HIGH-DENSITY interconnection technology requires

Transcription

1 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 31, NO. 2, JUNE Effect of 0.5 wt% Cu in Sn-3.5%Ag Solder to Retard Interfacial Reactions With the Electroless Ni-P Metallization for BGA Solder Joints Application Mohammad Ohidul Alam, Senior Member, IEEE, and Y. C. Chan, Fellow, IEEE Abstract Among the most advanced microelectronic packages, ball-grid-array (BGA) technology are expected to have increasing applications because of their higher input output connection density achieved through area-array solder joints. In this study, the role of 0.5 wt % Cu in the interfacial reaction between the Sn-3. 5%Ag solder and the electroless Ni-P metallization on the BGA substrate was investigated. Sn-3.5%Ag and Sn-3.5% Ag-0.5%Cu solders were reflowed on the electroless Ni-P layer at the peak temperature of 240 C and the duration above 220 C was 0.5 min. After reflowing, the samples were aged at 150 C temperature for different times ranging from 24 h to 400 h. It was found that Cu addition retards the reaction rate with the electroless Ni-P layer significantly especially during reflow soldering. However, among different layers formed by interfacial reactions, the P-rich Ni layer grew at a slower rate when Cu was present in the solder. Higher solder reaction rate for the Cu free alloys was explained in term of higher Ni dissolution and rapid formation of Ni 3 Sn 4. Due to a higher reaction rate, the amorphous electroless Ni-P layer was transformed to the crystalline phases of Ni-P compounds rapidly. The presence of 0.5 wt% Cu in the Sn-3.5%Ag solder alloy reduces Ni dissolutions in the liquid solder and changes the driving force of interfacial reactions forming (Cu,Ni) 6 Sn 5 instead of Ni 3 Sn 4. The growth of (Cu,Ni) 6 Sn 5 requires less Ni compared to the growth of Ni 3 Sn 4 and thus reduces the consumption of the electroless Ni-P as well as the growth of the P-rich crystalline Ni layer. Retardation of the interfacial reactions, in particular, a thin P-rich crystalline layer reduces the chance of solder joint failure, thus improves the reliability of solder joint with electroless Ni-P metallization. Results from other concurrent investigators were also compared and discussed to understand the findings from this study. Index Terms Ball grid array (BGA), electroless Ni, interfacial reaction, intermetallic compound (IMC), reflow, solder joint. I. INTRODUCTION HIGH-DENSITY interconnection technology requires that the solderable pad surfaces on the ball grid array (BGA), chip scale package (CSP) and printed circuit board (PCB) should be smooth. While hot air solder leveling or Manuscript received April 21, 2005; revised July 25, This work was supported by RGC Competitive Earmarked Research Grant (CERG) CityU 1106/04E (CityU internal ref ). This work was recommended for publication by Associate Editor P. McCluskey upon evaluation of the reviewers comments. M. O. Alam was with the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China and is now with the School of Computing and Mathematical Science, University of Greenwich, London SE10 9LS U.K. ( m.o.alam@gre.ac.uk; ohidul.alam@gmail.com). Y. C. Chan is with the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China. Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TCAPT other methods provide uneven surfaces, electrolytic Ni/Au and electroless Ni/Au provide flat solderable surface finishes for the solder pads on area array packages such as BGA or CSP and printed wiring board (PWB) [1] [6]. However, densification of the input/output (I/O) pads restricts the space needed for bussing the electrolytic Ni/Au plating [1]. Thus, electroless Ni/immersion Au (ENIG) is often a preferred choice because of its low cost and fine pitch compatibility. Although, electroless Ni-P metallization has being successfully used for many electronic products, some reliability problems of electroless Ni-P metallization have been reported by electronics industry consortia and the individual company [2] [5]. An unpredicted open or fractured solder joint sometimes appears after board assembly on a PCB/BGA pad. When the BGA component is