Vias in a Single Pad
Vias are circular copper pads with plated through holes that connect traces on one side of a PCB to those on the other. Vias provide a critical role in the interconnection of layers and help to reduce signal reflection and loss. When they are designed correctly, vias can improve signal integrity and enhance thermal performance. However, overuse of vias can add significant cost and complexity to a PCB design, which can lead to manufacturing issues like voids and solder wicking.
When a designer inserts multiple vias in a single pad, it is called “via in pad.” When the design is properly executed and fabricated by your contract manufacturer, it can significantly reduce manufacturing defects and cost. During the fabrication process, the vias will be capped and filled with copper or conductive epoxy. This prevents solder wicking and voids that can cause shorts in the board. It also allows the manufacturer to make sure that the correct amount of copper is used, which can increase reliability and lower manufacturing costs.
There are three types of vias: blind, buried and microvia. Blind vias are mechanically drilled and start on an external layer and go part-way through the inner layers. Buried vias are a bit more complex and have a structure similar to a frustum. They are used when connections need to span more than two layers. Microvias are smaller and use a laser to create them. They are more like a slit than a hole and they are commonly used in HDI PCB designs.
Can You Have Multiple Vias in a Single Pad?
While it is possible to force most PCB design systems to place a via in a pad, the resulting board will likely have many design rule checking errors that may require manual inspection and correction by the manufacturer. The most common issue is that the buried or blind via in the pad will not have enough clearance between it and the component. This will result in the solder paste wicking into the via during the reflow process and possibly shorting the pad to the component.
The other problem is that the pad for a via in pad will need to be tentated with soldermask which can reduce yield and performance. This is especially true when using high density components such as fine pitch BGAs. Via in pad also plays a pivotal role in multilayer PCB designs. With the increasing complexity of electronic systems, multilayer boards are now commonplace.
The ability to use via in pad in such designs allows for efficient interlayer connectivity without sacrificing valuable surface area. This is particularly important for designs requiring high pin-count components, such as ball grid arrays (BGAs) or chip-scale packages (CSPs). However, stacking or staggering vias in multilayer designs introduces additional challenges, such as maintaining alignment and ensuring robust connections across layers. Advanced simulation and testing are often necessary to address these issues effectively.
When deciding to use via in pad, the decision should be made early on in the layout. This will help to minimize the number of changes that will be required to the layout and the Gerber files that are submitted to the manufacturer for production.