AND8432 - Thermal Considerations for a 4x4 mm QFN

Overview The 4x4 QFN package is becoming popular for many signal and interface applications. To enable optimal thermal performance, this package has an exposed heatsink designed to be soldered directly to a metal pad on the application board. The purpose of this article is to show how application board thermal properties dominate the typical system in which a small QFN package might be used. Therefore, external design choices will, in most cases, make the difference between what would otherwise be reasonable, plausible or impossible applications, at least when viewed from a strictly thermal perspective. In broad terms, the junction−to−back−of−flag thermal resistance of the 4x4 QFN is on the order of 6−8°C/W (the lower value being obtained when referencing the temperature at the hottest point on the board−side of the exposed mounting pad, the higher value when referencing the perimeter of the exposed pad). On the other hand, because of the small footprint of the package, board−to−ambient thermal resistance (which is controlled by board size, materials, layout, and thermal boundary conditions), can easily range from about 15°C/W up to in excess of 120°C/W. JA (junction−to−ambient, the sum of the junction−to−board contribution and the board−to−ambient contribution) thus may vary nearly an order of magnitude, from the most optimistic, thermally aggressive value of about 20°C/W, up to 130°C/W. Conceivably, even lower values could be achieved by the use of forced−convection cooling applied to external heatsinks on the back side of the application board, yet on the other hand, values much worse than 130°C/W could be realized if insufficient board area is provided for the device. The latter possibility may arise indirectly and inadvertently, for instance, if the heat−spreading area is disrupted by injudicious gaps, or if other heat sources exist in close proximity to the device in question, effectively “stealing” some of its intended convection area.