removed, black colored pads are observed at the affected pad sites. This infant mortality is commonly known as a black pad problem, which is related to several processing issues of the ENIG [1], [3] [6]. Additionally, the composition of the deposited electroless Ni layer influences the reliability of the solder joint. We have investigated in our earlier studies how P content in the electroless Ni layer affected the reliability of Pb-containing solder joints and Pb-free solder joints [7] [9]. Electroless Ni is usually deposited from an acidic hypophosphite bath that introduces atomic P in the deposited electroless Ni layer and makes the layer amorphous at P-level higher than 10 at.% [10]. During the reflow soldering, Au dissolves away quickly and molten solder reacts with the electroless Ni metallization. When Ni dissolves into the solder and forms Ni Sn intermetallic compound (IMC) at the solder interface, P accumulates at the remaining electroless Ni layer as the solubility of P in the Sn-rich solder is nearly 0 and there is no chance of forming intermetallics of P with the solder components. It was found in our previous study that after a prolonged reaction, tiny crystals of Ni P, Ni P, Ni P, and NiP were formed in this P-rich Ni layer from the amorphous electroless Ni-P [7]. During this crystallization process, stress is generated in the crystalline P-rich layer which leads to fracture and eventually a weak layer is formed in between the IMC and the Cu layer [8], [9]. Moreover, Kirkendall voids also generate due to the excessive depletion of Ni in the P-rich layer [10] [17]. The condensation of the Kirkendall voids results a continuous crack along the interface of the P-rich crystalline layer to the underneath Cu layer [8], [9]. In our studies on the electroless Ni-P layer with the Pb-Sn solder and Sn-Ag solder, we found a relatively low shear /$ IEEE

2 432 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 31, NO. 2, JUNE 2008 strength after a prolonged soldering reaction which is due to the appearance of this P-rich crystalline brittle-type layer at the joint interface. However, growth of this brittle P-rich layer is entirely dependent on the reaction rate between the solder alloy and the electroless Ni-P metallization, which in turn, is related to the composition of the solder alloy as well as the P content of the electroless Ni-P metallization [9], [10]. It was found that Sn-3.5%Ag solder reacted very quickly than that of the traditional Sn-Pb solder and the electroless Ni-P layer with a higher percentage of P content yielded a rapid appearance of the brittle P-rich layer at the solder interface. Among the Pb-free solder, Sn-3.5%Ag-0.5%Cu solder has found to show better properties in terms of reduced Cu dissolution (during soldering process from the Cu metallization), improved mechanical strength etc. [18]. Recently, we have found that Sn-3.5%Ag-0.5%Cu solder reacts with the Ni metallization in a different manner than that of the binary Sn-3.5%Ag solder both in the liquid state (during soldering) and in the solid state(during aging) [19], [20]. Pre-doping a small amount of Cu in Sn-3.5%Ag solder has showed an enormous influence on the interfacial reactions with the Au/Ni metallization. Along with our previous studies, this study is intended to find any such influence of Cu pre-doping on the interfacial reactions with the electroless Ni-P metallization. II. EXPERIMENT The copper bond pads on the flexible substrate of the BGA package were used as the base for electroless deposition of Ni-P. An immersion Au plating was immediately layered on top of the electroless Ni-P to avoid oxidation of the nickel surface. An electroless Au plating was also carried out to obtain a thicker Au layer. The thicker Au layer is required for gold wire bonding on the Die-down BGA substrate used for this study. The average thicknesses of electroless Ni, immersion Au and electroless Au were 4, 0.1 and 0.6 m, respectively. Commercially available BGA solder balls of Sn-3.5%Ag and Sn-3.5%Ag-0.5%Cu were used to compare the effect of the pre-doped Cu on the interfacial reactions with the electroless Ni-P metallization. The compositions are given