Thickness of Microvia Aspect Ratio

A microvia is a hole in an HDI PCB that can be filled and plated with copper, forming an electrical connection between layers. PCB manufacturers can use a wide range of dielectric materials for microvias, from conventional glass-reinforced epoxy to ultra-thin unreinforced dielectrics such as resin coated copper foil (RCC). PCB designers should consider the ideal aspect ratio for a microvia when planning the structure of an HDI board. Deviations from this ratio can cause problems during the manufacturing process and compromise reliability.

The ideal aspect ratio for a microvia is determined by the pad size, the laser drill size and the finished press-out thickness of the dielectric. The laser drill dimension is critical to the fabrication process since it determines the shape of the via and how easily the plating solution will flow inside. The finish layer thickness is important because it enables the plating of copper and provides a strong connection between the microvia and the surrounding copper.

Stacked and staggered microvia aspect ratio are used in HDI PCBs to allow routing across multiple layers without using standard through-holes. A common application is BGA breakout routing in fine pitch BGAs. However, the high aspect ratio of these vias makes them susceptible to failure during reflow cycles. This failure is caused by stress concentration in the neck area of the via. The high aspect ratio also makes the vias difficult to metallize.

Factors That Determine the Thickness of Microvia Aspect Ratio

For the best microvia performance, a designer should limit the diameter to less than the pad size. This will ensure that the via will be easy to route, but not so small that it is at risk of cracking or delamination during reflow. It is also important to avoid using buried microvias. These are holes that start in one layer and run through the innermost layers of the PCB. They must be completely filled with conductive material to prevent them from leaking and contaminating adjacent pads.

The final factor that determines the microvia aspect ratio is the voiding factor, which is influenced by the volume and shape of the via as well as the aspect ratio. While voiding is the primary contributing factor to microvia failure, it can be mitigated by using an appropriate design approach, such as staggering or stacking the vias.

Aside from the considerations outlined above, there are a few other issues that should be addressed when designing an HDI circuit board with microvias. For example, an aspect ratio that is too large can make it difficult for the drill to penetrate the entire dielectric layer. This can lead to a gap in the hole and create a weak point that can fail during reflow. This can be avoided by using a smaller dielectric layer and by choosing a larger drill size for the microvia. Similarly, an aspect ratio that is too small may result in the copper plating being thinner than required, which can affect reliability. This can be solved by choosing a larger drilling diameter or by adding an annular ring to the via.

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