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Thermal design of a package


Lower thermal resistance maximizes the intrinsic performance and lifespan of semiconductor devices, helping meet market needs for higher performance as well as saving costs by curtailing the need for heat dissipation measures.

This article introduces various actions that can be taken to lower the thermal resistance of a package, taking two representative package types, QFP and BGA, as examples.

Lowering thermal resistance of QFPs
The thermal resistance of QFPs can be reduced through appropriate material selection and lead frame design.
1. Development of thermally enhanced resin
Because mold resin is the lowest thermally conductive material among package materials, thermal enhancement of the mold resin can significantly lower the thermal resistance of packages. One way to do this is to replace the existing filler with high thermal conductivity filler.

Further, the thinner the resin on the chip, the shorter the distance over which heat is transmitted. Thus mounting a heat sink on the surface of the package enhances the heat dissipation effect to a greater extent in the case of TQFPs, which have a thinner resin thickness than QFPs.
2. Lowering thermal resistance through lead frame design
It is a well-known fact that the thermal resistance decreases if the material of the lead frame is changed from 42 alloy (Fe-Ni) to a copper alloy.

Thermal resistance can be lowered by designing the die pad on which the chip is to be mounted as large as possible.

To further decrease thermal resistance, a heat spreader may be attached to the die pad so that heat is transmitted almost throughout the entire package. 		Low Thermal Resistance Design of QFP
Figure 4: Low Thermal Resistance Design of QFP(Lead frame design)
Figure 4: Low Thermal Resistance Design of QFP(Material) Figure 4: Low Thermal Resistance Design of QFP(Package structure)

Lowering thermal resistance of BGA
The thermal resistance of BGAs can be lowered by addressing three aspects: material, package structure, and substrate design. As in the case of QFPs, the thermal resistance of BGAs can be decreased by using material with a higher thermal conductivity for the filler. For package structure and substrate design, the following measures may be taken.

1. Use of thermal balls as a low-cost solution
In terms of package structure, a heat dissipation path is secured from the rear side of the chip to the solder balls immediately beneath the chip by providing a large number of solder balls on the rear side of the chip and thermally connecting these solder balls to the die pad via through-holes (thermal vias). These balls at the center of a package are electrically grounded and commonly called “thermal balls" as they play a thermal dissipation role by conducting heat to the printed wiring board.

This is the cheapest way to dissipate heat. The thermal balls also serve as ground pins and neighboring balls can be assigned as signal pins, meaning that the actual number of pins can be increased.
2. Use of two inner layers of package substrate as ground layers
Generally, a printed wiring board having four or more layers, including power and ground layers, is used as the package substrate for BGAs to ensure satisfactory electrical characteristics. Recently, however, the use of a 4-layer structure to lower the thermal resistance, rather than improve electrical characteristics, has been increasing.

In this case, heat from the chip is transmitted to the inner layers via the die pad's through-holes, which also serve as grounds, and two out of the four inner layers are used as grounds to secure a heat dissipation path.

To further decrease thermal resistance, a substrate with a thick embedded metal core layer is used.


3. Use of embedded heat spreader
If the combination of thermal balls and a 4-layer package substrate still fails to satisfy the thermal resistance requirement, a heat spreader can be embedded in the package. Such a heat spreader serves to diffuse the heat transmitted through the mold resin to the surface of the package. However, the reduction in thermal resistance that can be achieved this way is limited because the heat spreader is not in direct contact with the die pad and the chip.

4. Modification of printed wiring board design
The thermal resistance of a BGA can also be lowered by modifying the design of the printed wiring board. As shown in figure, the thermal resistance changes according to the number of thermal via holes, the number of layers of the package substrate, and the presence or absence of a heat dissipation path. Reassessment of the thermal design of the entire package, including the printed wiring board, can result in a low-cost package that meets thermal resistance requirements.
Some Measures for Lowering Thermal Resistance of PBGA
Figure 5: Some Measures for Lowering Thermal Resistance of PBGA
5. Selection of cavity-down type PBGA
Cavity-down type BGAs, in which the chip is flipped and attached to the heat spreader, which is exposed at the surface, is the most effective solution for lowering thermal resistance. This also holds true for QFPs. Packages of this type include TBGAs, ABGAs, and FCBGAs. Heat is directly conducted from the chip to the copper plate on the package surface, thereby achieving low thermal resistance.
Low Thermal Resistance Design of BGAs
Figure 6: Low Thermal Resistance Design of BGAs

Measuring the junction temperature of a device is also important.
Measuring the temperature of the chip after it has been installed in a system may be considered pointless because the package has already been selected at that stage. Nevertheless, such measurement data are useful for estimating the power consumption of devices to be developed in the future. Furthermore, if the power consumption of the system is known to be considerably above the value estimated at the design phase, it is very important to know the junction temperature of the device under actual use in the system.

NEC Electronics collects information on various thermal parameters, such as Ψjt, for each type of package to allow cooperation with customers regarding actual measurements. This information is available upon request. We also provide thermal analysis services. For details, contact an NEC Electronics sales representative.

Thermal characteristics