Do Blind Vias Cost More?

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Blind vias are used to increase the pad density and wiring of a high density interconnect circuit. Designers often combine blind vias with buried vias and microvias to implement their designs without sacrificing precious real estate on their circuit board. However, the use of blind vias can add to the overall cost of the board.

In general, blind vias can be affordable to work with as long as the designers is creative enough. In other words, blind via technology will not reduce the cost of working with HDI PCBs. It will, however, enable the miniaturization of the board that may not be possible using conventional thru-hole technology.

There are many ways where technologies like blind vias can reduce the overall manufacturing cost.

high density interconnect

Layer Count Reduction to Widen BGA Breakout Channel

The trace escape for BGAs will not go smoothly using through-hole vias. You can use blind vias to widen breakout channels on the inner and bottom layers.

Here’s how it works. Suppose your board has 8 layers and a BGA with 300 pins. Half of the pins will be power and ground, and the other half will be just ground.

These grounds would usually pass through vias which can lead to routing congestion. However, we can use one blind via to connect layers 1 to 2 and a second blind via to connect layers 7 to 8.

Adding these blind vias now opens up additional routing channels and would allow you to reduce the number of layers from 8 to 6. The removal of these layers will lower your manufacturing costs because you have fewer materials to work with. Another plus point is that you can reduce crosstalk and electrical interference.

Reducing Aspect Ratio of the Circuit

PCBs often have BGA components with different pitches. For example, there may be a 0.3mm pitch BGA and a 0.8mm pitch BGA on a circuit that is 4mm thick. We use the drill size and aspect ratio to determine the minimum via hole.

The aspect ratio is the PCB thickness divided by the diameter of the drilled hole. Aspect ratios higher than 10 often incur an additional manufacturing cost. It may be a challenge to keep the aspect ratio low if through-hole vias are being used.

To keep the aspect ratio on the low end of the spectrum, the micro via PCB thickness can be reduced by replacing through-hole vias using microvias and blind vias. The cost of using blind vias is lower than the material cost of working with aspect ratios higher than 10 or 12.

These were just a few scenarios where blind vias can be cost-effective for working with HDI PCBs.

This isn’t to say that blind vias are always affordable. There are scenarios where it can incur additional manufacturing costs, which can be troublesome if you are working on a tight budget. You can, however, use appropriate drilling technologies to keep costs low.

Here are some technologies to drill blind vias in your PCB.

Laser Drilled Blind Vias

These blind vias are created before etching and plating the outer layer and after laminating all layers of the micro via PCB. An ultrashort pulse laser is used for the ablation of copper on the outer layer and the insulating material between the first two layers.

Designers can choose from two types of lasers in this process:

  • Eximer laser
  • CO2 laser

CO2 laser is more powerful and has the ability to quickly drill the right holes. The only problem is that the laser's wavelength will not be sufficient for the removal of copper on the first layer.

Not only does it add another processing step but it also results in an alignment problem because the photomask must be properly aligned with pads on layer 2 that are not visible when the laser is in use.

An eximer laser can drill through the dielectric material and copper to form a blind via at the same time. This is why eximer lasers are chosen by designers because they don’t require the copper layer to be predrilled.

However, designs must exercise caution when setting the laser up to ensure that the hole passes through the underlying dielectric and outer layer copper without cutting the copper pad found on layer 2.

Sequential Lamination Blind Via

Sequential laminate blind vias are created by processing laminate through all the steps involved in making a double layer circuit. The laminate cycle will define the features of the side that will form layer 2 of the micro via PCB. The other player is kept as a solid sheet of copper and will be used to form layer 1 of the circuit.

This is followed by lamination of all the other layers of the circuit. The resulting lamination is then processed through all of the steps involved in creating the external layers of an HDI PCB.

Sequential blind vias are the most expensive way of forming blind vias because of the extra steps required. There is also a high yield loss associated with handling the laminates. Sequential blind vias should be used as a last resort when all other processes are not available.

Alternative to Blind Vias: Back Drilling Vias

An affordable alternative to blind vias in HDI PCB design are back drilling vias. While blind vias require additional drilling and plating applications for inner layers, it is much cheaper to drill and plate all layers of a micro via PCB.

This approach isn’t without its disadvantages. It comes with undesirable cross talking and electrical interference caused due to the unconnected parts of the vias. These parasitic effects can be eliminated by drilling out unused via stubs within a defined depth at the final phase of board assembly. This process is known as back drilling.

Here are a few tips of working with back drilling vias: 

  • Define the maximum admissible stub length in your multi PCB design software
  • Define drill hole specifications
  • Specify drill layer and drill diameter     

Pro tip: Experiment with multiple design software to find the right one for you.

Wrapping Up

So do blind vias cost more? It really depends on the application and how you created the blind vias. Skilled designers can find ways of saving costs while working with blind vias.
For more information, get in touch with the experts at Hemeixin HDI PCB.

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