Microvia Sizing in Your Next Multi-layer PCB

October 4, 2018 Altium Designer

Plated vias and traces on a PCB

 

If you’re not familiar with PCB design or electronics in general, you probably wonder how newer electronics can pack so much functionality into progressively smaller devices. It takes more than just Moore’s Law, it takes creative PCB design methods to pack all that functionality into a single device.

 

Vias make their mark in multi-layer PCBs and are used to route connections between board layers. PCB designers should consider via inductance, via crosstalk, and noise coupling when designing new PCBs. All of these effects are dependent on the via aspect ratio and arrangement on the board. It is also important to consider your manufacturer’s capabilities when choosing the correct via size.

 

In HDI designs, your PCB will likely require the use of microvias, especially on smaller boards. A large number of vias can take up valuable real estate, and sizing and arranging your microvias appropriately can ensure that all your components will fit on your board while conforming to your packaging. The size of your vias, including microvias, can affect your trace impedance and should be considered in high speed design.

Microvia Sizing

Microvias generally refers to vias with diameters less than 6 mils. Microvias are commonly used as blind vias on the outer signal layer or as buried vias connecting between inner layers. High density interconnect (HDI) multi-layer PCBs commonly use microvias to route signals between layers.

 

Higher pin density components, like BGAs, inevitably require that vias be used to route traces away from the pins. As components FPGAs can have hundreds of pins with low pitch, the typical dog bone fanout strategy is no longer useful and the BGA must be attached directly to via pads. BGA pads that are less than 0.5 mm can no longer accommodate mechanical drilling, and plated laser-drilled microvias are required to mount these components on your PCB.

 

Once the size of your board reaches a certain thickness and layer count, standard through-hole vias are no longer useful for routing. As an example, think about the case of BGA breakout routing using through-hole vias. Drilling limitations would require that you use vias with wider holes in order to breakout from the BGA. This then requires the use of a BGA with coarser pitch and consumes valuable real estate.

 

Via connected to gold contacts on a PCB

Via connected to gold contacts on a PCB

Microvia Fabrication

Laser drilling is useful for fabricating microvias with a microvia size that is a small diameter, but the depth of focus of a laser beam only allows these microvias to cross a single layer. These vias typically have 1:1 or smaller aspect ratio, depending on the drill depth and the diameter of the via hole. These low aspect ratios allow the use of blind vias at the surface layer, or buried vias in the inner layers.

 

In comparison, mechanical drilling can be used to place high aspect ratio vias, such as through hole vias. However, the smallest available via diameter will be limited in mechanical drilling. As this can consume significant space in HDI boards, microvias would be preferable for accessing the inner layers of a multi-layer PCB. Blind and buried microvias can be stacked to form a long via structure with high aspect ratio, allowing designers to reach multiple layers with laser drilling.

 

HDI fabrication can already be expensive, but using microvias instead of standard through-hole vias in a multi-layer PCB can help reduce layer counts, thus reducing overall costs. Judicious placement of blind and buried microvias rather that through hole vias allow you to increase trace density in your signal layers, and you can reduce your overall layer count.

 

As the board size becomes thicker, drilling through hole vias with smaller diameter consumes more time. It also consumes more drill bits as bits with smaller diameters tend to wear out and break rather quickly. So as the layer count increases, your through hole vias need to be made wider in order to keep fabrication costs down. Using laser-drilled microvias gives you more routing flexibility for similar costs. You can still route through multiple layers by stacking blind/buried vias through multiple layers.

 

CNC drilling in a green PCB

CNC drilling in a green PCB

Via Capacitance and Inductance

Vias with different sizes will have different inductance values, and the inductance depends on the fill material, whether the vias are tented, and their aspect ratio. At low speed and/or in short traces, signal reflection and via inductance can be neglected as the entire interconnect will not act as a transmission line.

 

Parasitic capacitance becomes important at lower switching speeds and frequencies. Just like a pair of adjacent traces has some parasitic capacitance, so does a pair of adjacent vias. The overall size of the via becomes as important as the arrangement of two vias, as these vias form an equivalent capacitor. Two adjacent microvias with small aspect ratio will have a small projected surface area between them, reducing the parasitic capacitance compared to two vias with large aspect ratio.

 

Eventually, the speed of signals in a PCB will reach a level where signal reflection and EMI become significant issues. Assuming an FR-4 PCB and a via with 6:1 aspect ratio, you’ll find that the inductance of an ordinary through-hole via is on the order of tens of nanohenries. The inductance of a microvia is similar, despite its smaller aspect ratio. While this may seem like a small number, this level of inductance is significant at 1 Gbps data rates.

 

When considering the effects of parasitic capacitance and inherent inductance in groups of vias, it is a good idea to use differential signalling to suppress crosstalk between neighboring microvias and the traces connected to them. This is especially important for any high speed or high-frequency signals that must be routed through microvias.

 

When working through manufacturing documentation, you’ll want to annotate annular ring, pad, solder joint, plating, and hole size in your files. Thankfully, the advanced CAD tools in Altium Designer 18.1 make it easy to define via sizes and arrange your vias in your next multi-layer PCB. You can download a free trial and find out if Altium Designer is right for you.

 

If you want to learn more, talk to an Altium expert today.

 

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