Evaluation of Pb-free BGA Solder Joint Reliability on Ni-based Surface Finishes using Alternative Shear and Pull Metrologies

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1 Evaluation of Pb-free BGA Solder Joint Reliability on Ni-based Surface Finishes using Alternative Shear and Pull Metrologies Kuldip Johal and Hugh Roberts Atotech USA Inc., Rock Hill, SC Sven Lamprecht, Thomas Beck, Hans-Juergen Schreier and Christian Sebald Atotech Deutschland GmbH, Berlin, Germany ABSTRACT During the relatively short history of the electronics packaging and assembly industry, few innovations have impacted the design and manufacturing sectors as much as the ball grid array (BGA). Widespread acceptance of the BGA has occurred for most electronics applications. In the case of BGAs, the nature of the solder bond was quite different from that of the earlier leaded through-hole and SMT components. As a result, new testing methodologies were required to augment the data resulting from more traditional solder spreading and wetting evaluations. Consequently, the ball shear test and cold ball pull test represent two relatively fast and simple methods for measuring the effectiveness of bonding achieved through reflow of BGA solder balls. However, there have been some recent developments that may raise questions regarding the applicability of previously obtained test data. Most of the global electronics producers are now actively responding to the newly effective Pb-free requirements. The electronics manufacturing chain, including material suppliers, IC substrate fabricators, EMS/ODM companies, and OEMs, is continuing to learn about Pb-free assemblies. In some cases, parameters for testing methodologies are also changing in response to the combined effect of continued miniaturization and Pb-free requirements. This technical paper summarizes the findings and conclusions from a comprehensive examination of Pbfree solder ball bonding, comparing ball shear and cold ball pull testing results. The paper compares results obtained from conventional (slow) speed ball shear and cold ball pull testing with the results from newer high-speed test parameters. Relationships between ball shear and cold ball pull results are examined. Findings are also compared for two different PWB surface finishes: a process-of-record electroless nickel / immersion gold finish (ENIG), and an electroless nickel / electroless palladium / immersion gold finish (Ni/Pd/Au). Testing variables include surface finish deposit thickness, solder ball diameter and resist opening, ball shear speed and cold ball pull speed. Test results are presented as graphical representations of ball shear and cold ball pull forces and graphical representations of fracture modes. Analyses of results include determinations of the fracture modes for the various test methods and deposit conditions. Conclusions are offered regarding the two test methods and parameters, the performance of the two surface finishes and their interaction within Pb-free applications. BACKGROUND The ball grid array (BGA) has been universally accepted as a reliable component packaging methodology, as illustrated in Figure 1. With Pb-free alloys for both BGA solder balls and paste now becoming the dominant bonding medium, the importance of solder joint integrity has become even more significant. Pbfree joints can be particularly susceptible to brittle fracture at the ball-to-pad interface. Such failures can occur at any stage in the full life cycle of the solder joint including manufacturing, handling, testing and end-using. Widespread adoption of Pb-free fabrication and assembly is further impacted by the continuing demand for finer circuitry and interconnect features. Fig. 1 Projected growth of flip chip PBGA market (above) and wire bond PBGA market (below) from 2003 to (Source: Prismark Partners) The increasing use of high-speed bond testing is the result of considerable research under a U.S.-based consortium focused on meeting the growing demand for a tool that can measure solder bond reliability. According to one test equipment manufacturer, the essence and advantage of the high-speed machine is that at high strain rates the applied force is transferred to the bond itself. When testing at traditional shear speeds and pull speeds, the dominant failure mode tends to be within the solder ball. Such results only indicate that the bond strength is greater than that of

