THE EFFECTS OF PLATING MATERIALS, BOND PAD SIZE AND BOND PAD GEOMETRY ON SOLDER BALL SHEAR STRENGTH

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1 THE EFFECTS OF PLATING MATERIALS, BOND PAD SIZE AND BOND PAD GEOMETRY ON SOLDER BALL SHEAR STRENGTH Keith Rogers and Craig Hillman CALCE Electronic Products and Systems Center University of Maryland College Park, MD, USA ABSTRACT There has been a vast amount of literature published on the characterization of the shear strength of solder balls attached to ball grid array (BGA) substrates. This has allowed for the establishment of a quasi-defacto specification of 1000 gramsforce shear strength on packages with 30 mil diameter solder balls and 25 mil pads for BGA components. There is significantly less data on the expected shear strength values of the solder ball to printed wiring board (PWB) attachment. This prevents contract manufacturers and their customers from benchmarking the robustness of their material sets and reflow processes. An extensive amount of shear testing has been performed to quantify the performance of the solder ball joint at the PWB, under a variety of materials and designs. These include different plating materials (immersion tin, immersion silver, organic solderability preservative and electroless nickel/immersion gold), plating suppliers and bond pad dimensions and geometries (round vs. square). In this paper, solder ball shear strength variation at the PWB interface as a function of some of these parameters is presented, along with a discussion on the relevant failure types. Key words: solder ball, ball shear, shear strength BACKGROUND The present trend towards higher-performance, smaller and lighter products with increased functionality has resulted in an increasing demand for smaller component packages and/or higher pin counts. Due to this, the semiconductor packaging industry has experienced a shift from traditional peripheral leaded devices to area array technology, specifically ball grid array (BGA) technology. The BGA package has become one of the high performance semiconductor packages of choice for advanced applications and is expected to have an increasing share in the microelectronics market in the future. Two disadvantages of using BGAs are inspectability for interconnection cracks and individual ball - only peripheral balls/columns can be inspected easily, and lack of individual solder ball re-workability. To preserve the solderability of PWBs to which the BGAs are attached to by reflow, it is necessary to protect the copper surface mount pads during storage with a solderable surface finish. This is because the copper pads are easily oxidized. The most common finish has been eutectic tinlead alloy by hot air solder leveling (HASL) method, because it has desirable properties of an ideal PWB surface. Unfortunately this coating does not meet the requirement of soldering pads planarity; a fundamental factor for fine pitch components such as ball grid arrays (BGAs), and contains lead, one of the toxic metals. Alternatives to the tin/lead (Sn/Pb) HASL finish include co-planar coatings such as: Electroless nickel/immersion gold (ENIG) Immersion matte tin (ImSn) Immersion silver (ImAg) Organic solderability preservative (OSP) To evaluate the reliability of the BGA interconnections, the strength of the solder balls (or solder bumps) attachment not only to the BGA, but also to the printed circuit board is one of the perquisite data. To characterize the integrity or strength of the solder ball connection at the PWB interface, companies have experimented with varying parameters including solder pad size on PWB, solder pad geometry (on PWB), solder ball size, plating bath chemistry, surface finish, cleaning method and storage condition. There are two test methods to assess the interconnect strength of the ball to pad interface on the PWB: (1) solder ball shear testing and (2) solder ball pull testing. Currently, the most popular method to evaluate the interconnect strength is the ball shear test. In July of 2000, JEDEC established a standard for BGA tests, JESD22-B117. The stated purpose of this test is to determine the ability of the BGA to withstand mechanical shear forces that may be applied during module manufacturing and handling operations. The test applies to shear force testing of the solder balls on the BGA, prior to second level attachment to the PWB. This method can also be used to assess the shear strength of the solder ball at the PWB interface as opposed to the BGA substrate interface. It has been observed under combined thermal cycling and vibration that the solder ball attachment can fail at either the solder ball to PWB interface or the solder ball to BGA substrate interface [1]. Figure 1 shows a photo where there are separations in the solder ball attachment at both the component and PWB interface after 82 thermal cycles (-40 to 125 C) and vibration (0.10 G 2 /Hz) testing. The probability of failure at each interface depends on a number of geometric factors including relative size of solder pads, if

