To find the ideal balance between circuit board cost and performance for a given application. Let’s start at the beginning. The most common PCB fabrication material is FR-4. It is cost-effective, highly flexible, and mechanically strong with bond strengths at temperatures up to 130ºC.
However, FR-4 does not work well for high-speed or high-frequency (greater than 1 GHz) applications circuit board where signal precision is critical and any signal loss has a significant impact on performance. Quite frankly, these applications are quickly becoming more the norm than the exception. These applications require PCBs with a low dissipation factor (Df) and very stable dielectric constant (Dk). Since the dielectric constant of FR-4 varies from one lot to another, as well as with frequency, PCB manufacturers are forced to consider other alternatives for high frequency applications.
High-frequency laminates are one such alternative that offer much more stable Df characteristics and have considerably less loss at high frequencies. Selecting PCB materials for high-frequency applications is often a tradeoff between ease of fabrication and addressing the needs of a particular application. Although high frequency laminates provide improved electrical performance, as well as consistent performance over a large circuit board dynamic thermal range, they are more expensive than traditional FR-4 materials.
Therefore, hybrid multi-layer PCBs using a combination of circuit materials are becoming more popular. circuit board In a hybrid PCB, the digital and analog circuitry can be placed on one or more FR-4 layers, and the RF components can be placed on a high-frequency laminate layer that is ideal for frequency dependent circuitry. In general, the type of application and its performance requirements circuit board will dictate the choice of PCB materials.
For example, in RF applications where devices are becoming smaller and more complex, such as my shiny new iPhone 5s, the selection of a PCB laminate is a limiting factor of the circuit design. So much so that even though high-frequency laminates satisfy the performance requirements at RF frequencies, they do little to address the need for circuitry miniaturization required by emerging RF applications.
As a result of these limitations, continued research into alternative PCB materials circuit board has led to recent innovations in PCB technology, such as silicon circuit board (SiCBs), that are capable of meeting both the performance and density requirements of RF devices. SiCBs are similar to traditional PCB except they are made with a silicon substrate rather than FR-4. Since a SiCB is fragile, it is usually mounted on an FR-4 PCB for ruggedness and to facilitate connectivity to other electronics.
The SiCB is the brainchild of Bob Conn, circuit board who first developed the idea while working at Xilinx (see Figure). He realized that silicon provides excellent signaling characteristics, and that a SiCB could provide the computing power of 150 to 200 CPUs in a reduced footprint of about 2 in x 3 in while requiring far less power than an equivalent FR-4 PCB.
Although SiCBs promise to offer a powerful alternative to traditional PCB for applications such as RF design, they are not yet in commercial use. While it is possible to use existing PCB layout tools for SiCB design, they are not ideal since they do not support IC output and dimensions of less than a micron.
In fact, the same can be said for the entire supply chain required for SiCB design. The fabrication process used for traditional PCB design will work, but it is not yet optimized to make commercial availability of SiCBs a reality circuit board.
Like all great innovations, it takes time for related processes and technologies to catch up. I have to admit that it is exciting to be a consumer in these times as technology takes leaps and bounds towards the future circuit board. Would you considering using a SiCB in a future project?
