A Novel Non-Solder Based Board-To-Board Interconnection Technology for Smart Mobile and Wearable Electronics

A Novel Non-Solder Based Board-To-Board Interconnection Technology for Smart Mobile and Wearable Electronics Sung Jin Kim, Young Soo Kim*, Chong K. Yoon*, Venky Sundaram, and Rao Tummala 3D Systems Packaging Research Center Georgia Institute of Technology Atlanta, Georgia 30332-0560 USA *UNID Corporation, LTD Corporate R&D Center 40 Sagimakgol-Ro 9Beon-Gil, Jungwon-Gu Seongnam-Si, Gyeonggi-Do, 462-807, Korea Abstract In this paper, the first demonstration of a novel-concept for an ultra-high density and low-cost, non-solder based interconnection technology, which can be applied for miniaturized board-to-board (BTB) as well as 2nd level package-to-system board interconnections, is reported. This is an array type, dry-fit interconnection technology that is modeled, simulated, designed and fabricated. Such a technology is expected to have many applications in small consumer systems such as smart mobile and wearable electronics. This paper presents three key innovations. The first innovation is the introduction of non-solder based, dry-fit interconnection concept. The second innovation is the demonstration of the reduced interconnection height of 0.45 mm, compared to a typical 0.7~1.0 mm mating height in most miniaturized BTB connectors in the market [1]. This thin mating height is one of the important features in portable electronics applications [2]. The third innovation is in the manufacturing processes. Unlike conventional BTB connectors, simplified and standard manufacturing processes were applied to build this innovative interconnection structure. Introduction Recent technical advances in smart mobile and wearable electronics require advances in interconnections at various levels, including from board-to-board (BTB). Industry has introduced a variety of narrow-pitch BTB interconnection technologies with minimal size and volume [3]. However, there is a continued need to improve this technology by three factors: miniaturization, manufacturability and cost. All of the BTB interconnection technologies reported to date are dual-in-line connectors with their intrinsic limitations in I/Os and size [4]. Conventional BTB connectors faced a couple of major technical challenges in size reduction or miniaturization. First is the signal integrity at high frequencies due to high insertion loss and high loop inductance. Second is the assembly yield during solder reflow especially due to solder bridging leading to electrical shorting on terminals in narrow I/O pitches. To overcome these critical technical challenges, this paper describes technical innovations in ultra-slim and miniaturized BTB connectors with many advantages such as: 1) higher I/O density, 2) smaller X-Y area, 3) low stacking height with smaller solder volume, 4) relaxed or coarser SMT pitch for improved manufacturability 5) simpler manufacturing process flow. This paper consists of four major sections: 1) modeling, 2) simulation, 3) fabrication, and 4) characterization. In the modeling section, the basic concept of miniaturized BTB test vehicle structure and materials are described and modeled. Simulation section describes various structural design and geometry studies together with multiple finite element modeling (FEM) and simulation results [5]. Fabrication section covers the final prototype test vehicle fabrication. Characterization section covers the degree miniaturization of the final test vehicle with the actual scaled comparison with the BTB connectors which were used for one of the most recent smart phones in the market. Finally, the results are summarized in the last section. 1. Modeling The test vehicle of this study, a 64 I/O, 8 x 8 array type miniaturized BTB connector, consists of two major interconnection parts female and male. An innovative structure and its fabrication processes are proposed to enhance contact the area with coarser connector pitch, both favorable to improve manufacturability and lower cost. The basic concept of the test vehicle structure in this study is shown in Fig. 1. Fig. 1. Cross-sectional view of BTB test vehicle structure before mating, and after mating. 978-1-4799-8609-5/15/$31.00 2015 IEEE 1122 2015 Electronic Components & Technology Conference

