Comparative study of the dissolution kinetics of electrolytic Ni and electroless Ni P by the molten Sn3.5Ag0.5Cu solder alloy

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1 Microelectronics Reliability 43 (2003) Comparative study of the dissolution kinetics of electrolytic Ni and electroless Ni P by the molten Sn3.5Ag0.5Cu solder alloy M.N. Islam, Y.C. Chan *, A. Sharif, M.O. Alam Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong Received 28 January 2003; received in revised form 25 May 2003 Abstract Lead-free solders have high Sn content and high melting temperature, which often cause excessive interfacial reactions at the interface. Sn3.5Ag0.5Cu lead-free solder alloy has been used to identify its interfacial reactions with two-metal layer flexible substrates. In this paper we investigate the dissolution kinetics of Sn3.5Ag0.5Cu solder on electrolytic Ni/electroless NiP layer. It is found that during 1 min of reflow electroless NiP layer dissolves slightly lower than the electrolytic Ni due to the barrier layer formation between the intermetallic compounds (IMCs) and electroless NiP layer. Faster nucleation of IMCs on the electrolytic Ni layer is proposed as the main reason for higher initial dissolution. The appearance of P-rich Ni layer acts as a diffusion barrier layer between the solder and electroless NiP layer, which decreases the dissolution rate and IMCs growth rate than that of the electrolytic Ni layer, but weaken the interface and reduces the ball shear strength and reliability. After acquiring certain thickness P-rich Ni layer breaks and increases the diffusion rate of Sn and as a consequence both the IMCs growth rate and dissolution rate also increases. It is found that 3 lm thick electroless NiP layer cannot protect the Cu layer for more than 120 min at 250 C. In electrolytic Ni shear strength does not change significantly and lower dissolution rate and more protective for Cu layer during long time molten reaction. Ó 2003 Elsevier Ltd. All rights reserved. 1. Introduction Soldering is the most important method for joining of mechanical components in electronics. The traditional tin lead soldering method has been used for many years. However, Pb containing solder is become a conscious issue of manufacturing due to environmental and health concerns. Because of the lack of information regarding reliability in practice, a lot of reliability test are needed before lead-free solder can replace traditional tin lead solder. One of the most influential factors in the solder joint quality of a BGA component is the metal surface finish * Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). on the pads. The pads, which are Cu, easily oxidized, need to have a protective surface finish to ensure solderability during assembly. The most common surface on BGA is electrolytic Ni/Au or electroless NiP/Au plated over the copper pad. Electrolytic Ni/electroless NiP layer acts as a diffusion barrier layer and Au layer acts as an oxidation barrier layer between the Cu and the solder materials [1,2]. The thickness of intermetallic, composition, microstructure, mechanical properties and reliability of solder joints are strongly dependent on surface finish layers. Interaction and interdiffusion behavior between solder and Cu has been studied elsewhere. It is found that at the Sn-containing solder/cu interface, tin reacts rapidly with Cu to form Cu Sn intermetallic compounds (IMCs). The strength of the solder joint decreases with increasing thickness of IMC formed at the interface [3 5]. Many works have been reported that the growth rate /$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi: /s (03)

2 2032 M.N. Islam et al. / Microelectronics Reliability 43 (2003) of intermetallic compounds is lower in the Ni/solder and NiP/solder system than in the Cu/solder system [6 9]. Thus electrolytic Ni layer/electroless NiP layer are recognized to act as a diffusion barrier between Cu and solder in order to sustain the long period of service. The Sn Ag Cu alloy is one of the promising lead-free alloys. Among the promising candidates for the lead-free solder alloys; the Sn Ag Cu ternary eutectic system has the lowest melting temperature (217 C) and better interfacial properties than the binary alloys. The metallurgical behaviors of Sn3.5Ag0.5Cu solder in joints with different surface finishes and the related joints reliability have not been sufficiently studied yet. Therefore, the present work was carried out to investigate the dissolution kinetics of electrolytic Ni and electroless NiP metallization for Sn3.5Ag0.5Cu solder. We also studied the reaction kinetics of the solder joint. (SEM). The chemical and microstructural analyses of the gold-coated 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 dissolution of electroless NiP and electrolytic Ni diffusion barrier layer are determined by measuring the remaining layer thickness after each reflow condition Shear strength The shear