2 the sheared solder ball, making it very difficult to compare the effects of different pad finishes and solder alloys. Extensive research has now shown that testing at much higher speeds will result in significantly higher incidence of bond failures, allowing the performance of different pad finishes and solder alloys to be compared [ 1]. Traditionally, ENIG has been accepted as a surface finish for both IC substrates and PWBs for BGA applications. However, with the recent introduction of Pbfree fabrication and assembly requirements, alternative finishes are being examined. The suitability of the Ni-Pd-Au finish for both wire bonding and soldering has been previously reported [ 2, 3]. In further studies, ball shear and cold ball pull investigations of several alternative finishes were performed, but only at the more traditional (slower) test speeds [ 4]. The results of these investigations indicated that the Ni/Pd/Au process provides improved bonding results for meeting the more demanding BGA soldering requirements of the Sn-Ag-Cu alloy composition. A major design of experiment (DOE) was conducted to compare the solder joint reliability of two different surface finishes, electroless nickel / immersion gold (ENIG) and electroless nickel electroless palladium / immersion gold (Ni/Pd/Au). ENIG has long been used as a surface finish within the PWB and IC substrate fabrication sectors. The Ni/Pd/Au finish was introduced more than ten years, but is receiving renewed attention because of its suitability for both Pb-free soldering and wire bonding applications. DESCRIPTION OF TESTING To further evaluate the reliability of Pb-free solder joints on nickel-based surface finishes, two different testing methodologies were used, the ball shear test and the cold ball pull test. The evaluation included comparison of results for both conventional and higher shear and pull speeds. IN addition to surface finish deposit thicknesses, variables also included solder ball diameter and solder resist opening. Test Vehicle Figure 2 shows an example of the BGA test vehicle used for these investigations. Also shown are the locations of the solder balls tested in both examinations. As shown, four balls are located at each of the remote corners of the grid array and the remaining 14 balls are located diagonally across the center of the array. Fig. 2 BGA test vehicle for solder ball shear and ball pull testing showing location of solder balls tested and detail of four-point anchor design (under solder mask). The objective of the DOE was to evaluate solder joint reliability of alternative surface finishes. Therefore, it is important to note that the test vehicle, including both the base material and pad design, was intentionally designed to be conducive to interfacial fractures between solder and pad finish (or failure within the bulk solder). To accomplish this objective, FR4 was selected for construction of the test vehicle. Typical buildup material for IC substrate applications does not provide sufficient adhesive strength to withstand the aggressive forces of the ball shear and ball pull bond testing. As a result, most failures occur as a loss of adhesion between the copper pad and the underlying dielectric; thus, the bond at the surface finish is never fully tested. FR4 was selected as the dielectric because of its inherently higher adhesive strength (approximately 50-70% greater than build-up materials). Furthermore, all pads were designed to include a four-point anchoring feature (shown in detail in Figure 2). This construction method also promotes the occurrence of interfacial failures by firmly securing the test pads to the underlying dielectric. Surface Finish The test vehicles were fabricated with two different surface finishes: Electroless nickel / immersion gold (ENIG) and Electroless nickel / electroless palladium / immersion gold (Ni/Pd/Au). Table 1 presents a summary of the surface finishes included in the investigations. As shown, two versions of the ENIG surface finish were examined, differing primarily in terms of the content of co-deposited phosphorus in the nickel layer and the resultant achievable immersion gold thickness. Several variations of the Ni/Pd/Au finish were included in the test matrix, incorporating different deposit thicknesses and/or compositions.

