Ball grid array

A ball grid array is a type of surface-mount packaging used for integrated circuits. It is descended from the pin grid array (PGA), which is a package with one face covered (or partly covered) with pins in a grid pattern. These pins are used to conduct electrical signals from the integrated circuit to the printed circuit board (PCB) it is placed on. In a BGA, the pins are replaced by balls of solder stuck to the bottom of the package. The device is placed on a PCB that carries copper pads in a pattern that matches the solder balls. The assembly is then heated, either in a reflow oven or by an infrared heater, causing the solder balls to melt. Surface tension causes the molten solder to hold the package in alignment with the circuit board, at the correct separation distance, while the solder cools and solidifies. The composition of the solder alloy and the soldering temperature are carefully chosen so that the solder does not completely melt, but stays semi-liquid, allowing each ball to stay separate from its neighbours.

The BGA is a solution to the problem of producing a miniature package for an integrated circuit with many hundreds of pins. Pin grid arrays and dual-in-line surface mount (SOIC) packages were being produced with more and more pins, and with decreasing spacing between the pins, but this was causing difficulties for the soldering process. As package pins got closer together, the danger of accidentally bridging adjacent pins with solder grew. BGAs do not have this problem, because the solder is factory-applied to the package in exactly the right amount.

A further advantage of BGA packages over leaded packages (i.e. packages with legs) is the lower thermal resistance between the package and the PCB. This allows heat generated by the integrated circuit inside the package to flow more easily to the PCB, preventing the chip from overheating.

The shorter an electrical conductor, the lower its inductance, a property which causes unwanted distortion of signals in high-speed electronic circuits. BGAs, with their very short distance between the package and the PCB, have low inductances and therefore have far superior electrical performance to leaded devices.

A disadvantage of BGAs, however, is that the solder balls cannot flex in the way that longer leads can, so that bending and thermal expansion of the PCB is transmitted directly to the package. This can cause the solder joints to fracture under high thermal or mechanical stress. BGAs are therefore unpopular in certain fields, such as aerospace and military electronics. This problem can be overcome, at a cost, by matching the mechanical and thermal characteristics of the PCB to those of the BGA.

Another disadvantage of BGAs is that, once the package is soldered down, it is very difficult to look for soldering faults. X-ray machines and special microscopes have been developed to overcome this problem, but are expensive. If a BGA is found to be badly soldered, it can be removed in a rework station, which is a jig fitted with infrared lamp, a thermocouple and a vacuum device for lifting the package. The BGA can be replaced with a new one, or can be refurbished or reballed. Packets of tiny ready-made solder balls are sold for this purpose.