When it comes to high density PCBs where you cannot fit all your connections on one layer, vias are extremely useful.
Simply put, vias are barrel shaped vertical conductive holes that help make connections between multiple layers of a PCB.
Blind and buried vias
The two vias are popular when you need to make connections between the multiple layers of a printed circuit board that are blind and buried vias.
To put it succinctly, a blind via connects the outermost layer of the board to one or more inner layers, but does not go through the whole PCB. Blind vias free up space and works well for fine pitch BGA components. They also help reduce signal stubs through the drilling process as the via terminates at the last connected layer.
A buried via, on the other hand connects two or more inner layers but doesn’t penetrate through to the outer layer. As their name suggests, they are hidden beneath the layers and free up surface space without impacting the traces or surface components on the top or bottom layers.
Both blind and buried vias are of tremendous use in High density interconnect PCBs. Such PCBs offer multiple advantages including but not limited to:
- High layer density
- Better power delivery
- Ability to use smaller pitch devices
Types of blind vias
There are broadly four types of blind vias
- Photo defined blind vias.
- Sequential lamination blind vias.
- Controlled depth blind vias.
- Laser drilled blind vias.
Photo Defined Blind Via
This is created by laminating a sheet of photosensitive resin to a core. The layer of photosensitive material is covered with a pattern that covers the areas where the holes are to be created. It is then exposed to light that causes the material on the PCB to harden. After this, the PCB is immersed in an etching solution. Post the etching process the copper is plated in the hole and on the outer surface. These vias are typically used to create BGA packages.
Sequential Lamination Blind Via
This is created by processing a thin piece of laminate. The laminate is drilled, plated and etched. The other side is left as a solid sheet of copper. The subassembly is then laminated. The combined lamination is processed to create the outer layers of a multilayer PCB. This process needs some extra steps, hence the price of this kind of via is high. Additionally, there is also yield loss that comes with handling thin laminates.
Controlled Depth Drilled Blind Vias
These are created like through-hole vias. The drill penetrates only someway through the PCB. Care is taken to see that no features below the drilled hole come in contact with the drilled hole. Copper is hen plated in the drilled hole and the through-hole vias. These vias are least expensive. However they come with a limitation that the holes must be large enough for the mechanical drills to create them and the area below them must be kept clear of circuits.
Laser Drilled Blind Vias
These are created once all the layers in the PCB are laminated and before the outer layer is plated and etched. With the use of either CO₂ laser or Eximer laser the copper on the outer layer is ablated.
The CO₂ laser is extremely powerful and can quickly drill the holes. However, with this laser the drilling step has to preceded by an etching step. The eximer laser can drill through both the copper balance in PCB and the underlying dielectric material to form a blind via.
The Advantages of Blind Vias
Blind vias help to reduce parasitic capacitance. This is done by shortening the length of the vias and by reducing their diameters. Blind vias are therefore a great way to connect signal lines operating above 4.8 Gb/s.
Some of the limitations of blind vias include:
- A blind via is a blind hole in the PCB. In order to ensure that the plating is done correctly, the diameter of the hole should be equal to its depth. This requires an aspect ratio of 1:1 or less. Some fabricators require a diameter that is 1.5 times the depth.
- With a blind via the drilling needs to stop at the intended layer. With a laser drill, while the drill needs to go through copper on layer 1 in should not drill the copper connecting pad that is in layer 2. This requires careful calibration.
- Another issue is to do with soldering a component to a pad into which the blind via is placed. When solder paste is applied to the pads, the air in the blind via is trapped under the hole that cannot weaken the joint.
Aspects to keep in mind in making vias
Vias can be made before or after multilayer lamination. One or more cores are drilled and through holes are plated. The stack is then built and pressed.
In creating blind vias, it is important to be aware of the drill depth. If the hole is too deep it can impact the integrity of the signal and if the hole is too shallow the connection wouldn’t be proper.
It is also important to plug the vias with either metal or thermal/electrical epoxy and plate it with copper. In the absence of this, bubbles can move upwards and impact the solder joint.
The other aspect to keep in mind is that the vias must cut through an even number of copper layers. Also, if the via is completely enclosed within the terminal point it can lead to a significant increase in costs.
While discussing Blind & Buried Vias it is also important to understand what stacked & microvias are:
A stacked via further helps in improving the size & density considerations of the PCB. It therefore greatly aids miniaturization and high-signal transmission speed.
If you have blind vias with an aspect ratio of greater than 1:1 or if your drilling needs span multiple layers, a stacked via is needed. These are laminated blind or buried vias. Not only do stacked vias save space and help in high-density, they also offer greater flexibility when it comes to the inner connections along with better routing capacity and less parasitic capacitance.
A microvia is a small via that offers more routing space on the board and lower parasitic capacitance. However, such Vias requires more drilling time.
To sum up
The main benefits that blind and buried vias offer, include:
- You can meet the density requirements and constraints for lines and pads on your design without increasing the overall layer count or the size of your board.
- They help in saving space and meeting extremely tight design tolerances.
- They help designers manage the hole-to-pad ratio and restrict chances of breakout.
The issue with blind and buried vias include:
- They increase the cost of the board. This is because the board is complex and many manufacturing steps are added. Also testing and precision checks need to be added. Between the two, however, buried vias offer a lower cost.
It is important to choose the right electronic contract manufacturing partner who has the requisite experience with blind and buried vias.
Technotronix is one of the leading PCB manufacturers based in the USA. We have adopted advanced methods and technologies, which can give you the finest PCB manufacturing services. Our ability to deliver, unparalleled quality as well as customer service, is what makes us stand out. Both on-time delivery and price competitive products are our USP. The most important element of our success, however, has been the relationships we have developed with our customers.
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