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Additive Dielectric Microstructures
Keywords: Aerosol Jet, Interconnect, 3DIC
Additive, printed, 3D electrical traces are needed to achieve the desired advances in packaging. Recent advances in silver inks in some cases provide resistivities of 2x bulk silver (3.2E-6 Ohm-cm) when sintered at 120 C, and laser sintering can provide similar performance on substantially room-temperature substrates. Compared to wire bonds, printed traces have smaller heights (no loops) and the trace cross sections can be tailored to carry larger currents. In RF applications, printed traces can be shorter, have lower inductance and crosstalk, and be shaped to form substantially impedance-matched connections. Additive dielectric microstructures are often required to support the additive printing of conductors on 2.5D and 3D electronic structures. For example: • A fillet, or at minimum a wall coating, is needed on the conductive sidewall of an unpackaged IC die that is to be connected to an underlying PCB by a printed trace. Large fillets (ca 350 um) are needed on typical single die, <50 um fillets on stacks of thinned die, and <20 um fillets are needed on flexible die. • Filling the inter-die gap is necessary when interconnecting closely spaced die. • Filling the gap between the substrate dielectric and die is needed when the die is mounted in a cavity in the substrate. Cavity mounting is sometimes used when the floor of the cavity is a thick metal layer that acts as both a ground plane and a heat sink. Dual-cure (UA and/or heat) resins are desirable for creating microstructures as the resins can be “pinned” in-situ to create structures that would normally be impossible due to the flow of the resin. A secondary heating step is normally used to cure any areas shadowed from the UV. These resins can shrink on curing by as much as a few percent causing delamination from underlying structures or stress in the final parts. Much like applying multiple thin coats of paint to avoid the cracking that results from applying one thick coat, applying multiple thin coats of dielectric with simultaneous UV or UV exposure between the coats allows the polymer to be deposited with low stress and without delamination. For example, a method for creating fillets around thick die by multiple layering has been described by Hines et al. It is also possible to create structures with low-shrinkage/CTE materials, but these materials are often compromised in other metrics. Creating micro dielectric structures by conventional syringe dispense methods is difficult due to the size of currently used dispense needles. Even the 190 um OD of a 34 ga needle, which is too small to be used with filled resins, is poorly suited to forming 45-degree fillets on a stack of 35 um-tall die. This work discusses some of the practical aspects and results of creating these micro dielectric microstructures and printed traces using Aerosol Jet® technology.
Kurt Christenson, Senior Scientist
Saint Paull, MN

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