There are two primary categories of vias, depending on where they are in the PCB layers — the blind hole and the buried hole.
In a blind hole, the hole penetrates the top or bottom layer of the board but stops before any of the internal layers. Blind holes are named as such because you cannot see through them when you hold a board up to the light. The process involved with blind-hole creation can be difficult because you must know when to stop drilling into the board. As such, many PCB manufacturers avoid hole plating of this type.
Another type of via is the buried hole, which might appear through one or more internal layers. Since the buried hole is sandwiched between layers, it cannot be seen by the naked eye. For a multi-layer board to have a blind layer, the hole plating on the internal layer must be completed early on during PCB assembly, before the top and bottom layers are applied to the board.
The most obvious type of via is the plating through-hole, which penetrates all the layers in a multi-layer board. Through-holes are typically larger than blind holes and buried holes and are also a lot easier to identify with the naked eye. When you hold a board up before the light, the light will pierce right through a plating through-hole. Through-holes are also easier to make because you can simply drill right through all the layers, unlike with blind holes, where you have to be careful how deep you make the hole.
Through-hole technology has been around since the mid-20th century when it replaced point-to-point construction. Through-holes were most common between the 1950s and 1980s when nearly all the features found on a printed circuit board were attached to a through-hole.
In the early days of through-hole technology, PCBs had their tracks printed only on the top side. As technology advanced, the printing appeared on both sides. Eventually, multi-layers boards came into use. At this point, through-holes were updated to plated through-holes to enable contacts between conductive layers. Today, plated through-holes are used to connect the different layers in a PCB. Through-holes are usually used to facilitate components with wire leads. Axial-leaded components are placed through these holes and used to connect across short spaces.
One of today’s more popular designs for printed circuit boards involves the application of vias on ball grid array (BGA) pads, alternately known as via-in-pad. In the via-in-pad design, the vias are placed on the BGA pads of the PCB. The design has become popular because it makes it possible for manufacturers to minimize the amount of space required for vias.
The via-in-pad design is especially convenient for routing because the holes connect straight to the layer that underlies the component, making it possible to route signals without the risk of going outside the perimeter of the device’s footprint.
In a standard via layout, solder mask can be applied to stop the flow of solder into the via. With the via-in-pad design, you cannot leave the barrel free of filling because air could get trapped, causing out-gassing while the PCB is in production. Therefore, the vias need to be filled when placed on BGA pads. For the design to work, you must also have a flat planar surface, as this will make it possible to connect fine-pitch BGAs with little, if any, complications.
In PCB design, vias that are under 150 microns are known as microvias, which are used on many high density interconnect (HDI) boards. Designers prefer microvias because of the small size of the hole, which consumes far less space on the board than the holes that require more drilling. In a microvia, the layers are connected to one another with copper plating.
Microvias are created in the shape of cones, which makes it easy for the via sides to be copper plated. A microvia can go through two adjacent layers but not any further. If a board design requires a via through multiple layers, multiple microvias would need to be stacked accordingly.
From a manufacturing standpoint, creating stacked microvias can be a costly and time-consuming process. On boards that require one microvia over another, the most common design will feature two microvias. As far as stacking is concerned, the ultimate limit is four microvias. However, four microvias are rarely applied in PCB manufacturing because of the high costs.
An alternative option to the stacked microvia is the staggered microvia, which is set up like a staircase where a second or third microvia is placed a step forward below the one above. As with stacked microvias, staggered microvias can be difficult and costly to produce on a multi-layer PCB.
A microvia that connects two inner layers in a PCB is known as a buried microvia. For a buried microvia to be included in a PCB design, the layers that contain the holes must first be drilled before the outer layers are applied. With a buried microvia, you can connect two inner layers on a printed circuit board. The drilling can be accomplished with a mechanical tool or, alternately, with a laser.