in weight percent. A commercial water-soluble flux was screen printed on the solder mask defined areaarray Au/Ni-P/Cu bond pads of the BGA substrate. The solder mask-opening diameter was 0.6 mm and the diameter of the BGA solder ball before melting was 0.76 mm. Solder balls were placed on the pre-fluxed Au/Ni-P/Cu bond pads and reflowed in a N atmosphere oven. The schematic diagram of the soldering process on the BGA substrate was shown in [18]. Solder balls of Sn-3.5%Ag and Sn-3.5%Ag-0.5%Cu were placed side by side to experience identical conditions of reaction. The peak reflow temperature was 240 C and the time above 220 C was about 0.5 min. Immediately after the reflow, few samples were prepared for the cross-sectional study and other substrates were subjected to high temperature aging at 150 C for time up to 400 h. After the aging test for each readout point, the cross-sectioned samples were prepared by a standard metallographic method [7] [9], [18] [20]. Interfacial microstructures were studied by a Philips XL 40 FEG scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis (EDX). The back-scattered electron (BSE) mode of SEM was Fig. 1. SEM-BSE image of a cross section of the Au coated electroless Ni-P deposit on Cu layer. used to examine the morphology of cross-sectioned samples and to measure IMC thickness. The average thickness of the IMC layer, the P-rich Ni layer and the unreacted original electroless Ni layer were determined by measuring the layer thicknesses at 20 equally spaced points from each solder joint as described in [18]. At least five solder joints were used for final average value. III. RESULTS Fig. 1 shows a typical SEM image of the cross-section of an electroless Au/immersion Au/electroless Ni-P/Cu layer before the soldering process. From this cross-sectional micrograph, it is seen that the Au layer thickness is higher than 1.5 m (while it is originally below 1 m). This is due to the smearing of the soft Au layer during metallographic polishing. Underneath the Au layer, the electroless Ni-P layer is visible on top of the Cu layer. It is also interesting to see that most of the scratches in the Cu layer were not extended into the electroless Ni-P layer. This is because of higher hardness of the electroless Ni-P layer than that of the Cu. It was also found that electroless Ni-P layer was amorphous before soldering [7]. From the EDX analysis, it was found that the P content in the as-deposited electroless Ni-P layer was about 10 at%. Careful observation of the as-deposited Au/Ni-P/Cu reveals a thin dark layer between the Au layer and the electroless Ni-P layer. EDX results also confirm a higher percentage of P in this dark layer, however, because of the limitation of the resolution of EDX, it was not possible to get an exact composition. During the immersion Au deposition, a galvanic displacement reaction occurs by a spontaneous exchange of Ni and Au atoms. The Au cation from the solution takes electrons from the Ni atom thus Ni is ionized and separated from the electroless Ni-P layer and a thin, protective Au layer forms over the exiting electroless Ni layer. Due to the Ni dissolution, the surface of electroless Ni-P beneath the thin Au layer becomes enriched with P. It has been reported elsewhere that this thin P-rich layer which is detected before reflow consists of Ni and Ni P precipitates [6], [21], [22]. Fig. 2 shows two most representative SEM images of Sn-3. 5%Ag solder interface with the Au/electroless Ni metallization just after reflow soldering. The difference between these two types of images lies on the IMC spalling characteristics both