2 the solder pads are solder mask defined, the shape of the solder attachment at the interface, and the presence of voids at the interface. Given that the disadvantages of using BGAs include limited inspection to only peripheral balls and lack of individual solder ball re-workability, and that separation could occur at either interface, both interfaces of the solder balls attachment should be analyzed. To assess the strength of the ball attachment to the PWB, ball shear tests were conducted using a wire pull, ball/die shear tester (see Figure 3). The shear cartridge used to evaluate the strengths measures a maximum load of 2 kilograms at a resolution of 5 grams. Figure 2: Plot of the time versus temperature shows the solder reflow profile that was used during the experiments Figure 1: This photo shows separations in the solder ball attachment at both the component and PWB interfaces after 82 thermal cycles (-40 to 125 C) and vibration (0.10 G 2 /Hz) testing [1]. EXPERIMENTAL PROCEDURES Most of the previous studies regarding solder shear strength of solder balls/bumps have characterized the shear strength of the solder ball/bump attachment at the solder ball to the component (usually a BGA) interface. Since most of earlier work focused on the BGA solder connection to the BGA substrate, CALCE decided to investigate the robustness of the other interface; the solder ball to PWB interface. Eutectic solder balls - 63Sn/37Pb - of 25 mils ± 1 mil (635 microns ± 25 microns) were reflowed onto pads of different sizes, geometries, plating materials, cleaning chemistries, exposure conditions and laminate manufacturers. The balls were reflowed using eutectic solder paste applied to the pads on the PWBs with a stencil designed specifically for each different test board layout and pad geometry. The stencils had a thickness of 6 mils. The reflow conditions were: Ramp up time 180 seconds Time above 183 o C 50 seconds Max temperature o C Time at max temperature - 10 seconds Cool down time seconds A plot of the time versus temperature for the reflow profile is given in Figure 2. This profile was used for all of the tests. Figure 3: Ball shear test equipment Shear testing is a method of determining bond strength that has been in use for many years. A simple tungsten chisel tool is positioned behind the bump and, as the tool moves forward at a predetermined shear height and velocity, it

3 performs the test (see Figure 4). The peak force reached during the test is recorded, and the sheared pad or interface is then examined to determine the failure mode and mechanism. The parameters used for the shear testing were a shear velocity of 100 microns/sec (6mm/min) and a shear tool height location of 50 microns above the board. This height is specified in JESD22-B117. The height of the reflowed solder balls was approximately microns. Figure 5: Shear strength of solder bumps, reflowed on Au/Ni/Cu as a function of the relative shear tip height [2]. Figure 4: Shear setup showing the tungsten chisel tool placement prior to test initiation Research by Choi has shown that to compare ball shear strength results with varying parameters, it is important to keep the shear velocity constant as well as the tip of the shear tool lower than 30% of the reflowed solder ball height [2]. JESD22-B117 specifies that the shear tool height must be equal to or less than 25% of the reflowed solder ball height [4]. Choi also shows that shear strength of the solder ball can decrease if the shear tip, positioned above the substrate prior to the solder ball shear test, is higher than half of the reflowed solder ball height (see Figure 5). Tests by Choi have also revealed that shear strength is approximately linearly related to shear velocity; increasing as the shear velocity increases (see Figure 6). Since the shear strength varies with shear tool tip height and shearing velocity, it is imperative that these two parameters be kept constant, to evaluate the effect of the other factors under investigation. For each parameter variation investigated, a minimum of 96 solder balls were reflowed and shear tested. Four balls were reflowed on each of at least 28 test boards, one ball on each corner (see Figure 7). Figure 6: Shear strength of solder bumps, reflowed on Au/Ni/Cu as a function of the shear velocity [2]

4 A. Solder balls attached to 270 µm diameter pads Figure 7: This photo shows an example of one of the test board designs, with each of the four reflowed solder ball locations indicated. EFFECT OF PARAMETER CHANGES ON SHEAR STRENGTH Solder pad size Test boards that were similar except for solder pad diameter (sizes of 270 and 475 microns) were used to evaluate the effect of pad size on shear strength. After the solder balls (25 mil diameter) were reflowed, the size of the reflowed balls on the larger pads was notably larger than those on the smaller pads due to a greater volume of solder paste adhering to the larger pads (see Figure 8) Comparing the area of the pads with different diameters reduces to the ratio of the square of their diameters: 2 2 A2 πr2 475 = = = A r π 1 B. Solder balls attached 475 µm diameter pads Figure 8: These photos show the difference in reflowed solder ball size, due to different amounts of solder paste stenciled unto the respective pads. The ratio of the areas of these pads is 3 to 1. The plot of shear strength values for these pad diameter sizes also shows an average of ~3 to 1 (~900 vs. 300 grams of force, see Figure 9). This suggests that shear strength is linearly proportional to wetted contact area between the solder ball and the pad. Pad geometry Two boards, with all parameters equal except for the pad geometry, were designed so that the effect of the pad geometry on solder ball shear strength could be evaluated. One design had the typical circular pads while the other had square pads, but with equals areas (see Figure 10). The diameter of the circular pads was 650 microns (area of 0.33 mm 2 ), while the dimension of the length and width for the square pads was 575 microns (area of 0.33 mm 2 ). Figure 9: Shear strength variation for two different pad sizes, averaging approximately 300 and 900 grams of force for the 270 and 475 micron diameter pad sizes respectively.

