There are a few external influences that affect the thermal resistance between the junction and ambient temperature. If the system is going to be designed within an enclosure, a new additional thermal resistance must be accounted for. Now, what was previously the thermal resistance between junction and ambient air, is only the thermal resistance between the junction temperature and the air within the enclosure. Any changes in ambient temperature will have a greater impact on the junction temperature. It is important to offset the additional thermal resistance that an enclosure provides by improving the thermal resistance of other areas of the system.

One way to improve the thermal performance of a system within an enclosure is to introduce air flow. By including a fan on a cutout of the enclosure, the thermal resistance that is created by the enclosure is minimized. Additionally, air flow in general will decrease the thermal resistance of external surface area such as the package lid or the PCB itself.

Attaching a heat sink is a general solution to address thermal issues. However, it should be noted that using a heat sink only decreases the thermal resistance between junction and case temperature. A PCB with significant thermal dissipation flaws will not be fixed simply by adding a heat sink.

For best thermal performance:

• Use an enclosure that is optimized for thermal performance
• Include fans in the enclosure design
• Use a heat sink attached to the BGA Case lid
• Maintain consistent air flow to the system