3 ALAM AND CHAN: EFFECT OF 0.5 WT% CU IN SN-3.5%AG SOLDER TO RETARD INTERFACIAL REACTIONS 433 Fig. 2. SEM-BSE images of the as-reflowed Sn-3.5%Ag solder with the Au/electroless Ni-P metallization. Fig. 3. SEM-BSE image of the as-reflowed Sn-3.5%Ag-0.5%Cu solder with the Au/electroless Ni-P metallization. types of image have been found randomly. A dark P-rich layer is noticed in between the IMC and the original Ni-P layer for both the interfaces. Fig. 3 shows a typical interfacial microstructure of the Sn-3.5%Ag-0.5%Cu solder just after reflow soldering. Compared with Fig. 2, it is clear that a thinner IMC and a thinner dark layer are formed at the Sn-3.5%Ag-0.5%Cu solder interface. A quantitative measurement of the thicknesses was also carried out to compare the extent of interfacial reactions during relfow soldering. Fig. 4 shows this comparison of the variation of thicknesses of the original Ni-P layer, the P-rich layer and the IMC layer for Sn-3.5%Ag and Sn-3.5%Ag-0.5%Cu solder as a bar chart. From this comparison, it is clear that the consumption of original Ni-P layer is slower for the Sn-3.5%Ag-0.5%Cu solder compared to the Sn-3.5%Ag solder. Thus, the resultant P-rich dark layer and the IMC layer are also thinner for the Sn-3.5%Ag-0.5%Cu solder. From this study of solder reactions, it is seen that the addition of 0.5 wt% Cu to the Sn-3.5%Ag solder reduces the interfacial reaction rate with the electroless Ni-P metallization significantly. From EDX analysis, it was found that only one IMC of Ni Sn formed at the Sn-3.5%Ag solder interface, whereas, for the Sn-3.5%Ag-0.5%Cu solder interface, no such binary IMC was detected, rather, it was a complex IMC of Au, Cu, Fig. 4. Bar chart diagram depicting the variation of the dissolution of the original electroless Ni-P layer during reflow by Sn-3.5%Ag solder and Sn-3.5%Ag-0.5%Cu solder a new P-rich Ni layer and an IMC layer also grew at different proportions. Ni and Sn with a Ni percentage of at %. It was also noticed that some grains away from the interface have a lower percentage of Ni (Au:5-8 at %, Ni:5-8 at%, Cu:30-40 at% and Sn:44-46%). From the typical stoichiometry of Cu Sn IMC and knowing the fact that Au and Ni can replace Cu from Cu Sn IMC [19], [20], [23] [28], we tend to believe that the complex IMC formed at the Sn-3.5%Ag-0.5%Cu solder interface is (Au,Ni,Cu) Sn. The P content in the dark P rich layer for the Sn-3.5%Ag solder interface was confirmed to be near 25 at.%, however, a lower value of the P content was found for the Sn-3.5%Ag-0.5%Cu solder interface. It is not yet clear whether this is due to the thin dark layer (i.e., related to the resolution of the SEM-EDX) or to a lower percentage of P accumulation at the Sn-3.5%Ag-0.5%Cu solder interface. Fig. 5 shows typical SEM-BSE images of both types of solder interfaces after aging at 150 C for 100 h and 400 h. The interface of the IMC to the Sn-3.5%Ag solder was very irregular even after aging at 400 h. Whereas, the IMC interface for Sn-3.5%Ag-0.5%Cu solder was found to be smooth after 400 h aging. On the other hand, the interface of the IMC to the electroless Ni-P became progressively smoother for the Sn-3.5%Ag

4 434 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 31, NO. 2, JUNE 2008 Fig. 5. SEM-BSE micrographs of Sn-3.5%Ag solder (a, and c) and Sn-3.5%Ag-0.5%Cu solder (b, and d) interfaces with the electroless Ni-P after aging at 150 C for 100 h (a and b) and 400 h (c and d) (all figures are in the same magnification). Fig. 6. High magnification SEM-BSE images of the Sn-3.5%Ag solder: (a) and Sn-3.5%Ag-0.5%Cu solder and (b) interfaces with the electroless Ni-P showing an additional layer of NiSnP for only the Sn-3.5%Ag solder. solder but not for the Sn-3.5%Ag-0.5%Cu solder. This indicates that the reaction interface proceeded for the Sn-3.5%Ag solder towards the electroless Ni-P, whereas, the reaction interface for the Sn-3.5%Ag-0.5%Cu solder proceeded towards the solder side. Fig. 6 compares high magnification BSE-SEM images of the Sn-3.5%Ag solder and Sn-3.5%Ag-0.5%Cu solder interfaces with the electroless Ni-P. An additional layer of NiSnP in between the IMC and the P-rich layer is visible only for the Sn-3.5%Ag solder. From Fig. 6, again it is clear that the interface of IMC to P-rich layer for the Sn-3.5%Ag solder is smooth, whereas, it is very rough for the Sn-3.5%Ag-0.5%Cu solder. Fig. 7 shows the consumption of the original electroless Ni-P layer with aging time for Sn-3.5%Ag and Sn-3.5%Ag-0.5%Cu solders. There was relatively little consumption of the original electroless Ni-P layer noticed for the Sn-3.5%Ag-0.5%Cu solder during aging time investigated. On the other hand, in the initial

