Initially, surface mount devices, especially ICs, came as perimeter-only packages, with pins for soldering placed along the edge of the device. As ICs became more complex, they needed more pins for external interfacing, which made the packages larger. Manufacturers soon realized there was a large unused real estate that lay just under the package. Therefore, they made the ball grid array (BGA) packaging, which, in place of pins, had solder balls aligned in a grid under the device. Soldering BGAs involves melting these solder balls onto pads on the PCB.
Using BGAs leaves a considerably larger area free on the PCB. Compared to mounting a package with pins on its perimeter, BGAs offer better thermal and electrical properties, and this has made the format popular, following the continued miniaturization of electronics.
Since their introduction, although their basic concept has remained the same, BGAs have changed in dimension and now come with far smaller pitches and smaller outlines. There are varieties as well, with some packages having connections only on the periphery and none at the center, while others have the connections distributed evenly across the bottom of the package.
For simpler BGAs, routing traces on the PCB is simple as the balls are placed well apart or there is space in the middle of the device. However, with increasing pin counts and decreasing pitches, routing between the pins becomes more difficult, resulting in increasing the layers of the board, thereby increasing the cost and reliability concerns.
As BGAs become increasingly more complex, designers have to depend on vias to connect the BGA with the rest of the circuitry on the PCB. Vias are small holes drilled through the multilayer PCB and plated with copper to provide connection between pads and traces on different layers. Some vias are through-hole types, meaning they start and end on the two extreme layers of the PCB, and may connect to other layers in between. Other vias can be blind types, starting from one of the outermost layers and ending on an internal layer, possibly connecting other layers in between. Blind vias are not visible on the PCB surface as they start and end at different internal layers, and may connect other internal layers as well. However, all the above require great precision while manufacturing, and are expensive processes.
Ordinarily, PCB designers prefer not to use vias on a pad, as during soldering, vias can wick solder from the pads leaving the joint in a dry and unsoldered state. However, with BGA pitches getting increasingly smaller, designers do not have much choice, but tenting is offering a way out. Tenting allows filling the via hole with an insulating material and covering the top with a layer of copper, thereby preventing wicking.
As the BGA pins lie in between the device body and the PCB, traditional soldering methods such as hand soldering and wave soldering are no longer useful, and assemblers rely on infrared heating or reflow ovens to solder BGAs to a PCB. This requires a pick-and-place machine placing the BGA package precisely on the pads and uniformly heating the area to form the actual connections.