2ND LEVEL INTERCONNECT RELIABILITY OF CERAMIC AREA ARRAY PACKAGES

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1 2ND LEVEL INTERCONNECT RELIABILITY OF CERAMIC AREA ARRAY PACKAGES Shingo Sato, Noriyuki Shimizu*, Shin Matsuda, Shoji Uegaki and Sachio Ninomiya Kyocera Corporation Kyoto, Japan Biography Noriyuki Shimizu received his B.S. degree in Material Science from Hiroshima University in He joined Kyocera corporation in 1990 and had been working as a process engineer for multilayer ceramic packages and R&D engineer for the development of interconnect technology for ceramic packages. He is currently a Senior Engineer of Semiconductor Components Research and Development Division and is engaged in the development of interconnect technology for ceramic surface mount packages. During temperature cycling test (TCT), CBGAÕs failure usually occurs in the form of an electrical open resulting from the thermal fatigue of the solder connection. The Coefficiency of Thermal Expansion (CTE) for alumina ceramic is 7.0 x 10-6 / k. FR-4 for Printed Wiring Board (PWB) has CTE values in the range of 14 to 18 x 10-6 / k. The CTEÕs mismatch between the CBGA and the PWB would cause a shear displacement, and this displacement applies shear stress to each solder interconnections. This stress is proportional to the distance from the geometric center of the package to the respective I/O. Abstract A new Ceramic Ball Grid Array (C-BGA) package called Dimpled Ball Grid Array (D-BGA) has been developed as the methodology to improve second level reliability of Package to Printed Wiring Board (PWB). Characterization of D-BGA with factors which affect to second level reliability were performed. Data A New Ceramic BGA called D-BGA has been developed to improve the reliability of conventional CBGA technology which the innovations of physical structure and material composition were created. The physical BGA structure change was the addition of an extra layer of ceramic, with mm diameter by 0.20 mm deep holes in the ceramic sheet. This cylindrical hole, which is called a dimple, is screened with solder paste and the ball sits on the dimple. This structure protect solder balls from deformation caused when share displacement force is applied during TCT. Figure 1 shows cross-section of D-BGA. Thermal Fatigue Life Improvement

2 Solder Pad Ceramic Wall Solder Ball With respect to the PWB, physical dimensions, layer count and construction, TCE, warpage, component mounting location, PWB pad size, solder mask vs. non-solder mask defined pads, etc. are other considerations as factors affect to second level reliability. Figure 1. Structure of Dimpled BGA The material composition of solder balls for the D-BGA utilizes a monolithic solder system with three parts metallurgical composition which melts at eutectic solder temperature. This system was chosen so as not to disturb conventional surface mount processing and to maximize self- alignment capability of D-BGA. Therefore D-BGA components can be soldered to the PWB along with other compound technology modules and passive components. Of course standard eutectic solder paste can be utilized for mounting D-BGA as well as other modules and components. Reliability Test Configuration IC Size Interconnect Material Solder Ball Stand-off Height Material Lid With/Without Material PWB CTE Pad Size / Style LGA Size Thickness Structure(Dimple) Figure 2. Factors to Second Level Reliability Other factors to be considered are in the assembly processes. The second level attachment process needs to be monitored, characterized, and controlled. Processing variables such as reflow temperature profiles and atmosphere, printing solder volume, solder paste and fluxes (including their cleaning methods), are all important for the evaluation of second level reliability. There are many factors related to second level reliability which are driven by physical structure, as illustrated in Figure 2. In relation to the BGA package itself, the solder connection interfaces at both the solder-to-substrate and solder-to-pwb need to be considered. Various ball geometryõs and the utilization of stand-offs are being evaluated. Both package size and thickness have significant effects on reliability. In addition, a lid also affects reliability. Metal lid has been developed to optimize second level reliability. Lastly, the reliability testing method and criteria need to be clearly defined for baseline evaluation purposes. This includes the evaluation test methodology, such as temperature and power cycle profiles and conditions. The dwell, ramp rate, and transfer times all need to be stated for meaningful evaluations and comparisons of related technologies. The test system itself for electrical continuity is also important how many nets of electrical connection exist, whether in-site monitoring is utilized, and the actual measurement method of resistivity is considered.

3 cycles. The following reliability data is based upon the same test conditions. The PWB used for the evaluation is standard FR-4 material with a measured TCE of 14 / 16 (X-axis/Y-axis) x 10-6 / k. The daisy chain electrical connection is provided between D-BGA and PWB. The pads for mounting on PWB are defined by soldermask and their size is 0.6 mm diameter. The electrical continuity was monitored to determine the disconnection of solder. Typical disconnection occurred during TCT which is thermal fatigue failure is shown in Figure 3. Cumulative Failures(%) mm sq. Conventional CBGA 21mm sq. Conventional CBGA.1 BGA : 1.2mmT w/o Lid PWB : 1.6mmT/FR Number of Cycles 35mm sq. Dimpled BGA Figure 4. The Reliability Test Results of Conventional CBGA and D-BGA Package Body Size and Thickness 21mm sq. Dimpled BGA Figure 3. Disconnection after TCT (Dimple Side) Baseline Test Result Figure 4. shows the reliability test results of conventional type CBGAs and D-BGAs. The ball size for both packages is 0.7 mm diameter. High Melting Point (HMP) solder balls were attached with eutectic solder paste on 0.7 mm pad diameter for CBGA. Both packages with body size of 21 mm square and 35 mm square were mounted on FR-4 boards. TCT conditions were -40 to 125 degree C. First failure for the 21 mm square D-BGA package occurred at 1,200 cycles and at 600 cycles for CBGA package. First failure for the 35 mm square D-BGA package occurred at over 500 In order to evaluate the second level reliability of D-BGA by package body size and thickness, TCTs were performed on a variety of package body size and thickness. TCT condition for these evaluation was degree C which is more applicable to in-house use (like PC/US) applications. In the evaluation by body size of D-BGA, first failure occurred at 1,800 cycles for the 35 mm square package, 2,500 cycles for the 25 mm square package, 2,800 cycles for the 21 mm square package as shown in Figure mm sq. 21mm sq mm sq. 1 BGA : 1.2mmT w/o Lid.1 PWB : 1.6mmT/FR Number of Cycles Cumulative failures (%) Figure 5. The Reliability Test Results by Size of D-BGA

