Via stands for Vertical Interconnection access. Vias help in transferring signals from one layer to another layer of a PCB. Let’s suppose the tracks exist on the x-axis and the y-axis of a PCB while vias are present on the z-axis. Generally, they consist of tiny vertical pipes with a coating of conductive layer and put in holes. These holes also use different kinds of filling, which ensure high electrical and mechanical strength.
In a traditional method, if a pin is used to connect vias, it creates tracks that help in routing the signal far away from the pads. Moreover, this method does not suit well for Surface-mounted dense PCBs, BGAs, ICs, and QFN.
This article helps in understanding the Via-in-pad in detail. It describes the vias types depending on their depth. Moreover, this article will surely help you in understanding the development and usage of via-in-pad in various devices and applications.
Introduction
Gordon Moore, the CEO of Intel, in 1956, indicated that the use of transistors in one silicon chip would double every couple of years. However, it was only the beginning. Over the passing years, the component size keeps decreasing.
However, in the late 1980s, the innovation of I’d and the constantly decreasing size of electrical components gave rise to the uselessness of manually designed PCBs. This led to the manufacturing of PCBs with multilayer, flexible variants and high power to boost efficiency.
Printed Circuit Board offers a platform and mechanical support to place components over them and create a connection between them. Copper tracks help in creating connections between components in a single-sided PCB while it needs via for transferring signals from one surface layer to other surface layers.
Different Types of Vias On Pads
Vias come in variant forms depending on their usage and the type of connections they create. Let’s dive into the details of some most commonly utilized vias.
1. Through-hole Via
Through-hole via helps in shorting almost all PCB layers at a certain point. It creates a hole through drilling and then uses conductive copper to coat it. This helps in connecting all PCB layers from the outer layer to the other outer layer, all from top to bottom.
Because of its simple construction and structure, through-hole vias come in the most common category in the manufacturing of a PCB. Among all types of vias, through-hole vias are the simplest and cheapest to create.
However, a revolution in the advancement of accurate machines developed special types of vias that allowed designers to leave certain layers of a PCB. These specialized vias comprise buried, blind, via-in-pad, and microvia.
2. Blind Via
Blind Vias help in creating connections between the external layer and more than one internal layer. These types of vias can only be visible from one side while invisible from PCBs’ other side because blind via cannot go through the whole PCB. This gives them their name.
Blind via seems more expensive and complicated to make than through-hole via.
3. Buried Via
Buried Vias help in connecting more than two internal PCB layers. These types of vias do not make connections with the external layer. Thus, one cannot see it from the outer side. Buried vias are only used over the surface of a PCB with more than four layers. This also makes it more difficult to create them than blind vias.
4. Microvia
Microtia refers to traditional vias but in small sizes with certain changes in the structure. Microvias give a similar look to a frustum instead of creating a cylindrical shape. The radius of a micro via keeps decreasing with its transitioning from one PCB layer to the other.
Designers create micro vias to specifically connect two or more layers. However, for creating connections of multilayer PCB, manufacturers place micro vias over the top of one another and stack them. Buried micro vias typically use filling like epoxy conductive substances, which enhance their durability from mechanical pressure and stress.
What Is A Via In Pad?
Via-in-pad, as the name suggests, is put inside the pad of electronic components on the PCB surface. They create connections between external layers to all other PCB layers.
However, to understand the Via-in-pad necessity in a PCB, let’s dive into applications with high-frequency and HDI PCBs that use ICs along with a package of BGA.
Applications of Vias On Pads
PCB designers mainly use vi-in-pads in the manufacturing of PCBs when saving space concerns them the most. Via-in-pad is also widely used with tiny BGA package components as they do not allow other vias to fit in it.
Moreover, BGA packages help in fixing and placing the tiny and compact surface-mount components with various amounts of pins. Components pins in the packages of BGA arrange in a systematic matrix order in which each pin has a tiny solder ball. Contrary to other widely used packages like Dual-in-line, the BGA package allows higher packing ruggedness and efficiency.
The solder ball size and the distance among the closeby BGA pins must be as tiny as a millimeter fraction. This makes the routing of surfaces complicated. Moreover, tracks with smaller trace widths are put out from each pin in an appropriate place for creating vias. Although, this does not seem to be an ideal approach.
In electrical components in which the distance among the pins is smaller than the secure track clearance, then it may lead the PCB design system to give warnings.
Regardless of routing all pins far away and creating a via away from an electrical component, you can directly put the via right under the BGA pad. This helps in carrying the signals through the pins of components to a specific layer of PCB.
Advantages Of Vias On Pads Routing
Via-in-pad offers various benefits, which include.