5 ALAM AND CHAN: EFFECT OF 0.5 WT% CU IN SN-3.5%AG SOLDER TO RETARD INTERFACIAL REACTIONS 435 Fig. 7. Consumption of the original electroless Ni-P layer with the aging time. Fig. 10. Variation of the composition of the complex IMC (Au,Ni,Cu) Sn layer with the aging time for the Sn-3.5%Ag-0.5%Cu solder interface. Fig. 8. Growth of the P-rich layer at the solder interfaces with the aging time. between these solders is significant for the as-reflowed condition. With an increase of aging time, the IMC thickness increases for both the solder systems, however, a faster rate is noted for the Sn-3.5%Ag-0.5%Cu solder. For the Sn-3.5%Ag solder, Ni Sn growth slowed down with time; whereas, for the Sn-3.5%Ag-0.5%Cu solder system, the growth rate of (Au,Cu,Ni) Sn was found to increase even after 100 h of aging. A compositional analysis at each read out point indicates that the percentage of Ni decreases with aging time. Fig. 10 shows the compositional variation of the (Au,Cu,Ni) Sn with aging time in an area-type chart. From this chart, it is interesting to see a nearly constant percentage of Sn over the entire aging range, whereas the percentages of Cu and Au increase. The Cu migrates to the interface with aging time and reacts with the existing (Au,Cu,Ni) Sn where it is involved only in replacement of Ni atoms. During reflow, (Au,Cu,Ni) Sn was formed instead of Cu Sn by replacing some of the Cu sites with the dissolved Ni atoms. During aging, with the diffusion of Cu atoms towards the interface, the IMC becomes thicker and Ni atoms are distributed evenly in the IMC, resulting in a lower percentage of Ni in the IMC. This analysis also proves that there is less Ni diffusion from the P-rich under layer and/or the original electroless Ni-P layer to the IMC layer. Fig. 9. Growth of the IMC layer at the solder interfaces with the aging time. stage of aging, the consumption of the electroless Ni-P layer by the Sn-3.5%Ag solder was found to be faster with a larger standard deviation which in turn progressively reduced with time. Fig. 8 shows the growth of P-rich layers as a function of aging time. The difference between these two solders is very significant. The P content in the P-rich layer was also found to be higher at the later stages of aging in the Sn-3.5%Ag solder interface. Fig. 9 shows the growth of IMC thicknesses with the aging time. As mentioned earlier, the difference in IMC thicknesses IV. DISCUSSIONS During the soldering reaction, the electroless Au and the thin immersion Au (both of them comprises only around 0.7 m thickness) dissolve within a few seconds. Indeed, it has been reported that solder component, especially Sn starts to react with the Au layer before the melting of the whole solder ball [29]. After the Au-Sn intermetallic formation and its subsequent spalling from the interface, Ni starts to dissolve in the liquid solder from the amorphous Ni-P layer leaving behind a P-rich layer. It has been reported that amorphous electroless Ni-P crystallizes during solder reaction assisted P accumulation. In our earlier study, we have found that the extent of P accumulation depends on the extent of the solder reaction [7] [9]. It is well known that lead-free solder which is essentially Sn-rich, reacts

6 436 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 31, NO. 2, JUNE 2008 at a faster rate with the metallization of the bond pad than does the traditional Sn-Pb solder during reflow soldering process. We have also reported that during prolonged reflow soldering time, a thicker P-rich layer (which is composed of tiny crystals of Ni P, Ni P, Ni P, and NiP ) forms at the Sn-3.5%Ag solder interface compared to the Sn-37%Pb solder interface. Other investigators have reported that the growth rate of the P-rich layer during solid state aging is also higher for the Sn-3. 