High-density interconnect (HDI) circuit board designs have higher routing and pad density than traditional PCBs, as well as smaller trace widths and space. They require advanced PCB technology such as blind vias, buried vias and microvias. Due to the complicated construction process involved in the manufacturing process, HDI PCBs are generally more costly than traditional PCBs.

Although HDI PCB manufacturing can be expensive, in some cases, blind vias, buried vias, and microvias can provide cost-effective solutions. The following are several situations where advanced PCB technology can actually reduce overall manufacturing costs.

Scenario 1: Reduce the number of layers by adding a broadband BGA distribution channel with blind holes to reduce the number of layers of the bottom layer and inner layer channels with blind holes.

Consider the picture below. The top circuit board has three rows of through holes, leaving only four traces between the first and third rows.

In contrast, in the case of the bottom circuit board, the middle via has been replaced with a blind hole. The same channel now allows 6 traces, which is equivalent to an increase of 50%. This technology allows you to reduce the number of PCB layers, thereby reducing costs.

Scenario 2: Eliminate the electrical layer by replacing vias with microvias. Microvias have the smallest pad size (as low as 0.008 inches), which helps maximize the wiring channel width. As in the previous situation, replacing vias with microvias can alleviate BGA breakthroughs and reduce the number of PCB layers. When removing the internal signal layer, sometimes its reference plane can also be removed, thereby removing two layers.

The figure below shows a technology that can be used to maximize the breakthrough between 1.0mm pitch BGA microvias:

If the microvia is placed to obtain the maximum breakthrough, the gain will be 67%. If the micro-holes are placed on the same grid as the through-holes, the breakthrough gain will be 33%.

Scenario 3: Use blind vias and buried vias to reduce the aspect ratio of the PCB. Generally, a PCB will have several BGA components with different pitches. For example, there may be a 1.27mm pitch BGA and a 0.8mm pitch BGA on a 4.0mm (160mil) thick PCB. The smallest via is not only determined by the size of the hole, but also on the aspect ratio, that is, the thickness of the PCB divided by the diameter of the hole:

Aspect ratios higher than 10 or 12 usually incur additional manufacturing costs. (Please note that the finished hole size is approximately 2 mils smaller than the drilled size; this is due to the width of the barrel plating.)

If through holes are still needed, maintaining a low aspect ratio may be a challenge even after using blind vias/micro vias/buried vias. An aspect ratio of 12 on a 4 mm board means that the minimum finished via size is 0.3 mm (12 mils), which corresponds to a hole size of 0.36 mm (14 mils) and a land of 0.56 mm (22 mils) size. This is no problem for 1.27 mm pitch BGA, but it will cause concerns about 0.8 mm pitch BGA, because when 0.4 mm (15.7 million) BGA pads are used, the gap between the via pad and the BGA pad is only 0.09 ( 3.4 mils).

In order to meet the aspect ratio requirement, the PCB thickness must be reduced as follows:

If you reduce the number of layers by replacing through holes with blind vias and micro vias, you can achieve a smaller PCB aspect ratio and avoid any cost loss caused by the use of blind vias/micro vias.

These are just three situations where advanced PCB technology may be cost-effective for your design. With the continuous advancement of PCB manufacturing equipment and materials, more unique situations will soon be discovered.

— Pi Zhang is a senior design engineer at Nuvation Engineering, a design company specializing in new product development.

Fact: Blind and buried via technology will not reduce the cost of the circuit board. It just makes the manufactured circuit board take up less space, otherwise it cannot be built using standard through-hole technology.

Thanks for your comment! As the article describes, using blind and buried vias can reduce the number of layers and aspect ratio (just to name a few benefits), which will have a great cost impact on high-level TH PCBs. For a 16-layer PCB, HDI technology may not be cost-effective, but when the number of PCB layers reaches 20+, I found that a) the use of small TH vias is limited by the board aspect ratio b) BGA with a pitch of 1.00mm or larger is correct Required for breakthrough. This leads to a larger PCB size. Blind vias/micro vias/buried vias can play a role in reducing the number of layers and PCB size.

The cost model of HDI PCB is not as simple as TH PCB because it is closely related to HDI layer stack design and differs between boards with the same number of layers but different HDI layer stacks. Although HDI technology may be the most cost-effective choice for a product, you are absolutely right that HDI technology itself is more expensive. I think the more accurate title is "The cost-effective option for high-level TH PCBs using blind, buried, and microvias."

Thank you for your response. I totally agree with everything you said in your reply. This article is well written (except for the title) and solves the aspect ratio problem, which is an important issue for high-density boards using micropores. For the average PCB designer, the issues surrounding the aspect ratio have not been fully resolved or explained in sufficient detail. Thank you for taking the time to reply.

My own blind hole method is driven by another requirement:

We must get a sealed sensor module to prevent external moisture. In fact, the track on the top layer does not work well when using O-ring seals:(

The solution is blind holes-no water leakage and no additional cost of plugging the vias.

I hope we start using blind vias (4-layer PCB) soon, because it will soon become clear that they provide you with a lot of space to route on the unaffected 2-layer. If you don’t need a complete power/reference layer, you can actually have 2 separate 2-layer boards for wiring,

It will never be gone again.

Good summary! Just a little supplement. According to my experience, for a BGA pin fanout with a pitch of 1.0mm or smaller, the maximum layer reduction caused by blind holes or microvias actually depends on the order of the vias, although the line spacing increases more. For single-stage blind vias based on secondary lamination or single-stage micro vias from HDI, the number of layers will only be reduced by the signal layer or a few layers.

Therefore, simply reducing the number of layers to save costs is usually not enough to make HDI solutions cost-effective. For the effect of aspect ratio, it uses PCB thickness exceeding 3 mm or more than 24 layers than 24 layers and 1.0 or less pitch BGA applications.

For other application scenarios, as well as storage devices such as double-sided mirror layout BGA, in order to maximize the layout space.

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