Thermal Design for Packages

Lower thermal resistance enables the devices perform consistently and ensures lifetimes, 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.

Renesas Electronics Co. offers various QFP types shown in the diagram below to meet power-consumption requirements of devices. QFPs with built-in heat spreaders have been produced for decades for devices that consume larger power. In response to increasing power consumption of devices, QFPs with exposed die pads (HQFP) have been lined up (see below). Soldering the exposed die pad on a PWB will enhance its thermal conductivity (see diagram below). For smaller pin count range, where smaller package size is required, full variations of QFNｓ are available. They also have exposed die pads; therefore, soldering the pads is beneficial to enhance the thermal conductivity as well.

Figure 5 Thermally enhanced QFP variations

The thermal resistance of BGAs can be lowered by addressing four aspects: thermal balls, higher layer count of substrate,metal core design and PWB design.

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 PWB.

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.

Use of two inner layers of package substrate as ground layers

Generally, a 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.

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.

PWB design

The thermal resistance of a BGA is significantly affected by the PWB design, such as the number of thermal via holes, the number of PWB layers, and the occupancy rate of Cu-trances on the PWB surface. Reassessment of the thermal design of the entire system would offer a low-cost solution that meets thermal resistance requirements.

The picture below shows examples of measureｓhow to lower the thermal resistances of BGAs.

Figure 8 Low Thermal Resistance Design of BGAs

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.

Renesas Electronics collects information on various thermal parameters 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 a Renesas Electronics sales representative.