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BGA Design

A BGA package is more advantageous at design layout because it provides increased functionality at the same package size compared to the Quad Flat pack (QFP). It improves the ratio of pin count to board size, hence providing more connections and increased area for routing. A BGA package also offers better thermal and electrical performance. It does that by providing ground planes for a low impedance power system and controlled impedance on traces.

BGAs also offer reduced package thickness than those with pins. Moreover, BGA balls are considerably stronger than QFP leads and therefore can tolerate rough handling. This can potentially reduce the cost of ownership by virtue of their re-workability.

BGA packages come in a variety of pitches and sizes. Pitch is defined as the distance between the center of one ball to the center of the next one. Currently, 0.75 mm ball pitch and smaller is being implemented due to increasing device complexity and OEMs’ growing demand to use smaller components

Ball layout in each package dictates the number of layers required for effective routing of these packages. If several other technologies and components are already designed on the board, system cost is factored in with every added board layer. A PCB designer’s intent is to optimize the number of layers required to route these packages, considering both cost and performance.

The escape routing technique takes into account such factors as ball pitch, land diameter, number of I/O pins, via type, pad size, decoupling capacitor placement, trace width/spacing, and the number of layers required to escape the BGA.

The first three factors are device specific. Manufacturers have detailed datasheets for their devices that provide recommended land diameters for both masked and unmasked lands. BGA pin assignment and pin grouping can also be modified to enable efficient routing of the board with an optimum number of required board layers.

Placement of decoupling capacitors is also very critical. A decoupling capacitor must be placed as close as possible to the BGA pad. Otherwise, a higher inductance path than necessary is created. Ideally, the designer wants to maintain the shortest possible distance between the two.


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