5%Ag solder compared to the Sn-36%Pb-2%Ag solder [11]. They reported that the P-content in the P-rich layer increases with the aging time. Compared with the shear strength of the SnPbAg, they found that the shear strength of the SnAg solder decreases rapidly after 250 h of aging. Fracture surface analysis to understand the reason for the lower shear strength of the SnAg solder, suggested that fracture occurred at the interface of the Ni-P layer and the Cu metallization. They explained that this was due to a higher reaction rate of SnAg solder with the electroless Ni-P metallization which introduces Kirkendall voids and cracks at the interface of Ni-P and Cu metallization. From our earlier work [8], [9], we are confirmed that when all the original Ni-P layer has consumed and the P-rich layer met with the underneath Cu metallization, the solder joint become very unreliable because of large voids and cracks in the P-rich layer, however, that finding was not on a solid state aged case as was Ahat et al. s study [11] or the present study. We have shown for both the Sn-3.5%Ag solder and the Sn-37%Pb solder that the P-rich crystalline layer is the weakest layer which leads to a brittle fracture between the solder and the Cu metallization. It has been found that this brittle fracture appears earlier for the Sn-3.5%Ag solder compared to the SnPb solder. It has also been found that a higher P content in the original electroless Ni-P layer leads to a faster reaction and also that brittle fracture appears earlier. It is thus recommended that a low P content electroless Ni-P layer could be used to retard the appearance of the P-rich weak layer. Wang and Liu [30] studied the liquid state reaction between the electroless Ni-P and Sn(Cu) solders, with a variation of Cu content. They found that a thick P-rich layer grew for the pure Sn-solder. The IMC formed at the pure Sn-solder interface did not adhere to the interface. They have also reported on another layer composed of Sn, Ni and P in between the IMC layer and the P-rich layer for the pure Sn solder. Whereas, for the Cu-bearing solder, no such layer of SnNiP was found in their study and the IMC adhered well to the interface. They also explained how the localized wedging of the Sn-Ni-P layer in a pure Sn solder interface is responsible for spalling the IMC from the P-rich layer as they found a higher growth rate of the Sn-Ni-P at the edge of the IMC grains. In this study, the observation of an additional SnNiP layer for the Sn-3.5%Ag solder has also been made, however, during the shorter reflow soldering times no such wedging effect of the SnNiP layer has been noticed. During the solid state aging, a distinct uniform layer of SnNiP was always noticed [see Fig. 6(a)], where, no wedging effect and also no further spalling of the IMC in the solid solder were detected. The novelty of the present study is that by adding only 0.5 wt% Cu in Sn-3.5%Ag solder, it has been possible to substantially slow down the reaction rate during reflow soldering as revealed in the microstructure of the interface in Fig. 2 & 3 and the bar chart in Fig. 4. From the systematic quantitative measurements during the solid state aging, while IMC growth was noticeable, no appreciable growth was found for the P-rich Ni layer for the Cu-doped solder. Obtaining a very thin P-rich layer for the Sn-3.5%Ag-0.5%Cu solder with a very slow growth rate indicates that there is a lower chance for the concentration of defects for the Sn-3.5%Ag-0.5%Cu solder compared to the Sn-3.5%Ag solder. Thus, 0.5 wt% Cu addition could eventually increase the joint reliability. TEM observations by Torazawa et al. [14] showed a thinner P-rich layer for the Sn-3.5%Ag-0.75%Cu solder compared to the Sn-3.5%Ag solder both for as-soldered and solid state(after thermal cycling) aged samples between a Ni-8wt% P UBM and the solder. Hiramori et al. [31] also reported a thinner P-rich layer for the Sn-3.5%Ag-0.75%Cu solder compared to the Sn-3.5%Ag solder. Their work is rather interesting in that they tried to correlate the interfacial reactions with the thickness of the Au layer on the electroless Ni-P metallization. With an increase of Au layer thickness, a significant change in the Ni Sn morphology is noticeable from their published microstructures. Ni Sn grains became needle-shaped and spalled off from the interface with an increase of Au content in the interfacial reaction zone. The effect of Au on the morphological changes has also been reported by Park et al.