island-shaped compound [labeled C], (c) solder ball, and (d) the magnified image of D region in (c). has also been recommended as a lead-free solder for replacing Sn Pb eutectic solder. 8,9 Moreover, the addition of Bi to Sn Zn near eutectic solder can improve the soldering properties by lowering melting temperature to roughly C With the results mentioned above, the soldering temperature of Sn Ag Cu could be decreased with the introduction of Sn Zn Bi solder paste. The lowering in reflow temperature with the intermediate Sn Zn Bi paste could inhibit the excessive growth of IMCs by Sn Ag Cu. The purpose of this study is to investigate the interfacial interactions of the materials when the commercial Sn Zn Bi solder paste is introduced to the Sn Ag Cu soldering system for single reflowed and multiple reflowed specimens. In addition, the effect of the different soldering temperatures on the interaction behavior of the solder alloys will be examined for multiple reflows. II. EXPERIMENTAL PROCEDURE Commercial Sn 8Zn 3Bi solder paste and Sn 3.2Ag 0.5Cu (mass%) solder balls were used in this study. The solder paste and solder balls were reflowed simultaneously on Cu/Ni/Au metallized ball grid array (BGA) substrates. The solder paste was composed of activated flux and Sn Zn Bi solder particles, which were m in diameter. The solder balls were 760 m in diameter. The thickness of the gold layer was less than 1 m, while the nickel and copper layers were about 7 m and 30 m thick, respectively. A stencil printing machine was used to apply solder paste to each Cu/Ni/Au metallized solder pad on the substrate, and then a solder ball was located on each paste-covered pad, as shown in Fig. 1. The specimens were divided into two sets; one set was reflowed at 210 C while the other set was reflowed at 240 C. Within each set, separate groups were reflowed 1, 5, and 10 times. The reflow experiment was performed in an infrared (IR) furnace using a protective atmosphere of 90% N 2 10% H 2. The reflow profile consisted of activation at 170 C with a peak temperature of 210 or 240 C for 30 s and afterward descended to room temperature through 3 4 min. Multiple reflows of the solder balls were conducted by repeating the reflow temperature profile for the desired number of cycles. The BGA specimens were mounted, polished with 0.3- m Al 2 O 3 powder, and then etched with a 5% HCl 95% H 2 O solution for interfacial investigation by scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), and electron probe microanalysis (EPMA). Moreover, to easily identify the crystal phases of the IMCs of BGA samples observed in either 210 or 240 C soldering systems, the x-ray diffractometer analysis was carried out by employing the Cu/Ni/Au foils instead of J. Mater. Res., Vol. 20, No. 1, Jan

