Computer cooling is about cooling the individual components, often just improving air flow is all that is required. If after maximizing air flow in and through the case there are still some components suffering from heat exhaustion, then you should consider adding heat sinks as well.
For most of the popular cards, chips and CPUs there are commercially available heat sinks. These are often the best choice, but also expensive. You could also design your own heat sink using such formulas as: P = a F(T1-T2). Where: P= dissipated power in watts, a = coefficient of convection (W/m² K), F= interface surface area (m²), T1= Temperature of heat sink fins (°C or °K), T2= Temp of ambient air (°C or °K). If you really need peak performance from your heat sink this might be a good idea, but for most a simpler and cheaper method is required..
Deciding which chips need extra cooling can involve extensive research and temperature measurements. Getting the thermal specs for a chip is not a bad idea but I prefer an easier method. I feel the surface of the IC case. If the surface is very hot (I call this 'ouch hot') then extra cooling is most certainly required. If the surface is very warm then it should be OK but if you're overclocking then a heat sink might be called for. If the surface is barely warm or at ambient then extra cooling is not required. The touch test should be done while the chip is being heavily used, for example when checking the Voodoo II card be sure to be running a very 3D intensive game.
If you don't feel comfortable removing the case cover then you should not put your hands in a running computer. I have heard of at least one person who has died while messing around with a computer. Be warned! Following my procedures or suggestions can result in death and will result in computer destruction (eventually). If this happens to you please drop me a line and we can cry in our beer together (virtually anyway).
Looking at this S3 video card you can see one main chip. It is the chip closest to the centre of the card. While running complex AVI files I found this chip to get warm to very warm. The other four large chips did not feel particularly warm, this is because they are memory chips. The hottest chips will be those doing calculations or processing work.
This Voodoo II card has three main chips with 3Dfx written on them. I found the chip cases to be very warm, almost ouch hot. One other chip on this board felt quite warm. That chip is in the lower left hand corner. I determined that a small heat sink on this chip wouldn't hurt.
Before starting you should collect everything together to be sure you can complete the task. Some of the tools you will need include: Hacksaw, Buck knife, large hammer, superglue, heat sink compound, measuring stick, file and a clean electronics work bench. Start hammering the buck knife into the table whenever anyone walks by. This should result in fewer interruptions. ;-)
Over time, I've collected a number of CPU heat sinks, most leftover from other projects. I have found that CPU heat sinks are cheap (esp. used heat sinks) and easy to adapt to many applications. When making a heat sink from scratch, there are many things that should be considered. For example the base of the heat sink should be on the thick side if the item being cooled is subject to short quick bursts of work (or heat). This 'thermal density' helps keep the item cool on the short term but can result in a 'damping effect' when trying to dissipate the heat to the air. In my case the only real thing that is of any concern is fin height. The heat sinks should not protrude from the card too far. I always keep the PCI slot next to hot cards empty but I still want lots of 'air' room around the card. I found that an older blue Pentium Pro heat sink was shallow enough for my purposes yet large enough to make four heat sinks of the size required.
When trying to get a minor improvement (as in this case) it is not necessary to make the heat sink much larger than the chip being cooled. In fact I some times make the heat sink smaller if the wattage is low and I have a little chunk of heat sink that will work. If you want to check the performance of your heat sink I would suggest getting an indoor/outdoor thermometer from Radio Shack. Tape the probe to the back of the card, directly behind the chip being cooled. If it isn't clear by the circuit layout it will be by feeling for the hot spots. Take before and after temp readings for comparison. A heat sink will keep the whole card cooler.
The heat sink should touch nothing except the ceramic case of the chip being cooled. Watch out for capacitors and other parts that stick up out of the board. Some people will electrically ground the heat sink by running a wire from the heat sink to the board or case ground. I hope to do a few tests on this as I think it is a good idea but until I test it I won't say it helps cooling or overclocking.
Once you have measured and cut the heat sinks to size, attachment will be the next step. This image shows some heat sink compound on each chip to be cooled. Don't use too much compound. One way to see if you have too much or too little is to put a dab in the center of the chip. Press the heat sink onto the chip and wiggle the heat sink around. Very little wiggle, all you want is to let the compound squish out to its thinnest. After pressing quite hard for a bit of time (say 30 secs.), remove the heat sink and check how much the compound has spread. You will have to pull very slowly to remove the heat sink or even slide it to one side. A perfect spread will have a very thin circle of compound just barely reaching the edges of the ceramic. If the compound has spilled over the edge then too much compound has been used. If the spread is no larger than shown in the picture then more is required. The compound is really just to fill the voids in the interface between the ceramic and metal. Air is a poor thermal conductor.
There are many different types of heat sink compounds. They have different temperature ranges and applications. Some people suggest things like regular motor grease. Sounds crazy but it would help over nothing. Myself, I'll stay with proper compound or nothing. The stuff is cheap and a small tube of Dow Corning 340 will last a life time. Other compounds include General Electric G641, American Oil and Supply 300, and Wakefield Engineering 120. A very convenient compound comes in the form of double sided tape. Don't use regular double sided tape as it will act like a thermal insulator resulting in higher chip temps. Even the thermal tape has some insulating qualities but this is offset by ease of use. I don't like it myself as it can be very hard to remove and doesn't lend itself to reuse. Still if you can get some, it is the method of attachment I would recommend. Glue and heat sink compound all in one is very handy for low power requirements.
My preferred method of attachment is superglue. Just a small drop in each corner will do. If the correct amount of heat sink compound has been used, the glue and compound will not contact each other. If everything is clean and not too much glue or compound has been used the attachment becomes permanent. You may still be able to remove the heat sink but it could take a chunk of ceramic with it. Glue all heat sinks on a given card at the same time. I find it easier that way as I lay a chunk of copper bus bar (left over from yet another project) over the whole card. If all the heat sinks are the same height then the even pressure makes for complete contact between each heat sink and chip case. If the heat sink has been properly attached then you should be able to pick up the whole card with just the heat sink. It should also survive a little shaking. If the glue doesn't hold, clean everything off and start again. Let the glue set for at least 1/2 hour with constant pressure.
This image shows the Voodoo II card with the four heat sinks installed. Notice that lower left heat sink does not cover the chip case. The heat being given off by this chip was not as great as the 3Dfx chips. The fins run horizontally to take advantage of the air flow in the case. This card is positioned in front of the case fan which blows at the back of the card (DRAM end, no connectors). This air flow is further enhanced by leaving the slot next to the card open and the slot cover off the case so the air can exit. A heat sink without proper air flow is useless. If needed add a small fan inside the case to direct air at the heat sinks. My set up has lowered the temps to the point where any further heat control would be pointless. If your case is quite warm (+35°C) I would suggest using larger heat sinks and maybe a fan per heat sink should be considered as well.
Here the heat sink is larger than the chip case. Two reasons for that, one is that I already had this heat sink made up for my last card. The other is to increase the heat dissipation. This card does not have as large a fan blowing on it.
Don't forget about air flow. In fact air flow is more important or at least as important as heat sinks. Before making heat sinks you should improve air flow through your case and around the cards and hard drives. Try to keep case temps as low as possible. If overclocking I would suggest using the computer in a desk top configuration and leaving the case cover off. Get a case fan that blows air into the case and a power supply that exhausts. An indoor/outdoor thermometer is very handy for determining if your modifications are resulting in cooler temps.
Cost of this project is less than $10 yet results in dramatically lower chip temperatures. Some cards will require this kind of treatment to operate reliably when overclocked. The same method can be used to cool other chips like the LX and BX chips on the motherboard.