Vias on a PCB are holes for electrically connecting stacked pads on different circuit layers. A plated hole wall allows electrical signals to travel from one circuit layer to another. Although all vias essentially perform the same function, there are different types depending on their appearance on the PCB surface.

Covering or Filling Vias

To improve the assembly yield or the thermal performance, READA PCB often performs additional treatments on vias. These include filling, plugging, covering, capping, or some combination.

Such additional process steps help eliminate several assembly issues. These include shorting between component pads and a via pad or solder wicking down through the hole of a via. An appropriate via treatment often helps in eliminating expensive troubleshooting and rework.

Tented Vias: A non-conductive solder mask covers both pads at the end of the hole, effectively sealing the openings. Tenting was a popular process when fabricators uses dry film solder masks. The dry film, with its 4-mil thickness, was adequate for covering even large holes effectively, and did not break down much.

However, not many PCB fabricators use dry films now, as its thickness is not suitable for soldering SMT components. Modern LPI solder mask is only one tenth the thickness of dry films, and it cannot form a true tent.

Plugged Vias: Plugging the via on one end or both ends with a non-conductive epoxy paste prevents solder flow-through or wicking during soldering. To allow the epoxy plug the hole effectively, READA PCB limits the diameter of the via to a maximum of 20 mils. Fabricators often cover the plugged via with solder mask.

Filled Vias:

What Is Via Filling?

Via filling involves completely filling a plated hole (via) with either conductive or non-conductive materials. This process enhances the reliability of the PCB

• Preventing      the entry of impurities, reducing the risk of contamination or corrosion.
• Improving the      mechanical strength of vias and pads.
• Enabling the      placement of SMT components on via holes (with resin filling).
• Strengthening      pad attachment.
• Reducing the      risk of solder wicking.
• Preventing      trimming of silkscreen printing.
• Enhancing      thermal conductivity and current capacity of a via (with conductive      filling).

Types of Via Filling

1.    Conductive Polymer Paste: Offers high conductivity and reliable performance, suitable for applications where electrical performance is critical.

2.    Non-Conductive Resin: Provides mechanical strength and is ideal for applications where electrical conductivity is not required

Fabricators often fill a regular or encroached via with non-conductive epoxy paste. These filled vias have the solder mask stopping short of the pad by a few mils. This is a good compromise for medium density PCBs as the presence of the solder mask reduces the likelihood of solder bridging between the via and the neighboring pad.

READA PCB uses active pad processing, where a vacuuming process draws the non-conductive paste through the via hole, filling it entirely. We then planarize the ends, and cover it with solder mask.

For via-in-pads, where the via appears directly on the pads, we use active pad processing. However, instead of covering the ends with solder mask, we plate the surface with copper. This not only hides the via opening, but also makes the surface of the pad indistinguishable from other SMT pads that do not have a via underneath them.

Conductive Filled Vias: READA PCB often fills micro-vias with a conductive paste to enhance their conductivity. This filling can be epoxy resin with copper, although sometimes we also use pure copper for the filling.

Conductive filling allows effective transfer of electrical signals from one side of the board to the other, while enhancing its thermal transfer properties as well. In fact, conductive filling is applicable to all other types of vias also.

Advantages of Conductive filled Vias

Copper filled micro-vias have several advantages. Some of them are:

ü  High Routing Density

ü  EMI Reduction

ü  Better Thermal and Electrical Conductivity

Being small, micro-vias allow high routing density on the PCB, allowing substantial reduction in overall PCB size and lower number of layers. All this reduces the cost of the PCB.

Manufacturing Process

·         Plating: The hole wall is plated to make it conductive.

·         Filling: The barrel is filled with the chosen material.

·         Capping: A cap is applied to seal the via.

·         Planarization: The surface is smoothed to ensure uniformity.

At READA, approximately 10 million holes are filled every month, demonstrating the scalability and efficiency of the via filling process.

Considerations and Challenges

·         Complexity: The via-filling procedure can be intricate, requiring precise control to ensure quality.

·         Cost and Time: The choice of via-filling material impacts the board’s cost and production time.

·         Manufacturing Equipment: Not all manufacturers have the specialized equipment for via filling, which can limit options.