3 Specialized PCB Via Options: Boomerang, ELIC, VeCS
Fabrication capacity for high density PCBs and packaging is currently increasing, especially in North America and Europe. These platforms still use standard via styles, namely through-hole vias, blind and buried vias, and core vias in HDI PCBs. But the electronics assembly and packaging world is no stranger to innovation, and there are alternative via options that can offer much more flexibility in routing and layout.
Some packaging manufacturers and HDI PCB manufacturers offer more advanced via constructions, sometimes which are combinations of standard vias or totally different structures. If you are looking for alternatives to standard vias, even in HDI PCBs, then consider these three options for your routing.
Boomerang Vias
Boomerang vias are groups of vias used to make deep layer transitions while eliminating any stub on the routing path. Boomerang vias combine a through-hole via with a blind-buried stack or skip via in order to complete a route between opposite sides of the board. For example, if a route starts on the top layer, and needs to route down to an internal layer, this would be done with a large stack of blind-buried vias or a through-hole via. The boomerang via eliminates the through-hole stub and it eliminates the need for blind-buried stacks.
An example of a boomerang via is shown below. For example, suppose a route starts on L1 and needs to reach L6. If the leftover stub on the L6 land would be too long for this route, the through hole can be combined with a blind/buried/skip via back to L6.
Boomerang via.
When should this style be used? This should be used when channels start to reach very high frequencies such that moderately long leftover stubs will create excess insertion loss. In this example, the route could have ended from a through-hole via directly to L6 while leaving behind the stub. However, if this stub was too long, controlled depth drilling could have trouble fully removing it from the route. Using Boomerang vias overcomes this problem because there will be no stubs on the via.
Every Layer Interconnect (ELIC)
In ELIC (also called any-layer PCB), the conventional rules about stacked blind buried vias go out the window. ELIC is used when we want to route into any layer using blind-buried via stacks, but we want to eliminate core vias in the internal part of the board. This approach is taken in high layer count HDI boards without a thick core and in high layer count packaging in order to reach the package ball out.
An example of the cross section of an ELIC via stack is shown below. And this via stack, many blind and buried views are stacked directly on top of each other spanning all the way through the stackup. If there are any mechanically drilled core vias, they are very short due to the thinness of the internal core layer. Essentially, all of the laminated layers are HDI buildup layers that use blind and buried vias.
Side view of an ELIC PCB stackup.
Because the via stack spans through every layer, it can connect to any layer directly using only blind and buried vias (stacked or staggered). In simpler HDI stack-ups, this would not be so simple and would essentially require the use of core vias, through-hole vias, skip vias, and possibly blind-buried stacks all combined.
While ELIC is convenient to design, it carries fabrication requirements that are more specialized than traditional through-hole or i+N+i stackups. If a board is planned for ELIC construction, manufacturing needs to be contacted to ensure the via size appropriately and a stackup can be fabricated reliably. These boards are also much more expensive due to the number of HDI build-up layers required, so designers might want to consider simpler HDI stackups (Type 1, Type 2, etc.) if cost is a factor in the final product.
Vertical Conductive Structures (VeCS)
Another option for an advanced via style that completely eliminates the inductance of a via is VeCS. In this vertical routing architecture, a via is initially placed to allow vertical routing through the stackup. This could be done as a through-hole via or a blind/buried via. Once it is drilled and plated, the sidewalls are routed out to leave a portion of the vertical interconnect along the whole wall. This leaves a small slot in the circuit board where there would have been a circular via hole.
An example of VeCS routing is shown below. This graphic was provided by an overseas PCB manufacturer and it illustrates the types of direct-to-pad connections that can be made with VeCS.
Stackup with VeCS routing. (Source: NextGIn Technology)
VeCS offers a benefit in terms of signal Integrity as it can be a bit easier to control the parasitics in the structure thanks to the elimination of a cylindrical interconnect. The downside is that the fabrication process is specialized and is offered by very few PCB manufacturers. There may also be size limits because there is a mechanical routing procedure needed to pull away part of the hole and place a small slot. These structures will not be appropriate for every via in a board and instead must be targeted to specific interconnects with the approval of your manufacturer.
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