5 The elemental analysis showed that there may be a thicker gold layer over the nickel in sample A, as compared to sample B, since the electron beam penetrates through the outer layer of gold before hitting the nickel at the same operating kv. For a given material, the depth of electron penetration into the sample is proportional to the operating voltage. In sample C, the organic solderability preservative over bare copper, the electron beam penetrates through the coating, allowing copper to be detected. A small amount of chlorine, a possible contaminant is also detected during the elemental analysis. A. Circular pad design layout E-SEM image of surface plating (sample A) B. Square pad design layout Figure 10: Optical photos show the two designs with equal pad area; the circular pads in shown A, while the square pads in shown in B. Results of the shear testing showed that the geometry of the pad had a negligible effect on the shear strength values. The average shear strength value for the circular pads was 1.73 kilograms compared to 1.80 kilograms for the square pads. This difference is around 4%. Plating materials EDS spectrum of surface plating (sample A) The effect of plating materials on shear strength was characterized by testing reflowed solder balls on three Figure 11: E-SEM image at 5000x and EDS spectrum for surface finishes. The surface platings investigated were (A) the surface plating on sample A ENIG - electroless nickel/immersion gold, (B) PNS - post nickel strike/immersion gold and (C) OSP organic solderability preservative over bare copper. Table 1: Compositional analysis of plating Prior to reflow, an energy dispersive spectral (EDS) analysis and a 5,000x magnification environmental scanning electron microscope (E-SEM) image were acquired for each of the three surface finish types. The E-SEM was operated at 20 kv for both the imaging and elemental analysis of the three surface finishes (see Figure 11, Figure 12, Figure 13 and Table 1). Sample Elemental weight percent Au Ni Cu Cl Si O A B C

6 150 o C are given in Table 2. The results show that except for the organic solderability preservative, the finishes show virtually no degradation after 8 hours of exposure to 150 o C. E-SEM image of surface plating (sample B) E-SEM image of surface plating (sample C) EDS spectrum of surface plating (sample B) Figure 12: E-SEM image at 5000x and EDS spectrum for the surface plating on sample B A plot comparing the shear strengths of the balls reflowed onto the three surface finishes is given in Figure 14. Stabilization bake According to MIL 883, Method , a stabilization bake may be used to determine what effect storage at elevated temperatures could have on microelectronic devices without electrical stress applied. This test may also be used in a screening sequence or as a preconditioning treatment prior to the conduct of other tests. To evaluate the effect of storage conditions on the reliability of the solder joints, several finishes were subjected to an 8-hour bake at 150 o C. The shear strengths of reflowed solder joints on the exposed surfaces of these boards were evaluated and compared to a similar set of boards, which received no exposure. The solder balls were attached after the high temperature exposure. The finishes evaluated were Immersion silver (ImAg), Organic solderability preservative (OSP), Immersion tin (ImSn), hot air solder leveling (HASL) and Electroless nickel/ immersion gold (ENIG). The results of the shear strength values comparing the various plating finished in the as-plated samples to those exposed to EDS spectrum of surface plating (sample C) Figure 13: E-SEM image at 5000x and EDS spectrum for the surface plating on sample C Table 2: Comparison of shear strength values for the various plating types and high temperature exposure Ave shear strength (kilograms) Sample As plated 8 hours at 150 o C. ImAg OSP ImSn HASL ENIG Manufacturing process Boards plated with immersion tin from five manufacturers were characterized to assess the difference in ball shear strength as a function of the manufacturing process after reflow. The shear strength results for the five companies represented as U, V, W, X, Y and Z are shown in Figure 15.

