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JULY 2019 I DESIGN007 MAGAZINE 77 Figure 2 shows the process flow to make the two-layer printed ink interconnect structure on PET substrate using filled microvias. The first step was to screen print fiducials for lat- er fabrication processes, including via drilling and screen printing. Reference or anchor pads were also printed at this step. By using the ref- erence pads, we can easily check the sizes and positions of the drilled microvias with regard to the reference pads. For microvia drilling, mechanical and la- ser drilling were evaluated initially. Mechan- ical drilling yielded more consistent results in terms of the via diameter and position with sharp via walls. Accordingly, mechan- ical drilling was used to fabricate the mi- crovias for this project with two hole sizes: 150 µm and 250 µm, using a precision drill- ing system. The printed ink fiducials were used for the alignment of the microvias dur- ing drilling. After drilling, positions and diameters of the microvias relative to the ref- erence pads were inspected under optical microscope. After microvia drilling, conductive ink was printed on the bottom of the substrate using the fiducials made in the first step. The PET substrates are precoated for better printing quality. A polyester screen was created with a mesh count of 305, thread diameter of 34 µm and thread angle of 45 degrees. Screen printing parameters were as follows: • Pressure 14–16 Kg • Speed: 35 mm/second • Gap: 4 mm After printing, the ink was then cured at the elevated temperature (120 o C for 10 minutes). The substrate was afterwards flipped over and the conductive ink was printed on the top side. The ink was then cured at 120 o C for an- other 10 minutes. During the screen printing process, the conductive ink was expected to be automatically filled through the microvias, making connection between the top and bot - tom inks. For this initial trial, the dielectric ink was not printed. The next step is to attach the electronics components on the printed substrates. Several approaches were evaluated in parallel: aniso- tropic conductive paste (ACP), thermocom- pression flip-chip bonding with silver bumps and non-conductive paste (NCP), soldering re- flow process, and electrically conductive adhe- sive (ECA). ECA is mainly used for attaching the passive devices, while the other processes can be used for attaching both passive and ac- tive devices. Only the development work on the reflow process is presented and discussed in this article. One big advantage of the reflow process is that it is compatible with the con- ventional PCB assembly process, so we can fully utilize current manufacturing infrastruc- ture that is already in-house. From our previous work, we have dem- onstrated the solderability of a specialty for- mulated conductive ink using low-tempera- ture solder Sn42/Bi57.6/Ag0.4 with a melt- ing temperature of 138 o C. By using low temperature solder, we can minimize the damage to the PET substrates. For this ini- tial feasibility study using low-temperature solder and printed conductive ink, only the microprocessor packages were attached and evaluated. Figure 3 shows the reflow profile for the assembly process with a peak temper - ature of 155 o C. Figure 2: Process flow to make a two-layer interconnect structure.