Metallurgical reaction and mechanical strength of electroless Ni-P solder joints for advanced packaging applications

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1 JOURNAL OF MATERIALS SCIENCE: MATERIALS IN ELECTRONICS 11 (2000) 587±593 Metallurgical reaction and mechanical strength of electroless Ni-P solder joints for advanced packaging applications K. C. HUNG, Y. C. CHAN, C. W. TANG Department of Electronic Engineering, City University of Hong Kong, Hong Kong We have studied the metallurgical reaction and mechanical strength of the electroless Ni-P solder joints as a function of re ow time at 220 C. It is found that both Ni 3 Sn 4 intermetallics and Ni 3 P are formed due to the solder reaction-assisted crystallization. However, after the rst 15 min of re ow, an unusual depression of Ni 3 Sn 4 growth has been observed. A detailed description of the diffusion mechanism has been presented to explain the prohibition of the Ni 3 Sn 4 growth. It is found that the growth of Ni 3 Sn 4 and Ni 3 P may have a mutual effect on each other during the solder reaction since there is a direct correlation between the depression of the Ni 3 Sn 4 growth and the ending of Ni 3 P growth. The characteristic of the mechanical strength of electroless Ni-P solder joints has been demonstrated. A correlation between the mechanical strength and the interfacial metallurgical reaction has been discussed. Also, it is found that different re ow times will result in different fracture interfaces of the sheared electroless Ni-P solder joints. The detailed explanation of the fracture surface morphology has been explored. # 2000 Kluwer Academic Publishers 1. Introduction To achieve the requirement of high density interconnection and lighter weight, advanced packaging technology with an area array of solder joints such as ip chip will be a dominant trend of electronics packaging in the near future [1]. In the past, the material widely used in the under bump metallization (UBM) and substrate metallization for advanced packaging applications was copper (Cu) [2, 3]. However, due to the rapid formation of Cu-Sn intermetallic compounds (IMC) at the liquid Pb-Sn solder/cu interface during solder reaction, the reliability of this type of solder joint is a serious concern [4±7]. Nickel (Ni) has been recognized as a diffusion barrier in the Au/Cu metallization because Ni-Sn compounds show a very slow IMC growth rate [8] and Ni has a relatively low diffusion rate through Au and Cu [9, 10]. Of all the Ni plating processes, the most cost effective option should be electroless nickel-phosphorus (Ni-P) and it receives much attention and widespread use in PCB fabrication and UBM for ip chip technology [11± 13]. Normally, electroless Ni-P deposit is crystalline at lower P contents ( at %), while at higher P levels ( at %), the deposit is an amorphous phase [14]. It has been found that amorphous Ni-P alloy will undergo a self-crystalline transformation to Ni and Ni 3 P at temperatures above 300 C [15]. Recently, Jang et al. [12] have found that the solder reaction will assist crystallization of electroless Ni-P UBM in ip chip packages even if the re ow temperature is well below the self-crystallization temperature. The crystallization of electroless Ni-P to Ni 3 P is induced by the depletion of Ni from electroless Ni-P for the formation of Ni 3 Sn 4. For advanced packaging applications using electroless Ni-P metallizations, it is indeed crucial to understand the effects of the solder reaction-assisted crystallization on the interfacial mechanical properties of the solder joints. Yet, no precise study on the relationship between the metallurgical reaction and mechanical strength of these electroless Ni-P solder joints has been made. In this paper, we attempt to study the metallurgical reaction of the electroless Ni-P solder joints and investigate the correlation between the metallurgical reaction and the mechanical strength of the electroless Ni-P solder joints. 2. Experimentation The test substrate was a custom-made FR4 printed circuit board (PCB) in which electroless Ni-P was deposited on the Cu pads with a diameter of 0.75 mm and a gold (Au) ash was deposited on top of that electroless Ni-P in order to avoid oxidation of the nickel surface. The thickness of the top Au layer on Ni-P was measured by using a Rutherford backscattering spectrometer (RBS) and the surface oxide or interface oxide in the Au/Ni-P/ Cu pad substrate were detected by using oxygen resonance with a 3:04 MeV He ion beam. Eutectic Pb-Sn solder balls with a diameter of 0.75 mm were placed on the pre uxed Au/Ni-P/Cu pad substrates and re owed at a temperature of 220 C for different re ow times from 0.5 to 90 min. The ux used in this work was 0957±4522 # 2000 Kluwer Academic Publishers 587