4 substrate. Thickness of the Ni layer is 3 m, and that of Au is less than 1 m. Ammonia solution was used to etch away the Cu for x-ray analysis. FIG. 4. The SEM images of the IMCs formed for reflow at 210 C, (a) 5 reflow cycles, (b, c) 10 reflow cycles. III. EXPERIMENTAL RESULTS Identifications of the IMCs with the interaction of the materials The cross-sectional SEM image of the single reflow specimen soldered at 210 C is shown in Fig. 2(a). Elemental mapping analysis [Figs. 2(b) 2(h)] reveals that discontinuous discrete particulate IMCs formed at the interface between the Cu/Ni/Au metallization layers and solder. These IMCs consist of Ni, Sn, Cu, and Zn. It is likely that Ag and Au are incorporated with these compounds. Meanwhile, the needle-shaped Ag Zn Sn and Ag Zn compounds are observed in the solder region. Moreover, a relatively thick layer compound composed of Ag, Au, Cu, and Zn with varying elemental distribution forms near the interface. It seems to show that Ag Sn IMC particles are embedded within this compound layer. The Bi distributes uniformly in the solder and does not associate with any compound. It is noticed that the region between the Ag Au-Cu-Zn and the Ni-Sn Cu-Zn compound layers consist of Sn and Bi only. Zn of the Sn 8Zn 3Bi solder paste migrates outward and inward completely to form the IMCs. Figures 3(a) 3(b) show the IMCs formed between the solder and the metallized pad. The energy dispersive spectroscopy (EDS) results show that composition ratio of the compounds A and B are, respectively, Ag:Au:Cu:Zn 10:7:20:63 and 7:17:12:64 (at.%), which correspond to the EPMA results for Ag, Au, and Cu elemental distributions of the layer IMCs in Figs. 2(b) 2(d) and 2(f). The composition of the island-shaped compound of Fig. 3(b) (labeled C) is Ag:Au:Cu:Zn 10:6:22:62 (at.%), and the needleshaped compounds are identified as Ag 5 Zn 8. Figure 3(d), a magnification of region D in Fig. 3(c), reveals that there are Ag 3 Sn particles in the upper part of the solder ball. Reflow resulted in counter diffusion between the Sn Ag Cu solder ball and the Sn Zn Bi solder paste. The two originally separated solders mixed together and did not show visible interfacial boundary, as shown in Fig. 3(c). The Ni Sn Cu-Zn compounds (roughly Ni:Sn:Cu:Zn 27:43:13:16, at.%), shown in Figs. 4(a) and 4(b), slightly grew upon multiple reflow. While the layer Ag Au Cu Zn compounds observed in the single reflow specimen break into smaller pieces and detach from the underneath Ni Sn Cu Zn compound after 5 reflow cycles at a soldering temperature of 210 C, as shown in Fig. 4(a). The layer Ag Au Cu Zn compounds shrink and almost disappear after 10 reflow cycles at 210 C, as shown in Fig. 4(b). The Ag 3 Sn grows and coarsens significantly after 10 reflow cycles, as comparing the results 222 J. Mater. Res., Vol. 20, No. 1, Jan 2005

5 FIG. 5. The elemental analysis at the interfacial area of the specimen soldered at 210 C after 10 reflow cycles (a) backscattered electron image, (b) Zn, (c) Ag, (d) Au, (e) Sn, (f) Cu, (g) Ni, and (h) Bi. J. Mater. Res., Vol. 20, No. 1, Jan

6 FIG. 6. The Zn containing compounds shrink after multiple reflow, (a, b) single reflow cycle, and (c) 10 reflow cycles. FIG. 7. The SEM images of the IMCs formed at the interface after reflow at 240 C for (a, b) single reflow, (c) 5 reflow cycles, and (d) 10 reflow cycles. 224 J. Mater. Res., Vol. 20, No. 1, Jan 2005

7 FIG. 8. The elemental analysis at the interfacial area of the specimen soldered at 240 C after 5 reflow cycles (a) backscattered electron image, (b) Zn, (c) Ag, (d) Au, (e) Sn, (f) Cu, (g) Ni, and (h) Bi. J. Mater. Res., Vol. 20, No. 1, Jan