3 Table 1 Summary of Surface Finish Specifications ENIG 1 ENIG 2 Ni Pd Au 4.8 µm P= 8-9% 4.8 µm P=11-12% µm µm NiPdAu µm 0.05 µm µm NiPdAu µm 1.0 µm µm NiPdAu µm 3.0µm µm Deposit thickness variations: Ni: ± 5%, Pd: ±10% Au: ±10% Assembly Issues Table 2 summarizes the key information regarding assembly of the solder balls to the BGA test vehicle. As noted, a Pb-free Sn-3.0Ag-0.5Cu (SAC305) alloy was used for all investigations. Prior to solder reflow, all test vehicles were thermally pretreated for 12 hours at 155 C to simulate actual assembly conditions. All bond testing was performed within four hours of reflow. The solder reflow profile that was used for all investigations is shown in Figure 3. Table 2 Solder Ball Reflow Specifications Solder balls: Solder Flux: Senju Sn3Ag0.5Cu Kester TSF6502 Reflow profile: TSF6502 Pb-free linear profile; air atmosphere Peak temperature: 258 C Time to peak: 241 sec Time above liquidus: 57 sec Table 3 presents a summary of the important parameters related to the ball shear investigations. Table 3 Solder Ball Shear Test Parameters Equipment (low speed) Dage Series 4000 Equipment (high speed) Dage Series 4000HS Shear speed (low) 0.3 mm/sec Shear speed (medium) 20 mm/sec Shear speed (high) 1000 mm/sec Shear height 50 µm Solder Resist Openings 450µm/550µm/650µm Solder Ball Diameters 600µm /760µm Solder ball composition Sn-3.0Ag-0.5Cu All solder balls were mounted on solder resist-defined pads. Solder resist openings of 550 µm and 650 µm were used in conjunction with solder balls of 760-µm diameter. Solder resist openings of 450 µm were used with solder balls of 600-µm diameter. Slow-speed shear testing (0.3 mm/sec) was performed on a Dage Series 4000 Multi-Function Bond Tester, while higher speed testing (20 mm/sec and 1000 mm/sec was performed on a Dage Series 4000HS High-Speed Bond Tester. For each test condition, investigations included the shearing of 30 Pb-free solder balls (as located in Figure 2). Equipment used for solder ball shear testing is shown in Figure Temperature ( C) C Time (sec) Fig 3. Pb-free solder reflow profile (air atmosphere) Test Procedures - Solder Ball Shear Solder ball shear testing was performed at different shear speeds and with different solder resist openings (SRO) and solder ball diameters. Each test condition was specifically defined by the following: One of five surface finishes and plating thickness configurations (ENIG or NiPdAu) One of three shear speeds One of three solder resist openings /ball diameters Fig. 4 The Dage Series 4000 Multi-Purpose Bond Tester (left) and Dage Series 4000HS High-Speed Bond Tester An illustration of the ball shear test mechanism and a detailed view of the high-speed ball shear device are shown in Figures 5 and 6, respectively Shear rate: 0.3 to 1000mm/sec. Shear height: 50µm +/- 10µm. Fig.5 Illustration of ball shear test mechanism

4 Test Procedures - Cold Ball Pull Cold ball pull investigations also included the testing of 30 Pb-free solder balls, as located in Figure 1, for each test condition. Similar to the shear testing, cold ball pull testing was performed at different speeds and with different solder resist openings (SRO) and solder ball diameters. Each test condition was specifically defined by the following: One of five surface finishes and plating thickness configurations (ENIG or NiPdAu) One of two pull speeds One of three solder resist openings/ball diameters Fig. 6 Detailed view of the solder ball shear device on the Dage Series 4000HS High-Speed Bond Tester Ball shear failures were categorized according to the type and location of fracture. For ball shear, the preferred type of failure occurs entirely within the solder ball, indicative of a ductile fracture. In contrast, brittle fracture occurs primarily at the interface between the solder and the surface finish, often in combination with shearing of a portion of the solder. Failure by pad pullout (Mode 1) only indicates that the bond of the solder ball to the pad was stronger than the pad-to-substrate adhesive strength. Figure 7 illustrates the location of the five different ball shear fracture modes within the solder ball joint. Similarly, Figure 8 shows examples of the fracture interface for failure modes 2 though 5. All solder balls were mounted on solder resist-defined pads in the same fashion as previously described for the ball shear investigations. Cold ball pull testing at the slower speed (5 mm/sec) was performed on the Dage 4000 Multi-Function Bond Tester, while the higher speed pull testing (100 mm/sec) was performed on the Dage 4000HS High-Speed Bond Tester. Table 4 presents a summary