Microvias on a Printed Circuit Board Handle Thermal Expansion

Printed circuit boards are increasingly using microvias pcb, small holes in the board that interconnect layers. These microvias, also called blind or buried vias, save space and enable higher component density in the PCB. However, microvias have different properties than traditional through-hole vias that must be considered. One of these differences is their thermal expansion. The different coefficients of thermal expansion between the copper and the substrate material can lead to problems with the vias, including voiding, open circuits and intermittent failures.

A microvia is a much smaller hole in the surface of a PCB than a regular through-hole via, but it has the same electrical characteristics. The smaller size offers a lower resistance path for heat, which makes the components on the board run cooler and can increase the performance of high-speed signals. The smaller size of a microvia also means that it has a thinner wall, which can offer improved signal integrity and less crosstalk.

In order to use microvias, the designer must carefully consider their placement and size. They must be placed to minimize copper skew, while ensuring that the PCB fabrication process can reliably plate them. This is because a hole with a high aspect ratio, or width-to-diameter ratio, can be difficult to plate. A hole that spans ten or more layers is particularly challenging for the PCB fabricator.

How Do Microvias on a Printed Circuit Board Handle Thermal Expansion?

To avoid these issues, the design engineer should keep the hole-to-diameter ratio below 1:1. If the aspect ratio is too large, then the microvia can be difficult to plate and may suffer from a variety of defects, such as bare spots, incomplete plating, or poor copper connections. These defects can result in intermittent failures or open circuits, especially under stress during assembly and usage.

The best way to avoid these issues is to work closely with the PCB manufacturer when selecting the materials used to fabricate the microvias. The engineers can help you select the right materials and provide guidance in implementing new designs with microvias. They can also advise you on non-destructive inspection methods to ensure that the microvias are fabricated correctly.

There are two primary technologies for producing microvias in a PCB – laser drilling and photoimaging. Both offer advantages, but each has its own set of unique challenges. Laser drilling can offer a tight microvia footprint, with capture pads/annular rings as small as 0.003 inches to maximize routing space for traces. It can also achieve depth-to-diameter ratios of up to 15:1 and can drill through thick multilayer boards.

Photoimaging, on the other hand, requires a more sophisticated imaging system. This can allow for smaller via sizes, but it is more difficult to maintain consistent hole dimensions and alignment. This is because of the difficulty in controlling the positioning of a photoreactive metal layer that is used to deposit copper onto each hole. To overcome these limitations, a number of processes have been developed to automate the placement and alignment of the copper layer for photoimaging.