2 CLEANLINE 2 LR721H2 BGA no-clean ux. For interfacial microstructure examination, the samples were mounted in epoxy and then sectioned using a slow speed diamond saw. The cross-sectioned samples were ground, polished and etched with 10% HCI 90% water. The chemical and microstructural analyses of the cross-sectioned samples were obtained by using the Philips XL 40 FEG scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis (EDX). The crystal structures of the samples were investigated by X-ray diffraction (XRD) and a Philips CM 200 FEG high-resolution transmission electron microscope (HRTEM). The shear test was performed on re owed samples by using the Dage Series 4000 Bond Tester. The shear height of the shear test in this work was about 50 mm. 3. Results and discussion 3.1. Metallurgical reaction of electroless Ni-P solder joints The gold thickness of the Au/Ni-P/Cu pad substrate has been determined to be 84.2 nm. The RBS result shows that there is no surface oxide and interface oxide in this raw Au/Ni-P/Cu pad substrate. The chemical compositions of electroless Ni-P have been found to have 77 at % Ni and 23 at % P by EDX analysis and the structure of electroless Ni-P is con rmed as amorphous by XRD. Fig. 1 shows the SEM images of cross-sectioned electroless Ni-P solder joints after re ow at 220 C for (a) 0.5, (b) 15, (c) 60, and (d) 90 min. The intermetallic (IMC) layer at the solder/electroless Ni-P interface is con rmed to be Ni 3 Sn 4 XRD and EDX analyses. Both chunky-type and needle-type Ni 3 Sn 4 IMCs are present within the range of short re ow time. However, the chunky-type Ni 3 Sn 4 IMCS will grow at the expense of the needle-type as the re ow time increases. With the EDX analysis at the interface between the solder and electroless Ni-P of the crosssectioned solder joints, neither the Au layer nor the Au-Sn IMC layer is found at the interface and thus all the Au should be dissolved into the eutectic Pb-Sn solder. It is interesting to note that there is a thin dark layer between the Ni 3 Sn 4 IMC and the electroless Ni-P. This dark layer is con rmed to be Ni 3 P by EDX and TEM. It is shown in Fig. 1 that the Ni 3 Sn 4 IMC thickness is changed with the re ow time. A clear relationship between the thickness of Ni 3 Sn 4 IMC and the re ow time at 220 Cis plotted in Fig. 2. It is shown that the thickness of the Ni 3 Sn 4 increases rapidly during the rst 15 min of re ow at 220 C and then it becomes constant or increases slowly thereafter. Fig. 2 also shows the relationship between the thickness of Ni 3 P and the re ow time at 220 C. When the re ow time is short ( 5 15 min), Ni 3 P grows rapidly due to the solder reaction-assisted crystallization of the electroless Ni-P [12]. Then, the growth of Ni 3 P is stopped or even dropped after the rst 15 min Mechanisms of Ni 3 Sn 4 and Ni 3 P growth during solder reaction It is found that within the rst 15 min of re ow time, the Ni 3 Sn 4 thickness is linearly proportional to the square root of the re ow time. This means that the Ni 3 Sn 4 growth may be controlled by a diffusion process [16]. However, after 15 min of re ow, the growth rate of Ni 3 Sn 4 is dramatically reduced. Our results are quite interesting when compared to those published recently in [12] in which the Ni 3 Sn 4 growth is diffusion-controled throughout the whole range of re ow time. Therefore, it is undoubtedly essential to understand the mechanisms causing the depression of Ni 3 Sn 4 growth. In general, during the solder reaction, Ni 3 Sn 4 Figure 1 SEM images of cross-sectioned electroless Ni-P solder joints after re ow at 220 C for (a) 0.5, (b) 15, (c) 60, and (d) 90 min. 588

