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100 DESIGN007 MAGAZINE I DECEMBER 2021 e use of rigid-flex PCB, CoB implemen- tation, and wirebonding alone could lead to a slew of potential pitfalls or challenges: • Designing the rigid-flex with different stack-up between the rigid and flex sections • Wirebond routing in the miniature board area • Use of stacked vias for routing • Controlling the impedance of 100 Ohm • Extraction of the 3D wirebond in 2D (with all legacy tools) does not help with accuracy • Simulation time for extracting the wirebond profile with respective signals e CoB assembly process, commonly referred to as direct chip attachment (DCA) technology, refers to the assembly/technology process in which a die is directly mounted, elec- trically connected, and typically encapsulated with a silicone- or epoxy-based material in the final PCB. Figure 2 shows a typical implemen- tation of the CoB and the wirebonding. e rigid-flex design is best created using a PCB design tool with the ability to define and add multi-stackup and constraints for rigid- flex, including inter-layer and comprehensive spacing rules. When the CoB is a large chip with a greater number of pins than a typical PCB tool can handle, you will need to use the SiP capability of a package design tool. Figure 2 and Figure 3 show the final fabrica- tion layout images of the product and 3D view with wirebond profile. Aer the layout of the CoB in rigid-flex, cre- ate the MIPI/xTalk channel topology and per- form signal extraction and simulation of the channel. If a custom compliance kit/MIPI is available, perform compliance checks and gen- erate the compliance signoff pass/fail report. All of these are best performed using a fast extraction and simulation solution. Next, it is important to simulate the entire camera module including the rigid-flex, wire- bonding, channel, and the connector. is is typically done using the divide and conquer approach by extracting each component as a whole or in parts and merging them together manually in a SPICE file for simulation. is is no longer true with new extraction tools, which can extract the rigid-flex board, wire- Figure 2: Fabrication drawing.

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