As PCB designers face greater challenges with fine-pitch components, less real estate and greater need for thermal conductivity, the use of epoxy filled vias has become common.  With two very different options of non-conductive and conductive (silver filled) epoxy available, the question if which is best often leaves designers and engineers with a difficult decision to make.

Conductive (Silver Filled) Epoxy:

Conductive silver filled epoxy contains organic solvents which require storage at a temperature of less then 5 degrees C, and limit the shelf life of the material.  Because of this, many PCB factories do not stock this epoxy and purchase it in quantities needed for the order, which may cause delays in production.  The size of the silver balls in the epoxy can make filling smaller holes difficult and often lead to air pockets inside the hole, which will actually decrease the thermal conductivity of the hole.  If the air pockets are close to the surface, this can cause voids at the pad surface, leading to assembly issues.  As the CTE (Coefficient of Thermal Expansion) of Silver Filled epoxy can be very different from the CTE of high TG material used in lead free or ROHS assemblies, blistering of surface pads can be common, as well as lifted pads during high temperature soldering operations.

Silver filled epoxy reacts to process chemistry during metallization by expanding which will cause an uneven or swelled pad surface.  This creates the need to add an additional process of “planarization” of the pad surface which can be costly, time consuming and add to pad surface imperfections including dimples in the land pattern and voids that can outgas during assembly.

Non-Conductive Epoxy:

Non-Conductive epoxy is 100% solid epoxy material which usually yields good pad planarity for via-in-pad designs.  The lack of silver in the epoxy resin allows the process to be used on smaller holes, including micro-vias often down to as small as .004 inch.  The CTE (Coefficient of Thermal Expansion) of non-conductive epoxy is often very close to that of high TG material, meaning less issues with hole expansion during the assembly process.  This also allows Non-Conductive Epoxy to be used in sequential lamination processes to fill blind and buried vias.

Non-Conductive Epoxy usually exhibits very little shrinkage during the thermal curing process, which will yield good pad planarity and will not require a separate planarization process.   The flat surface of the pad after plating over the Non-Conductive Epoxy means less chance of lifting pads during the assembly process and greater rework ability.

As there are no temperature or shelf-life restrictions to Non-Conductive Epoxy fill, most PCB factories have a ready supply of the material in stack, thus eliminating production delays caused by material procurement.

Effects on Thermal Conductivity:

Improved thermal conductivity is often cites as the reason for choosing Conductive Epoxy Fill over Non-Conductive Epoxy fill.  It is true that pure silver has the highest thermal conductivity; however, when surrounded by epoxy the silver surfaces are insulated and there is no direct contact of the silver to effectively increase the thermal conductivity.  Additionally, the air pockets present when using Silver Filled Epoxy may also reduce the thermal conductivity.  A properly filled and capped plated through hole will have better thermal conductivity using Non-Conductive Epoxy fill in place of Conductive Silver fill.

Conclusion:

The development of improved formulations of Non-Conductive Epoxy fill have created an equal or superior bond to electroless copper and increased the Thermal Conduction of filled holes, while eliminating many of the manufacturing and assembly concerns with Conductive (Siler Filled) epoxy.  Due to the points raised above, PCB Solutions strongly recommends Non-Conductive Epoxy

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