Ball Grid Array (BGA)  - Page 2 of 2            

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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  

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