of the important parameters related to the cold ball pull investigations. Table 4 Cold Ball Pull Test Parameters Equipment (low speed) Dage Series 4000 Equipment (high speed) Ball pull speed (low) Ball pull speed (high) Solder Resist Openings Solder Ball Diameters Solder ball composition Dage Series 4000HS 5 mm/sec 100 mm/sec 450µm /550µm /650µm 600µm /760µm Sn-3.0Ag-0.5Cu Solder Figures 9 and 10, respectively, show an illustration of the high-speed cold ball pull test mechanism and a detailed view of the device. Mode 2 SM IMC Surface Finish Copper Pad SM Mode Mode 1 PWB - Substrate Fig. 7 Illustration of ball shear fracture modes and location within the solder ball joint Fig. 9 Illustration of high-speed cold ball pull test mechanism (Source: Dage) Fig. 8 Examples of ball shear fracture type showing Modes 2, 3, 4 and 5, from left to right

5 transferred from the solder ball to the bond between the ball and pad-interconnect, resulting in a test that examines bond reliability [ 5]. In comparison to ball shear, cold ball pull testing is a newer technique for determining solder joint reliability that the packaging industry is slowly adopting. This method uses a plain tensile load, as opposed to the traditional ball shear test that employs a combination of tensile and compressive loading. Cold ball pull testing offers another methodology for measuring interfacial bond strength in a relatively fast, low-cost manner. Fig. 10 Detailed view of the cold ball pull device on the Dage Series 4000HS High-Speed Bond Tester Similar to the ball shear investigations, cold ball pull failures were also categorized according to the type and location of failure. As was the case with ball shear testing, with cold ball pull testing the preferred type of failure also occurs entirely within the solder ball (Mode 2). A bond failure (Mode 4) is indicative of brittle fracture, although in the case of cold ball pull testing the complete IMC is typically exposed. Any combination of IMC and solder at the plane of fracture is treated as a bond failure. Again, pad failure or pad pull-out (Mode 1) only indicate that the bond of the solder ball to the pad was stronger than the pad-to-substrate adhesive strength. Ball extrusion (Mode 3) may indicate a problem with the clamping force of the device or that the solder is too soft. Figure 11 illustrates the location of the four different fracture modes within the solder ball joint. TESTING RESULTS The DOE reflected in this paper resulted in a significant amount of data, requiring both compilation and statistical analysis. In particular, much effort was devoted to screening the various parameters associated with the Ni/Pd/Au finish. To determine the influence of each test parameter for the Ni/Pd/Au finishes, linear regression analyses of failure mode were performed with ball shear and ball pull results using the factors of nickel thickness, palladium thickness and solder resist opening. For this examination, failure modes 4 and 5 were included in the ball shear analysis, while failure mode 4, alone, was used for the cold ball pull analysis. While palladium thickness and SRO were found to significantly impact test results in terms of both ball shear and cold ball pull testing. However, the analysis also indicated that the nickel deposit thickness had only a relatively minor effect on the results of both the ball shear and cold ball pull testing. This finding is presented in Figure 12 by the two leverage plots showing the effect of nickel deposit thickness on the occurrence of brittle fracture for the two test methodologies. As a result of these analyses, the subsequent evaluation of the results was limited to the testing variables of ball shear speed, cold ball pull speed, palladium thickness (0.1 µm, 0.2 µm and 0.3 µm) and SRO diameter. Fig. 11 Illustration of cold ball pull failure modes and location within the solder ball joint It has been reported that pull strength increases with test speed and that the rate of interfacial solder joint fracture increased with test speed. The same investigation noted that at high test speeds, a greater force is

