Via in Pads and BGA

Via is a pad with a small plated hole in a Printed Circuit Board (PCB) which serves as a connection between copper tracks on various layers of a PCB. There are two types of micro vias used on multi-layered and high density PCBs, blind vias that can only been seen on one side and buried vias which are invisible. Devices and components with very fine pitches are now vastly used in addition to smaller size of PCBs which has created advanced challenges. A recent technology of PCB manufacturing with a self-describing name “via in pad” is the modern answer to these challenges.

The “via in pad” technique is primarily used to lower inductance, for using packages with finer pitch and for high-density designs. As suggested by its name, vias are placed directly under the pads of a component which provides more space for routing and increases density. Space in PCB is saved, for example: four components are placed by conventional fan-out whereas six are placed in the same space by using via in pad.

A trade-off to get intermediate cost with intermediate density as compared to buried or blind vias is filled via in pad. Listed below are some important benefits of using “via in pad” technique:

Fine pitch BGAs with lesser fan out (less than 0.75mm)
Close component spacing requirements can be met
Has improved thermal management
High speed PCB design has its own constraints, such as lesser inductance which can be handled
Component location do not require vias to be plugged in
A straight surface is provided for components where they can be easily placed

Despite many pros, this technique has some cons. A new technology is always increases the cost of a product. The two key cost increasing factors studied by PCB vendors were the increased complexity in manufacturing process as well as the cost of the material used in the conductive fill.

Cost of via fill is directly proportional to the number of vias in a design and their sizes, additionally board manufacturing process increases by almost 8 to 10 stages. This increases cost and complexity which can be compensated by the reduction of layers in the design which ultimately reduces the cost of the process.