Technology & Engineering

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 desogn 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.

Blind Via is a copper plated hole that connects only one outer layer to one or more inner layers. A blind via never goes all the way through a circuit board.  In terms of design, blind vias are defined in a separate drill file.

Additional Benefit of Blind Vias:

Ability to widen BGA breakout channel (layer count reduction)

Buried Via is a copper plated hole that connects two or more inner layers, with no contact with the outer layer. It is impossible to detect a buried via as it is “buried” underneath the outer layer surfaces of a PCB. Buried vias also require a separate drill file.

Additional Benefits of Buried Vias

  • No impact to any trace or surface mount component on the top or bottom layers of the board.
  • Trace or an SMD pad placement on the outer layers directly over the buried via (added space on outer layers)

What is Impedance?

Impedance is the combination of the capacitance and inductance of a circuit when operated at high frequency. Though also measured in Ohms, it is somewhat different than resistance which is a DC characteristic. Impedance is an AC characteristic, meaning that it is related to frequency, resistance is not.

What is Controlled Impedance?

Unless you have carefully designed the trace and its environment, impedance is typically “uncontrolled”, meaning that impedance will vary in value from point to point along the trace.

At high frequencies, PCB traces do not behave like simple connections, controlled impedance helps us ensure that signals are not degraded as they route around a PCB.

Essentially, controlled impedance is the matching of substrate material properties with trace dimensions and locations to ensure the impedance of a trace’s signal is within a certain percentage of a specific value. Controlled impedance boards provide repeatable high frequency performance.

What Determines Controlled Impedance?

The characteristic impedance of a PCB trace is typically determined by its inductive and capacitive reactance, resistance, and conductance. These factors are a function of the physical dimensions of the trace, the dielectric constant of the PCB substrate material, and dielectric thickness. Typically PCB trace impedance can range from 25 to 125 ohms. The impedance value generated from the PCB structure will be determined by the following factors:

– width and thickness of the copper signal trace (top and bottom)

– thickness of the core or prepreg material on either side of the copper trace

– dielectric constant of the core and prepreg material

– distance from other copper features