Via hole protection – what is it and when can it be used

Via hole protection is a PCB manufacturing technique in which the via hole is filled with solder mask or epoxy. This process partially or completely closes the via hole using a conductive or non-conductive filling material.?

Types of Via Plugging?

Overall, there are two types of via protection available depending on the material used in the process; non-conductive via filling and conductive via filling. Out of these two, the most common and widely preferable is non-conductive via filling.?

Conductive Via Protection?

For PCB designs that require to transfer high amount of heat or current from one side of the board to another, conductive via filling is a handy solution. It can also be used to dissipate excessive heat generated underneath some components. The metallic nature of the fill will naturally wick heat away from the chip to the other side of the board in many ways like a radiator.?

Advantages:?

• Heat sink or transfer where other conventional methods are impractical e.g. underneath the chip component.?
• Increased current carrying capacity due to higher thermal conductivity (between 3.5 to 15 W/mK) of the conductive material.?

Disadvantages:?

• High instability of copper pad and copper plating inside the via hole barrel. This occurs due to the difference in CTE (coefficient of thermal expansion) value of conductive material and laminate surrounding it. When the PCB goes through thermal cycles, metal will heat and expand more rapidly than surrounding laminate, which can cause a fracture between pad and via hole barrel and lead to an open circuit?
• Thermal conductivity is not too high (compared to electroplated copper that has thermal conductivity of more than 250W/mK) so it is possible to add a few more vias and avoid this process to more reliable non-conductive via plugging?
• More expensive than non-conductive via protection?
• Not in high demand so minimal manufacturers can supply?

Non-Conductive Via Protection or Epoxy Resin Protection?

This is the most common and popular method of via plugging, especially for Via In Pad process. The barrel of via hole is filled with non-conductive material. Selection of the material depends on the CTE value, availability, specific design requirements and type of process. Thermal conductivity of non-conductive material is normally close to 0.25 W/mK. A common misconception about non-conductive via treatment is that the via will either not pass any current or only a weak electrical signal, which is absolutely not correct. The via will still be plated as normal before non-conductive material is filled inside. It means vias will work as normal as in any other standard PCB.?

Advantages:?

• Prevents solder or any other contaminants from entering into via.?
• Provides strength and structural support to active pads (Via In Pad process).?
• Offers better stability and reliability of pad and via due to close match of CTE between filling material and surrounding laminate when comparing the same with conductive material.?

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