Blind Vias

Blind vias are plated through hole connections that pass from an outside layer and end on an internal layer. They do not pass all the way through the PCB like a conventional via hole. Blind vias aren’t cost effective, unless used for a technical reason such as permitting the fan out of tracking from a fine pitch foot-print. The use of blind vias just to reduce the PCB layer count will not save costs.

This is an example of a 4 layer PCB with blind vias between layers 1 and 2 as well as 3 and 4, together with through holes connecting all layers from 1 to 4. We can produce a wide variety of blind via pcbs. We recommend that your proposed construction is checked for compatibility with our manufacturing process before design finalisation. Send us details and ask us to quote.

Newbury Electronics will advise on track widths, copper layer spacing, copper weights, and the base materials to achieve the impedance required. We use industry standard software to assist with impedance calculations and selection of appropriate PCB construction.

We design suitable test coupons and verify calculated impedance with actual production outcomes using Time Domain Reflectometry (TDR) measurement instruments. Our high resolution digital microscope allows us to physically measure all relevant PCB features in all axes and verify the construction.

There are two basic ways to manufacture PCBs with blind vias. These are either by laser drilling or sequential layer build. Blind via holes may be drilled by a laser through a thin layer of insulating dielectric, usually stopping where the laser beam meets an internal copper pad. Laser drilled blind vias have limitations because the drill hole is small, its depth can be no more than its diameter to ensure adequate penetration of the copper plating chemistry. Therefore, it can commonly only penetrate through just one layer starting from the outside layer.

In the sequential build method, pairs of layers are drilled and plated before bonding. Because these pairs of layers are plated with the hole open at both ends, the plating chemistry penetrates the hole quite easily and the blind via can be designed to pass through multiple layers. By combining the appropriate sequence of bonding, drilling and plating, a wide variety of blind via constructions is possible. This depends on if the blind via can pass through an even number of layers, starting with an outside layer.

Buried Via PCB

Buried vias are plated connections between the inner layers of a multi-layer PCB that do not emerge on either of the outer layers. Buried vias are created by drilling and copper plating the inner layer core (pairs) of a multi-layer PCB in the same way as a standard 2 layer or indeed for a multi-layer plated through hole PCB.

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There is no problem with the depth of the buried via because at the time of plating, the hole is open at both ends and the plating chemistry can freely penetrate. If the buried via PCB is designed with buried vias passing between even numbered pairs of inner layers, construction is straightforward. It is not recommended to design buried vias passing between odd numbers of inner layers.

Sequential build up of multi-layer PCBs using multiple printing, bonding, and plating sequences allow complex inter-connection structures to be built.

If careful consideration is taken with the practicalities of fabrication, then blind vias, buried vias and through hole vias may be combined on the same PCB.

Buried vias are less costly to manufacture than blind vias, but as a guide, the use of buried vias is unlikely save costs unless the overall layer count is reduced by at least 4 layers.

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