Using vias in a PCB design allows the designer to shorten the distance that a trace must be routed in order to complete its connection. Vias are metallic lined holes connected to the metal circuitry of a PCB that conduct an electrical signal between the different layers of the board. Although vias can vary in their size, pad shapes, and hole diameters, there is only a handful of different via types or structures:
Thru-hole via: This is the type of via that is used most often in a circuit board. The holes are drilled all the way through the board with a mechanical drill bit and can get down to 6 mils in size.
Buried via: This via only connects internal layers of the board and is useful for PCBs with very dense routing.
Blind via: This via starts on either the top or bottom of the board but doesn’t go all the way through it.
Microvia: For hole sizes smaller than 6 mils, a laser-drilled microvia is used. These vias connect only two adjacent layers of the board and can be on the surface or buried within the board layer stackup. Microvias are extremely versatile and can be stacked together, or on top of a buried via, but have a higher fabrication cost associated with them.
Via-in-pad (VIP): These vias can either be standard thru-hole vias or microvias, but their position in a surface mount pad makes them unique. If a standard mechanical drill is used, the via will require extra fabrication steps to prevent solder on the pad from flowing down through the hole. Microvias on the other hand don’t have this problem, but they can be more difficult to fabricate due to the tighter trace and space tolerances in a high-density design.
Ultimately, which type of via to use depends on the technology of the printed circuit board, the circuitry needs, and the targeted cost of PCB fabrication. For instance, a microvia is very desirable to use because of its smaller size, but that doesn’t necessarily make it the best choice. A microvia has more steps involved in its fabrication and, therefore, is more expensive compared to a mechanically drilled thru-hole via. But if you are designing a high-density interconnect board, the microvia becomes the better choice. Here is a breakdown of the common uses of vias in PCB design:
Signal routing: Most circuit boards will use a thru-hole via for signal routing placed on a grid. Denser boards, however, may also use blind or buried vias, while very dense boards will need microvias.
Escape routing: Larger surface mount (SMT) components can usually have their escape, or fanout, routing done with thru-hole vias. In some cases, blind vias or microvias will be used, and on very dense packages such as high pin-count BGAs, a via-in-pad will be used.
Power routing: Since the vias used for power and ground nets will conduct more current, they are usually restricted to larger thru-hole vias, although blind vias may be used as well.
Stitching vias: These vias are used to provide multiple connections to a plane and are, therefore, thru-hole or blind vias. For example, a sensitive area of circuitry may be surrounded by a strip of metal with vias stitched in it to connect to a ground plane for EMI protection.
Thermal vias: In this case, a via is used to conduct heat from a component out through the internal plane layer that it is connected to. This usually requires a larger thru-hole or blind via and these vias are often in the pads of these devices as well.