Analog Angle Article

Me and my (thermal) shadow

We all know that smaller is better: reduced IC and circuit footprint, lower component profiles, and smaller I/O are not only good, they are essential to meet the market and marketing requirements. It's not just for portable, handheld products either; even larger products such as rack-mounted, room-housed servers are feeling the literal and figurative pinch.

This point became very clear to me when I met with engineers at Linear Technology Corp., who talked about their LTM series of integrated, non-monolithic dc/dc modules. Although these LGA-packaged units do have a very small footprint (typically 15 × 15 mm), one of their other key attributes is that they have a low profile of just 2.8 mm. This is a fraction of a millimeter lower than the external capacitor needed when using a semi-discrete design, according to LTC.

Why is this fractional millimeter difference important? One of the main markets for this unit is providing distributed power on server blades housed in a rack, where OEMs are mounting components on both top and bottom of the PC board. Mounting on the bottom means that not only do you have to watch the board-to-board spacing for physical contact between the top one board and the bottom of the adjacent one, but you also have to worry about airflow path.

Shaving a fraction of a millimeter from this part's height means it can be mounted on the bottom, which gives OEMs some more layout flexibility, certainly. Also important, it also reduces the impedance to airflow, whether convection or forced, and this airflow is not easily modeled or managed. A minor change in height makes a big difference in how a component “shadows” the airflow onto adjacent components.

Today's PCB and system designer has many issues to simultaneously worry about, of course. But board spacing is so tight, and the interboard channel so narrow, that standard airflow and thermal analysis is no longer enough. You still have to be concerned about system-level thermal loads and cooling capacity, as macrothermal issues. Now, in addition to bulk air flow, you increasingly have to model these highly constricted airflows and do airflow boundary-layer analysis. You have to be alert for numerous subtle, localized microthermal hot spots, and worry about which components are moving into your neighborhood and the shadow they cast.

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