4 In the evaluation by the difference of package thickness of D-BGA, the 3.0, 2.0 and 1.6 mm thickness of the 35 mm square package were performed. Cumulative failures are charted as shown in Figure 6. This data suggests that increase of package thickness makes decrease of second level reliability. square package with a thickness of 1.6 mm with a ceramic lid, metal lid, and without lid. By comparison between a package without and with a ceramic lid, it indicates a 2 to 1 fatigue life ratio. Meantime, since Alloy 42 has almost same TCE value as ceramic and also the thin structure does not generate additional strain to solder ball, the equivalent strain and fatigue life ratio is approximately the same as the package without lid. Therefore the metal lid for the use of lid is recommended. Cumulative failures [%] PWB:1.6mmT/FR-4 3.0mmT 2.0mmT 1.6mmT Structure Equivalent Strain by Simulation Ceramic Lid Metal Lid* No Lid 30.0sq 30.0sq T= P1.27 (mm) *Alloy Number of cycles Figure 6. The Reliability Test Results by Thickness of D-BGA Fatigue Life Ratio at Evaluation Table 1. Metal Lid for Strain Reduction As a summary of these evaluations, smaller and thinner package configuration is better for enhancement of second level reliability. Metal Lid for Strain Reduction A ceramic lid attached to the package has a detrimental effect on the reliability. because ceramic lid gives additional stiffness to CBGA in structure, which cause the same effect at thicker package, and this increases the strain of solder ball. A metal (Alloy 42) lid has been tooled to assess the strain reduction. Table 1 shows the simulated data on the relationship between the equivalent strain of the furthermost distance-to-neutral point (DNP) ball and fatigue life ratio for a 35 mm Higher Stand-off for Strain Reduction Three types of standoff configurations were evaluated using the standard 35 mm square and 1.6 mm thick package. Three different sizes of solder ball which are 0.7, 0.8, and 0.8 mm with a standoff ball were evaluated (shown in Table 2). The first two columns with the 0.7 and 0.8 mm balls show the resulting standoff with the normal self-control and collapse dynamics of the melting sphere. Recall that D-BGA technology has an additional 0.2 mm of solder within the package body. Stand-off as referenced is the dimension between the PWB and package body. The 0.8 mm diameter ball results in a stand-off increase of 0.03 mm, (0.56 mm) compared to the 0.7 mm ball (0.53 mm). The fatigue life ratio between these two

5 configurations indicates that the 0.8 mm ball achieves a 10 % improvement in reliability. The 0.8 mm diameter ball with 0.8 mm noncollapsing metal ball (Cu) results in a 0.75 mm stand-off. The fatigue life ratio between the 0.8 mm ball with stand-off and 0.8 mm without-standoff is a 20 % improvement in reliability when a stand-off is used. Cumulative failures TCT 0-100C mm 0.65mm Number of Cycles Figure 7. The Effect of Larger Dimple Diameter 0.7mm Dia. Ball 0.8mm Dia. Ball 0.8mm Dia. Stand off Ball Conclusion Structure Equivalent Strain bysimulation Fatigue Life Ratio at Evaluation P (mm) Table 2. Higher Stand-off for Strain Reduction Larger Dimple Diameter The diameter of dimple is 0.55 mm in the above evaluations. The effect of larger dimple diameter was also evaluated. Standard 33 mm square and 2.0 mm thick packages were used for this evaluation. The diameter of dimple were 0.55 and 0.65 mm. The first failure occurred at 1,300 cycles for 0.55 mm and 1,800 cycles for 0.65 mm mm diameter can be applied for 1.27 mm ball pitch package. The result is shown in Figure 7. The data obtained through above evaluations could characterize D-BGA. The structure and solder material system provides that thermal fatigue life is enhanced with D-BGA. FEM analysis and temperature cycling data supports the reliability of the dimpled solder pad. Test data and analysis demonstrates that there are many factors which influence second level reliability. Package and PWB design are key factors to consider for the optimization. Kyocera takes into consideration for the customer application and ensures that the design is optimized to extend the reliability to the fullest extent possible. References 1) Shin Matsuda, Koichiro Sugai, and Nobuyuki Itoh, ÒLatest Development and Reliability of Ceramic BGAÓ, Proceeding of International Society for Hybrid Microelectronics (ISHM) Singapore, January 17-19, 1995

6 2) Shin Matsuda, Kazuhiro Kawabata, and Nobuyuki Itoh, ÒHighly Reliable Ceramic BGAÓ, Proceeding of International Society for Hybrid Microelectronics (ISHM) Los Angeles, California, pp October 24-26, ) Rao R. Tummala and Eugene J. Rymaszewski, Microelectronics Packaging Handbook, Van Nostrand Reinhold, New York, pp. 36, ) John H. Lau, editor Ball Grid Array Technology, McGraw-Hill, Inc., New York, pp ,