What is a Via in Pad?
The miniaturization of PCB boards has led to the proliferation of countless innovations such as high density interconnect circuits, vias, and via-in-pad technology.
Via in pad is used to increase density, minimize inductance, and facilitate conductor traces for fine-pitch components such as BGAs. This article takes a more in-depth look at via in pad technology and its advantages to electronic engineers.
Unraveling How Via in Pad Technology Works
Via in pad methodologies allow designers to place vias directly on the copper pad of SMT components. This improves routing and allows higher component density. It also allows designs to free up essential PCB space. Via in pad is a great solution in cases where the pin pitch is limited from a pad to a via.
Some of the biggest advantages of using via in pad technology include:
- Placing components, such as bypass capacitors, in close proximity to each other.
- Help with grounding on high frequency parts
- Improve the circuit’s heat tolerance
- Make routing easier when working with fine pitch BGAs
A major disadvantage of using via in pad design is it increases the board’s manufacturing expenses and time of assembly. Another problem with via in pad design is it leads to a wicking problem. When vias are left open, the solder tends to wick into the hole. The effect is worse in vias that have larger diameters.
A quick fix solution is to cover, or cap, via in pads during the manufacturing process. Moreover, the caps used for via in pad designs must be made of a material that you can solder on because it is part of the pad that the component is attached to.
The trade off of using caps for via in pads is that they can trap air within the via holes. This leads to off-gassing that can damage the performance of the HDI PCB by breaking the solder joint.
Thermal expansion may also lead to the caps popping off due to thermal expansion and create voids in the solder joint.
How Via in Pads are Created
Via in pads can be created with the help of laser technology or by using a mechanical drill. The type of via depends on the specificities of the board, the fabricator’s capabilities, and the overall budget.
Mechanically drilled via in pads can be useful to the HDI PCB as long as it supports the pad size. The annual ring in the hole should be large enough to support the drill bit. This places a restriction on the types of components that designers can work with. For example, it may not be possible to use fine pitch BGA pads because they don’t support regular sized vias.
Mechanically drilled via in pads must also be capped with epoxy - which creates additional assembly steps such as:
- Drilling additional holes
- Plating vias with a conductive material such as copper
It is common to choose epoxies with low electrical conductivity and high thermal conductivity since the copper plating of the cap is a decent conductor of electricity.
Note that the process will take quite a bit of effort, especially for HDI PCBs that incorporate a large number of microvias and fine pitch BGAs. This could easily increase the price of the board by an additional 30% and increase the assembly time by about 3 to 4 more days.
Boards without via in pad routing are cheaper and require less time in assembly. The specific time and cost of via in pad routing is specific to each project, so make sure to consult with your manufacturer for more accurate quotes and timelines.
Choosing the Epoxy
The epoxy is selected after carefully studying the coefficient of thermal expansion (CTE) of the laminate and fill material.
It is important to consider CTE because aggressive thermal cycles during stackup may result in fractures and electrical circuit breaks. A popular choice is to use non-conductive epoxies that have similar CTEs as laminates. This option is relatively affordable. However, the main factors for choosing the epoxy depend on the design and application of the HDI PCB.
Microvias support smaller pad sizes and work with components that have fine pitched pads. They provide a good solder surface because of the copper filling. Micro via PCBs also rules out the risk of off-gassing.
The trade off is that microvias require laser drilling, which is more expensive because of the smaller trace widths and spacing used.
With that said, via in pad technology is almost always beneficial to any application.
Best Practices for Via in Pad Routing
There are a few guidelines that can be used for via in pad routing in circuits that use SMT components. These include:
- Making sure to follow design recommendations that are provided by the fabricator when it comes to via capping, via filing, and component placement.
- Using microvias on one layer of the HDI PCB
- Making sure to plug via holes because it prevents oxidation effects and increases the durability of the HDI PCB.
Why Should You Use Via in Pad Routing?
The main reason for using via in pad routing is to optimize circuit real estate. There are several restrictions that designers have to circumvent, including the size of the board, extremely small connection points on design components, and other routing problems.
Via in pad routing allows designers to overcome all of these problems. The real estate savings provide a compelling reason to use via in pad technology.
Manufacturers may also recommend via in pad routing with thermal pads to help with cooling. It is common to connect these pads on the ground plane of the circuit because of the large surface area, allowing more efficient heat dissipation.
Board miniaturization and power demands make via in pad routing convenient for designers, escape when it comes to improving the thermal characteristics of an HDI PCB.
The experts at Hemeixin PCB are experienced with via in pad technology and can help you with your project. Click HDI PCB here for more information.