Gold Finger PCB FR4 BGA IPC ENIG 1u' 6 Layer PCB

Are there different types of via hole plugging or protection? Are there variants when calling for a ‘plugged’ via? Yes there are. In fact there are seven types of via hole ‘protection’. Some are recommended and some are not, some are necessary for certain technologies, and all have different ‘names’. In this text, we will explain some of them.

What is a via?

A via is a plated through hole (PTH) in a PCB that is used to provide electrical connection between a trace on one layer of the printed circuit board to a trace on another layer. Since it is not used to mount component leads, it is generally a small hole and pad diameter. The following are the two processes that can achieve this.

No 1: Soldermask covered (Tented)

A via tenting is nothing more than covering its annular copper ring with solder resist, also known as LPI (Liquid Photo Imageable) ink. PCB designers need to remove the solder mask opening from its via in their design, which enables via tenting. This is why it’s considered standard and won’t increase the price of the PCB. In this process, we can only ensure that the annular copper ring is covered with solder resist ink. The surface of the hole may or may not be covered with the solder resist ink.

It is very important to note that the smaller the via hole size, the better the result will be. Suggested is a via >=0.20mm. Via hole sizes less than 0.3mm have the best chance of getting filled, while between 0.3mm to 0.5mm sizes, filling results may vary. Because this is an uncontrolled process, it is not recommended when holes need to be closed. Advantages:

• No cost involved because via filling is achieved during the standard PCB process (screen printing process costs extra).
• See Table 1 for the main benefits, degree of certainty and cost drivers.

Disadvantages:

• Not suitable if a design demands 100% guaranteed filled via.
• Not suitable for Via In Pad process (for active via) and therefore not recommended for highly-complex designs having a fine pitch BGA.

No 2: Soldermask plug

Compared to tented vias, via holes are also filled with solder resist ink (LPI) in this process.

Screen printing process

In this process, a drilled ALU sheet is used to push standard solder resist ink (LPI) into the via holes that need to be filled. The normal solder mask process is carried out after this screen printing process. 100% guaranteed result is assured in this process. Advantages:

• Less expensive in cost compared to via plugging (conductive or non-conductive) process.
• This makes it ideal if it’s just about filling vias with 100% certainty.
• See Table 1 for the main benefits, degree of certainty and cost drivers.

Disadvantages:

• Not suitable for Via In Pad process (i.e. for active via)

Via Plugging (Via In Pad – conductive or non-conductive)

To manufacture products that are more and more compact and advanced, electronic engineers are facing a challenge to design circuit boards that are smaller without compromising performance. Therefore, BGA packages with smaller pitch or clearances are becoming more popular. Instead of using the standard “dog bone” footprint, where signals are transferred from the BGA pad to a via and then from the via to other layers, the via can be drilled directly into the BGA pad. This makes the routing of track work tighter and easier in designing PCBs as the surface of via itself becomes BGA pad allowing it to be treated as a normal SMD pad for soldering. This process is called “Via In Pad” while the pad is called an “Active Pad”.

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

Conductive Via Plugging

For PCB designs that require to transfer high amount of heat or current from one side of the board to another, conductive via plugging 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 plugging
• Not in high demand so minimal manufacturers can supply

Non-Conductive Via Plugging or Epoxy Resin Plugging

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 plugging machine. Thermal conductivity of non-conductive material is normally close to 0.25 W/mK. A common misconception about non-conductive via plugging 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 plugged inside. It means via 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