The thermal performance of an electronic component is sometimes measured by mounting it on a board in a wind tunnel and testing it at a variety of air flow rates. Although at higher speeds, forced convection will clearly dominate, as the air speeds get lower, the effect of buoyancy will become more and more significant. Since wind tunnels are made in both horizontal and vertical configurations, the question is:
"How much effect does the tunnel orientation have at any particular speed?"
Here are two very simple little FLOTHERM analyses to look into that question. The configurations that have been examined are:
a 208L PQFP on a low conductivity test board; and
a socketed CPGA with a heatsink on a low conductivity board.
In each case, existing validated FLOTHERM models have been used to save time.
This analysis was carried out in support of work being performed by the EIA (Electronic Industries Association) JEDEC JC15.1 committee which is looking at the standardization of thermal testing methods for electronic components. For more details about the work of the JC15.1 committee, take a look at the JEDEC Home Page.
208L PQFP Tests
The first analysis used a 208L PQFP (Plastic Quad Flat Pack) mounted on a low conductivity test board (equivalent to the JEDEC Standard JESD 51_3 board) in a wind tunnel. This is the same general arrangement as has been shown elsewhere in these Web pages, and has also been discussed in detail at a number of conferences. 
Figure 1-1: The 208L PQFP in the Wind Tunnel
Here are a typical set of results for the component: 
Figure 1-2: Typical Results (208L PQFP; Power=1W; Air Speed=1m/s)
Here are all the results: 
Table 1-1: Summary of the Results for the 208L PQFP
Tmax = Junction Temperature (°C); Thja = Theta(ja) (°C/W)
Figure 1-3: Summary of the Results for the 208L PQFP
Above 1m/s, it's clear that the orientation of the tunnel is having little effect on the Theta(ja) results measured. However, below 1m/s, the results diverge, depending on both tunnel orientation and the power of the component.
CPGA + Heatsink Tests
In the second analysis, the PQFP was replaced with a cavity-down CPGA (in this case, using the FLOTHERM model of the 66MHz PentiumTM CPGA package supplied with the standard installation) and a heatsink. Whilst the majority of the heat from the PQFP in the previous example enters the board (see the analysis of the PQFP heat flux budget), for the CPGA, the majority of the heat will pass to the air through the heatsink. The two cases might therefore therefore represent different ends of the spectrum of components that could be tested in this way. 
Figure 2-1: The CPGA+HS in the Wind Tunnel
Figure 2-2: Typical Results (CPGA+HS; Power=16W; Air Speed=1m/s)
Table 2-1: Summary of the Results for the CPGA+HS
Tmax = Junction Temperature (°C); Thja = Theta(ja) (°C/W)
Figure 2-3: Summary of the Results for the CPGA+HS
Interestingly, although the thermal behavior of this component is very different to the PQFP (the majority of the heat being passed to the air through the heatsink rather than through the board), the results are remarkably similar. Once again, above 1m/s the orientation of the tunnel has little effect on the results and below 1m/s the results for the horizontal and vertical tunnel configurations diverge. (end)
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