This question covered it for enclosures. However, from the point of view of the fan attached to a heatsink does it matter whether air is blown through the fins, or sucked through the fins. In other words, is the pattern of airflow different enough to matter?
Electronic – Fan and heatsink – suck or blow
fanheatsink
Related Solutions
It is possible to determine the thermal resistance of a heatsink that is at hand, but for which there is no data sheet. This can be done relatively simply and without iteration. First, weigh the heatsink, then heat it to some uniform steady temperature in an oven, finally remove from the oven and allow to cool. Cool down time will be related to the overall thermal resistance and mass of the heatsink.
To see how heatsink temperature change is related to mass and thermal resistance to ambient, use an analogous RC electric circuit. The least complicated circuit that's useful is a parallel RC with initial voltage condition (\$V_o\$) on the capacitor. In the thermal analog of the RC circuit, resistance becomes thermal resistance between the sink and ambient (\$ \Theta _{\text{SA}}\$) in \$\frac{\text{${}^{\circ}$C}}{W}\$. Heat stored in the heatsink can be mapped into the capacitance as \$m\$ \$C_p\$, where \$m\$ is heatsink mass and \$C_p\$ is specific heat capacity of the material (~0.9 \$J\$/\$g\$/\$\text{${}^{\circ}$C}\$ for aluminum). An equation for heatsink temperature (\$T_{\text {hs}}\$) can be written for the thermal circuit as:
\$T_{\text {hs}}\$ = \$\left(T_{\text{hso}}-T_{\text{amb}}\right) e^{-\frac{t}{m C_p \Theta _{\text{SA}}}}+T_{\text{amb}}\$
Rearranging, thermal resistance is:
\$ \Theta _{\text{SA}}\$ = \$\frac{t}{m C_p \text{Ln}\left(\frac{T_{\text{amb}}-T_{\text{hso}}}{T_{\text{amb}}-T_{\text{hsf}}}\right)}\$
Thermal time constant for the heatsink is:
\$\tau \$ = \$m C_p \Theta _{\text{SA}}\$
It is convenient to use \$\tau\$ to set the target heatsink temperature (\$T_{\text {hsf}}\$) to terminate the measurement, because with the measured time the only remaining unknown is \$ \Theta _{\text{SA}}\$ which can now be calculated.
Method with more detail and example numbers
- Weigh the heatsink. Let's just say you get 100g.
- Install thermal probe. Attach probe where a device would be mounted.
- Put heatsink, on thermal insulator (like a piece of wood), in oven and heat to elevated temperature. While waiting for readings to stabilize calculate target cool down temperature \$T_{\text {hsf}}\$ by setting \$t\$ to \$\tau\$ in \$T_{\text {hs}}\$ equation. For example, using \$T_{\text {hso}}\$ = 100\$\text{${}^{\circ}$C}\$ and \$T_{\text {amb}}\$ = 25\$\text{${}^{\circ}$C}\$, \$T_{\text {hsf}}\$ will be 52.6\$\text{${}^{\circ}$C}\$.
- When heatsink temperature stabilizes, remove insulator and heatsink from oven (don't burn yourself) and place in ambient environment. Record time when heatsink reaches target temperature. For this example that's 52.6\$\text{${}^{\circ}$C}\$ and \$\tau\$ would be 225 Sec.
- Use recorded time and equation for \$ \Theta _{\text{SA}}\$ (or even \$\tau\$) to calculate heatsink thermal resistance. For this example \$ \Theta _{\text{SA}}\$ = 2.5\$\frac{\text{${}^{\circ}$C}}{W}\$.
When deciding how high a temperature to use as heatsink initial condition, use a temperature that makes sense for the application. 100\$\text{${}^{\circ}$C}\$ is probably as high as you should ever go (it would mean that the junction of whatever device was mounted to the heatsink would be at 110\$\text{${}^{\circ}$C}\$ or more, and that's hot).
Maybe, maybe not, but I'd ask why you are not correlating hot chips with power supply currents, and why you're not putting a temperature sensor on the heatsink. If the thermal path from the die to the heatsink is impaired you'll get a different temperature differential between the die and the heatsink. Likewise, if the chip is drawing more current you should be able to predict the final temperature of the die based on normal thermal behavior. And measuring the heatsink temp doesn't require a dedicated contact sensor: a temporary one will do, or a non-contact IR unit should work, since the emissivity of the heat sinks should be pretty uniform.
As to why the maybes, consider the following model:
simulate this circuit – Schematic created using CircuitLab
If the thermal resistance from the die to the heatsink is much larger than the thermal resistance of the heatsink to ambient, and the thermal capacity of the die is much less than the capacity of the heat sink (and I would guess both to be true), the latter is the dominant factor in determining the thermal time constant of the heatsink, and thus of the die. In this case, increases in the die/HS thermal resistance will have only small effects on the time constant of the die, but will cause the die to get hotter. You'll have to figure the values for your board to see if this is the case.