test was performed on reflowed samples by using the Dage Series 4000 Bond Tester. The shear tool height of 100 lm and shear speed of lm/s were used. A total of 20 solder ball joints were sheared for each condition. The fracture surfaces after the ball shear test were investigated thoroughly by SEMin secondary electron mode as well as by EDX. 2. Experimental procedures 2.1. Solder ball attachment The solder joints were made by soldered between two-metal layer flexible substrates with Cu/Ni/Au, Cu/ NiP/Au coated pads and lead-free solder. The solder balls were placed on the substrate as shown in Fig. 1 and then reflowed at 250 C. After as reflow, all sample were kept again in 250 C for 5, 10, 30, 120, and 180 min. The composition of the solder ball is 96% Sn, 3.5% Ag and 0.5% Cu. The size of the solder ball was mm. The flux used in this work was CLEANLINEe Characterization To investigate the microstructure of the samples, the samples were sectioned by using a slow speed diamond saw and then mounted in epoxy. The cross-sectioned samples were grounded and polished very carefully and then gold coated for scanning electron microscopy 3. Result and discussion 3.1. Mechanical strength andfracture mode During soldering Ni, Sn and Cu atoms inter-diffuse and the initial formation of intermetallics ensured a good metallurgical bond and strengthened the interface. 100% fracture within solder is required in shear mode to have the best solder joint integrity. Fig. 2 depicts the solder ball shear testing results of electrolytic Ni/solder and electroless Ni P/solder joints. Fig. 2 shows that initial average shear strength of the electrolytic Ni/solder joints (1.754 kgf) is higher than the electroless NiP/solder joints (1.470 kgf). The maximum shear strength is found kgf in electrolytic Ni after 120 min but kgf in electroless NiP layer after 10 min of molten reaction. The average shear strength of the solder joints after 30 min of molten reaction is about kgf for electrolytic Ni and kgf for electroless NiP layer. So electroless NiP layer form very weak interface at this stage. The minimum shear strength is Solder ball Cu Micro via Solder mask Flux Au layer Ni/NiP layer Cu pad Polymide Shear load, gmf Electrolytic Ni Electroless NiP Heat sink Fig. 1. Solder ball attachment on two metal layer flexible substrate Molten time, min Fig. 2. Average shear strength of electrolytic Ni/solder joints and electroless NiP/solder joints.

3 M.N. Islam et al. / Microelectronics Reliability 43 (2003) Fig. 3. Fracture surface of (a) electrolytic Ni/solder joint, and (b) electroless NiP/solder joint after 180 min of molten reactions. found kgf for electrolytic Ni and kgf for electroless NiP layer after 180 min. In the electrolytic Ni ball shear strength does not decrease or increase significantly with the increasing of molten time. Fig. 3 shows the fracture surfaces of solder joints after 180 min molten reaction. Closed observation after ball shear test (Fig. 3a) and EDX analysis, it is revealed that ductile fracture occurs within the solder and solder IMCs interface. But electroless NiP shows ductile and slightly brittle failure occurs during ball shear testing after as reflow. The brittleness of the solder joints increases with the increase of reflow time and fracture occurs within the IMCs and P-rich Ni layer. Fig. 3b shows the fracture surface is almost flat. Failure mode is very brittle and according to EDX analysis, fracture occurs within the IMCs and/or dark P-rich Ni layer. The IMCs of electroless NiP seams to be weaker and brittle than the electrolytic Ni IMCs. These results demonstrate that electrolytic Ni/Au plating has greater solder joint integrity compared to the electroless Ni P/Au plating which also reported by other investigators [10]. The reasons for such different trends of mechanical strength for electrolytic Ni and electroless NiP/solder joints could be explained by in-depth study of the interface Dissolution kinetics andcross-sectional studies of the interface To investigate the relation between the shear strength and the reaction kinetics of the electrolytic Ni/solder and electroless Ni P/solder deposit, a detailed cross-sectional studies are carried out by SEM. During reflow, molten solder absorbs the entire Au layer into solution, allowing Sn, Ag and Cu from the solder to react with the Ni layer/ NiP layer and form different type of intermetallic compounds. The thickness of the intermetallic compound layer depends on a number of factors, such as temperature/time, volume of solder, property of solder alloy and morphology of the deposit [11] Interface after as reflow In electrolytic Ni/solder joint (Fig. 4a), the thickness of intermetallics is about lm. According to EDX analysis, the IMCs form on the electrolytic Ni layer is composed of Sn Ni Cu. No Ag is detected in the Fig. 4. Interface structure of (a) electrolytic Ni/solder joint and (b) electroless NiP/solder joint after 1 min reflow at 250 C.

