Have you been considering a PCB design with via-in-pad? Via-in-pad design is becoming increasingly popular, and if you’re not familiar with the idea of vias on BGA pads, it may be in your best interests to explore this new printed circuit board design option that seems to be growing in popularity.

WHAT IS A VIA-IN-PAD?

A via-in-pad design, as the name indicates, is a printed circuit board design with the vias directly on the BGA pads. The main benefit of a via-in-pad design, also called VIP design, is that you reduce the area needed for the vias, making it easier to manufacture miniaturized PCBs and dramatically minimizing the amount of board area you need for signal routing. With via holes connected directly to layers beneath the component, you can have signal routing without escaping the device footprint perimeter.

IS IT CONSIDERED BAD PRACTICE TO PUT VIAS ON BGA PADS?

Is this a good practice or a bad practice? Why isn’t everybody doing it? In fact, many people are. It is becoming a common practice to put vias on BGA pads. Why don’t all designers do it? The main reason is that if you put a via in pad, you have to fill it — either with copper or a copper-covered non-conductive material. Non-conductive fill is most popular and more price competitive.  If you do not, the solder will flow away from the pad and you will not get a functional electrical connection.

Filling the vias is an extra step, and some designers may not want to incur the cost and lost time required to do it. Putting vias in pad also affects the drill diameter you will need. Nevertheless, there are many good reasons to opt for a via-in-pad design, which is why many PCB users do call for them, despite the slightly added cost and time commitment.

Next, what are the advantages and disadvantages of VIP vs. traditional via placement?

TRADITIONAL VIAS VS. VIP TYPES ADVANTAGES AND DISADVANTAGES

As mentioned, in a traditional via layout, you can simply apply solder mask to prevent the solder from drawing into the barrel of the via and creating electrical connection problems. But when you have via-in-pad, this will not work. You must completely fill the vias so there will be no air entrapment with resulting outgassing in the assembly phase. You also need a flat planar surface in order to attach fine-pitch BGAs as well as components effectively.

How can you fill these in-pad vias? After mechanically drilling and plating your in-pad vias, you must fill them with epoxy. Alternatively, you can laser-ablate your vias and fill them with copper. Which you choose to do will depend on your specific application and needs when it comes to your printed circuit boards as well as the size of the via. The main issue when deciding your process will be pad diameter. You need to make sure the pad size is large enough for the via diameter while still being able to accommodate manufacturing tolerances and meet the minimum IPC Class 2 or 3 annular ring requirement.

The VIP advantage is that once you effectively place the vias in-pad, you will enjoy some incredible space savings, and this can not only increase your efficiency but may also be required for certain modern applications. If you have a revolutionary application that necessitates space flexibility, VIP can be the ideal choice and may, in fact, be the only choice.

The advantages of via in pad

There are a lot of advantages of via in pad PCB. First of all, It’s good for increasing density, using finer pitch packages, as well as lessening inductance. What’s more, in the process of via in pad, a via is directly placed below the contact pads of the device, which can achieve greater part density and superior routing. So it can save a great quantity PCB spaces with via in pad for PCB designer.

Compared with blind vias and buried vias, there are many advantages for via in pad as follows:

• Apply to fine pitch BGA;
• Lead to higher density PCBs and promote space saving;
• Improve thermal dissipation;
• Provide a flat and coplanar surface with component attachment;
• Lower inductance due to no dog-bone pads with traces;
• Increase voltage capability of the via;

However, you need to confirm that your PCB manufacturer is well-equipped to fabricate your PCBs because it may cost more. If you are not able to place via in pad, putting directly and using more than one can assist in decreasing inductance.

When should I use via in pad?

It will lead to design rule check errors because of trace width, annular ring, as well as size limitations while trying to route and escape component packages with sub 0.5mm pitch with traditional routing methods. For the small pitch components, only to route them with capped via in pad can it make the circuit board routing to be as compact as possible.

And it can simplify routing for complex BGA and LGA packages as well with capped via in pad. And the components like bypass capacitors to be placed as close as possible with minimizing the surface routing so that minimizing parasitic inductance. In addition, the paths to power and grounds planes are short, which will be good for minimizing EMF emissions of high frequency designs.

Vias in thermal pads also can play an effect on heat management. In general, high power surface mount parts have a thermal pad that mounts to the circuit board. So you’d better drop vias through the board to the other side of PCB to increase the copper area for heat release.

Via in pad application for SMD pad

1. Plug the via by resin and plated it flat by copper

It is compliant with small BGA via in pad;

First of all, the process is filling the via hole with a conductive or non-conductive material, and then plating the via on the surface, which provide a smooth flat for solderable surface;

There are used in via in pad designs where it can mount the component over the via, or extend the solder joint to the via connection.

2. Microvias and via in pad plated over

A microvia is a hole with a diameter of less than 0.15mm based on IPC. It can be a through via hole ( related to a aspect ratio ), however, in normal the microvia is regarded as blind vias between 2 layers;

There are a majority of drilling the microvias with laser but some PCB manufacturers are also drilling them with a mechanical drill bit, which is slower, however the holes have a clean and nice cut;

The microvia cooper fill process is an electrochemical deposition process used for the multilayer PCB fabrication process, it also known as capped vias;

Although the process is complicated, it can fabricate HDI PCB that most PCB manufacturers will get the copper filling of microvias.

3. Plug the via by solder mask

It’s free and compliant with big solder SMD pads;

The standardized LPI solder mask process can not form the fill vias without the risk of exposed copper in the hole barrel. In general, it can deposit UV or thermally curable epoxy solder mask into the holes to plug them after using the second screen print;

It is known as via plugging. Via plugging is used to plug via holes with a solerrsist material so that protect air from leaking as testing the boards, or prevent the components near the board surface from shorting.

Via in pad PCB

In PCB design, via is a pad with a small plated hole in a printed circuit board which are used for a connection between copper tracks on a various layers of a board. There is a via known as micro vias, which have apparent blind vias only on a single surface for high density multilayer PCB or invisible buried vias on either of the surface. What’s more, there is a new challenge after bringing in and being widely used the high density pin out parts, as well as the need for small dimension PCB. So the better solution to meet the kind of challenges is using the latest but popular PCB manufacturing technology called “ via in pad”.

It needs to quickly use via in pad in the current PCB design due to the constantly reducing pitch of part footprints, as well as the need for miniaturizing PCB form factor. What’s more, it can achieve signal routing in as little an area of the PCB layout as possible, in most instances, even avoiding escaping the perimeter of the device footprint.

Via in pads is mostly useful in high speed designs as they reduce trace length and consequently inductance. You’d better check whether your PCB manufacturer is well equipped to fabricate your board or not, because it may spend more money on it. However, if you can’t place via in pad, directly putting and using over one to decrease inductance.