Vias are the essential component of a multi-layered PCB. From a design perspective, a via consists of two main parts: the drill hole in the center and the pad area surrounding the hole, and the sizes of these two parts determine the via's dimensions. In the PCB manufacturing process, there are five different methods to handle vias: via soldermask opening, via covered with soldermask, via filled with soldermask, via filled with resin, and via filled with copper paste.
Via soldermask opening refers to the condition where the pad of a via is left exposed without soldermask coverage. This treatment is similar to the process used for plated through-holes for mounting components, where the holes are typically tinned. Via soldermask opening is usually employed when the designer has specific requirements, such as the need to perform measurements using a multimeter on the via.
Tip: The effect of via soldermask opening is the same as that of through-hole. It is unnecessary to disable Via Window when converting Gerber files.
Figure 1. Via soldermask Opening
Via covered with soldermask means that the pad of the via are not exposed and covered with soldermask, preventing it from being exposed. Most circuit boards choose via covered with soldermask to ensure good electrical performance while mitigating the risk of shorting with other components.
Tip: It is not recommended that the pore diameter of the via hole cover soldermask design be larger than 0.5mm. If the diameter is too large, the soldermask collected in the via will have a certain quality hidden danger. When converting PCB design files to Gerber photoplot files, the soldermask opening must be disabled for via covered with soldermask, otherwise, the via would be left uncovered.
Figure 2. Via Covered with Soldermask
Via filled with soldermask is an essential supplement to via covered with soldermask, referring to the practice of completely filling the via with soldermask, and finally effectively sealing the via. This prevents solder from flowing into the via through the solder mask dams, thereby achieving the goal of preventing the via from becoming yellow. The standard for good via filled with soldermask is that the via should be non-translucent, meaning that when viewed from one side, it should not be possible to see through to the other side.
Tips: Smaller vias are easier to fill, as a result, vias intended for filling should not be too big, with a maximum size of 0.5mm. If vias exceed 0.5mm, it may be challenging to ensure that the soldermask covers the entire via, leading to the risk of the via hole and its walls being left unfilled. Similarly, when converting design files to Gerber format for via filled with soldermask, it is necessary to disable the soldermask opening.
Figure 3. Via Filled with Soldermask
Via filled with resin refers to the process of filling the inner space of vias with resin, followed by flattening the solder pads through plating. This method is applicable to any type of via with opening on one side of the board or a via in the center of the board with openings on both sides. The purpose of resin filling can be understood from both process and design perspectives.
From the process perspective: For blind vias, which are drilled before lamination, if the holes are not resin-filled, it may result in the press-fit PP flowing into the holes during lamination, leading to delamination issues and causing voids in the laminate.
From a design perspective: When vias are drilled on the solder pads, failure to resin-fill the holes before electroplating may lead to insufficient soldering areas, resulting in poor soldering quality and potentially affecting the overall reliability of the solder joints.
Via filled with copper paste refers to the process of filling the via with high thermal conductivity copper paste after copper plating the hole walls. Subsequently, the filled via is dried, leveled through grinding, and then capped with surface electroplating. This method is applicable to any type of via, whether it is a through-hole with opening on one side of the board or a via in the center of the board with openings on both sides.
Figure 4. Via Filled with Copper Paste
The difference between via covered with soldermask opening and via filled with soldermask: via covered with soldermask refers to the process of covering the solder pad of a through-hole with solder mask, ensuring that there is complete green solder mask coverage on the outer rim of the via pad. via filled with soldermask refers to the process of filling the entire via with soldermask, effectively sealing the via, resulting in a non-translucent appearance.
The difference between via covered with soldermask and soldermask opening: as mentioned before, via covered with soldermask refers to the process of covering the solder pad of a through-hole with solder mask. Soldermask opening is the opposite of that. In the case of soldermask opening, the solder pad of a via is left exposed without soldermask coverage. If surface treatment like tin plating is applied, the exposed via pad can function similarly to a surface-mount applied with tin, which allows the via pad to be used for soldering components or other interconnections on the PCB.
The difference between via covered with soldermask opening and via filled with soldermask: soldermask opening means the solder pad of the through-hole is left exposed, and the via is visible allowing light to pass through the hole; via filled with soldermask means the entire via is covered with solder mask, fully sealing the via, and making it non-translucent.
The difference between via filled with resin and via filled with copper paste: the filling materials are different: for via filled with resin, epoxy resin is used as the filling material, while for via filled with copper paste, high thermal conductivity copper paste is used as the filling material. The copper paste has a high thermal conductivity coefficient of 8 W/m·K.
Note: Due to the inability to distinguish between vias and through-holes in Gerber files, if your files are in Gerber format, we will process them according to your settings in files.