8 this layer and moved into the solder after 5 reflow cycles. The elemental mapping analysis also shows similar results, as shown in Fig. 8. Meanwhile, as found in the 210 C cases above, the Ag Au Cu Zn compounds also decompose into smaller pieces circled. For 10 reflow cycles at a 240 C soldering temperature, Ni Sn Cu Zn compounds reach a thickness of 4 9 m with some pieces of these compounds suspended in the solder, as shown in Fig. 7(d). Figure 9 shows the x-ray diffraction (XRD) data for specimens reflowed at 210 and 240 C for 1 and 10 reflow cycles. Similar results are obtained for the phase peaks of Ni 3 Sn 4, CuZn, AgZn 3, and AuZn 3 compounds at 210 and 240 C for the single reflow soldering system, while the peaks of AgZn 3 and AuZn 3 compounds are not distinct after ten reflow cycles. FIG. 9. XRD patterns of the interfacial compounds formed between the solder and the substrate at different soldering temperature for desired reflow cycles. of Figs. 3(d) and 4(c). Figure 5(a) shows the crosssectional image of the specimen reflowed for ten cycles at 210 C. It shows a segregation behavior of Bi in the solder near the interface, as shown in Fig. 5(h). This segregation behavior is not in association with any other elements. Besides this, Bi was seen to distribute through the solder layer. Zn forms the Ni Sn Cu Zn IMC at the interface. The elemental analysis indicates that Zn seems to form IMC with Ag in the region closing to interface. This IMC is likely to have Au, Cu, and Sn dissolved in it. However, Zn seems to dissolve in the Ag 3 Sn compound in the bulk solder, as revealed in Fig. 6. Moreover, Zn atoms gradually migrate out of the dispersive Ag Zn Sn compounds during multiple reflow and seem to lead to the coarsening of the Ag 3 Sn compounds, as shown in Figs. 6(a) 6(c). Figures 7(a) and 7(b) show the cross-sectional image of the single reflow specimen soldered at 240 C. As with the 210 C soldering system, the island-shaped and the elongated compounds, whose compositions are quite close to those in 210 C single reflow specimen, are also observed. Figure 7(c) shows that the IMCs located at the interface are mainly Ni Sn Cu Zn compounds (Ni:Sn: Cu:Zn 26:44:12:17, at.%), which formed in a continuous layer (about 3 5 m thick) and some detached from IV. DISCUSSION Based on observations and analysis of the interfacial IMCs, a possible interaction between the two solders and substrate can be described, as shown in Fig. 10. Figures 10(a) and 10(b) and Figs. 10(c) and 10(d) are respectively the schematic diagrams of the interfacial IMCs formation between the two solders and Cu/Ni/Au metallized solder pads at 210 and 240 C soldering temperatures. Previous studies indicated that the activation energy of diffusion of Sn in a Cu matrix (177 kj/mol) is much higher than that of Cu in a Sn matrix (33.02 kj/ mol). The diffusion constant of Cu or Zn atoms in Sn crystals is significantly larger than that of the selfdiffusion of Sn. Meanwhile, the magnitude of diffusivity of Cu in a Sn matrix is 1,000 10,000 times larger than that of Zn. This difference enables Cu atoms to move through the Sn matrix during the soldering process and leads to the formation of Cu-containing compounds near the interface between the solder and the Cu/Ni/Au metallized pads, as shown in Figs. 10(a) and 10(c). It was also indicated 4 that Au reacts easily with Zn to form Au Zn intermetallic compounds. Hence, according to the XRD experimental results and the binary phase diagram of Ag Au, Ag Zn, and Au Zn, the Ag Au Cu Zn compounds may consist of AuZn 3 and AgZn 3 compounds with Ag and Au substituting freely. On the other hand, the solubility of Zn in Cu at 200 C is at most 33 at.%. 18 Au and Cu also have a solid solution at 200 C. 19 However, Cu and Ag are hardly interact with each other. 20 The Ag Au Cu Zn compound could be composed of the (Ag, Au)Zn 3 and a solution containing Zn, Au, and Cu atoms of which the proportions vary by location. The CuZn peaks appeared in the XRD results of the single reflow samples. Hence, CuZn compounds may exist in Ag Au Cu Zn or Ni Sn Cu Zn IMCs or both. The detailed microstructure of the Ag Au Cu Zn compounds needs to be further investigated. Figures 10(a) 226 J. Mater. Res., Vol. 20, No. 1, Jan 2005

