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Thermal considerations for a UCSP package

Posted: 12 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:UCSP? PCB? heat sink?

The UCSP is a packaging technology that eliminates the traditional plastic package used to encapsulate ICs. Soldering the silicon directly to a PCB saves board space, but at the sacrifice of some of the advantages of a traditional package, especially heat dissipation.

Most packages used for audio amplifiers have some type of exposed pad that allows the substrate of the IC to be connected directly to either a heat sink or the PCB's ground plane. This design provides a low-thermal impedance path for the heat transferred from the IC to its surroundings, and thus keeps the device from overheating.

With a UCSP package, however, the IC is directly soldered to the PCB using bumps on the bottom surface of the device. While there are direct paths from the substrate to the PCB through the ground bumps and these bumps have a low thermal impedance, their area is much less than a typical exposed pad. Consequently, thermal dissipation is reduced. Non-ground bumps also help dissipate heat, but at a reduced capacity compared to ground bumps. Top-side thermal dissipation through a heat sink is impractical due to the space constants of most systems using UCSP devices. The UCSP package is, moreover, not as mechanically robust as other packages typically used with heatsinks. A UCSP can actually become damaged while in contact with the heat sink. The thermal capabilities of a UCSP device are therefore determined by combining the heat dissipated by the grounded bumps with the rest of the bumps on the device.

The amount of power that an audio amplifier will dissipate is primarily limited by its package and external heat sink (whether a copper plane on the PCB or a metal heat sink). While more efficient amplifiers like Class D do not dissipate as much as conventional Class AB amplifiers, all amplifiers dissipate some power as heat. This application note discusses the power-dissipation capabilities of the UCSP package, and how that package can limit dissipated output power compared to other package options.

View the PDF document for more information.

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