Hi, 1moreamp:
What product(s) from the web link for Digikey (for thermal tapes/sheets), above, are you suggesting for the Orion amps (I have others besides this one)?
And how wide does the strip have to be? (I was estimating 1 inch.)
I will take a look and use your language to see if I can find something but if you can tell me which part number like what Perry stated that would be appreciated. It looks like there is some difference in thickness between those tapes/sheets and the Kapton tape mentioned, which is 1-2 mil. Is "sil-pad" referring to the adhesive on the thermal tape/sheet?
(There is some kind of white paste used to attach some small components on the board to the side of the aluminum hold down bar on the transistors (not on the transistors). I did remove the board from one of my HCCA 250 amps and you are right, there is no paste of any sort between the transistors and the heat sinks.)
Sil-pad is Brand name used to describe silicon based pads. Which are used by many manufacturers of car amps. I use the term loosely to describe typically what you find in car amps. Pads can be silicon based, Graphite based, etc...typically you will see silicon based products in car amps due to cost reasons mostly.
The adhesive used varies but but in kapton tapes its typically silicon based adhesives. The adhesive serves two purposes. One to hold the pad in place temporarily till the assembly has be clamped to spec. And 2 it can offer similar benefits as silicon grease does by aiding in the bonding of the surfaces so as to fill all void space to improve heat transfer.
I myself use both kapton tape and sil-pad type materials since I stock both and have both on hand at all times. I also have mica, and ceramic based AL2O3 products. I stay away from Beryllium based products for heath hazard and cost reasons.
The materials physical size can be what ever is needed to do it job. You don't need to cover any more then the mating surfaces between the transistors and the heatsink face, so using excessively larger amounts only equates to excessive costs to your pocket. Most per-cut and sized products are slightly over-sized to facilitate fast assembly. This is that small amount you see sticking outside of the transistor all around its case dimensions.
All of the insulators you have listed in the link list the "thickness" of the material. This spec varies by material and usage, and clamping pressure requirements. Your Orion clamps are more then capable of dealing with most any material you may select, barring that you do not use too thick of a insulator to begin with.
I stick to the 1 to 5 mil range of thickness unless a job specific assembly requires otherwise. I use 2 mil kapton tape but also have 1 mil on hand. In the case of car amps the 1 mil thickness variance will likely not show and negative effects to the amps operations. These are simple car audio amps, they won't be going into orbit around the earth so generally speaking its just not that critical IMHO. Stuffing your car amp inside of a hot trunk will cause more issues then the insulator spec will.
Any white greasy pasty materials you will likely find inside of a car amp is most likely silicon based heat sink compounds. The white color comes from the addition is alumina compounds in most cases. It can also come from beryllium based compounds but that is rarely used by anyone because it is lightly radioactive and it represents a inhalation health hazard if you breath the dry product into your lungs. Also try to stay away from the super compounds you see used for CPU's these contain pure copper and Silver and both are electrically conductive and can cause short circuits to the case. CPU's are isolated from there sink surfaces, always have been. Transistors and Fets are directly bonded to the metal tabs used in there cases unless spec'ed otherwise at the time of manufacture. Yes they can be ordered in isolated packing, but in most cases it reduces the devices power dissipation significantly.
Perry Babin's experience is top notch in my book. If he says you should use a particular device then I would take it as golden information. He is the only person I would send my gear to if I could not work on it myself.
