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Deformable and Remateable Interconnects between Devices and Flexible Packages
Keywords: interconnects, deformable, remateable
The advent of flexible and wearable electronics has created the need for interconnection materials that can undergo severe bending, twisting, deformation and mechanical strain, and yet remain reliable. Such flexible interconnection materials must meet the required electrical and mechanical performance depending on the applications. From the manufacturing standpoint, these materials should be processed at low temperature and cost with high-throughput innovative curing techniques, and have high fatigue reliability. The desired electrical properties include low resistance at the maximum operational strain, and small change in resistance over the range of applied strains during the lifetime of the device. For several emerging applications in wearable and textile electronics, the interconnections should be reworkable or remateable (able to disconnect and reconnect), a property that allows replacing and repairing of devices or removing them during a sensitive operation and re-inserting them later. Furthermore, the approaches must be scalable to pitches of less than 100 microns, for seamless integration and miniaturization. Two types of interconnection technologies are primarily used in flip-chip on flex (FCOF): 1) conventional C4 (controlled collapse chip connection) solder melting with low-melting-point and low-modulus solder bumps with low-temperature processing (ex. Photonic curing by NovaCentrix and asymmetric heating by others); 2) adhesive bonding using isotropic or anisotropic conductive films or adhesives [1]. Current assembly technologies do not provide tolerance to bending, small radius of curvature, twisting and deformation during the intended use of e-textiles. For example, both solder and adhesive (NCF and ACF) films present reliability challenges when assembled on flex substrates. Isotropic conductive adhesives with soft printed silver composite bumps are also being developed but offer limited remateability, performance and pitch scalability. Interconnections with nanosilver pastes and inks provides opportunities to address these challenges, but do not provide reworkability and mechanical reliability. In this paper, deformable and remateable interconnects are demonstrated to attain high conductivity as well as ability for deformation. We show flexible elastomer composites for easy reconfiguration and rearrangement of the material under deformability. The compressible and deformable nanowire composite bumps are printed into microsockets under the substrate bonding pads so that deformation is volumetrically distributed. This leads to lower interfacial and bulk strains during deformation and hence more robustness. For remateable interconnects between devices and flex substrates, devices are inserted into cavities inside flexible substrates. Elastomer inserts is utilized in the cavities to create press-fit connections between devices and substrates. The elastomers are compressed inside the cavities during assembly and thus hold the devices under compression. The contact from device termination to substrate traces are through the deformable bumps and the lining inside the cavities. This assembly will enable reworkability with vacuum pen or manual techniques, and still have the desired functionality after re-assembly. Thin flexible elastomer composites allow for easy reconfiguration and rearrangement of the material for deformability. Such composites were obtained from a proprietary partner. The paste is printable on a variety of permeable, flexible and textile substrates. It has excellent adhesion to many substrates and is also resistant to wash cycles. The metal content is about 70 wt.% and the median particle size is ~1 microns. Two- terminal passives are chosen for these initial demonstrations. The first step in the process is to drill sockets in the flexible substrate. Cavities are drilled with a CO2 laser. A CAD file is designed to create cavities and sockets in the substrate that match with the footprint of surface mount devices that are used as the test components. The next step is to fill the sockets with deformable nanowire or nanoflake paste. A stencil is used to squeeze the paste into the drilled cavities. Printed silver flake composite bumps are also created on the terminations of the passive devices. The substrate and passive device are aligned and assembled using a pick-and-place tool. The assembly is cured at 125 C for 30 min. Remateability is achieved with nanowire-elastomer composites for the interconnects between the device termination and substrate pads. Strong contact is achieved with elastomer gap- fillers that enable compression-based press-fit interconnections. The device with nanowire-elastomer printed terminated connections is then assembled into the cavity. The compressive force holds the device in place. The assembled substrate was bent to a radius curvature of 1 cm without resistance shift. The device also can be removed and re-inserted multiple times, demonstrating remateability. Microstrip and coplanar waveguides were utilized to characterize the interconnect losses between the device the package traces using a technique described by the authors in their earlier work [2]. A transmission line of finite length was characterized with a vector network analyzer for transmission (S_21) and reflection (S_11) measurements, providing for the baseline losses from the conductor traces. The measured losses were 0.2 dB/cm in the 2-3 GHz range for the flexible conductor traces. The same was implemented with a region replaced by the deformable interconnects. Interconnection parasitics of less than 50 milliohms and RF losses of less than 0.1 dB at 2-5 GHz is seen from the initial results. The full paper will describe the fabrication process along with mechanical and electrical characterization.
S Y Been Sayeed,
Florida International University
Miami, FL

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