Best Ways to Use Skip Vias in an HDI PCB

One of the biggest design requirements in HDI PCBs is designing via stacks. Vias in an HDI PCB give you much more freedom than a standard board with through-hole vias, but the vias are also a big driver of manufacturing costs. These are also a big driver of reliability in the finished HDI build, especially when you look at challenges with stacked microvias. It is a PCB designer's job to make sure the selected via style allows dense routing without creating a point of failure in the finished assembly.

If you only need a small number of HDI layers, one option to enable routing in these designs is to use skip vias. When an HDI board does not require extreme complexity and only a few outer build-up layers are needed, skip vias are a good option, especially when you look at cost and reliability. If you want to shave some costs from HDI manufacturing, then make sure you use skip vias the right way.

How to Use Skip Vias

Skip vias have several different names, but they all refer to the same type of structure. Skip vias are blind vias that skip a layer in the HDI stackup. Instead of landing on layer 2, the skip via lands on layer 3 or deeper in the stackup. They are used in two types of HDI builds:

• Type 1, which is the standard usage of skip vias to reduce cost

• Type 3, which can eliminate the need for via stacks

Skip vias are shown in the Type 1 stackup below. The stack shown below only uses two outer layers for the HDI build, but could use more layers with additional skip vias or buried vias to make a non-standard HDI buildup.

Type 1 HDI stackup.

For the moment, let us focus on just the skip vias spanning between layers 1 and 3 using two HDI buildup layers. In the above image, the skip via does not land on layer 2. The only transition from layer 1 to layer 2 is through the blind via or through the through hole via spanning the entire stackup. This means anytime a transition is needed between layers 2 and 3, it must be implemented with a through-hole via.

Skip Vias Eliminate Stacking

A skip via is a good option for an HDI stackup construction because it allows a transition from layer 1 to layer 3 without requiring the use of stacked vias. Stacked microvias can have reliability challenges, where there is potential for the via stack to fracture or separate after reflow cycles. The stresses on via stacks during reflow are problematic for some manufacturers who produce HDI PCBs. These manufacturers like skip vias because they help ensure quality.

The other instance where skip vias might be used are in a Type 3 stackup, which is shown below. Here we have blind and buried on to HDI build layers, as well as a skip via from layer 1 to layer 3.

Type 3 HDI stackup.

In this stackup, you have blind and buried vias, but you do not have a requirement to stack them. Instead, the blind and buried vias could be staggered so that you do not have the well-known reliability problem. Once a transition is needed from layer 1 to layer 3, the skip via can be used so that a blind-buried stack is not needed. With a Type 3 stackup, this pattern could be repeated beyond two HDI buildup layers, but there are some DFM rules to obey when using skip vias.

How to Design Skip Vias

Skip vias have many of the same DFM rules as standard blind and buried vias. These via can only be fabricated reliably when they are within aspect ratio limits. This means the outer layers cannot be too thick. This also limits the number of buildup layers you can use, which will typically be capped at two buildup layers.

If you plan to use skip vias, the routing path also has to be planned appropriately. The skip vias will most likely appear in a BGA fanout area which will place some constraints on the design:

• Outer layers may need to be very thin in order for the skip vias to have the right aspect ratio underneath the BGA

• Routing through the skip vias will not be able to access layer 2, so the routes have to be carefully planned

In the end, if the skip vias are placed and routed correctly, the big benefit is elimination of a process step from the HDI manufacturing process. Two sequential laminations are needed, but the laminated HDI stack can be drilled directly to form the skip vias, rather than drilling the buildup layers individually. Reduction in HDI process steps is a big driver of per-unit costs and it can help make the final product more cost competitive.

Whatever board complexity you might face, you can create advanced electronics assemblies when you have CAD tools that make it easy. Multi-disciplined design teams rely on the best set of PCB design features in Allegro PCB Designer from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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