Ball Grid Array (BGA)             

Ball Grid Array, or BGA, is a surface-mount package that utilizes an array of metal spheres or balls as the means of providing external electrical interconnection, as opposed to the pin-grid array (PGA) which uses an array of leads for that purpose.  The balls are composed of solder, and are attached to a laminated substrate at the bottom side of the package.  The die of the BGA is connected to the substrate either by wirebonding or flip-chip connection.  The substrate of a BGA has internal conductive traces that route and connect the die-to-substrate bonds to the substrate-to-ball array bonds.

The main advantage of BGA as a packaging solution for integrated circuits is its high interconnection density,  i.e., the number of pins (or balls, rather) that it offers per given package volume is high.  A related advantage arising from this high I/O density is its small board space occupation. 

Figure 1. Examples of BGA packages; the leftmost photo is a top view image

In addition, assembly of BGA onto circuit boards is more manageable in comparison to its leaded counterparts of the same pin count, mainly because the solder needed for board mounting already come from the solder balls themselves, which are factory-applied in precise form and size during the assembly of BGA itself.  Balls also tend to 'self-align' to their attachment sites during board mounting. 

The BGA is attached to the circuit board using a reflow oven, which melts the solder balls.  The solder balls are already matched in position with their respective attachment sites on the circuit board as this happens. The surface tension of the molten solder ball keeps the package aligned in its proper location on the board, until the solder cools and solidifies.  Good control of the board soldering process and temperature is required to prevent the solder balls from shorting with each other.

Figure 2. Cross-section of a wirebonded PBGA package

Another advantage offered by BGA is the lower thermal resistance between itself and the circuit board due to the following reasons: 1) the relatively short distance between them; 2) the excellent thermal properties of the substrate; and 3) the use of thermally-enhancing features such as thermal vias within the substrate and thermal balls under it.  These allow the heat generated by the device inside the BGA to flow more freely to the board, resulting in better heat dissipation for the device that helps keep it from overheating.

The shorter path provided by the BGA between the die and the circuit board also leads to better electrical performance, since the shorter path introduces lesser inductance, in effect minimizing distortion of signals in high speed applications.  Power and ground planes may also be designed into the substrates to reduce ground and power inductance.

<Proceed to Next Page>