9 FIG. 10. The sketches of the IMCs formed at different soldering temperature for (a) 210 C-single reflow, (b) 210 C-10 reflows, (c) 240 C-single reflow, and (d) 240 C-10 reflows. and 10(c) show similar results except that some Au nodules did not diffuse away from the interface in single reflow due to the various diffusivities affected by a 30 C difference in soldering temperature. After 10 reflow cycles in both soldering systems, the Ag Au Cu Zn compounds seem to decompose into small dispersed particles. The reasons for the decomposition of Ag Au Cu Zn compounds are not clear. Ag might diffuse to enhance the growth of Ag 3 Sn, as shown in Figs. 4(c) and 6(c), and Au might dissolve into the solder after multiple reflow cycles because of the high solubility of Au in the molten solder. 21 Moreover, the Au Sn compounds could have formed, 22 but they are not observed in this study. The Cu and Zn atoms diffuse into the interface causing significant growth of Ni Sn Cu Zn compounds after ten reflow cycles, as shown in Figs. 4(b) and 7(d). On the other hand, the decrease in the Zn content of the Ag Zn Sn IMCs may relate to the growth of the Ni Sn Cu Zn IMC. The Ni Sn Cu Zn compounds, formed closing to the interface after single reflow cycle, mainly contain Ni and Sn. In addition, Ni Sn Cu Zn compounds grow faster at higher soldering temperatures, and some even spall into the solder. According to the XRD data, the detecting incident x-ray moving through the Cu/Ni/Au foil first, the AgZn 3 and AuZn 3 almost disappear due to the significant decomposition of Ag Au Cu Zn compounds, while the CuZn peaks are obvious in ten reflow samples. It may be attributed to that the Ni Sn Cu Zn IMCs consist of Ni 3 Sn 4 and CuZn. Previous studies indicated that Cu could react with Sn to form Cu Sn compounds. However, in this study, Cu Zn compounds are observed instead of Cu Sn compounds. The Gibbs free energy of compound formation of Cu Zn and Cu Sn are 11.5 to 12.3 kj/mol and 6.6 to 7.78 kj/mol, respectively. 8,28 From this point of view, Cu Zn compounds are more stable than Cu Sn compounds. The Gibbs free energy J. Mater. Res., Vol. 20, No. 1, Jan

10 may not absolutely govern the interfacial reaction during soldering as the reaction does not reach equilibrium. However, the formation of Cu Zn compounds, but not Cu Sn compounds, is also indicated by the thermodynamic analysis results. Other interfacial reaction results in the Sn Ag/Ni, Sn Cu/Ni, and Sn Ag Cu/Ni systems 7,23 25,29 are compared with those in this study. In the case of the Sn 3.5 (3.9) Ag solder system, the IMC phase formed at the interface was Ni 3 Sn 4. In the Sn 4.0Ag 0.5Cu solder system, the IMC formed at the interface was composed of Cu, Ni, and Sn. It means that a small amount of Cu in Sn Ag Cu alloy significantly affected interfacial reaction. In the case of Sn 4.0Ag 0.5Cu and Sn 0.7Cu solders, the IMC phases found at the interface were respectively (Cu, Ni) 6 Sn 5 and (Ni, Cu) 3 Sn 4, with the involvement of Ni or Cu in Cu 6 Sn 5 or Ni 3 Sn 4, completely different from the Ni 3 Sn 4 IMC observed in Sn 3.5 (3.9) Ag solder. A comparison between the results of the present study and those reported that the introduction of Zn tends to change the compound formation. Zn becomes more important than the Sn in compound formation. This behavior was also observed in the Sn Zn solder system. 9 V. CONCLUSIONS The Sn 3.2Ag 0.5Cu BGA soldering temperature is successfully lowered down to 210 C with the introduction of commercial Sn 8Zn 3Bi solder paste during multiple-reflow tests. The island-shaped Ag Au Cu Zn (near the solder) compounds and the Ni Sn Cu Zn (near the metallized pad) compounds were formed at the interface after single reflow at 210 and 240 C. Layered Ag Au Cu Zn, needle-shaped Ag 5 Zn 8, and Ag Zn Sn compounds are also observed within the solder near the interface for single reflow cycle. After ten reflow cycles, the Ag Au Cu Zn compounds significantly shrink, while the Ag 3 Sn and Ni Sn Cu Zn compounds grow. 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