6 both the phosphorus content in the ENIG nickel deposit and the palladium thickness of the Ni/Pd/Au finish impacts. Furthermore, the use of the highest ball shear speed increases the incidence of brittle interfacial fracture and would appear to provide a more demanding test of solder ball bond strength. Fig. 12 Leverage plots for ball shear results (above) and cold ball pull results (below) showing relatively minor influence of nickel thickness on failure mode. Testing Results Solder Ball Shear The guidelines and graphics legend for qualifying the five types of ball shear failures are as follows: Mode 1: Pad pull-out Mode 2: Greater than 95% within the bulk solder Mode 3: 5-25% interfacial, remainder in bulk solder Mode 4: 25-90% interfacial, remainder in bulk solder Mode 5: Greater than 90% interfacial fracture The effect of increasing shear speed on the solder ball shear test is graphically presented in Figure 13. This investigation was limited to the SRO diameter of 650 µm. As expected, there is a distinct trend of increasing shear forces with increasing shear speeds. Likewise, according to the failure mode section of the chart, there is also a trend of increasing incidence of interfacial fracture with increasing shear speeds. However, measurable differences among the finishes are evident within each shear speed segment. For all shear speeds, the ENIG-2 deposit yielded better results than the ENIG-1 (with a medium phosphorus content). More importantly, the Ni/Pd/Au surface finishes with thicker palladium deposits (0.2 µm and 0.3 µm) can withstand measurably greater shear forces and provide a significantly higher percentage of ductile (Mode 2) failures compared to the other surface finishes. This fact is best demonstrated at the highest shear speed, reflecting more disparity in both shear force and failure mode results. Generally, these findings indicate that shear force and failure mode results are affected by Fig. 13 Summary of solder ball shear forces and fracture modes by surface finish for 0.3 mm/sec, 20 mm/sec and 1000 mm/sec ball shear speeds (SRO = 650 µm). The effect of the SRO diameter was also examined for the different surface finishes as part of the ball shear investigations. Figure 14 presents the results of this examination for the three SROs examined. As expected, higher shear forces are required with increasing SRO diameter. However, the shear force segment of the figure shows a clear trend of higher shear forces for the Ni/Pd/Au finishes in comparison to the ENIG finishes. This same trend is also indicated with respect to failure mode, with a higher incidence of brittle fracture with the ENIG finishes. It is significant to note the dominance of brittle fracture (Modes 4 and 5) for the 450 µm SRO in the failure analysis segment of the chart, although the Ni/Pd/Au finish with thicker palladium did perform better. Also, the Ni/Pd/Au surface finishes with thicker deposits of palladium (0.2 µm and 0.3 µm) again result in a higher percentage of the preferred Mode 2 failures. For the two larger SROs there is essentially no significant difference between results for the NiPdAu 1 and NiPdAu2 surface finishes at the higher shear speed testing used in this analysis. However, at the smaller SRO of 450 µm, the additional palladium thickness provides noticeably improved bond strength, as shown by both the shear force and failure mode analysis.

7 The effect of pull speed on the cold ball pull results is presented in Figure 16. Similar to the ball shear results, the higher pull speed (100 mm/sec) resulted in somewhat higher pull forces for all surface finishes tested. Also, as was the case with ball shear testing, the Ni/Pd/Au finishes performed better than the ENIG finishes at both pull speeds. Overall, although some tendency toward increased brittle fracture mode can be seen by comparing the results of the higher pull speed to the lower, the contrast is not as significant as demonstrated by the ball shear investigations (refer to Figure 13 for comparison). One explanation is that an even higher pull speed (greater than 100 mm/sec) may be required to sufficiently test the bond integrity. However, the increased incidence of pad pull-out (shown at the higher speed) suggests that speeds greater than 100 mm/sec may only lead to increased numbers of pad pull-out and provide little additional information. Fig. 14 Summary of solder ball shear forces and fracture modes by surface finish for SROs of 450 µm, 550 µm and 650 µm (Shear speed = 1000 mm/sec). Figure 15 presents a failure mode analysis of the same ball shear results for each SRO according to the surface finish tested. This chart more clearly illustrates the significantly better performance of the Ni/Pd/Au surface finishes that included thicker deposits of palladium (0.2 µm and 0.3 µm). Fig. 16 Summary of cold ball pull forces and fracture modes by surface finish for 5 mm/sec and 100 mm/sec ball pull speeds (SRO = 650 µm). Fig. 15 Summary of solder ball shear fracture modes according to surface finish for SROs of 450 µm, 550 µm and 650 µm (Shear speed = 1000 mm/sec). Testing Results Cold Ball Pull The guidelines