I’ve got this awesome four-speaker stereo system with a subwoofer in my apartment that my neighbors love to hate. As much as I enjoy listening to music on this thing, the only part I hate is the mess of audio cords that hide behind the speakers. The last time I tried to clean behind my system, I nearly ripped out some of the audio cords. If only routing my high density of audio cords was as easy as routing traces on an HDI PCB.
Vias are important in multilayer boards as they allow designers to route electrical connections between layers. This is especially important with fine pitch SMT components and BGAs that require a high density of traces to make the required connections. Via-in-pad designs are an efficient method for saving board space, as there is no need to route a short trace between the pad and the via.
When to Use Via-in-pad Design
Some designers will tell you to avoid via-in-pad design. The reality is that in-pad vias, just like any other via structure, are useful tools in some situations. The metal pad around the edge of the via dissipates heat and helps with thermal management. The pads also allow you to make connections with passive components SMTs, or ICs, and the nearby via hole allows routing to deeper layers and helps keep component density high.
Aside from increasing interconnect density and allowing routing in lower layers, via-in-pad design also reduces inductance at connections. Via-in-pad design is typically used with fine pitch BGAs and offers some advantages over the usual dog bone-type fanout. The space savings offered by via-in-pad design can also help designers reduce the layer count.
Via-in-pad plated over (VIPPO) design is one technique that allows soldering to in-pad vias with a small cross-section. Vias can be plugged with a cured epoxy material in VIPPO design. First, a standard plating process is used to coat the inside of the via. After copper plating and filling with epoxy, the filled hole is capped with a copper pad. Components can then be soldered directly to the VIPPO pad.
The terms via-in-pad and VIPPO are sometimes used interchangeably. Via-in-pad design used for direct soldering should be filled with an epoxy to prevent wicking through the via hole, just as in VIPPO. The epoxy used to plug the via can be either conductive or non-conductive.
Fine pitch BGA
A conductive epoxy has an advantage in terms of thermal management due to its higher thermal conductivity. Since VIPPO uses a copper pad at the connection point, it also offers better thermal management due to its higher thermal conductivity. Combining VIPPO with a conductive epoxy offers even better heat dissipation away from the connection point. The best thermal management strategy requires that the interior of the via be completely plated with copper.
Via-in-pad and VIPPO designs both increase the number of manufacturing steps required, leading to higher manufacturing costs. The actual costs depend on the via size and the total number of vias on the board. But via-in-pad design can allow a creative designer to be more efficient with their routing and even reduce the required layer count. This may offset the increase in manufacturing costs.
Solder masks have been used to plug open via-in-pad holes in order to prevent solder from wicking into the via hole (known as “tenting”). Fine pitch components should instead be used with VIPPO designs due to the small pad size. The plating in VIPPO blocks solder from flowing through the via hole and creating a mess on the bottom layer during assembly.
Hand soldering may be appropriate for attaching components to open via-in-pad designs as human dexterity can help prevent solder wicking. With wave or reflow soldering, VIPPO should be used as you will lose control over wicking. One exception is fine pitch components: VIPPO with the appropriate fanout strategy should always be used on these components, especially with fine pitch BGAs.
High density PCB rework station
One drawback to complete filling with copper is the difficulty in evenly plating the interior of the via. Fluids and/or air can become trapped in the interior of the copper fill, even though the hole may look sealed. This results in outgassing during soldering. Your manufacturer should have proper quality control procedures in place if you elect to fill a VIPPO entirely with copper.
The built-in CAD tools in Altium Designer 18.1 make it easy to use in-pad vias to route connections. The ActiveRoute tool helps you route signals throughout your PCB. Talk to an Altium expert today to learn more.
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