Ball Grid Array - BGA

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.

All packages have drawbacks, and the BGA is no exception. Its disadvantages include: 1) the inability of the solder balls to flex, such that thermo-mechanical and flexural stresses from the circuit board can easily be transmitted to the package and its joints, leading to potential reliability issues; and 2) the difficulty of inspecting the balls and solder joints for defects once the BGA has been soldered onto the board.

Figure 3. Solder Balls on a BGA package

Plastic ball grid array (or PBGA), is a type of BGA that either has a plastic-molded or glob-top encapsulated body. It was originally developed by Motorola in the late 1980's for applications with space and weight limitations. PBGA body sizes range from 7 to 50 mm, with ball pitches of 1.00, 1.27, and 1.50 mm. PBGA pin counts, as of this writing, range from 16 to 2401 pins.

The laminated substrate of a PBGA is usually composed of glass-reinforced organic material that has excellent thermal properties (high Tg, high temperature stability, and low heat resistance), such as Bismaleimide-Triazine (BT). The conductive traces within the substrate are usually in the form of etched copper foils bonded to it.

The assembly of PBGA's is usually accomplished on a per substrate strip basis, with each strip holding several package sites.

Figure 4. Example of a strip of future BGA packages

A die is attached to every die pad or flag on the substrate strip, and then electrically connected to its substrate's routers either by wirebonding or through the bumps on its bond pads is flip chip connection is employed. The die and wires are then encapsulated either by cavity molding with epoxy molding compound or by glob-topping with a liquid encapsulant. The glob-top material is usually contained to its specific form and volume with a dam.

After encapsulation, solder ball preforms are placed on the solder pads of the bottom surface of the substrate strip, which are then reflowed to form the final solder balls under the PBGA package. Once the solder balls have been formed, the packages are singulated from the strip either by shearing with a carbide-tipped tool, by routing with a programmable router, or by cutting with a diamond wheel.

Figure 5. Example of an automated

routing machine for singulating BGA's

See Also: PBGA; CBGA; FPBGA or FBGA; LFBGA; TFBGA; VFBGA;

Die Attach; Wirebonding; Molding; Sealing; Marking;

Flip Chip Assembly; TAB Assembly; IC Manufacturing; Assembly Equipment;

Solder Paste Printing

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