Thermal pad confusion boogaloo - Page 1
Forum section: electronics / projects

author: sahko123
author_rank: Frequent Contributor
timestamp: May 24, 2020, 06:16:14 pm
content: I'm building an amp that needs a heat sink. I want a very thin thermal pad but i have no idea what to search up because the only ones that ever show up are silicon pads. Also it has to be electrically insulating.

author: Benta
author_rank: Super Contributor
timestamp: May 24, 2020, 07:28:36 pm
content: Well, that's because silicone pads are what's used. Earlier, mica insulators plus thermal grease were popular, but I haven't seen those in years, except for special high voltage applications.

author: sahko123
author_rank: Frequent Contributor
timestamp: May 24, 2020, 07:57:51 pm
content: ill probably end up having to use those but i know i have some thin grey pads that work well in my component bins

author: coppercone2
author_rank: Super Contributor
timestamp: May 24, 2020, 08:01:12 pm
content: the thermal conductivity of the mica is better but its like fragile and you need good quality for it to be non dangerous (they can fracture).The silicone pads, to use them1) cut carefully on table with exacto knife2) use a hole punch to make a hole (optimal) if necessary for a screw (better alternative to screw is to use a clamp of some sort to compress it3) apply thermal compound to your part and the heat sink4) align thermal pad with back of the part5) mate part with heat sink6) carefully wipe away excessAs for reuse, they 'dry up', so if you use old pads, expect worse results. The people who make them show decay graphs on their thermal properties.There are also plenty of youtube videos that analyze them, because they are useful for computer processors and other chips used for high end gaming systems where thermals are carefully managed.If you want to restore gear, to get original performance, you would need to replace them.

author: sahko123
author_rank: Frequent Contributor
timestamp: May 24, 2020, 08:14:29 pm
content: Thanks for the tips but i found what i was looking for. Its called Silicone Rubber Coated Glass Fiber Cloth which was what i was looking for because they're a lot thinner than the silicone pads which are like 1 to 2mm thick whereas these are cleaner and are like 0.3mm or there-abouts

author: coppercone2
author_rank: Super Contributor
timestamp: May 24, 2020, 08:34:43 pm
content: what product is it? I did not know they made it for components. I know its used for industrial heater coatings. That's like a material for mechanical protection IMO>Also I don't know where you are getting your thick ass sil pads from. 2mm is ridiculous. I don't think your thing will work well because it does not conform well, and its rather thick.https://www.digikey.com/product-detail/en/panasonic-electronic-components/EYG-A091201F/P19031-ND/58448580.01mmThick ones exist for exotic reasons like HV or RF isolation (less capacitance). And glass is a good insulator,you want stuff other then glass in there

author: Benta
author_rank: Super Contributor
timestamp: May 24, 2020, 09:29:08 pm
content: Quote from: sahko123 on May 24, 2020, 08:14:29 pmYou've lost me completely. What have you been searching for?Normal transistor silicone insulation pads are ~0.3 mm and can be bought everywhere. Just an example (first Google hit):https://www.tme.eu/gb/details/fix-to-3/radiators-equipment/fix-fasten/It's called a silicone heat transfer pad.I've never seen 1...2 mm thick pads in my life.

author: Benta
author_rank: Super Contributor
timestamp: May 24, 2020, 09:36:36 pm
content: Quote from: coppercone2 on May 24, 2020, 08:34:43 pmErm... your suggested product does NOT provide electrical insulation. In fact, quite the opposite.

author: Benta
author_rank: Super Contributor
timestamp: May 24, 2020, 09:41:04 pm
content: OP, perhaps you should understand the difference between "silicon" and "silicone" first...

author: nctnico
author_rank: Super Contributor
timestamp: May 24, 2020, 10:47:16 pm
content: Quote from: coppercone2 on May 24, 2020, 08:01:12 pmSorry but this is all wrong. First of all: buy silicone (rubber) pads which match the package. Secondly silicone pads don't need thermal compound. They are flexible enough to fill all the voids. Using thermal compound makes things messy and (worse) adds an extra insulating layer.

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2020, 03:49:36 am
content: refraining from getting into untested rule of thumb crazy shit, and assemblers choice preferences,Erm... your suggested product does NOT provide electrical insulation. In fact, quite the opposite.https://www.digikey.com/product-detail/en/laird-technologies-thermal-materials/A14692-30/926-1157-ND/24454860.01mmnot that that one happens to be a phase change pad, which melts

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 25, 2020, 09:34:52 am
content: Quote from: Benta on May 24, 2020, 09:29:08 pmSeen them plenty of times. They're used to make up height differences. Say you have a flat heatsink covering a "landscape" of components, some 1.5mm, some 3mm high you can have direct contact with the 3mm high component and a 2mm pad on the low component (heatpads require some compression to work well).Used to be like that with per example laptop graphic card modules. Direct contact with the GPU die, pads to the memory and power conversion.

