Aug 22, 2019

Breaking PCB Via Design Rules with Additive Manufacturing

pcb-prototypingThe story of electronics is one of miniaturization and greater integration of disparate functionality into a single device. This has progressed at the integrated circuit level through continued scaling of transistor architecture to smaller sizes. This has also occurred at the PCB level through higher routing density, which is driven by the creative use of vias in multilayer boards that go beyond traditional PCB via design rules.

Just like transistors are reaching the limits of Moore’s Law, further scaling of conventional via structures in PCBs are reaching the limits imposed by traditional subtractive manufacturing processes. Laser drilling and stacking of blind and buried vias have helped increase routing density. However, in contrast to the semiconductor industry, fabrication costs have gone up rather than down. This is where new manufacturing methodologies and systems are required to continue scaling vias and to easily implement new via architectures for next-generation PCBs.

Standard vias and traces in a planar PCB; additive manufacturing helps designers break traditional PCB via design rules.

You won’t be limited to standard PCB via design rules with additive manufacturing.

Subtractive vs. Additive Via Fabrication

There are several subtractive methods for placing vias in a PCB. As the term suggests, subtractive methods require milling a hole in a PCB, followed by plating the interior of the via with copper or another conductor to form a conductive channel through a multilayer board. This requires using a drill for vias with large aspect ratio or using a high-power laser for vias with a lower aspect ratio. Note that laser drilling can be used to fabricate via structures through multiple layers by stacking blind and buried vias in HDI routing. This has enabled denser routing in multilayer boards, although this requires many more fabrication steps than conventional drilling.

In an additive process, vias are co-deposited with the substrate material in a layer-by-layer printing process. This ultimately requires fewer fabrication steps as the board does not need to be removed, drilled, and plated once substrate deposition completes. When placing buried vias in an additively manufactured PCB, the savings in terms of fabrication and assembly are even greater. Any type of via can be deposited directly during the printing process without an increase in fabrication time or complexity.

This should illustrate the advantages of using an additive system to fabricate a PCB, especially a PCB that requires a large number of vias to make connections between layers in a multilayer board. The fabrication time does not increase with board complexity.

Note that not all additive systems can provide this advantage. Depositing vias with unique architectures require an aerosol or inkjet system that can be used to build up a substrate and deposit conductors simultaneously. In other additive processes that cannot deposit multiple materials simultaneously, the number of fabrication steps increases with complexity, and some via architectures may not be manufacturable.

The conventional via architecture has always been constrained by PCB via design rules. Most notably, vias in planar PCBs fabricated in the traditional multilayer process must have a vertical architecture that runs orthogonal to the direction of traces in the surface and interior layers. The drilling and plating processes used for fabrication also place specific constraints on via pad sizes, annular rings, and plating thickness.

Engineering working on a 3D printer.

Now you can use an additive manufacturing system for full PCB manufacturing.

PCB Via Design Rules for Unconventional Interconnects

The implication is of these differences in subtractive and additive manufacturing techniques for vias in PCBs is that board engineers have greater freedom to design unique interconnect architectures without the constraints of traditional PCB via design rules. The traditional subtractive techniques for via architecture have driven miniaturization of PCBs and allowed more functionality to be packed on a single board, but going further requires new interconnect architectures that provide greater routing density.

Advances like every layer interconnect (ELIC) and vertical conductive structures (VeCS) have helped increase routing density and/or reduce layer counts, but these techniques are still confined to planar boards through the traditional subtractive PCB manufacturing process. The use of an additive manufacturing system for PCB fabrication makes these structures easier to implement with reduced fabrication time.

Via designers can go beyond these multilayer interconnect structures and implement any interconnect architecture they like within the resolution limits of their additive manufacturing system. The layer-by-layer process does not limit via architectures to vertical structures that run orthogonal to traces in interior layers; these interconnects can run in any direction, and they can even be curved. Conventional via structures like filled vias (whether with the insulating substrate material or conductor), via-in-pad, or via-in-pad plated over (VIPPO) can be easily fabricated in a PCB without any increase in time or costs.

These same benefits, where the geometry of the via is not confined, also extend to the substrate. The layer-by-layer printing process used in additive manufacturing allows a PCB to be fabricated with non-planar substrate geometry, as well as with unique routing architecture. Designers can easily adapt a PCB to have any form factor they like, allowing a board to easily fit within any enclosure. Conductive components like electromagnets and non-planar antennas can also be embedded in these boards. These capabilities offer designers many more options for designing new electronics without being constrained by standard PCB via design rules.

Working with an additive manufacturing system that is specially designed for the fabrication of complex 3D-printed electronics frees designers from the conventional PCB via design rules. The DragonFly LDM system is designed for fabricating planar and non-planar electronics with any level of interconnect complexity. You’ll be able to stay competitive and produce novel devices with faster cycle times and lower costs. Read a case study or contact us today to learn more about the DragonFly LDM system.

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