Thermal conductivity of silicone
Thermal conductivity of silicone
(OP)
I intend to encapsulate some electronic components (4 resistors) in silicone. Power dissipation for each will be in the area of 100mW. The material I intend to use has a thermal conductivity of 0.2 W/mC (they specified it in C rather than K, but no biggie).
While looking at charts on the web, I came across a few interesting figures...the thermal conductivities of Aluminum (typical material for heatsinks) at 205 W/mK, ice at 1.6 W/mK, and air (0 degrees C) at 0.024 W/mK.
If I'm reading that chart correctly, the silicone will conduct heat away from the resistors better than cold air, correct? If that's true, then this is the stuff I need to be using rather than letting the components hang in the open air.
While looking at charts on the web, I came across a few interesting figures...the thermal conductivities of Aluminum (typical material for heatsinks) at 205 W/mK, ice at 1.6 W/mK, and air (0 degrees C) at 0.024 W/mK.
If I'm reading that chart correctly, the silicone will conduct heat away from the resistors better than cold air, correct? If that's true, then this is the stuff I need to be using rather than letting the components hang in the open air.





RE: Thermal conductivity of silicone
RE: Thermal conductivity of silicone
j=junction
c=case
s=silicone
a=air
The only time the series resistance works in your favor is with a heat sink, since the series resistance of the aluminum is mitigated by the fact that the convection coefficient of the heat sink is significantly better than that of the IC package itself. Thus, the net series thermal resistance is lowered, even though the immediate resistance is increased somewhat.
TTFN
RE: Thermal conductivity of silicone
As I mentioned before, each package will be dissipating around 100mW from a standard surface mount package (0805 size). While not a painful amount of heat, it can still be a bit uncomfortable. The silicone should insulate the "user" from direct contact anyway, but I'm more worried about the effects of the resistor's health due to a lack of heat dissipation.
Can either of you provide an idea and/or web source that would help me determine the derating needed for the resistors? The ones I'm currently using (in open air) are 1/4W, but if I need to derate them further I need to know now.
RE: Thermal conductivity of silicone
1) Derating to 50% is the rule for many companies.
2) I don't know how the maximum power rating for a resistor is determined by the manufacturer but I suspect it is for one resistor mounted on a vertically mounted PC board with the resistor attached to fat traces.
2) My experience is that resistors running at or near their power rating are quite hot, hot enough to eventually scorch the the PC board and hot enought to eventually crack the solder pads from thermal cycling.
3) The thermal environment for the resistor is not exactly known. It may be located near another hot device or shrouded from air flow. In your specific case, the four resistors are going to be right next to each other.
RE: Thermal conductivity of silicone
I suppose I could move up to 2010, 1/2W devices, but would the minimal change in surface really make that much of a difference when still buried under silicone? I would prefer to stay away from the larger (and more expensive) packages unless I have to. The resistors will reside on oppositie ends of the board in pairs, so there will be some heat sharing between two devices, but other than that they will be relatively free from other heat-producing components.
RE: Thermal conductivity of silicone
Exactly why do you feel the need to cover the resistors with silicone?
RE: Thermal conductivity of silicone
Is your recommendation to stick with the 1206's from experience? I will be running some experiments on my own to make sure it's OK, but I'd like to run as few tests as possible. I also feel it will be OK, but with this kind of money at stake (going into production), I can't afford to make a mistake (literally)...lots of tests to follow.
Thanks!
RE: Thermal conductivity of silicone
Some engineering organizations follow the rule "It's too hot if you can't put your finger on it." In your case, going to 8 paralled 0805 resistors would only be, say, 4 cents in parts cost.
You also seem to be saying that you are going to "Conformal Coat" the PC board for environmental reasons. May I suggest Humiseal, it is the most commonly used conformal coating.
www.humiseal.com
RE: Thermal conductivity of silicone
Also, the conformal coating needs to be transparent as there are LEDs involved. One of the engineers at Dow Corning has suggested a few silicones to try, but I'm currently having difficulties getting one of their local reps to return my calls. I would like to pick up some sample silicones to try before committing to a production run. I figure a test run at max current for 24-48 hours should help me determine if the heat is going to be a problem.
I'll be sure to check out what products HumiSeal has to offer.
RE: Thermal conductivity of silicone
Anyone trained in thermomechanical or thermal engineering knows that there is rarely a single answer to any cooling problem, as there are a million variables (conduction vs. convection, free convection versus forced, radiative concerns, fluctuations in ambient temps, time constants, fluctuations in device performance over time, etc., etc.) that can make a rough rule-of-thumb estimate wildly divergent from what happens in reality. This is why you see the huge derations and factors of safety in these designs. This book just might make you think of something you've overlooked, and at the very least, it should make you chuckle.
RE: Thermal conductivity of silicone