author: mikerj
author_rank: Super Contributor
timestamp: May 25, 2020, 01:35:21 pm
content: Quote from: Benta on May 24, 2020, 09:29:08 pmGap filling pads are pretty common, you can get them up to around 6mm or so. The ones I've used are a dense/sticky sponge like material saturated in silicone oil (which tends to leach out under compression). e.g.https://www.stockwell.com/thermal-gap-filler-pads/Obviously they are not suitable for clamping power devices to a heatsink, where you want the thinnest, lowest thermal impedance consistent with required voltage breakdown.

author: jmelson
author_rank: Super Contributor
timestamp: May 25, 2020, 03:50:01 pm
content: Quote from: sahko123 on May 24, 2020, 06:16:14 pmBergquist is the manufacturer of a wide range of these products. They have "gap pads" in a variety of thicknesses to accomodatethings that don't fit closely together. That is NOT what you want. You want their "Sil Pad" line. I use a bunch of the BER207 that I get fromDigi-Key, it is .007" (0.18mm) thick. That specific part is for TO-220 transistors, but they make it for all the other sizes, too. Thermal conductivity is 0.9W/m-K.Jon

author: bill_c
author_rank: Regular Contributor
timestamp: May 25, 2020, 05:09:11 pm
content: Quote from: coppercone2 on May 24, 2020, 08:01:12 pmAgree with most everything but the 'dry' up' as the reason even though the result is the same.Silicone doesn't 'dry up', what will happen is compression set, and most silicone suffers from this. Once you compress it beyond a certain point, it will not rebound to its original size. Even if you don't reach that level of compression, silicone will not rebound even when lightly compressed for a long time.While re-use is a bad idea (even if you just installed it 5 minutes ago), I have silicone that has been in service 30+ years (not a sil-pad, but same silicone) indoor, outdoor, and in machines, none has shown to 'dry up' or harden even when kept at 300F for years at a time.

author: Benta
author_rank: Super Contributor
timestamp: May 25, 2020, 05:52:23 pm
content: Quote from: coppercone2 on May 25, 2020, 03:49:36 amRefraining from commenting on your foul language, but an electrical conductivity of 10000 S/cm does not sound like an insulator to me.

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2020, 08:27:08 pm
content: FEATURES AND BENEFITS• High breakdown voltage of 4,000 – 9,000 range VAC10^14 ohm volume resistivityis your digikey broken? where do you even get that figure in seimens?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2020, 09:20:51 am
content: You're looking for Gap Pad and similar products. These usually place over 1 W/(m.K), which is alright in thin layers and wide areas, and the higher conductivity ones are good to lower aspect ratios, of course. Some products are over 10 W/(m.K) which is pretty crazy, and they still maintain good flexibility, it's not like it's a chalky mess.Examples:https://www.digikey.com/product-detail/en/taica-north-america-corporation/COH-1706-200-05/1880-1038-ND/8074046https://www.digikey.com/product-detail/en/t-global-technology/TG-A4500-100-100-0-5/1168-TG-A4500-100-100-0-5-ND/10464940A search:https://www.digikey.com/short/zmtdf4Beware that these materials are soft, and may crush down quite a lot. Apply clamping evenly, from spring pressure if possible, and shop for a fiberglass or similar reinforced material to help maintain a minimum thickness. As mentioned above, mind the electrical conductivity; some are ceramic filled and excellent insulators, others are graphite filled (or actual graphite) and only good for thermal transfer, no electrical isolation. Check the datasheet.Regarding the pyrolytic graphite ones specifically: they fill a niche case, where the absolute thinnest possible material is demanded, and lateral heat spreading is required. They have incredible conductivity in-plane, but unremarkable conductivity thru-plane -- this makes them unsuitable for thermal interface pads (nevermind they don't crush down), but useful for, for example, taking the heat from a cellphone's CPU and GPU, and distributing it throughout the enclosure, behind the display, whatever. A cellphone might dissipate 5 or 10 watts, which would simply start a fire in the local area of one of those chips. It must be distributed, and there's no room for heatsinks or fans on a cellphone!(In non-space-constrained applications, you get identical performance from a copper foil of 2-5 times thickness, and spend a tiny fraction of the cost.)Tim

author: lisafig
author_rank: Contributor
timestamp: December 01, 2022, 01:20:33 pm
content: Phase-change thermal conductors (become liquid around 50-60°C ) seems quite interesting (but they need to be pressed, eg. between a heatsink and a CPU). The PTM7950 from Honeywell is becoming popular:https://community.frame.work/t/honeywell-ptm7950-phase-change-thermal-pads-sheets-application-tips-and-results/20245Someone made a video of it:https://youtu.be/2BhKx0iQ4K8

author: coppercone2
author_rank: Super Contributor
timestamp: December 01, 2022, 06:12:17 pm
content: Quote from: bill_c on May 25, 2020, 05:09:11 pmI saw a graph or figure somewhere from a silpad manufacturer that specifically sited dry up, comparing cheap counterfeit to real silpads. They wrote that it turns hard because of chemical changes to the composition (and that is a reason why you should not re-use them professionally). maybe henkel or 3m. I also noticed a texture/ridigity change in old sil pad taken from 15y+ returned equipment IIRC