and graphics legend for qualifying the four types of cold ball pull failures are as follows: Mode 1: Pad failure (pad pull-out) Mode 2: Solder ball failure Mode 3: Solder ball extrusion Mode 4: Bond failure The effect of SRO in relation to cold ball pull testing is presented graphically in Figure 17, evaluated at a constant pull speed of 5 mm/sec. As expected, the similar trend of higher pull forces associated with increasing SRO is continued. Also, the overall performance of the Ni/Pd/Au finishes was significantly better in comparison to the ENIG finishes in terms of both pull force and failure mode. Similar to the results of the ball shear testing, the highest incidence of brittle fracture was reported for the SRO diameter of 450 µm. The extreme occurrence of pad pull-out for all Ni/Pd/Au finishes in combination with the 450-µm SRO is not

8 readily explainable. It may relate to a combination of stress factors specifically associated with the smaller SRO and the 760-µm solder ball, since this same solder ball was used with the SRO of 650 µm. As shown, with the larger SRO, the preferred Mode 2 failure is the dominant result. incidence of pad pull-out that was reported for the Ni/Pd/Au finishes coupled with the 550 µm SRO. Testing Results Intermetallic Examination The results presented to this point indicate that the Ni/Pd/Au finishes with thicker palladium deposits achieved superior ball shear and cold ball pull results in comparison to both ENIG finishes and to the Ni/Pd/Au finish with thin palladium. To determine the reason for this apparently improved performance, the intermetallic compounds (IMC) of the finishes were examined. Regarding ENIG, it was reported in previous studies that fractures for the finishes similar to ENIG-1 (nickel with medium phosphorus content) generally occurred closer to the IMC, as opposed to further into the bulk solder in the case of the finishes similar to ENIG-2 [ 6]. Because the formation of nickel-tin IMCs with respect to ENIG finishes is already well documented, the remaining evaluation presented here will concentrate on the Ni/Pd/Au finishes, specifically with 0.05-µm and 0.3-µm palladium deposits. Fig. 17 Summary of cold ball pull forces and fracture modes by surface finish for SROs of 450 µm, 550 µm and 650 µm (Pull speed = 5 mm/sec). As another means of interpreting the data, Figure 18 presents a failure mode analysis of the same cold ball pull results for each SRO according to the surface finish tested. Fig. 18 Summary of cold ball pull fracture modes according to surface finish for SROs of 450 µm, 550 µm and 650 µm (Pull speed = 5 mm/sec). This chart dramatically illustrates the superior performance of the Ni/Pd/Au surface finishes with thicker deposits of palladium (0.2 µm and 0.3 µm). Again, no apparent explanation is seen for the extremely high Fig. 19 SEM images of cross-section after brittle fracture due to bond failure showing detail of (Cu,Ni) 6 Sn 5 IMC, which forms fracture interface. Figure 19 presents SEM photographs of a solder joint after bond testing, showing the result of major interfacial fracture. As shown in the higher magnification, the fracture clearly occurs along the plane where the bulk solder meets the surface of the (Cu,Ni) 6 Sn 5 IMC. This

9 finding was typical of other solder joints that were examined with this defect. Figure 20 shows cross-sectional SEM photographs of the nickel-tin IMC created after one Pb-free solder reflow of the Ni/Pd/Au finish with 0.05 µm palladium. As shown, a portion of the nickel deposit has been converted to create the (Cu,Ni) 6 Sn 5 IMC. Also present are a thinner NiSnP IMC and a phosphorus-rich Ni 3 P layer, located directly above the originally plated Ni-P deposit. Fig. 21 SEM images of cross-section at 2000X (above) and 5000X (below) showing formation of (Cu,Ni) 6 Sn 5 IMC on Ni/Pd/Au finish with 0.3 µm Pd, following one reflow. Fig. 20 SEM images of cross-section at 2000X (above) and 5000X (below) showing formation of (Cu,Ni) 6Sn 5 IMC on Ni/Pd/Au finish with 0.05 µm Pd, following one reflow. For the Ni/Pd/Au finish with 0.3 µm palladium, the nickel-tin IMC created after one Pb-free solder reflow is shown in Figure 21. In comparison to the previous figure, little or no (Cu,Ni) 6 Sn 5 IMC is evident. Only the relatively thin NiSnP IMC can be seen, even at the higher magnification. The remaining Ni-P layer is also somewhat thicker, while the phosphorus-rich Ni 3 P layer is noticeably thinner. Fig. 22 SEM images of cross-section at 2000X (above) and 5000X (below) showing formation of (Cu,Ni) 6 Sn 5 IMC on Ni/Pd/Au finish with 0.05 µm Pd, following five reflows.

