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All Systems Go! Simulating Wirebonded CoB on Rigid-Flex

There are many good reasons to use a chip on board (CoB) implementation. When this is combined with wirebonding and the use of rigid-flex PCB, challenges mount. An application that demands all three—CoB, wirebonding, and rigid-flex PCB—is a camera module that goes into a mobile application, the sample design used to illustrate the design and analysis challenges in this article. If you are not aware of and prepared for the potential pitfalls, it is highly likely that your project could fall short or even fail.

If the camera module goes into a mobile application, cost, performance, footprint (in all three axes), and time to market become requirements that must be met. Cost and footprint justify the use of CoB implementation due to the elimination of a package; performance and cost justify the wirebonding of the image sensor die directly on to a rigid-flex PCB, since the short wires with the eliminated package improves performance and reduces cost; the restricted footprint within a mobile device calls for the use of a rigid-flex PCB.

Figure 1 shows a rigid-flex PCB with an image sensor implemented using CoB and the MIPI signals driven to the connector on the other end of the rigid-flex PCB. The impedance of the MIPI signals traversing from the wirebonded chip to the connector must be a consistent 100ohm path.

The use of rigid-flex PCB, CoB implementation, 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 100ohm
• 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

The 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, electrically connected, and typically encapsulated with a silicone- or epoxy-based material in the final PCB. Figure 2 shows a typical implementation of the CoB and the wirebonding.

The 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 fabrication layout images of the product and 3D view with wirebond profile.

After the layout of the CoB in rigid-flex, create the MIPI/xTalk channel topology and perform signal extraction and simulation of the channel. If a custom compliance kit/MIPI is available, perform compliance checks and generate 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, wirebonding, channel, and the connector. This 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. This is no longer true with new extraction tools, which can extract the rigid-flex board, wirebond, and the connector together, and perform that in 3D. The extracted 3D wirebond profile needs to be brought into the simulator, which is typically a manual process with legacy tools. This also has changed with new tools, which perform faster and yet accurate 3D extraction of the design, and directly read the 3D wirebond profile from a PCB file. The new tools perform much faster simulation of the design, including return and insertion losses, and power-aware analysis with virtually unlimited scalability. This improves the accuracy of extraction, saves time, and eliminates the risk of introducing manual errors in the process.

Learn how a customer applied this methodology to solve their camera module design that went into a mobile device.

Brad Griffin is a product management group director for the Multiphysics System Analysis Group at Cadence Design Systems, Inc., and the author of The System Designer’s Guide to… System Analysis (a free eBook available for download).