Wire-Bonded and Flip-Chip Ceramic Ball Grid Array (CBGA)

Introduction

Ceramic Ball Grid Array (CBGA) characteristics:

Ceramic Substrate

Solder Balls for 2nd level interconnect

CBGAs are classified according to the method of attachment of the die to the substrate:

Wire-bonded CBGA, or

C4 Interconnect CBGA

Applications of CBGAs:

High power parts (for example, processors)

Military and space applications, in which reliability is critical

Figure 5-8. Ceramic Ball Grid Array

General Thermal Considerations

CBGAs have good thermal characteristics due to the presence of Ceramic substrate, which has a relatively high conductivity.

Wire-bonded CBGAs tend to have most of their heat flow through the solder balls to the board. This is due to the relatively high thermal resistance of the Encapsulant.

Figure 5-9. Wire-Bonded CBGA

Flip-chip (C4) CBGAs have either an exposed die, or a capped die. When a heatsink is placed on top of a bare die, the thermal path from the die to the heatsink has a low resistance, thereby allowing a large portion of the heat to go through.

Figure 5-10. Flip-Chip CBGA

Figure 5-11. Capped Flip-Chip CBGA

Modeling Options

Due to the high conductivity of the ceramic substrate, the substrate usually offers a relatively low thermal resistance.

The die and die attach are modeled using cuboids.

For wire-bonded CBGAs, the Bond Wires do not contribute significantly to the thermal behavior, hence they are ignored in FloTHERM PACK. However, for flip-chip CBGAs, the C4 bumps and the underfill are taken into consideration in the model.

The solder balls can be modeled in various ways depending on the level of accuracy desired in the results.

For capped CBGAs, the cap is modeled explicitly, with cuboids. The adhesive that links the cap with the die cannot be currently modeled directly in FloTHERM PACK. It is recommended that you add this as a collapsed cuboid (as spreading within it is usually negligible) after importing the model into FloTHERM.

Material Properties

Thermal conductivities:

Alumina: 18 W/(m.K)

Validation & References

Gary Kromann, Thermal Management of a C4/Ceramic-Ball-Grid-Array: The Motorola PowerPC 603 and 604 RISC Microprocessors, Proceedings of SEMITHERM, 1996.