One-Piece Heatsink vs Floating Lid

Designing Heatsinks and Thermal Solutions for Xilinx Devices (XAPP1377)

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When using a lidless with stiffener ring package, there are two choices of base type that should be considered: a one-piece heatsink and a floating lid. A one-piece heatsink typically consists of the heatsink base with one side containing a properly sized island or pedestal, and the other side consisting of the fins or another means to conduct the heat to where it will be dissipated. Another option is a floating lid which is more-or-less a dedicated heat spreader that is used to transport heat from the die to a flat surface where a separate heatsink that transports the heat can be attached. Floating lids are commonly used during the protype and bring-up phase of a product where a socket is used, the production heatsink has not been manufactured, or it must be operated as an open-box where the proper thermal solution is possibly not fully attached or operational. In some cases, a floating lid can be used in a production environment. However, in general, one-piece heatsinks perform better, are less prone to reliability issues such as high-G drops and vibration, and can be more cost effective for higher volume procurement. Due to this, most engineers start with a floating lid and then transition into a one-piece heatsink as they go to a higher volume production product.

Figure 1. Floating Lid Showing Island, Pre-Applied TIM, and Etch Pattern