7 The four significant failure modes are: Figure 14: Plots of the shear strength of the solder balls on the three surfaces is shown. The PNS has the lowest average strength, but also the smallest variability. The OSP finish has the largest average shear strength but also has the largest variability. Ball shear solder ball fracture through the bulk solder. In this ductile fracture, the pad is still covered with solder after the ball has been shear off. This is the expected failure mode and is typical of a robust solder joint. Intermetallic fracture fracture interface is at the intermetallic diffusion layer. This is a brittle fracture with minimal distortion of the solder ball and a clean separation of the solder ball at the pad surface. Pad lift solder pad lifts off the substrate. This occurs when the peel strength of the copper pad is lower than the bulk flow stress of the solder. Ball lift ball lifts off from the pad. This is due to insufficient wetting. The pad plating is visible after the ball is sheared off. Acceptable failure modes are solder ball shear, intermetallic fracture and pad lift, for which acceptable shear force requirements have been met [4]. The other three are unacceptable modes of failure (ball lift, solder ball sheared above center line and tool cutting into substrate before the solder ball is sheared). In the tests conducted for this study, virtually all of the solder ball failures were due to the ball shear failure mode where the fracture is through the bulk solder. The other few cases were due to pad lift, with shear strengths all above 1.5 kilograms. Figure 15: Plots of solder ball shear strength for boards plated with immersion tin for five different manufacturers The plot shows that manufacturers X and Y had lower shear strengths values than V, W and Z. There is also more variability in manufacturers V and W, compared to Z. The average shear strength values for the five manufacturers were 1.73, 1.89, 2.06, 2.08 and 2.10 kilograms for companies V, W, X, Y and Z respectively. There is a difference of 3,700 grams of force from the weakest to the strongest. FAILURE MODES There are basically six different failure modes, two of which can be attributed to setup errors. These errors due to setup are from incorrect settings of the shear tool. Setting the tool height too high may result in shearing off a section of the ball above its center line, while too low of a setting may allow the tool to cut into the substrate prior to contact with the solder ball. CONCLUSIONS Given the drawbacks of using BGAs, limited inspectability and lack of individual reworkability, both interfaces of the solder joint (at the component and the PWB) should be characterized to ensure strong interfacial bonds. If there is separation at either interface, the part could lose functionality. Investigations with varying parameters suggest the following: 1. Solder ball shear strength is linearly related to bonded surface area; solder pad geometry does not appear to affect the shear strength if the pad sizes areas are equal 2. For ImAg, ImSn, ENIG and HASL, high temperature storage at 150 o C for 8 hours did not cause any significant change in shear strength; less than 3%. For the OSP, the same conditions caused a 10% reduction in shear strength. 3. The variability in shear strength from different manufacturers of the same surface finish can exceed that of different finishes from one source. ACKNOWLEDGEMENT Thanks to Mr. Sarju Patel at Capital Electro-Circuits, Inc. in Gaithersburg Maryland for the use of his reflow equipment.

8 REFERENCES [1] H. Qi, A. Ganesan, M. Osterman, and M. Pecht, Accelerated Testing and Finite Element Analysis of PBGA Under Multiple Environmental Loadings, 2004 International Conference, IEEE Business of Electronic Product Reliability and Liability, pp , April 27-30, [2] Choi, Jin-Won, Choi, Jae-Hoon and Oh, Tae-Sung, Ball Shear Strength of 63Sn-37Pb Solder Bump with Test Conditions, Department of Metallurgical Engineering and Materials Science, Hong-Ik University, Seoul, Korea [3] Li, M., Lee K. Y., Chen W. T., Tan B. T., and Mhaisalkar S, Microstructure, Joint Strength and Failure Mechanisms of SnPb and Pb-Free Solders in BGA Packages, IEEE Transactions on Electronic Packaging, Vol. 25, No. 3, July 2002, pp [4] JEDEC, BGA Ball Shear: JESD22-B117, July [5] Hung, S. C., Zheng, P.J., Lee, S. C., and Lee, J. J., The Effect of Au Plating Thickness of BGA Substrates on Ball Shear Strength Under Reliability Tests, Proceedings of the IEEE/CMPT International Manufacturing Technology Symposium, 1999, pp [6] Erich, R., Coyle, R., Wegner, G., and Primavera, A. Shear Testing and Failure Mode Analysis for Evaluation of BGA Ball Attachment, Proceedings of the IEEE/CMPT International Manufacturing Technology Symposium, 1999, pp [7] Morawska, Zofia and Koziol Grazyna, Lead-Free Solderability Preservative Coatings of PCBs, Advancing Electronics, Volume 28, No. 3, May/June 2001.