Additive Manufacturing for Multi-chip Modules
Authors: Shen Zhenzhen , Reiderman Aleksey 
Published: December 2018

Abstract
The implementation of microelectronics, also known as multi-chip modules (MCM), is extensive in automotive, downhole and aerospace applications. MCMs have 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 in downhole applications. High non-recurring expenditures (NRE) charges increase development costs. Long substrate lead times prolong the 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 times is the complexity of substrate manufacturing processes. Together with assembly, MCM manufacturing comprises at least 11 steps, 6 different materials, 10 or more different machines, and requires a minimum of 6 supporting employees.
A new concept proposes a simplified process to reduce labor and expenses. With best implementation, this process would require only a single machine capable of cycling through 3-step process of dispensing, placement and cure. Despite the dramatically simplified process, the constructional complexity of circuits can still be very high, such as a 3D multilayer MCM. In this paper, this concept was evaluated, 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 methods are presented in the study.
Conductors are a critical part in microelectronic assemblies because they create interconnects and thermal dissipation paths for microelectronics. Three different conductor materials were tested for their dispensability, resistance, continuity at temperature, and coefficients of thermal expansion (CTE) compatibility with different materials under thermal cycling. For dielectric materials, the requirements were to create various assembly constructs. The characterization included dispensability, electrical insulation, breakdown voltage, high-temperature performance, and the effects of CTE. Different approaches with different materials were tested for feasibility for wire bonding replacement. The application needs fine feature size with medium resistance lines. Consequently, the criteria for the material selection are fine particle size and medium sheet resistance. For high-power devices 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 this approach requires a set of optimally matched conductive and dielectric materials. Three conductive materials (A, B and C) and three dielectric materials (D, E and F) were evaluated in this study.
Tested conductive epoxy A can be used for attachment of SMT components with non-tin terminals, short traces, and wire bonding replacement for 25-μm wires, but it is not ideal for fine lines(&lt;65um). Tested conductive epoxy B can be used for fine traces (58μm), and wire bonding replacement for 25-μm 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 sheet resistance is equivalent to 0.5Oz Cu.
For dielectrics, epoxy C can be used for crossovers, dielectric layers, and components staking. Epoxy D can be used for die edge insulation, but it is not ideal. Epoxy E 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 the dielectric forming the insulation around die edge, then the conductive wires dispensed on top of it. Feasibility was confirmed, a proof-of-concept was built, and some level of thermal stress was 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 with materials.