Best Answer
This is such a wide subject it really isn't one you can answer with a simple one is better than the other answer.
Standing alone, the blow side of a fan does produce a more concentrated, faster moving, and more turbulent "river" of air compared to the intake side where air is drawn almost equally from all directions. You can test this easily enough with pretty much any fan. Put you hand in front of the blow side and you will feel the airflow and cooling effect. Put your hand behind and the effect is much harder to detect.
The turbulence also greatly improves the efficiency of the heat transfer. Turbulence is in fact your friend.
So from those points of view alone, the blow side does appear the better cooling side.
However, it is not just about the fan.
The geometry of the heat-sink chosen also greatly affects the performance of the fan. A rotary fan slapped on top of your typical linear finned heat-sink will actually be quite inefficient. In fact the region directly under the centre of the fan will get virtually no air movement at all. This of course is unfortunate, since that is normally where the thing you are trying to cool is located.
Further, unless the fins are quite deep the airflow is badly distributed in general. Too shallow, and the resultant back-pressure can actually "stall" the fan. In those circumstances, installing the fan in the "suck" direction can actually improve the situation since the air will enter the sides of the heat-sink more linearly to fill the void in air pressure created by the fan.
Arguably, the heat-sink shown above might be more efficient with longer fins and the fan mounted at one end.
Better designs use radial heat-sinks like the one below. As you can see, the style here is radially symmetric to the airflow on the entire circumference of the fan and consequently delivers a more even heat transfer around the central core.
However, even with this style, the core itself is still badly ventilated. As such it is usually manufactured as a solid high thermal conductance core which acts as a heat-pipe. Even then, looking at the image below, the area around the core in the square section that touches the chip actually is an air void that is quite inefficient. A better design would have that area filled with metal in a rounded conical structure. However, that would of course be impossible to extrude.
If fact materials and surface preparations also make a huge difference in heat-sink design. Highly thermally conductive materials are obviously best, but the surface should also be smooth enough not to allow pockets of air to form or to grab at dust particles, but also not so smooth that air passes too easily over it.
One could of course spend years getting that little formula perfect, but in general you don't want a high polish chrome heat-sink. Sandblasted aluminum, or gold coated sandblasted copper, if you can afford it, would work a lot better.
Another serious issue is contamination.
Dust and dirt is going to get into your fan and your heat-sink. Over time this builds up and severely degrades the performance of the unit. It is therefore prudent to design your fan and heat-sink arrangement to be as self flushing as you can.
This is where a blower fan usually wins out. With controlled airflow and if the air coming in can be kept clean, it tends to blow dust out of the heat-sink. Which brings me to the next point.
Air Sourcing and Removal
You can spend thousands of dollars developing the perfect arrangement of fan and heat sink and it will all be for naught if you do not deal with the rest of the air around your cooling system, especially in a tight enclosure.
The heat not only has to be removed from your device to air, but that hot air then needs to be removed from the vicinity. Failing do to so will just recirculate the hot air and thermal failure will still occur on the device you are trying to protect.
As such your cabinet needs to be vented and you should also include cabinet fans to draw in cool air from outside the enclosure. These fans should always include removable mesh and or foam filters to control the amount of ambient dust sucked into the unit. Open grill type exhaust panels are acceptable, however, for best operation a positive pressure should be maintained within the cabinet so airflow is maintained in the out direction to again limit contamination entry.
Special Cases
Wherever the unit is to be installed in an extreme environment special measures need to be taken. High dust environments like floor mills etc., or high ambient temperature environments will require either ducted air direct to the chassis, or a sealed unit and a two stage, possibly liquid, cooling system.
Critical Cases
If your system is controlling something critical then it is prudent to include thermal sensing and possibly active fan control as part of your heat-sink system. Such systems should include the feature of going into a safe state and warning the user to clean the filters or otherwise reduce the ambient heat around the system when necessary to prevent critical failures.
One More Point
You can spend a half years development money getting the best heat-sink design in the world with expensive fans and a perfect air distribution system all locked down then burn out devices for the lack of 2 cents worth of thermal compound.
Getting the heat from the device you are trying to protect into the heat-sink can often be the weakest point in the system. Components not properly mounted to the heat-sink with an appropriate thermal bonding material kills more units than the rest of the issues combined.
Your manufacturing process and procedures should be developed to give those aspects first priority.
For example, if say you are using three or four TO220 style transistors mounted to a single heat-sink, it is prudent to mechanically mount them to that heat-sink, and if appropriate, the heat-sink to the board, BEFORE going through the soldering process. This ensures the thermal connection takes priority.
Either thermally conductive pastes, creams, gels and or electrically isolated thermal pads should always be included between device and heat-sink to fill any air gaps caused by non-flatness, or bumps on either the device or the heat-sink surface.
And keep it clean. A contaminate the size or a grain of salt, or even a stray hair, can cause thermal failure.