What is Via Filling?

Vias 

What is via filling in PCBs? What materials are used for Via Filling?

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Editorial Team - PCB Directory

May 20, 2020

A Via is used to connect multiple layers of a PCB so that it can perform its intended functions. Via Filling is a PCB manufacturing technique in which the via hole is filled with epoxy. This process partially or completely closes the via hole using a conductive or non-conductive epoxy filling material. Filling Via Holes results in more reliable surface mounts, provide better assembly yields and improve reliability of the PCB by decreasing the probability of trapped air or liquids in the Via and thus the PCB Board.

Note: Via Filling is very different from Via Plugging. Click here to read about via plugging.

There are two main options for Via Filling 

Conductive Epoxy Via Fill: Here, the copper particles are evenly distributed throughout the epoxy to provide extra thermal and electrical conductivity. It has a thermal conductivity of 3.5 to 15 W/mK. Sometimes, it offers the conductivity desired for specific applications. The main drawback of conductive fill is CTE (coefficient of thermal expansion) of the metal, as the copper particles present in the conductive fill heat up and expand faster than the surrounding laminate. The conductive fill is used when a PCB has to carry a large amount of current from one layer to another. High-temperature PCBs use this type of via fill.

Common Conductive Via Fill Materials include the DuPont CB100 and Tatsuto AE3030, both are ‘silver coated copper particles’ filled epoxy. The only difference is particle size and CTE value. CB100 has a higher CTE value than AE3030.

Manufacturers can use Silver or Copper conductive epoxy as a Via Fill epoxy. Though Silver epoxy is more cost effective, copper conductive epoxy is far better in terms of thermal conductivity. It is not advisable to trade-off between cost and electrical conductivity, especially when a PCB’s functionality is majorly contributed by the quality of current conduction between its internal layers.

Non-conductive Epoxy Via Fill: Low-shrinkage epoxy. It has a thermal conductivity of 0.25 W/mK. It is the misconception that the non-conductive fill does not pass any current or passes only a very weak electrical signal through the via. We must remember that vias are still be plated with copper, like any other via on the PCB. The only difference is the air in the space is replaced by the fill material. It is usually done to prevent solder or other contaminants from entering the via. It also provides structural support for a copper pad covering the hole in the case of a Via in Pad.

Common Non-Conductive Via Fill Materials include Peters PP2795 epoxy and San-Ei Kagaku PHP-900 epoxy. PHP-900 has a higher CTE than PP2795.

Note: It is always recommended to match the CTE values of the via fill to be used with the surrounding laminate to avoid future stress fractures due to contraction or expansion.

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