[32]. During reflow soldering on the BGA bond pad, when solder melts and Au starts to dissolve there is the possibility of obtaining a localized concentration difference of Au at the interface because of the formation of Au-Sn IMC and dissolution phenomena. Due to such localized differences in the Au content, two distinctly different interfaces for the Sn-3.5%Ag solder might be obtained just after the reflow as shown in Fig. 2(a) and (b). It is clear from the experimental results given here that the IMC phase that formed during reflow soldering for the Sn-3.5%Ag-0.5%Cu solder with the electroless Ni-P was not (Ni,Cu) Sn but (Au,Cu,Ni) Sn. Even though the Cu content was only 0.5 wt% in the solder, the formation of (Au,Cu,Ni) Sn at the solder interface of electroless Ni-P was also confirmed by a TEM analysis of Jeon et al.[25] who used Sn-4%Ag-0.5%Cu solder on an electroless Ni-12at%P similar to this study. Torazawa et al.[14] also found (Cu,Ni) Sn, with a Cu content of 0.75 wt%. In our interfacial reactions study [20] with the electrolytic Ni metallization, (Cu,Ni) Sn was found at the solder interface with the addition of 0.5 wt% Cu in the Sn-3.5%Ag solder, however, (Cu,Ni) Sn, grew at a faster rate than that of the Ni Sn. On the other hand, in this study with the electroless Ni-P layer in the as-reflowed condition, the thickness of the (Cu,Ni) Sn layer in the Sn-3.5%Ag-0.5%Cu solder interface is noticeably less than that of the Ni Sn layer found in the Sn-3.5%Ag solder interface. While, the (Cu,Ni) Sn layer needs less Ni compared to the Ni Sn, it is interesting to see a lower amount of (Cu,Ni) Sn in the Sn-3.5%Ag-0.5%Cu solder interface. Thus, it is clear that a very small amount of Ni is dissolved in the Sn-3.5%Ag-0.5%Cu solder from the amorphous Ni-P in the presence of Cu in the liquid solder. From the bar chart in Fig. 4, this is confirmed by obtaining a thin P-rich layer and a lower consumption of the original electroless Ni-P layer for the Cu-doped solder during reflow soldering. This has also been

7 ALAM AND CHAN: EFFECT OF 0.5 WT% CU IN SN-3.5%AG SOLDER TO RETARD INTERFACIAL REACTIONS 437 noted by others that the presence of Cu in the liquid solder reduced the solubility of both Ni and Cu [5], [33] [35]. This is why, only a small amount of Ni dissolved from the electroless Ni-P layer to the Cu-doped solder during the short reflow time. However, during aging, (Au,Cu,Ni) Sn grows relatively faster due to the diffusion of Cu from the bulk solder. It is also interesting to see a lower percentage of Ni in the (Au,Cu,Ni) Sn layer with the progress of aging which is clear from Fig. 10. From the literature, it is known that (Au,Cu,Ni) Sn is a metastable phase of Cu Sn with a higher percentage of Ni. In the equilibrium phase of Cu Sn, a few Ni atoms can be incorporated ( 2 at% Ni) [5], [34], [35]. Thus, during solid state aging due to the restricted supply of Ni from the P-rich layer (which is composed of Ni-P compounds), metastable (Au,Cu,Ni) Sn tends to transform to Cu Sn through Cu intake from the bulk solder. V. CONCLUSION This study showed a significant influence of Cu-doping in the Sn-3.5%Ag solder on the interfacial reaction with the electroless Ni-P metallization on the BGA substrate. During reflow soldering, Cu-doped solder reacts with the electroless Ni-P metallization at a very slow rate, whereas, Sn-3.5%Ag solder reacts at a very high rate. While a simple IMC Ni Sn is formed at the interface of Sn-3.5%Ag solder, a complex IMC is formed at the interface of Sn-3.5%Ag-0.5%Cu solder. In the interfacial reaction of the solder alloy with the electroless Ni-P metallization, a P-rich layer is inevitable because of the Ni depletion and the P accumulation during solder reaction. A P-rich layer has been known to have a detrimental effect on the reliability of the solder joint. However, in this work a comparatively thin P-rich layer has been found at the Sn-3.5%Ag-0. 