All About Staggered Vias

Posted April 13, 2024

In Manufacturing, PCB Design, Technology

Staggered and stacked microvias are standard in HDI or High-Density Interconnect boards. In fact, these boards achieve their unique density primarily because of these tiny vias and closely spaced, narrow traces. Eminent PCB manufacturers like Rush PCB Inc. create effective, efficient, and compact circuit boards by incorporating staggered and stacked microvias in the design of their boards.

A rapid change has been evident in the entire electronic industry in the last few decades. The form factor of almost all electronic devices and equipment has been steadily decreasing. Two factors are responsible for this remarkable achievement. The electronic component industry has been steadily reducing the form factor of their SMT components. This has prompted the PCB industry to pack greater numbers of SMT components into smaller boards. And the way they are doing this is by adopting HDI technology along with staggered and stacked microvias to reduce the size of their boards.

With HDI technology, manufacturers can pack a greater number of tiny SMT components within a small region of the board, and improve its performance capability with versatile routing, along with staggered and stacked microvias.

Microvias

Just like vias do in conventional multilayered boards, microvias also provide interconnections between circuits in different layers in an HDI multilayer board. However, compared to mechanically drilled vias, microvias have far smaller dimensions. Therefore, apart from providing interconnections, microvias enhance the routable area on a board. Although it is possible to think of a microvia as a miniature version of a conventional via, the former has a different structure.

For instance, unlike a mechanical drill creating a conventional via, a laser beam creates microvias. The laser beam vaporizes the drill area and creates the hole. Another difference is, that the laser beam drills to a depth of only one layer, so that every microvia is only one layer deep. In contrast, a mechanically drilled via can traverse anywhere between one layer to all layers, thereby passing through the entire thickness of the board.

Advantages of Microvias

Apart from making the board compact and light, microvias have their own advantages. For instance, microvias:

Improve signal integrity
Offer superior electrical performance
Lower crosstalk, EMI, and RFI
Lower the size and weight of the board
Optimize the cost of the board
Improve board reliability
Reduce overall design time

Laser Drilling of Microvias

Laser drilling is a suitable technique for fabricating microvias. Not only does it create tiny holes, but it also allows greater routing density because of the miniature structures. However, there are some critical parameters in this process—the size of the pad, the drill size of the laser, and the thickness of the dielectric.

The operator must keep the size of the laser drill proportional to the finished press-out thickness of the dielectric layer. This helps in plating the via adequately. After drilling the via, the operator plates the via using either electroless copper plating or electrolytic deposition technique.

Aspect Ratio

The aspect ratio is a critical factor during laser drilling. The aspect ratio is the ratio of the via’s hole depth to its diameter. The hole depth for a via is the sum of the outer dielectric thickness and the copper foil thickness.

When plating the via, the operator must keep its thickness proportional to the via’s aspect ratio. Increasing the plating thickness increases the risk of rupture, as the plating may expand more during the temperature rise during the assembly process.

Keeping the aspect ratio low ensures an even plating, leading to a superior electrical connection, and providing a stronger mechanical strength. Although, as per the IPC definition, perfect microvias are the ones with a 1:1 aspect ratio, the industry standard prefers them with a 0.75:1 aspect ratio.

HDI designs endeavor to achieve supreme quality and absolute reliability. Increasing the aspect ratio leads to the possibility of design failure during reflow because of reflexing. Therefore, the PCB industry prefers to lower the aspect ratio with either a larger via diameter, a thinner copper foil, or even both. For an HDI board to successfully withstand several thermal cycles, a 6-mil via diameter is the most optimum.

Blind and Buried Vias

Just as in conventional boards, HDI boards also may have blind and buried vias. However, their construction varies from those of conventional boards. Blind vias connect one circuit on one of the outermost layers to another circuit in at least one inner layer. Although a blind via pierces through one of the outermost layers, it does not pass through the entire board and all layers. Therefore, a blind via is visible only on the outermost layer and its other end remains invisible.

Buried vias do not appear on any of the outermost layers. As their name suggests, they remain buried within the board and are not visible outside. A buried via passes through at least one inner layer. They are present on a separate drill file, and the operator electroplates them.

Creating a multilayer HDI board with microvias interconnecting several layers requires placing the vias one atop the other, usually in a stacked or staggered form.

Stacked MicroVias

Stacked vias have the vias placed directly one atop the other. The operator drills each via and plates it before stacking it above another to connect various layers. They use two narrow annular rings, one on the top and the other, on the bottom. The upper annular ring helps in precise registration, while the bottom one makes the electrical connection.

The operator fills the stacked vias with electroplated copper. This not only provides structured support but also ensures a solid electrical connection. However, improper deposition can create defects like voids and inferior bonding between the base of the via and the target pad beneath it, hampering the reliability of the stacked vias.

While compactness is the primary advantage of stacked vias, in HDI boards stacked vias also ensure flexibility during practical routing. In addition, stacked vias help maintain properly controlled impedance of the signal from its source to its destination.

Exerting pressure from the z-axis of the dielectric on the microvia actually impacts the reliability. The reason for this is a mismatch in the CTE of the materials. For instance, consider the situation beyond the glass transition temperature. While copper expands only up to 16 parts per million, the dielectric expands nearly 200 parts per million.

As long as only a single layer is present, there is not much of an issue. The discrepancy really begins when the layer number increases to two and beyond. For instance, with a layer count of three or more, the mismatch crosses tolerance limits. It can lead to microvia failure due to its barrel or its corner cracking up. This led to designers staggering the vias rather than stacking them vertically.

Staggered MicroVias

Like stacked vias, staggered vias also connect different layers of the board. However, they are never in direct contact with each other as their drill axes are separate, which offsets their position on adjacent layers.

Staggering the microvias involves fewer design steps. Because the drilled hole is not directly above the one below it, laser-drilled staggered vias do not need copper filling. This makes the design less complicated.

While designing a staggered via structure, the spacing between the laser-drilled holes is the primary concern. The possibility of the staggered via design depends on the vertical distance between the centers of two adjacent microvias. A viable staggered design requires the vertical separation to be more than the diameter of the microvia.

Stacking microvias exerts pressure on the via walls. The pressure can detach the topmost via from its pad. Therefore, designers prefer a staggered microvia configuration when they have to interconnect more than two layers.

However, if the design has space constraints, staggered vias may not be the right choice. Although less complex, staggering makes them use up more space on the board. On the other hand, staggering has negligible crosstalk issues because of the offset. Staggering introduces more discontinuity in the signal path. This makes it difficult to maintain a uniform via impedance requirement in a high-speed design.

Conclusion

With a careful design, both staggered and stacked microvias can withstand several thermal cycles. Rush PCB Inc. makes complex and dense HDI designs with both staggered and stacked microvias, achieving remarkable signal integrity in high-speed designs.