My friends and I took a road trip over the weekend. When three people take a three-hour car ride in a small sports car, you come up with some creative ways to pack all your stuff (and yourselves) in such a small space.
The demand for faster boards with higher component density constraints has forced PCB designers to use a number of creative routing techniques in multilayer boards. Skip vias, dog bone traces, pad-in-trace, microvias, and back-drilling are all being used by designers to increase routing density while maintaining signal integrity requirements.
Vias are essential for making high density interconnects (HDI) on PCBs. Skip vias are similar to blind vias in that they allow connections between the top or bottom PCB layer and an internal layer. A blind via connects to the next PCB layer, while a skip via can connect through multiple layers.
Skip vias also come in the buried variety (called a core via), meaning the via can connect multiple interior layers in a PCB. These vias need to be filled, either with a conductive or non-conductive filler. As electrical conductors are good thermal conductors, devices running at high current density will generate significant heat, and vias with a conductive filler material may be useful for dissipating heat away from certain components.
HDI PCB with various vias
Plating and Lamination Challenges
The aspect ratio of any via will determine how easily it can be plated during manufacturing. Skip vias in a multilayer board will likely have higher aspect thanks to their depth. High aspect ratios are more difficult to plate, and they require a plating solution with a higher surface tension and lower viscosity. This will ensure that the plating solution can penetrate the via through capillary action and can thoroughly deposit an electrical contact on the interior of the via.
Placing skip vias in a PCB requires the same tight tolerances as placing through-hole vias. Skip vias are normally laser drilled, and slight lateral displacements during drilling can distort the interior of the via, creating problems with plating and even rendering the board unusable in extreme cases. The laser drilling process is moderately complex and, of course, the manufacturing price goes up.
Proper plating will yield a via with a deep continuous conductive layer that is more durable than other vias, as the deep plating on skip vias improves their adhesion to the board material. The primary point of failure is at the bottom of the via, as the laminate may be prone to cracking at this point.
Manufacturing of high density interconnect PCBs
Despite some challenges, judicious use of skip vias can actually improve the lamination process. For example, connecting directly from L1 to L3 using a single skip via is preferred over using stacked vias as it reduces the number of lamination cycles. Before moving to manufacturing, make sure to speak with your manufacturer and verify that they can accommodate your via requirements.
Via-in-pad Plated Over Skip Vias
A design technique known as via-in-pad plated over (VIPPO) can be used alongside traditional designs. Via-in-pad designs already reduce inductance and can provide a quick path directly to ground, which is beneficial in high-frequency circuits. Using a via-in-pad designs like VIPPO with a deep skip via can also improve the pad adhesion to the board.
Leaving your skip vias unfilled during soldering can allow the solder to wick down the neck of the via. This can prevent proper soldering during automated manufacturing, and even manual manufacturing becomes more difficult as it is tough to gauge how much extra solder is required to connect components to the pads. Offsetting the via in the pad only makes a real difference when the pads are large, and it may be worth routing a short trace between the skip via and your solder point.
For this reason, skip vias should be filled with either a conductive or non-conductive epoxy. VIPPO design places plating over the top hole of the via, which prevents wicking into the via capillary and can help ensure a secure connection with electronic components directly on the pad.
The first consideration when choosing an epoxy filler is to try to match the filler’s thermal expansion coefficient to that of the surrounding laminate material. Non-conductive epoxies have thermal expansion coefficients that are closer to that of most laminate materials. Non-conductive epoxy fillers have become very popular for this reason, and these epoxies are a less expensive option as well.
A great piece of PCB layout software like Altium Designer makes it easy to layout your next high density design. The ActiveRoute tool and built-in CAD tools make it easy to customize your via parameters and route your signals around your PCB. If you are interested in learning about how Altium Designer can help you build your next high-speed device, talk to an Altium expert today.
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