3 Figure 1 (Continued) 589

4 Figure 1 (Continued) intermetallics and Ni 3 P are formed according to the ideal equation of solder reaction-assisted crystallization: Ni-P 4 a 3b Sn?bNi 3 3 P a 3b Ni 3 3 Sn 4 1 where the chemical compositions of electroless Ni-P have a at % Ni and b at % P. Theoretically, more Ni 3 P than Ni 3 Sn 4 will be formed during solder reactionassisted crystallization in this work since 8/3 moles of Ni 3 Sn 4 formation will drive 23 moles of Ni 3 P crystallization from electroless Ni-P according to Equation 1. Moreover, with the increasing re ow time, Ni 3 P grain growth and an increase in Ni 3 P thickness may cause less Ni depletion from the electroless Ni-P to form Ni 3 Sn 4 since less Ni 3 P grain boundaries and a longer path for the Ni diffusion have occurred, respectively. However, this is not the case when you compare the thickness results of Ni 3 Sn 4 and Ni 3 P in Fig. 2. The growth rate of Ni 3 Sn 4 is much faster than that of Ni 3 P. So, the rst thing is to determine what mechanism causes the rapid growth of Ni 3 Sn 4 or slow growth of Ni 3 P during the rst 15 min of re ow. In our previous study [17], large losses of P to molten solder occurred during re ow. As a result, these losses of P will reduce the rate of Ni 3 P formation tremendously and hence increase the rate of Ni 3 Sn 4 formation. However, as the re ow time increases, the thick Ni 3 P layer may act as a barrier layer for Ni to diffuse to react with solder. Thus, the Ni 3 Sn 4 growth is depressed. Finally, in view of Fig. 2, it can be seen that the growth of Ni 3 P is also stopped in this regime. According to Equation 1, the reaction between solder (Sn) and Ni depleted from the electroless Ni-P to form Ni 3 Sn 4 will drive the Ni 3 P crystallization. If the Ni 3 Sn 4 growth is depressed, then the Ni 3 P crystallization may also be affected. Therefore, we think that the growth of Ni 3 Sn 4 and Ni 3 P may have a mutual effect on each other during the solder reaction since there is a direct correlation between the depression of the Ni 3 Sn 4 growth and the cessation of Ni 3 P growth. Figure 2 Plots of Ni 3 Sn 4 IMC and Ni 3 P thickness against the re ow time at 220 C. 590 Figure 3 Schematic diagram of the shear test con guration for the electroless Ni-P solder joints.