10 The investigations were continued in an effort to determine if the formation of the IMCs on both finishes would be further affected as a result of additional Pbfree solder reflow operations. As such, IMC investigations were also performed following five Pb-free solder reflow cycles. Figure 22 shows SEM photographs of the nickel-tin IMC created after five such reflows on the Ni/Pd/Au finish with the thinner palladium deposit. As can be seen, considerably more of the nickel deposit has been converted to form more (Cu,Ni)6Sn5 IMC, as well as a much thicker Ni3P layer. As a result, the originally plated Ni-P deposit has been significantly reduced in thickness to approximately 1.5 µm. Figure 23 shows the nickel-tin IMC created after five reflows on the NiPdAu-3 finish. As shown in this figure, a more definable Ni3P layer has been formed. Also, the thickness of the originally plated Ni-P layer has been reduced in comparison to the deposit shown after one solder reflow, although still measurably thicker than that of the NiPdAu-1 shown in Figure 22. It is important to note that there is still no evidence of (Cu,Ni)6Sn5 IMC growth. The NiSnP IMC does not look significantly different from the SEM images after one reflow (refer to Figure 21). Based on this evaluation, the presence of the (Cu,Ni)6Sn5 IMC appears to be a key factor leading to the occurrence of interfacial fracture. Furthermore, it appears that the thinner palladium deposit of the ENIG-1 dissolves relatively quickly into the bulk solder during reflow, allowing the (Cu,Ni)6Sn5 IMC to be more readily created. By comparison, the thicker palladium deposit apparently inhibits or delays the (Cu,Ni)6Sn5 IMC formation. This theory is also supported by the thinner phosphorus-rich Ni3P layer associated with the thicker palladium deposit, as well as the obviously thicker nickel layer that remains. Fig. 23 SEM images of cross-section at 2000X (above) and 5000X (below) showing formation of (Cu,Ni)6Sn5 IMC on Ni/Pd/Au finish with 0.3 µm Pd, following five reflows. As such, the palladium layer seems to act as a barrier between the underlying nickel and Pb-free solder ball. It is further proposed that the relative absence of the (Cu,Ni)6Sn5 IMC is directly related to the superior performance of the Ni/Pd/Au surface finishes having thicker palladium deposits in comparison to the other finishes examined. DISCUSSION Two different test methodologies were employed in these investigations. Both the ball shear test and cold ball pull test are now used in solder joint reliability examinations. In general, the results of these methods do follow certain basic trends in terms of forces and failure modes for the tested samples. However, as shown in this paper, the results obtained from both methodologies are very much dependent on specific test parameters, particularly the selected shear and pull speed. In October 2006, the JEDEC Solid State Technology Association published JEDEC Standard No. 22B117, Solder Ball Shear [7]. This document provides valuable and needed guidance for conducting such investigations and will help to standardize the performance of such tests. However, the standard does not provide specific details regarding shear speeds, other than to identify low-speed and highspeed shear by definition. As such, selection of this critical parameter is the responsibility of the entity performing the test or specified by the end user. To date, no such standard has been developed for the cold ball pull test method. The investigations presented in this paper are part of a comprehensive DOE focused on identification of preferred surface finishes for IC substrates (ball side and C4 side) used in flip chip BGAs primarily for CPU and MPU applications. However, the findings of these investigations suggest that the Ni/Pd/Au surface should be considered for evaluation in other Pb-free applications. As shown, this evaluation has concentrated on the solder joint reliability that can be achieved with the tested surface finishes. However, the Ni/Pd/Au surface

