Via Fill

As a refresher, a via is a copper plated hole that is used to connect two or more layers within a PCB together. Via Fill is a special PCB manufacturing technique used to selectively and completely close via holes with epoxy. There are many instances in which a PCB designer might want to have a via filled. Some key benefits are:

More reliable surface mounts

Increased assembly yields

Improved reliability by decreasing the probability of trapped air or liquids.

Via in Pad

Perhaps one of the greatest benefits of via fill is the option to implement Via In-Pad. This process is becoming more and more popular and preferred as opposed to using the traditional "dog bone" method to transfer signal from the BGA, through the via, and on to inner layers. In this process, also known as active pad, vias are filled, planarized, plated over with copper. While the Via-In-Pad process does increase cost there can be significant benefits over conventional through hole technology.

Some key benefits are:

Tighter BGA pitches

Increased thermal dissipation

Reduced layer count or board size, which ultimately may reduce cost

Improved routing density (higher density per layer)

Strengthening Pad attachment

Gives high frequency designs the shortest possible route to bypass capacitors

Overcomes high speed design issues and constraints such as low inductance

Conductive vs. Non-Conductive Via Fill

Non-Conductive Via Fill, sometimes confused with Via Plug, still has copper plated vias to conduct power and heat. The via, however,  is filled with a special low shrinkage epoxy specially formulated for this application. Conductive via fill has silver of copper particles distributed throughout the epoxy to provide extra thermal and electrical conductivity.

Non-Conductive fill has a thermal conductivity of 0.25 W/mK whereas Conductive pastes have a thermal conductivity anywhere from 3.5-15 W/mK. In contrast, electroplated copper has a thermal conductivity of more than 250W/mK.

So while Conductive via fill can offer needed conductivity in some applications more often than not it is possible to use non-conductive paste and add additional vias. Often this results in superior thermal and electrical conductivity with minimal cost impact.