Not to toot my own horn, but I just became a homeowner. The moving process will start soon: everything needs to be packed in boxes and creatively arranged in a truck on moving day. I only hope that I can minimize my number of trips between my old apartment and new house without damaging anything. Packing all the family belongings in a truck evokes images of packing more features and components into the limited real estate available on a PCB.
At some point, the only way to continue packing features into your PCB without increasing the board size is to use HDI design techniques. This requires judicious use of vias, creative trace routing, and a layer stackup strategy for your PCB. If done properly, your layer stackup and routing helps maintain signal integrity and prevent interference from EMI.
Standard HDI Stackup Strategies
Each HDI stackup strategy is given a standardized designation. The simplest HDI stackup is called “1+N+1”; in this arrangement, the top and bottom layers are arranged as high density signal layers. The remaining N layers in between the signal layers are arranged as alternating power and ground layers. Placing ground layers below signal layers creates a small current loop with lower inductance, and the presence of the nearby ground plane helps reduce EMI.
The next common stackup design is called “i+N+i”, where the design uses multiple signal layers. The outermost layers on each side of the PCB are designed as high density signal layers, and the interior layers are designed as power or ground plane layers. Just as was the case in the 1+N+1 stackup, the power and ground arrangement should also be designed to provide a short path to ground while still suppressing EMI.
The most complex HDI stackup arrangement is called “any layer”. This stackup arrangement allows high density routing to be placed on any layer and requires placing vias between each layer. This requires using a combination of through-hole, blind, and buried vias to reach each layer. This stackup strategy is reliable for devices with high pin density like CPUs and FPGAs.
HDI Stackup With BGAs
Newer FPGAs placed on boards with small form factor will require HDI stackup and routing techniques. Once you incorporate a BGA for use with a high pin density FPGA or similar component, your chances of moving to an any-layer HDI stackup strategy increase. BGAs with higher pin density will require more dense trace connections to handle escape routing. The i+N+i and any layer stackups are best used with BGAs for high pin density components.
Escape routing can be made easier by simply routing in multiple layers. Moving between layers for BGA fanout and breakout will require the use of vias, and the placement of vias depends on the fanout strategy. Dog-bone fanout is typically used when the BGA pitch is coarse. As the pitch becomes fine and pin density becomes large, microvia-in-pad should be used to route connections to inner signal layers.
Routing between BGAs with PCB design software
Vias in HDI Design and Manufacturing Difficulties
When the differences between conventional and HDI PCBs are compared, the essential difference is that HDI PCBs typically connect layers using blind and buried vias rather than through-hole vias. This is especially important in HDI boards with multiple signal layers. The extremely fine spacing between signal lines in an HDI PCB typically requires the use of laser microvias rather than drilled vias.
To save space on an HDI board, it is common to pick a via diameter and stick with it. Via plating can become difficult if your HDI board includes through-hole vias. As the board thickness increases, so does the via aspect ratio. Higher aspect ratio through-hole vias are more difficult to plate, as the concentration gradient in a plating solution makes the plating near the center of the via thinner than at the outside.
Some manufacturers can improve interior coverage using oscillation or pressing, which forces the plating solution deeper into the via neck. Nevertheless, the interior plating can remain relatively thin and prone to fracture if the board is used in a harsh environment. This underscores the advantages of using stacked vias in your HDI stackup instead of through-hole vias. Be sure to consult your manufacturer’s capabilities if you must use through-hole vias.
One option to prevent the above issues with through-hole vias is to use stacked vias on top of buried vias. The buried via can still span multiple layers and will have better structural integrity than a through-hole via with high aspect ratio. The blind vias connect between the uppermost signal layers and create a smaller impedance discontinuity between signal layers in an i+N+i stackup. This allows you to use a buried via with small diameter and moderate aspect ratio.
PCB manufacturing processing
Another technique mixing buried and blind vias is building “staggered vias.” This structure connects blind vias at each end of the buried via, but the blind vias are offset some distance away from the buried via. Blind vias in adjacent layers are also offset from each other. This arrangement forms a staircase shape and can give you more routing flexibility compared to a simple stacked via structure.
Your PCB design software should allow you to define any stackup in your PCB without requiring specialized tools. The advanced CAD, layout, and simulation tools in Altium Designer 18.1 make it easy to define your layer stackup. Download a free trial and find out if Altium Designer is right for you.
If you are interested in learning more about the design features in Altium Designer, talk to an Altium expert today.
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