The fabricated structure is cast-molded with liquid crystal polymer (LCP) molding compound to provide mechanical robustness of BTB connector. The Cu-bronze alloy is selected for the male wing material. All the electrical connections are made through every single male and female mating structure to form the connectors. The male metal consists of four functional and structural elements: etched wing, pillar, pillar post, and SMT pad for the system board mounting, as shown in Fig. 2. Female interconnection part is a simple array of solder-onpad (SOP) holes, widely used in packaging industry. Further design details for the female parts are elaborated in the fabrication section. can be obtained through multiple iterations by varying geometry parameters such as metal thickness, wing width and length, elliptical angle of the wing, pillar diameter that is attached to male wing, the insertion diameter of female element, and so on. The large displacement FEM requires active node modifications and iterative contact constraints by varying different parametric values. In addition, mating and separation force per contact can be quantified and derived from FEM simulations for different designs. A typical FEM result of a mated element of this study is shown in Fig. 5. Fig. 2. Cross-sectional view of the BTB interconnection structure: before mating, and after mating. 2. Simulation Multiple FEMs were performed to obtain the optimal interconnection geometry for BTB interconnection reliability. Through these rigorous parametric modeling studies, the optimum material, and design values were obtained for the yield point (σ y ) of the interconnection materials for this specific application. Cu alloy is considered as the best material candidate for male wing material. Specifically, phosphor bronze 10%D, UNS C52400 was selected and used both for FEM and test vehicle fabrication. Its basic nodal schematics are shown in Fig. 3. Fig. 4. Basic geometry structure and material (Phosphor bronze 10%D, UNS C52400) properties for the FEM simulation. Fig. 5. A typical FEM result of test vehicle. Fig. 3. 3D View of the interconnection structure. The detailed material properties for the FEM are listed in Fig. 4. FEM simulation requires non-linear, quasi-static, and large deformation analysis with flexible-to-flexible surface contacts. The fundamental mechanical mating conditions and parameters between male and female parts With multiple FEM results, a plethora of valuable design factors could be extracted and compared to form the complex mechanical mating structure. Friction force is one important parameter for the dry-fit interconnection. In this study, the following geometric parameters which impact the friction force are: 1) female insertion slope, 2) contact area, 3) wing width, 4) wing length, 5) etched-middle-slot (EMS) design, and 6) pillar diameter. Basically, friction force changes with contact area and thickness of wing metal. Reaction force can be one of the important factors that determine the performance. Generally, large displacement simulation requires active node modifications and iterative contact constraints during FEM which requires further optimization with more accurate FEM meshing algorithm. 1123

Before the optimal simulation with actual prototypes, all the values listed in Fig. 6 should be regarded as normalized rather than the absolute values. In the simulation of sloped (angled) female insertions, two major mechanical indexes were found for their parametric tendencies: 1) reduced contact force, and 2) increased friction force with sloped female insertion. The overall summary of five different design parameters is summarized in Fig. 6 for Cases 1-5. Maximum displacement before large deformation is representing the flexural robustness of the male metal wing after mating. In addition to contact force and friction force, reaction force would impact the strength of overall BTB connector mating force. c) 1 st Pillar plating d) Dry film PR lamination, lithography and development for the pillar posts e) Pillar post plating f) Conductive adhesive application g) Dry film stripping and male wing metal attachment h) Dry film PR lamination, lithography and development for the tie-bar removal, and i) Tie-Bar etching and dry film stripping Fig. 6. FEM results of mated interconnection structures. The final FEM simulation analysis showed the best geometry values to be 500µm and 100µm for the male wing length and width respectively. They also showed that the etched middle slot is effective in maximizing the flexural strength of the metal wing. A pillar diameter of 100µm is found to be the optimal value when a 360µm diameter female insertion was applied with 30µm wing metal thickness. The insertion diameter of female part should be 360µm to achieve a 500µm interconnection pitch, considering that 100µm space between insertions. 3. Fabrication Male Part Fabrication The full structure of male part was fabricated using typical package substrate manufacturing processes with preformed 30µm Cu alloy metal sheets. The detailed process flow with step-by-step process schematics are shown in Fig. 7. The brief process descriptions are as follows: a) Pre-etched wing panel preparation b) Dry film PR lamination, lithography and development for the pillars Fig. 7. Process flow to fabricate male part. Fig. 8. Top view of 8 x 8 U-Conn TM male wing pattern design. The plated male metal wing sheet is attached onto printed circuit board (PCB) with conductive adhesive, 1124

removing tie-bar by simple etching. To quantify the effectiveness of the EMS that is described in the previous simulation section, solid type male wings are also designed with the same design parameters, as shown in Fig. 8, to fabricate the test vehicles. Before the final singulation of metal wings, these etched wings should be attached together for metal pillar plating processes so tie-bars are added in between metal wings. Fig. 10. Etched and plated male metal wing array tilted, top view from male PCB plane. Female Part Fabrication A typical PCB process was utilized to build the female part of miniaturized BTB connector in this study. Detailed process flow includes: plated through hole (PTH) formation, electro-less and electrolytic Cu plating, double-side development, etching stripping (DES) process for patterning, solder mask formation on both sides, final hard Au plating for final metallization, singulation, and cast LCP molding for the connector chassis. Fig. 11 shows the conceptual view of female part as well as actual fabricated test vehicle. Fig. 9. Etched and plated male metal wing array top view from male PCB plane, bottom view from plated pillars and pillar posts, and (c) plated wing array - SEM photos. All the fabrication processes including thick Cu pillar plating with a two-step 115µm thick dry film photo resist (PR) lamination process, had been optimized and developed. As a result, uniformly distributed metal pillars were achieved as shown in Fig. 9. The right SEM photo of Fig. 9(c) shows the supportive micro metals to prevent deformation of metal wings during dry film PR lamination for the tie-bar removal process which is described in Fig. 7(h). The final test vehicle picture is shown in Fig. 10. 1125