4 2034 M.N. Islam et al. / Microelectronics Reliability 43 (2003) interfacial layer, so Ag is not directly involved in the interfacial reactions. In electroless NiP (Fig. 4b), the thickness of intermetallics is about lm. A very thin dark layer of about 200 nm is also found in between the IMC and the original electroless NiP layer. EDX analysis reveals that the IMC is composed of Sn Ni Cu and the dark layer is composed of P and Ni, which is also confirmed by other investigators [12 14]. The P content in the original NiP, which is about at.% before soldering, decreases to 8.75 at.% (average) from the top of original electroless NiP layer after soldering due to diffusion of P atoms into the dark P-rich Ni layer, similar results are also reported by other investigators [15]. The intermetallic growth rate and dissolution rate of the electroless NiP are lower than the electrolytic Ni solder joints due to barrier effect of dark P-rich Ni layer (Fig. 5). The shear strength of solder joints for electroless NiP is also lower for the presence of continuous P-rich Ni layer in the structure Interface after long time of molten reaction The reflow time for industrial applications over 10 min needs not to be performed in real situation. But in this study, we use up to 180 min. The main purpose of this unrealistic molten time or reflow time is to establish a database, which are helpful for prediction of life and for designing new components. After 10 min reaction in molten condition, it is seen that IMCs thicknesses of both samples grow gradually with the increasing of time. The shear strength of electroless NiP/solder joints is increased significantly but still lower than the electrolytic Ni/solder joints (Fig. 3). The increase in shear load of the solder joint may be related to the strengthening effect of the solder alloy due to the homogenization at the time of reflow [16]. After 30 min of reaction in molten condition (Fig. 6a), it is seen that IMCs thickness of electrolytic Ni increases whereas original Ni layer thickness decreases. The IMCs thickness is about lm. According to the EDX analysis, the IMCs are composed of Sn Ni Cu Layer thickness, µ,/min (a) Dissolution rate, µ/min (b) Electrolytic Ni Electroless NiP Molten time, min Electrolytic Ni Electroless NiP Molten time, min Fig. 5. (a) Electrolytic Ni and electroless NiP layer thickness with the increasing of molten time. (b) Dissolution of electrolytic Ni and electroless NiP layer. and near to the electrolytic Ni layer the IMC is composed of Ni and Sn, with a small amount (about 8 at.%) of Cu. Different type of IMCs can form easily at the interface of the electrolytic Ni/solder joints. The shear strength of solder joints has decreased slightly due to increase of IMCs thickness. But in electroless NiP (Fig. 6b), the thickness of IMCs does not increase significantly with molten time and it is about 4 9 lm and the morphology is very rough. The formation of IMCs and dissolution rate of electroless NiP layer is still lower than that of the electrolytic Ni layer (Figs. 5 and 6b). According to the EDX analysis, the IMCs are composed of Sn Ni Cu. The IMC near to the dark P-rich Ni layer is Fig. 6. Interface structure of (a) electrolytic Ni/solder joint, and (b) electroless NiP/solder joint after 30 min of molten condition at 250 C.

5 M.N. Islam et al. / Microelectronics Reliability 43 (2003) Fig. 7. Interface structure of (a) electrolytic Ni/solder joint, and (b) electroless NiP/solder joint after 120 min of molten condition at 250 C. composed of Sn and Cu, but there is also small amount (about 7.56 at.%) of Ni present, similar result reported by other investigators [14]. It is clear that Cu 5 Sn 6 IMC first form on electroless NiP layer. When Ni atoms diffused through the P-rich Ni layer then reaction occurs between Ni and Cu 6 Sn 5 and form Sn Ni Cu IMCs. On the other hand P-rich Ni layer moves gradually downward to the remaining original NiP layer and helps the formation of IMCs. The shear strength of the solder joints decreases significantly with the increase of molten time and brittle fractures occur. Brittle interfacial fracture did not occur in the electrolytic Ni sample because P is not present there. It seems that P is responsible for brittle failure, which also reported by other investigators [13,17,18]. In electrolytic Ni (Fig. 7a), the thickness of IMCs increases with the increase of molten time, but the IMC has started to break and diffuse into the molten solder from the top of IMCs. After 120 min of molten reactions, Sn Ni Cu IMCs are stable and are well adhered on electrolytic Ni layer. The shear strength of the solder joints has slightly increased (Fig. 3). The slightly brighter IMCs have formed on the top of the electrolytic Ni layer. The bright IMCs are composed of Ni and Sn with also small amount of Cu. Ni Sn IMC forms on the top of electrolytic Ni layer due to limited supply of Cu from the solder. In electroless NiP (Fig. 7b), IMCs thickness increases with time and has started to break. The thickness of P- rich Ni layer is also increased and it is about 1.34 lm, with a P content of about at.%. As the thicker P- rich Ni layer grows, the more kirkindal voids are generated, which help to increase the diffusion rate of Sn and Ni [15]. So, the growth rate of IMCs is increased. The IMCs on the electroless NiP break and diffuse rapidly into the molten solder than that of electrolytic Ni layer IMCs (Figs. 6 and 7). So more Sn atoms diffuse through the Sn Ni Cu IMCs and form slightly brighter Ni Sn IMCs with small amount (about 7.56 at.%) of Cu on the top of the dark P-rich Ni layer. It is due to lower percentage of Cu within the solder. At this stage the dissolution rate of electroless NiP is increased and becomes slightly higher than the electrolytic Ni layer (Fig. 5). Brittle fracture is observed because of the increase of IMCs thickness, together with microstructures coarsening, increases of P content in dark P-rich Ni layer and diffusion related micro-porosities at the interface. After 180 min reaction in molten condition (Fig. 8a), it is seen that the thickness of the slightly brighter IMC increases, but the darker IMCs thickness does not increase Fig. 8. Interface structure of (a) electrolytic Ni/solder joint and (b) electroless NiP/solder joint after 180 min reaction in molten condition at 250 C.

