With the levels of integration rising to unprecedented levels and the requirement of integrated circuits with a high number gate and pins increased day by day, there was a need to manufacture a package that would be convenient to use, would be will reliable and robust, would pose higher pin count, improved performance while simultaneously reducing package size and thickness — that is when BGA technology came into the picture.
Figure 1 – BGA Package: top, bottom and side views (courtesy: Amkor Technology)
BGA , or Ball Grid Array, is a kind of a surface mount package which is used in electronic products to mount integrated circuits such as microprocessors, FPGAs, WiFi chips etc. The pins are in the form of solder balls that are arranged in a grid like pattern on the undersurface of the package to increase the area used for the connections rather than just the periphery.
Figure 2: BGA package vs PGA package
BGA has evolved from the PGA package. In a PGA or Pin Grid Array, you would see an array of pins arranged in a grid of one, two or more rows and are used to connect the chip to the printed circuit board so that electrical signals may be conducted between them through the pins. In a BGA, however, these pins are replaced with a number of solder balls (surface mount technology or SMT) attached to the undersurface of the package with the help of pads and flux through a very precise automated process.
There are 2 types of methods used today to connect the silicon die to the substrate: Wirebond and FlipChip. A wireboned BGA package uses wires to connect the silicon die to the substrate. A flipchiped package utilizes bumps as shown in the following figure.
Figure 3: Flip Chip BGA internal construction
The substrate is a small PCB with traces designed specifically for your package. The substrate is used for connecting the bumps to the solder balls and consists of several layers to meet the design complexity. The substrate material needs is selected to meet the signal integrity requirements, for example for RF signals a special material should be used to ensure the substrate does not reduce the signal quality.
There are a multitude reasons why Ball Grid Array packages are increasingly being preferred in various applications, some of which are detailed below:
Higher pin density
In order to improve performance/features/price, we saw that manufacturers would increase the number of pins on a package but compromise on the quality as the pins would be placed too close together and the soldering joints would come out to be weak and unreliable in the process. With BGA packages, we obtain hundreds of pins on a single package without compromising on the quality of the soldering or the reliability of the package.
Lower inductance
Since the connecting conductors are short in length as compared to longer wires, it lowers the inductance of the signals which improves their electrical performance as lowering inductance.
Better heat conduction
Again, the decreased distance and inductance between the BGA package and the circuit board makes for lower thermal resistance between the two components which allows smoother and better flow and conduction of heat through the board. In addition to that – a very good lid to dissipate the heat directly from the die. This significantly reduces chip overheating and thermal damage events.
Increased performance
All of these factors ultimately result in a much better electrical performance by the Ball Grid Array as compared to other IC packaging technologies. Utilizing bumping technology with a suitable substrate material gives it superior performance at high speed.
Difficult inspection
While the soldering helps improve the performance of the package and the circuit as a whole, it does not come without its disadvantages, with the main issue being the inability to inspect the package once it has been soldered onto the circuit board. As such, it can be difficult to identify any potential faults and fix them since you cannot view the surface like you would be able to with a PGA package. To counter this package density, the likes of X-rays and CT scanning are used to inspect for any aberrations in the circuit or the connections, although the process may prove to be expensive.
Somewhat expensive
While there may be some IC package solutions out there that may cost you less, using BGA packages are relatively expensive. The bumping process and the substrate are cost adders compared to, for instance, a QFN package.
There is no doubt that BGA package type will be used for a long time. However, a few technology advancements have made it possible to develop more advanced types of BGA package which are cheaper and provide better electrical performance. The main two types are:
WLCSP – Wafer Level Chip Scale Package is essentially a bumped die. Read more about WLCSP here.
eWLB – embedded Wafer Level BGA is a bumped die with an interposer. Read more about eWLB here.
