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Additive manufacturing for multi-chip modules
Keywords: Additive manufacturing, multi-chip Module, harsh environment
Implementation of Microelectronics, also known as Multi-Chip Modules, is widely used in automotive, downhole and aerospace applications. MCMs had already demonstrated high temperature performance, step improvement in reliability, and the potential to reduce product cost through miniaturization and integration of more functions. However, there are barriers preventing wider adoption of MCM technology. High production NRE charges increase development costs. Long lead-time of substrates prolongs time to market. Lengthy design iterations make it difficult to apply lean startup methodology to accelerate innovation. The main factor that leads to high NRE and long lead time is the complexity of substrate manufacturing process. Together with assembly, MCM manufacturing is comprised of at least 11 steps, 6 different materials, 10 or more different machines, and requires minimum of 6 supporting employees. This paper evaluated a simplified process to reduce labor and expenses. It requires a single machine, 3 materials and a cyclic 3-step process of dispensing, placement and cure. Despite the dramatically simplified process, the constructional complexity of circuit can be very high - such as a 3D multilayer MCM. Micro-dispensing equipment was used to create basic circuitry blocks. Different materials to create conductive traces, isolation layers and wire bond replacement were evaluated. High temperature aging tests were conducted to monitor the electrical and mechanical performance under thermal stress. The feasibility of dispensing fine features using dispensing and jetting method are presented in the study. Conductor is a critical part in microelectronic assemblies, it creates interconnects and thermal dispassion path for microelectronics. Three different conductor materials were tested on their dispensability, resistance, continuity at temperature, CTE compatibility with different materials under thermal cycling. For dielectric materials, the requirements are to be able to create various assembly constructs. The characterization included dispensability, electrical insulation, breakdown voltage, high temperature performance, and effects of CTE. Different approaches with different materials were tested with feasibility for wire bonding replacement. The application needs fine feature size with medium resistance lines. Thus the criterial for the material selection are fine particle size and medium sheet resistance. For high power device where heavy gauge wires were used, jet dispensing is applicable. For other application with regular wire diameters, direct write is used. The over-all tests demonstrated the feasibility of using dispensed materials to replace wire bonds, which brings better reliability for shock and vibration, as compared to traditional wire bonds. The reliability of such approach requires a set of optimally matched conductive materials and dielectric. Tested conductive epoxy A can be used for attachment for SMT components with non-tin terminals, short traces, and wire bonding replacement for 25um wires, not ideal for fine lines(<65um). Tested conductive epoxy B can be used for fine traces (58um), wire bonding replacement for 25um wires. The resistance of that material is not ideal. Nano-silver paste can be used for long traces, heavy gauge wire bonding replacement, pads/polygons, the resistance is close to 0.5Oz Cu. For dielectric, epoxy C can be used for crossovers, dielectric layers, and components staking. Epoxy D can be used for die edge insulation but is not ideal. Epoxy E from Henkel can be used for crossovers and components staking. Epoxy F can be used for encapsulation and components staking The wire bonding replacement concept structure is established with dielectric forms the insulation around die edge, then the conductive wires dispensed on top of it. Feasibility is confirmed and proof of concept is built and some level of thermal stress has been tested on the samples. Particle size and viscosity are critical to achieve fine features for micro-dispensing conductors and dielectrics. Periodic evaluations must be conducted to follow up on industry’s progress.
Zhenzhen Shen,
Baker Hughes GE
Houston, TX

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