Package heatsinking

Finalize the physical design. This would include physical packaging within the product, heatsink design, and the PCB design. 

Power packages not mounted on a suitable heatsink can expect to dissipate less than five percent of the maximum specified power capability of the package. So 100 W devices will only dissipate 1 to 2W when they are free standing. This also includes using the PC board copper plating as a heatsink. Thus, great discretion should be used when cost is the most important issue. 

Copeland, D., Fundamental Performance of Heatsinks, ASME Journal of Electronic Packaging, 125(2), 221-225 (2003). 

Note that the packages shown in Fig. 1.21 do not use active cooling (fan cooling). Heatsinks with cooling fins and fan are commonly used to cool electronic microchips including Si CMOS microprocessors. They have thermal resistances < 0.5 K/W. The use of active cooling devices would reduce the power efficiency of LED-based systems and are therefore not used. 

Fig. 1.21 Thermal resistance of LED packages (a) 5 mm (b) low-profile (c) low-profile with extended lead frame (d) heatsink slug (e) heatsink slug mounted on printed circuit board (PCB). Trade names for these packages are Piranha ((b) and (c), Hewlett Packard Corp.), Barracuda ((d) and (e), Lumileds Corp.), and Dragon ((d) and (e), Osram Opto Semiconductors Corp.) (adopted from ref. [69]).

Frequently, thermal management solutions are built into the LGA stack. The heatsink, if constructed properly, can be used as the pressure plate. A thermal interface material such as graphite sheet or other thin thermal transfer material can be used between the heatsink and the IC package to enhance heat dissipation. 

The pressure plate or heatsink usually overhangs the LGA IC package. For this reason, tall components should be kept outside of the pressure plate/heatsink footprint to prevent mechanical interference with the LGA stack. 

For bolt-down heatsinks, a significant amount of the compressive load comes not from the heatsink weight but from the bolt down mechanism itself. Tolerance variations in package height, warpage, heatsink stand offs, and so on only make the applied load worse. The attachment mechanism should be such that it can accommodate these variations in tolerance without transferring excessive load to the package. 

To verify the impact of heatsink loading on package solder joint reliability, temperature cycling can be performed on packages with heatsinks and compared with control samples. However, there are two caveats ... 

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