11 finish also offers distinct technology and cost advantages in comparison to processes used for similar applications. The Ni/Pd/Au finish provides an opportunity to combine multiple bonding functions, including both soldering and both gold and aluminum wire bonding on one common surface. Traditionally, gold wire bonding has been accomplished with the use of other surface finish processes (electrolytic nickel-gold or electroless nickel-gold), more costly because of the precious metal consumption. Additional cost savings would be realized as a result of manufacturing process simplification. Ni/Pd/Au is an electroless type of process, meaning that no electrical buss is required, thus simplifying the process and reducing costs. More importantly, as an electroless process, it allows a product of finer features of to be produced. CONCLUSIONS Based on the investigations performed in this evaluation, the following conclusions are offered: 1. Both ball shear and cold ball pull test methodologies can be used to evaluate BGA solder joint reliability. In the respective tests, the selection of shear and pull speeds are critical in terms of the resultant findings. This finding is particularly true in terms of failure mode results. Strictly based on the parameters used in this evaluation, the more demanding test of solder ball bond strength would appear to be the high-speed ball shear testing performed at 1000 mm/sec. To obtain comparable results with cold ball pull testing, a higher pull speed than that used (100 mm/sec) may be required. 2. For the respective test methods, selection of the higher shear speed or higher pull speed will encourage failure modes more specifically related to brittle (interfacial) fracture. Additional investigations are required to determine the optimum range in speed that is most appropriate for either method. 3. For the samples tested, the diameter of the solder resist opening (SRO) had a significant influence on both ball shear and cold ball pull results. Generally, the solder joints with a larger SRO were able to withstand greater shear and pull forces and exhibited a higher percentage of preferred (ductile) failure mode. 4. For the two ENIG finishes examined, the phosphorus content of the nickel deposit impacts both shear force and failure mode results. For all shear speeds tested, the ENIG with higher phosphorus content in the nickel deposit yielded better shear force and failure mode results. 5. For all shear and pull speeds investigated, the Ni/Pd/Au finishes with thicker palladium deposits (0.2 µm and 0.3 µm) were able to withstand measurably greater shear forces and exhibited in a significantly lower incidence of interfacial fracture. It was also determined that nickel thickness variations (3.0 to 7.0 µm) did not significantly impact ball shear or cold ball pull results. 6. For the samples tested, solder joint failures primarily caused by brittle (interfacial) fracture were found to occur at the interface of the bulk solder to the (Cu,Ni) 6 Sn 5 IMC. SEM analysis indicated that the Ni/Pd/Au finishes with thicker palladium deposits (0.2 µm and 0.3 µm) showed little or none of this IMC in the solder joint following one and five Pb-free solder reflow cycles.

12 ACKNOWLEDGEMENTS The authors would like to acknowledge the contributions of Stephen Clark, Victor Morse and Bob Sykes of Dage Precision Industries. Their information and guidance regarding the bond testing equipment used in the investigations described in this paper was extremely valuable. REFERENCES 1. R. Sykes, Lead free BGA reliability: high-speed bond testing and brittle fracture detection ; Global SMT & Packaging, October K. Johal, S. Lamprecht and H. Roberts; Electroless Nickel / Electroless Palladium / Immersion Gold Plating Process for Gold- and Aluminum- Wire Bonding Designed for High-Temperature Applications ; SMTA 9 th Annual Pan Pacific Microelectronics Symposium; January K. Johal, H. Roberts and S. Lamprecht; Electroless Nickel / Electroless Palladium / Immersion Gold Process for Multi-Purpose Assembly Technology ; SMTA International Conference; September K. Johal, H. Roberts, K. Desai, Q.H. Low, R. Huemoeller; Performance and Reliability Evaluation of Alternative Surface Finishes For Wire Bond and Flip Chip BGA Applications, SMTA 11 th Annual Pan Pacific Microelectronics Symposium; January L. Peters; Lead-Free Brings Out Problem of Brittle Fracture ; Semiconductor International; September K. Johal, S. Lamprecht, H.J. Schreier and H. Roberts; Impacts of Bulk Phosphorous Content of Electroless Nickel Layers to Solder Joint Integrity and their Use as Gold- and Aluminum-Wire Bond Surfaces, SMTA 9 th Annual Pan Pacific Microelectronics Symposium; January JEDEC Standard Solder Ball Shear (JESD22- B117), JEDEC Solid State Technology Association, October 2006.