6 2036 M.N. Islam et al. / Microelectronics Reliability 43 (2003) with time due to breaking and diffusing into the molten solder. The electrolytic Ni layer is still intact (2.68 lm). About 2.32 lm of electrolytic Ni layer takes part in reaction with molten solder and has formed different type of Sn Ni Cu and Ni Sn IMCs. In this stage the dissolution rate of electrolytic Ni layer is lower than that of the electroless NiP layer (Fig. 5). In the case of electroless NiP (Fig. 8b), after 180 min of molten reaction it is seen that thicker P-rich Ni layer has broken and creates more channels. These channels are the results of the coalescence of the kirkindal voids formed during the diffusion process [6,13,19]. So, the diffused Sn atoms come in contact to the remaining thin electroless original NiP layer and then Cu pads at a faster rate through these channels and reacts to form Ni Sn Cu and then Cu Sn IMCs. Thus, the dissolution rate of electroless NiP layer is increased (Fig. 5). No original NiP layer is found in most of the interface. EDX analysis reveals that Ni Sn Cu and Cu Sn IMCs has formed under the broken dark P-rich Ni layer and P content in the broken P-rich Ni layer is about 31 at.% and some P has entrapped in the IMCs, similar result reported by other investigators [13]. At this stage, very lower shear strength and more brittle fracture is observed for this P entrapment in the IMCs. 4. Conclusion During molten condition of Sn3.5Ag0.5Cu solder formation of IMCs, dissolution rates of electrolytic Ni and electroless NiP layer and the morphology of intermetallic compounds are investigated. Initially the growth rate of IMCs and dissolution rate of electroless NiP layer are lower than the electrolytic Ni layer due to barrier effects of dark P-rich Ni layer. For long time ( min) molten reaction, breaking and diffusing of IMCs and increasing of kirkindal voids help to increase the diffusion rate of Sn atoms and formation of IMCs at the interface, so dissolution rate of electroless NiP layer increases and becomes higher than the electrolytic Ni layer. At first Cu Sn IMC form on the diffusion barrier layer and than react with Ni and form Sn Ni Cu IMCs. After sufficient amount of Sn Ni Cu IMCs formation, Ni Sn IMC also form on the top of barrier layer due to limited supply of Cu from the solder. The IMCs of electroless NiP/solder joints are weaker and brittle than the electrolytic Ni IMCs due to the presence of dark P-rich Ni layer and P entrapment in the IMCs. The shear strength of electroless NiP/solder joints is changed significantly with the increase of molten time. The average shear strength is significantly decreased within 30 min. In case of electrolytic Ni/solder, shear strength does not decrease significantly with molten time. Electroless NiP layer is not suitable for long time reflow due to higher dissolution rate and significantly lower the shear strength. It can be concluded that 3 lm electrolytic Ni layer can protect the Cu layer from lead-free Sn Ag Cu solder for more than the 180 min molten reaction at 250 C. The 3 lm electroless NiP layer cannot protect the Cu layer from the lead-free Sn Ag Cu solder for more than 120 min molten reaction at 250 C. Acknowledgements The work has supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China, RGC project (¼ CityU 1187/01E). References [1] Blackwell Glenn R. The electronic packaging handbook. Ó2000 CRC Press LLC. p [2] Zakel E, Reichi H. In: Lau JH, editor. Flip chip technology. McGraw-Hill; p [3] Chan YC, So ACK, Lai JKL. Growth kinetics studies of Cu Sn intermetallic compound and its effect on shear strength of LCCC SMT solder joints. Mater Sci Eng B 1998;55:5 13. [4] So ACK, Chan YC, Lai JKL. 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