As printed circuit boards get smaller, through-hole components are used less and less. It becomes increasingly difficult to justify the use of space for relatively large plated holes and pads. Surface-mount components should be used as much as possible. Surface mount technology is becoming more popular, and the majority of through holes are now plated on modern PCBs.

Through-holes on PCBs are used to conduct an electrical signal between circuit layers. There are many different types of vias and not all vias will look the same on the PCB. Each type of via must be clearly defined in the documentation, even though they all serve the same purpose. This will ensure that the PCBA is assembled correctly and operates properly.

Through-Hole Structures: Straight-Through, Blind-Through, Buried

A is the first type of visa.

The Via

It is a hole which goes from the top to the bottom. The hole is open on both ends so that the plating fluid can flow through it and coat the wall of the hole to make the conductivity. Visas do not need to be drilled, as long as all manufacturer rules are adhered to. These include minimum diameters, maximum aspect ratios (board thickness divided into hole diameter), as well as contiguity.

Next, mechanically drilled

Blind Vias

Blind vias are drilled through a PCB from either the top or the bottom layer, but stop at a certain point. Blind vias mechanically drilled can connect an outer-layer to an adjacent layer, and in some cases another layer below. However, careful planning is needed to achieve good results. Blind vias, unlike through holes, are only open at one end. This means that plating solution can't flow through. This can complicate the plating process.

Air bubbles trapped in the bottom of the holes can cause voids with no copper coating. For voids to be eliminated, use larger holes with smaller aspect ratios and more vigorous plating. This will allow the air bubbles in the hole to escape, exposing its walls and allowing reliable plating. When using vias, you should be aware of the design rules set forth by the manufacturer. High yields are achieved by combining good design with process control.

Next, mechanically drilled

Buried Vias

Only internal layer structures can be connected using these. The holes are drilled through the internal structure from top to bottom (such as L2-L7 on an 8-layer PCB), before they are filled and plated in preparation for final lamination.

Blind vias can be used to connect the inner layers only before lamination. Laser microvias can be used to connect small outer-layer features (such as BGA pads or QFP pads), with buried structures in certain high density applications. The "stacked" structure has the same effect as a through-hole, but takes up less space on the outer layer than a mechanically-drilled via.

Laser Micro Vias are the smallest visas, usually about 0.003"-0.004" in diameter. Micro vias are able to fit in very small pad areas. They can be used as via-in pads within SMT packages or BGA packages with a very tight pitch. After plating, the surface of the pad is restored to its original state. This allows it to be used for component welds.

Laser microvias have a maximum aspect ratio that is typically less than 1:1. For most applications it is therefore only feasible to connect one layer with its adjacent layers by using a thin dielectric sheet. They can be stacked in an orderly stack-up, similar to the way buried vias are stacked.

The vias must be laser drilled and then plated to create an interconnect between the outermost layer and the layer beneath it. After these steps, a second layer is laminated on the outside of stack. The new layer is blinded via drilling and plating after lamination. This is the process that connects the outermost layers to the next layer. If necessary, this process can be repeated three to four times.

How to Fill or Cover a Vial:

It is sometimes desirable to apply additional treatments to vias at a later stage in the production process to improve thermal performance. This may include an epoxy hole filling mask or an auxiliary welding shield.

These extra steps are designed to eliminate problems with assembly, such as shorting of the weld between component pads and via pad or welding migration down a via barrel that has been drilled in a component's welding area. These problems can lead to costly troubleshooting, rework and repair. By specifying the correct treatment, you can eliminate most problems.

The pads at either end of a tented hole are covered with a nonconductive welding mask. When the dry-film welding mask was in its prime, tenting was very popular. The 0.004 inch thickness of the film allowed it reliably to tent larger holes without cracking. Dry film masks, however, are no longer in use because they interfere with surface mount welding. Dry film masks have disappeared and are no longer used. Modern LPI welding helmets, which are about a tenth of the thickness and do not create a tent, can now be used instead.

The pad of a tented via is covered with a mask that covers most of it, but the mask is just a few thousandths away from the hole. This is a great compromise for medium-density boards, as it allows you to avoid doing nothing. A welding mask increases the distance between a via and a nearby solderable surface, which reduces the possibility of welding bridges from the pad into the via. The hole is not sealed, so there are no concerns about air pockets or contaminants forming inside the barrel. The size of the opening on the Gerber file for the affected hole should be the diameter + 0.004. Example: 0.010 via, 0.014 Gerber mask opening.

Filling holes is also known as button prints or plugged vias. The fillings prevent the welding from migrating (flowing) and ensure that the right amount of welding is applied to the pad when assembled. For these processes, non-conductive epoxy is recommended. To ensure that the epoxy can fill the hole, limit the diameter of the vias to 0.020 inch. To use these files, you will need to provide a welding mask that does not have the visa.

Active pads plug not only the hole, but also the board. They are also known as "covered-in pad" or VIPPO. If the pad used to drill a via is later used to weld an surface-mount component, then they will be needed. The via dissipates heat by drawing it to the opposite side of the board. This is useful to cool hot components.

In the active pad process, an insert is pulled through with a vacuum and then flattened. Then, it's over-plated. The surface of the SMT pad is often indistinguishable to other SMT pads which have not been drilled. To create this type of Gerber file, you will need to provide the openings for welding masks without any associated via.

How to Define Visa Requirements:

In order to avoid PCB problems, it's important that you clearly specify the requirements for via TYPE and TREATMENT within your manufacturing documentation. This will ensure you get the desired results. Provide a separate file in all cases for each via set used in your design.

Please Provide Your Manufacturer with the Following Information for TYPE:

Types of visa (througn, blind, or buried)

Via Diameter (we suggest between 0.008" - 0.020").

Tolerance (Usually +/- 0.003" although smaller holes or blocked vias can be +/- 000/hole diameter)

Connect two layers per drill file (if they are blind or buried)

Example: Drilling blinds 0.008" from the TOP layer to the GND layer. 0.008" finished + 0.000'/- 0.008".

Please Provide the Following Information When Contacting the Manufacturer for Treatment:

As per the Gerber file, do not fill any holes in all vias of _____ diameter. Use a welding helmet and dip them into pads.

Plug vias (partial Fill): Use IPC Class 4 or button print treatment with nonconductive epoxy to fill all holes of _____ diameter.

Use IPC Class 5 with non-conductive epoxy to fill all holes larger than _____.

Fill and cap all _____-diameter vias according to IPC Type VII. Fill the holes, cover them with epoxy, and level. Surface-treat pads.

Strive to Achieve the Best Results

Shenzhen Yt-electronic: A PCB manufacturing instructions document that is unclear can delay the quotation or affect the delivery of the bare PCB. It may also complicate the assembly process. The advent of high density designs has made it more important than ever to have clear requirements on via type and treatment. Remember that different PCB manufacturers use different equipment and materials. You could end up providing Gerber welding masks without via openings to one manufacturer and expecting another to deliver the same board. You should instead clearly specify all through requirements and make sure that your data files are compliant. By accurately describing your project, you can avoid unpleasant surprises.