How Microvias Influence Board Impedance Matching

Microvias Influence Board Impedance Matching

Printed circuit boards (PCBs) form the conductive pathways that connect the various components of a modern electronics device. A wide range of PCB design techniques enable manufacturers to pack more functionality into these compact devices, including high-density interconnect (HDI) techniques and microvias. These small structures let traces reach inner layers of the PCB and support HDI board designs.

Using microvias to reduce the size of PCB pads and holes is crucial for supporting the latest high-density component packages, such as fine pitch ball grid arrays (BGAs). Conventional through-hole fabrication technology cannot support these new geometries with physical pad sizes that are too large. Microvias help designers increase the number of connections per layer and meet IPC 6012 requirements for HDI designs, while reducing manufacturing costs and improving board performance, efficiency and reliability.

To make this possible, the copper foil used in a via must be of an appropriate thickness and the drilling process must be accurate to avoid issues with thermal expansion, stress concentration and other factors that can affect a via’s structural integrity. For this reason, it’s important to consult with your PCB fabricator before selecting the material set for your design. Epec engineers can help you select a suitable laminate and ensure that it is compatible with your chosen plating and drilling processes.

How Microvias Influence Board Impedance Matching

When designing a PCB, the first step is to define a stackup in the PCB design software. This stackup defines the layer pairs and materials for the entire PCB, allowing you to create your desired electrical connections. To avoid issues with manufacturability, it’s recommended that you build a proposed stackup and then send it to your fabricator for review before making any changes. This allows the fabricator to advise you on whether your stackup is feasible and how to best implement blind, buried and microvias in the board’s construction.

Once a stackup has been defined, the fabricator can drill the holes and use a variety of plating processes to fill them with conductive copper, forming what’s known as a via. The type of plating is an important consideration, as different types of copper have different conductivity and heat dissipation capabilities. For example, thicker copper foils can offer greater conductivity, but can also introduce challenges during the drilling and plating processes.

Microvias are most susceptible to failure at the via neck, where copper curves inward into the via’s barrel region. Stress can concentrate in this area when the via is exposed to repeated thermal cycling or strong vibration. This makes the use of microvias more challenging in high-temperature applications.

In addition to limiting the area of the copper plane where circuitous return paths can occur, microvias also offer improved signal integrity by lowering parasitic inductance and capacitance, as well as reducing ground bounce. This leads to better RFI/EMI performance and increases the overall signal quality of a PCB. However, this is only true if the size of the via is correctly matched to the signal frequency.

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