(c) Fig. 11. Female Part: 3D perspective view of female part prototype, fabricated female part, and (c) cross sectional view of vertical type insertion hole for the female part. To have electrical connection between top and bottom signal layer, PTHs should be applied for every interconnection node in a female part. The wetted solder volume amount is simulated for this case is shown in Fig. 11. Depending on the applications, this solder volume amount might not be enough to meet board-level reliability, so the alternative design option should be considered to make maximum solder wetting area for SMT process. Further detailed reliability test characterization with industrial standards will be reported in future. To maximize the wetting area of SMT pads, via-on-pad (VOP) design structure would be one of the best options. The VOP concept applied fabricated test vehicle is shown in Fig. 12. However, VOP process requires additional substrate process so the overall manufacturing cost would be relatively higher than standard PTH process. 4. Characterization Electrical, mechanical, reliability and assembly process characterizations are in progress with final test vehicles. To get the insight on the scale of miniaturization by comparing with one of the most widely used BTB connectors in current smart phone industry, 54 I/O, Dual in-line BTB connectors were compared with the test vehicles. Fig. 13 shows the area reduction benefit of this innovative technology, even with 10 more I/Os than standard connector. Fig. 14 shows the final test vehicle of female and male parts at the on-set-of mating to form the interconnection. Fig. 13. Comparison of conventional BTB and new BTB test vehicle in this study. (c) Fig. 12. Final female part test vehicles SMT side view, mating elements (insertion side view<left>, and SMT side view<right>, and (c) Final female part test vehicles. Fig. 14. Final male and female part at the time of mating. 1126

Unlike typical dual-in-line BTB connectors, the newly developed miniaturized BTB connector (U-Conn TM ) has array shaped interconnections resulting in reduced X-Y area, but increased SMT pitch to the system board which can significantly contribute to higher SMT yield and lower system board cost. More than 50% thinner mating height, a total of 450µm, compared to traditional BTBs was achieved. 5. Conclusions In summary, this paper reports advances in miniaturized BTB by modeling, simulation, design and fabrication to achieve slim, dry-fit interconnections. In modeling and simulations, optimal parameters were obtained through multiple FEM iterations for non-linear, quasi-static, and large deformation analysis along with flexible-to-flexible node and surface contacts. In fabrication, multiple and variable-geometry prototypes were fabricated consistent with and following FEM analyses. Optimal processes were developed for each process using minimal process steps and standard substrate processes to pave the way for easy commercialization of the newly-developed BTB connector. Continued research and development will be carried on to extend this technology and commercialize for smart mobile, wearable, and other electronics. Further in-depth electrical characterization, reliability testing and characterization are in progress and will be reported separately. Acknowledgements The authors would like to thank UNID Corporation R&D Center for their research funding and technical insights, reported in this paper. The authors would also like to thank Christ White and Jason Bishop from 3D Systems Packaging Research Center, Georgia Institute of Technology, Atlanta, for their help with materials and processes in this study. References [1] Slim Stack 0.40mm Pitch Board-to-Board Connectors, Molex Product Data Sheets for AS-503304-001 Series, October 2014. [2] SOP: What Is It and Why? A New Microsystem- Integration Technology Paradigm-Moore's Law for System Integration of Miniaturized Convergent Systems of the Next Decade, Tummala, Rao R, IEEE Transactions on Advanced Packaging, May 2004, Vol. 27 Issue 2, p241-249. [3] AVX Designs Insulation Displacement Connectors with Broadest Range of Contact Options Available, Tom Anderson, Garth Miller, Business Wire, May 23, 2012, 2pp. [4] 0.4mmPitch Vertical-Vertical SMT With metal tab Type, Kyocera Connector Products Data Sheet for 5806 Series, October 2014. [5] Analysis of the Influence of Geometric Parameters on the Stress Distributions in Adhesively Bonded Scarf Joints Using 2D Models Under Elastic Assumption, Cognard, J.-Y.; Leguillon, D.; Carrere, N, Journal of Adhesion 2014, vol.90, no.11, pp. 877-898. 1127