5%Cu solder interface. There was no NiSnP layer detected at the Sn-3.5%Ag-0.5%Cu solder interface like the Sn-3.5%Ag solder interface. During the aging test, no appreciable growth of the P-rich layer has been noticed. From the detailed microstructural study as well as the survey of the concurrent published works, it has been proved that the addition of Cu retards the interfacial reaction at the solder interface-especially during reflow soldering reaction. It has also been proved that the Ni content is decreasing in the complex IMC, (Au,Ni,Cu) Sn with the aging time, and there is no consumption of original electroless Ni-P noticed. REFERENCES [1] R. J. Coyle, D. E. H. Popps, A. Mawer, D. P. Cullen, G. M. Wenger, and P. P. 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8 438 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 31, NO. 2, JUNE 2008 [29] C. E. Ho, S. Y. Tsai, and C. R. Kao, Reaction of solder with Ni/Au metallization for electronic packages during reflow soldering, IEEE Trans. Adv. Packag., vol. 24, no. 4, pp , Nov [30] S. J. Wang and C. Y. Liu, Retarding growth of Ni P crystalline layer in Ni(P) substrate by reacting with Cu-bearing Sn(Cu) solders, Scripta Mater., vol. 49, no. 9, pp , Nov [31] T. Hiramori et al., Sn-Ag based solders bonded to Ni-P/Au plating: Effects of interfacial structure on joint strength, Mater. Trans., vol. 44, no. 11, pp , Nov [32] J. Y. Park et al., Influence of Au addition on the phase equilibria of near-eutectic Sn-3.8Ag-0.7Cu Pb-free solder alloy, J. Electron. Mater., vol. 32, no. 12, pp , Dec [33] J. K. Kivilahti, The chemical modeling of electronic materials and interconnections, J. Min. Metals Mater. Soc., vol. 54, no. 12, pp , Dec [34] K. Zeng and K. N. Tu, Six cases of reliability study of Pb-free solder joints in electronic packaging technology, Mater. Sci. Eng. R, vol. 38, pp , [35] T. M. Korhonen et al., Reactions of lead-free solders with CuNi metallizations, J. Electron. Mater., vol. 29, no. 10, pp , Oct Mohammad Ohidul Alam (SM 06) received the B.Sc. and M.Sc. degrees in engineering from the Bangladesh University of Engineering and Technology (BUET), Dhaka, in 1995 and 1997, respectively, and the Ph.D. degree in electronic packaging from the City University of Hong Kong in He joined BUET as a Lecturer in 1997 and was promoted to Assistant Professor in He worked in the EPA Center, City University of Hong Kong, as a Research Fellow until He has published more than 60 journal and conference papers in the area of electronic packaging and materials engineering. His research interests include electronic packaging and failure analysis, Pb-free solder, tin whiskers, and electromigration. Dr. Alam received the Marie Curie Incoming Fellowship from the EU (hosted in the Greenwich University, UK to conduct research on the Reliability Modeling of the Pb-free solder joint). Y. C. Chan (F 95) received the B.Sc. degree in electrical engineering, the M.Sc. degree in materials science, and the Ph.D. degree in electrical engineering from the Imperial College of Science and Technology, University of London, London, U.K., in 1977, 1978, and 1983, respectively. After eight years of industrial experiences, he joined the City Polytechnic of Hong Kong (now City University of Hong Kong) as a Senior Lecturer in electronic engineering in He is currently Chair Professor of Electronic Engineering and Director of the EPA Center, and Assistant Head for Applied Research and Industry Relations in the Department of Electronic Engineering. He has authored or co-authored over 150 scientific publications in peer-reviewed journals, over 70 international conference papers, and co-edited three books. His current research interests include advanced electronic packaging and assemblies, failure analysis, and reliability engineering. He is world renown in electronic product reliability, and has had extensive industrial connections in the local electronics and manufacturing industry. Dr. Chan served in the Executive Committee of the IEE Hong Kong for eight years and as Chairman in 1995 to He was Founding President (2001) and is presently Honorary Chairman of the Hong Kong Electronic Packaging and Manufacturing Services Association. He is currently serving as member of the HKIE Examination and Education committee representing the Electronics discipline. Worldwide, he has chaired numerous international technical conferences in electronic product reliability and green electronics, and participated as organizing committee members/international advisors in many others.