Introduction

Most reputable, customer oriented printed circuit board (PCB) manufacturers prefer to use microvias when building cost-effective products for their customers. High-density, multi-layer PCBs use microvias, as they represent the most affordable yet highly function-able interconnect solution.

Fabricators use stacked pads in multilayered PCBs that they puncture and connect electrically with a series of copper tube-lined holes. The conductive holes are the vias, and traditionally, there are three types of these—through, blind, and buried, as in Fig, 1.

Fig. 1: Traditional Types of Vias

1. Through vias: these connect two exposed surfaces and any inner layer, going through the entire board.
2. Blind vias: theseconnect one exposed surface with one or more inner layers, but do not go through the board.
3. Buried vias: these connect some inner layers with each other, but do not extend into the exposed top or bottom layer.

Application

Traditionally, PCB manufacturers drilled all vias with mechanical drills. However, with increasing density, and the advent of fine-pitch components such as BGAs, they needed very small diameter vias, and mechanical drills were rather fragile. This prompted fabricators to change over to lasers instead of using regular drill bits, as lasers could drill holes with diameters as small as a few micrometers.

Although lasers can drill holes of extremely small diameter, they have a shortcoming—the depth of the laser-drilled holes is limited to a single pair of layers. That means manufacturers have to fabricate layers in drill pairs—1-2, 3-4. 5-6. etc., so they use the laser to punch through two stacked pads to create the microvia, before they laminate the pairs of layers together, as in Fig. 2.

As evident from Fig. 2, now fabricators have the freedom to make any type of via structure shown in Fig. 1, using the new microvia technique. In fact, now they have the ability to mix and match the old technology with the new to arrive at the cheapest solution.

Accordingly, the board manufacturer can have staggered or stacked microvias, and even mix stacked or staggered microvias with buried vias, if they so wish. To connect several layers or those that are not adjacent, the fabricator merely sets up staggered or stacked microvias. Stacked microvias sit right on top of each other, while staggered although placed atop each other, do not maintain the same z-axis.

Advantages of Microvias

According to IPC standards, manufacturers have to make buried and blind vias less than 150 micrometers in diameter. Therefore, microvias are unbelievably tiny structures, allowing them to connect high-density layers of advanced PCBs.

The miniature size and comparatively improved capabilities of microvias are also a part of the reason for the increasing processing power of our digital age. By using microvias, manufacturers are able to bring down the number of layers used by a PCB, reducing its cost, thereby decreasing the physical size of electronic gadgets, at the same time improving its electrical characteristics and functionality.