february 2005

a recent printed circuit board (pcb) design application demonstrates how heat-dissipating components can preheat the air passing over downstream components, causing thermal problems that would otherwise not exist. although some rules of thumb can be employed in regards to the required volume and drafting effects, they often compete and do not provide accurate enough information for tradeoffs to be considered. by optimizing the component placement before design for manufacturing begins, the risk and cost of potential redesigns are significantly reduced.

the board under consideration is a single board computer designed as an upgrade to an existing chassis. the current platform was tested and shown to provide a slot flow rate of 200 lfm. the number of interconnects and processing speed are the major driving factors affecting component placement. this translates to the physical definition in the pin-out and the subsequent ability to route the board in the least number of layers. cost restrictions limit the pcb to 4-layers, making it necessary to maintain essentially a direct line of sight from the appropriate pin section to the target counterpart.

use of the flo/pcb board level simulation tool exposed a knock-on effect in which the cpu preheated the air passing over the dimm. if this were caught later, after the detailed design had begun, the options would be either to reduce performance to reduce the power dissipated or to add cost by implementing a specialist thermal solution such as an internal heat spreader for the dimm. by evaluating this in the concept phase, it was possible explore other options in a matter of minutes to reduce the risk.