· Space Saving
Saving space falls under the primary reason for utilizing via-in-pad in the PCBs. Regardless of sending signals away from a pad in traditional vias, via-in-pad makes a direct connection with the components under the pad. This leads to saving a lot of space. Moreover, it also eliminates the need to create dog-bone trails, which helps in routing optimization and significantly minimizes the whole footprint of a PCB.
· Easier Routing
Putting the via right beneath the pads directly not only save space on the PCB but also enables the process of routing simple. Particularly for tiny components with tiny footprints like the BGA package.
· Improved Heat Dissipation
Via-in-pad can effectively enhance the dissipation of heat for the power source, which mounts over the surface of a PCB. Nonetheless, putting a via closer to the heat or power source significantly enhances the thermal conduction between distinct PCB layers and components. Thus, it allows heat to dissipate faster.
· Reduce Parasitic Inductance
Via-in-pad helps in minimizing the associated inductance by eliminating extra connection parts. Additionally, via-in-pad also enables designers to put bypass capacitors nearby to the electrical components, which helps in reducing the inductance. Therefore, for high-speed interfaces and designs, it mainly helps in the enhancement of signal integrity.
Drawbacks of Vias On Pads Technology
Via-in-pad does have various downsides which affect the functionality of a PCB. Therefore, one must know them to create a functional and error-free PCB design. However, a PCB designer must go through these factors before considering any design for the via-in-pad.
· Higher Manufacturing Cost
The creation of a via-in-pad mostly needs more than a hole drilling and filling it with a layer of conductive material. Via-in-pad needs a type of filling, especially conductive filling or conductive epoxy, to enhance its mechanical strength. However, this may add up in the cost because of the material and some additional steps of the assembly process. The price of via-in-pad manufacturing does not go down with an addition in the manufactured units, which literally increases the connected cost.
· Surface Bumps
When the vias do not place and fill appropriately, it creates surface bumps. However, this surface regulation leads to complications in the components soldering process particularly in ting components with a clear connection. Therefore, while making via-in-pad, PCB manufacturers must make sure that surface bumps must be eliminated.
· Increased Design Complexity
Via-in-pad is quite hard to install and design.
Conventional Vias Vs. Via-in-pad
Although the process of installation of via-in-pad is quite similar to the installation process of the traditional via. However, designers must consider a few factors while designing the PCB and its fabrication process. These factors include:
· Verifying PCB Manufacturer’s Design Capabilities
Manufacturers of PCBs set up brief descriptions of their manufacturing process capabilities. However, before choosing a specific design of a PCB, a designer must go through the website of manufacturers to understand distinct parameters and factors. This includes the minimum size of an annular ring to prevent rule check of design or DRC and manufacturability design or DFM issues at the after stages.
· Extracting All The Required Files From The PCB Design Tool
Reviewing and evaluating the PCB design after exporting it from the EDA or Design Automation tool helps in solving issues and confusion. Additionally, the file which contains drill maps gives an extra file that contains a via-in-pad map. This ensures that conventional vias will not get filled and capped.
· Via Capping
Via tenting or capping is a type of coating process using a solder mask. This helps in resisting the solder paste or solder. Via tenting or capping does not consider an ideal choice for via-in-pad because solder masks cannot apply over pads. Therefore, while manufacturing PCBs, manufacturers need to create capped vias beneath the pad. This aids in preventing the solder mask from sliding inside the holes of the vias. For instance, in some cases, when vias are put beneath the thermal pad, this dissipates the heat better only if the via is uncovered or hollow.
· Creating Precision Vias On Pads In Your Design
You may force the design systems of a PCB to insert vias inside the pad. However, this may lead to various checking errors of design rules if the system does not set up to a specific setting according to the technology. You have to ensure that the design software must support this sort of via-in-pad technology, then make a via according to it. After that, set all the necessary rules and configuration of the via-in-pad inside the design software. This helps the systems to accept vias as you inset them.
To design an efficient design of modern PCBs, one may need a design system of a PCB. Make sure the system offers the necessary features to create various structures and shapes of vias. Also, it must have editing capabilities of rules to create your via-in-pad design.
However, PCB designer Allegro serves as an ideal choice for this sort of work. It has advanced features via pad stack. Additionally, it owns a constraint editor and fully functions features of design rules. This also allows you to use any type of technology in creating the design of your PCB.
Conclusion
Vias are mainly used for transferring signals electrically between the various layers of a PCB. Via can be put directly under the pads instead of putting a conventional via, which keeps the signal far away from an electrical pin. Via-in-pad helps in saving space, enhances thermal conduction, minimizes parasitic inductance, and simplifies routing.
These types of via cost much more than traditional ones. Therefore, a PCB designer needs to check the manufacturer of PCB’s design capabilities and make sure that they have all the necessary resources, advanced methods, and design tools. So that they offer high-quality products in the end.
