After I finished moving into my new (and somewhat smaller) apartment, I was left with a stack of boxes that looked quite out of place in the corner of my living room. Packing all my extra boxes in my small storage closet was like playing a game of Tetris. As PCBs continue to get smaller, you may be playing that same game of Tetris with your components. This means you will need routing techniques that can accommodate high-component and interconnect density.
With PCBs containing even more layers and becoming thinner than ever before, new techniques are used to increase interconnect density. Every Layer Interconnect (ELIC) is one technology that lets designers create very thin, flexible PCBs with high functional density. These advanced HDI PCBs contain multiple layers of copper-filled stacked in-pad microvias that enable even more complex interconnections.
Designing Multilayer Boards with ELIC
ELIC is sometimes referred to as any-layer HDI. When using ELIC on an HDI board, each layer has its own copper-filled, laser-drilled microvias. ELIC uses only stacked copper-filled microvias to make connections through each layer. This allows connections to be made between any two layers in the PCB once the layers are stacked. Not only does this offer an increased level of flexibility, but it also allows designers to maximize interconnect density.
Some general high-density design principles apply to HDI boards using ELIC. Since interconnections are placed so close to each other in HDI boards, care must be taken during routing to prevent crosstalk. Impedance control is also important during routing, especially since ELIC-connected HDI boards are used for high speed and/or RF applications.
The ELIC design process starts with an ultra-thin core with laser-drilled microvias and a solid copper-filled base. After the initial microvia is filled with copper, the next dielectric layer is added with lamination. Laser drilling is applied to the new layer to build the ELIC stack, followed by copper via filling. This is repeated until the desired stack is built with copper filled microvias. Since ELIC uses a copper-filled structure, plating techniques like VIPPO are not required.
PCB with SMD components
ELIC has found a home in PCBs used for GPUs and memory cards, but newer smartphones, tablets, and wearable devices can also be designed using ELIC. These applications tend to require components with high pin count and fine pitch. These boards also tend to use 10 or more layers. Using ELIC in these applications allows designers to route the required interconnects in boards with a small footprint.
ELIC has also become a mainstay in miniaturized rigid-flex PCBs. Package sizes have been reduced even further by combining ELIC-capable PCBs with folded rigid-flex boards in a single package. The standard design techniques for flex ribbons apply as they would in other applications, but the use of ELIC allows ribbons to be integrated into smaller PCBs.
Ribbon for rigid flex PCB
Routing, Layer Arrangement, and Impedance Control
In terms of routing, ELIC is unique in that any trace can be started or ended on any layer. Buried, blind, and through-hole vias are not needed anymore since all connections between the board are fabricated in the initial buildup. The sequential copper fill also improves the structural integrity of the board, so the overall thickness of the PCB can be reduced.
To control the ground impedance, multiple internal ground layers should be used in the stack. Two signal layers should not be adjacent in order to minimize crosstalk between them. Instead, signal layers should be separated by a ground layer. Signals in a given signal layer are then referenced to an adjacent ground layer. Careful trace width and spacing design, dielectric prepreg thickness, and layer arrangement give designers tight control over impedance.
An inner signal layer should be sandwiched between a ground and power plane. This includes high-speed signal layers. This technique effectively isolates nearby signal planes and has the dual effect of reducing crosstalk and EMI. Finally, the layer arrangement should be made symmetric throughout the device. Since ELIC naturally allows connections between any layer, building symmetric layer stacks is simple compared to using blind and buried vias.
Before finalizing the design of any multilayer PCB or a multi-board rigid-flex PCB, designers need to confirm the design can comply with standard EMC requirements. Most manufacturers opt for more layers in their boards to ensure that the final product will have sufficient immunity to EMI.
Building your next HDI design is easy with a great piece of PCB layout software like Altium Designer 18. The built-in CAD tools and the ActiveRoute feature make HDI routing using ELIC a breeze. Talk to an Altium expert today if you are interested in learning more about Altium Designer.
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