Thermal management in high layer count PCBs using sintered-paste-filled PTH

Abstract

Thermal management is becoming increasingly important throughout the electronic packaging industry. In the high-layer-count printed circuit board (PCB) market for applications such as servers and modems, plated through holes (PTH) are used for both electrical interconnection and as thermal drains. In order to take maximum advantage of the PTH as a thermal conductor, it is becoming increasingly common to fully plate up the PTH to closure. Although this provides excellent thermal conduction, this process is very lengthy, costly and prone to a number of pitfalls. In many cases, the requirement for the product is to exceed the thermal conduction of the more common epoxy-filled PTH, but the level of thermal conduction provided by the plated-closed PTH is far in excess of this requirement. Highly-metal-loaded (97% net by weight) transient liquid phase sintering (TLPS) pastes offer an attractive intermediate solution. In an experiment on a typical high-layer-count PCB configuration with 0.62 mm diameter holes on a 1.34 mm pitch with 0.013 copper thickness in the barrel and a 2.34mm board thickness, the fully- plated-shut copper architecture was modeled and calculated to be 67 W/mK effective thermal conductivity, the conventional epoxy-fill in PTH approach calculated as 4 W/mK, and the TLPS-paste-filled PTH was calculated to be 11 W/mK. The TLPS paste was then installed in the PCB using conventional pressure head hole-filling equipment, and the effective thermal conductivity was measured to be 10.9 W/mK - in excellent agreement with the model. It is also worth noting that the modeled performance of the TLPS paste in a non-plated through hole exceeded the modeled value for the conventional epoxy-in-PTH solution by 47%. The use of TLPS-paste-filled PTH thus offers a high performance solution at a fraction of the cost of plating to closure.