High Density Interconnect PCB
The high density interconnect pcb is one of the fastest growing designs that offer advantages like faster transmission times, more real estate for component placement and reduced signal loss. However, the design of these PCBs is not without challenges. High-density circuit boards require precise assembly processes to achieve sufficient solder joint reliability matching the advanced substrate. This requires innovation across adhesives, solders and programmable automation.
The core of a high-density circuit board is made from multiple layers of copper foil and dielectric insulator. These layers are separated by partially cured laminates and stacked together like a book. Precision drilling is performed to create microvias that connect the internal copper layers. Copper is then plated through the holes, forming the conductive pathways for the circuit. Photoresist is applied and exposed to UV light, defining the circuit patterns. The etched copper is then filled with one of several types of fill and capped. The finished product is subjected to rigorous testing to ensure quality and functionality.
In addition to the increased density, high density interconnect pcb must be produced with high quality materials and tight tolerances. This includes a good match between the laminate and copper thicknesses to reduce the coefficient of thermal expansion. It also includes the use of liquid photoimageable resist (LPI) and dry film photoresist, which are both capable of exposing the copper with greater accuracy. Additionally, the copper plating process must be able to provide sufficient coverage over the mask for adequate solderability and inspection.
Soldering in the High Density Interconnect PCB
To address the challenge of higher routing density, HDI circuit boards can be fabricated with via-in-pad technology, which uses via holes that are placed directly in the component pads. This eliminates the need for conventional through-holes and provides one of the shortest routes to bypass capacitors for high-frequency designs. However, the via-in-pad production process is often a 10- to 12-step process that requires specialized equipment and skilled technicians.
When manufacturing high-density circuit boards, careful consideration must be given to the pad and trace spacing, particularly in terms of avoiding tombstoning. This occurs when solder unintentionally connects two adjacent pads or pins during the reflow process. The resulting short can be difficult to identify and remedy. Proper pad size and spacing, optimized reflow profiles, and a well-designed solder paste stencil can help mitigate this issue.
The most significant limitation of high-density circuit boards is the physical space available for locating components. This is typically limited by the maximum amount of copper that can be plated through each hole. In addition, the thickness of each layer and the presence of vias can limit the number of layers that can be populated.
For these reasons, it is important to develop a proper stack-up, or series of layers, for your circuit board. This will reduce the number of blind and buried vias as well as free up routing space on the inner layers. This can be done by arranging the layer structure to minimize the number of through-holes or by replacing them with microvias, or both.