5 Figure 4 Plot of shear strength of the electroless Ni-P solder joints against the re ow time at 220 C Mechanical strength of electroless Ni-P solder joints Fig. 3 shows the schematic diagram of the shear test con guration for the electroless Ni-P solder joints. Fig. 4 shows the relationship between the shear strength and the re ow time at 220 C. The shear strength of the electroless Ni-P solder joints increases rapidly to a maximum during the rst 15 min of re ow. Then it suddenly turns to decrease and nally becomes constant after 30 min of re ow Correlation between metallurgical reaction and mechanical strength According to the SEM microstructural images in Fig. 1, at the very beginning of the metallurgical reaction (e.g. 0.5 min), Sn in the molten solder reacts with Ni to form needle-type Ni 3 Sn 4 IMCs. The interfacial strength of these needle-type Ni 3 Sn 4 IMCs is quite low compared to that of chunky-type, and thus the shear strength of the solder joints is weaker at the very beginning of re ow. As the re ow time increases, needle-type Ni 3 Sn 4 will grow to be chunky and consequently the interfacial morphology between the solder and Ni-Sn compound becomes extremely rough as shown in Fig. 1b. Thus, the shear strength of the electroless Ni-P solder joints attains a maximum value. However, after 15 min of re ow, more and more chunky-type Ni 3 Sn 4 grains appear and stack together, as shown in Fig. 1c. As a result, this stacking of Ni 3 Sn 4 grains will weaken the shear strength of the electroless Ni-P solder joints. Thus, the shear strength decreases after 15 min of re ow as shown in Fig. 4. Moreover, since the thickness of the Ni 3 Sn 4 IMC is nearly constant or increases slowly thereafter, as shown in Fig. 2, the shear strength of the electroless Ni-P solder joints will then maintain a constant value Fracture surface morphology of electroless Ni-P solder joints Fig. 5 shows the SEM images of the fracture surface of the electroless Ni-P solder joints after (a) 10 min and (b) 90 min of re ow at 220 C. Fig. 6 shows the EDX spectrum on the fracture surface of the electroless Ni-P solder joints at (a) point A and (b) point B in Fig. 5b. From the EDX analyses, the compound should be Ni 3 P and Ni 3 Sn 4 IMC at point A and at point B respectively. It is found from Fig. 5, the fractures occur at the compound layer of either the Ni 3 P or the Ni 3 Sn 4 IMC. In addition, Fig. 5a shows the grain-like structure of Ni 3 P which indicates that the crystallization of amorphous electroless Ni-P to Ni 3 P has occurred during the solder reaction. In Fig. 5, it is notable that the surface morphology structure of Ni 3 P is changing with the re ow time. With more thorough inspection of Fig. 5b, we can see that there are some cracks on the surface of Ni 3 P grains, which split the bigger Ni 3 P grains into smaller ones. This phenomenon can also be found in the cross-sectional SEM images of Figure 5 SEM images of the fracture surface of the electroless Ni-P solder joints after (a) 10 min and (b) 90 min of re ow at 220 C. 591

6 Figure 5 (Continued) Figure 6 EDX spectrums on the fracture surface of the electroless Ni-P solder joints at (a) point A and (b) point B in Fig. 5b. the electroless Ni-P solder joints in Fig. 1. After long re ow time, some black voids (BV) extended to the Ni 3 P surface are induced at the Ni 3 P grains as shown in Figs. 1c and 1d. In fact, the Ni 3 P grain will grow during the long time re ow process. This Ni 3 P lm growth will be generally accompanied by stress. As a result, cracks will be induced due to the stress, as observed in Fig. 1 and Fig Conclusion During solder reaction, Ni 3 Sn 4 intermetallics and Ni 3 P are formed due to the solder reaction-assisted crystallization. The thickness of the Ni 3 Sn 4 and the Ni 3 P increases rapidly within the rst 15 min of re ow at 220 C. After that, both the Ni 3 Sn 4 and Ni 3 P growth rates become constant. A detailed description of the diffusion mechanism is presented to explain the prohibition of the Ni 3 Sn 4 growth. Since the formation of Ni 3 Sn 4 will drive the Ni 3 P crystallization, the depression of Ni 3 Sn 4 growth has indeed affected the growth of Ni 3 P. In addition, the 592 characteristic of the mechanical strength of electroless Ni-P solder joints has been demonstrated. It is found that there is a correlation between the mechanical strength and the interfacial metallurgical reaction. Moreover, it is found that different re ow times will result in different fracture interfaces of the sheared electroless Ni-P solder joints. It is interesting to note that there are cracks on the surface of the Ni 3 P grains after long re ow times. This is due to the fact that Ni 3 P lm growth will be accompanied by stress and this stress will then induce cracks, thus splitting the bigger Ni 3 P grains into smaller ones. Acknowledgments The authors would like to acknowledge the nancial support provided by the Direct Allocation Grants (Project no ) and the Strategic Research Grants (Project no ) of the City University of Hong Kong.

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