HDI PCB design
Achieving precise characteristic impedance in multilayer HDI PCBs plays a critical role in the performance of electronic devices, particularly mobile / cellular phones, touch-screen devices, digital cameras, and 4/5G network communications. This requires careful planning of conductor width and spacing as well as selection of substrate materials with consistent dielectric properties.
HDI circuit boards feature narrower traces and smaller spaces, which require advanced etching and plating capabilities to maintain signal integrity and prevent stray currents. This is especially important in hdi pcb designs that feature multiple ground and power planes, which must be properly positioned to ensure the best thermal management.
Increasing component density also requires that circuit designers plan for tighter space between components and traces, which can be challenging to achieve without compromising on quality. This can be mitigated by using proper DRC settings in advanced design software, implementing thermal management strategies, and communicating with the fabrication shop early on to identify their capabilities.
What are the challenges in HDI PCB design?
High-density interconnections allow designers to fit more components in the same area, reducing overall board size and thickness while improving performance. They also offer better reliability, as the smaller aspect ratio of microvias compared to through holes leads to reduced signal loss and crossing delays.
With increased density comes additional challenges, such as the need to ensure that the underlying copper surface is clean and free from solder bridges and shorts. This can be difficult to accomplish, but ensuring that all exposed copper areas are fully covered with solder mask openings, as well as the correct solder paste spread rate and temperature, can help to reduce the risk of defects during assembly.
Another challenge is ensuring that the correct impedance value is achieved, which is dependent on several factors, including the metallization process, via diameter and pitch, and the layer stack-up arrangement. Improper impedance control can lead to signal degradation, decreased performance, and other issues. This can be mitigated by ensuring that the correct metallization process is used, and that the layer-stack-up arrangement is optimized for the required impedance.
Tight spacing between traces and components presents further challenges in HDI PCB design, which can be overcome by utilizing the appropriate DRC settings in advanced design software, minimizing the number of layers in the stack-up, choosing thinner dielectric layers, and leveraging techniques like conductive and non-conductive vias to minimize the need for through-holes. Moreover, using the right imaging tools and class 100 clean rooms during manufacturing reduces the chance of defects in the finished product.
When working with complex HDI designs, it is essential to communicate with the fabrication house to understand their capabilities and to verify that the design meets their minimum requirements. Otherwise, the board may be difficult or impossible to manufacture, which can result in delays and a longer time to market for the final product. Providing clear design notes to the fabricator that specify the minimum trace width, clearance, and aspect ratio can help to avoid this issue. Sierra Circuits utilizes advanced Hitachi drilling machines that can produce a 1m hole placement tolerance, known as drill true position, for HDI designs.