{"id":394,"date":"2012-10-07T17:13:34","date_gmt":"2012-10-07T22:13:34","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=394"},"modified":"2014-04-13T20:28:33","modified_gmt":"2014-04-14T01:28:33","slug":"everything-you-know-is-wrong-september-2002","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=394","title":{"rendered":"Everything You Know Is Wrong September 2002"},"content":{"rendered":"

Answers to those Doggone Thermal Design Questions<\/strong><\/p>\n

By Tony Kordyban<\/strong><\/p>\n

Copyright by Tony Kordyban 2002<\/p>\n

To the Thermal Explaining Guy,<\/em><\/p>\n

What’s the deal with thermal interface materials?\u00a0 I’m referring to the whole collection of goops, greases, silicone pads and gap fillers intended to be stuck between a component and a heat sink.\u00a0 They are supposed to lower the thermal resistance between the component and the sink, compared to a “dry” joint (one that has no interface material.)<\/em><\/p>\n

I guess they all work, by which I mean, they almost always are better than nothing at all.\u00a0\u00a0 But how do you predict the thermal resistance of an interface material in a particular application?\u00a0 When I go by the thermal conductivity from the material data sheet, I predict a much lower temperature rise than I actually get.\u00a0 I could live with a little exaggeration from the material vendors, but it’s even worse than that.\u00a0 I used the same material on two different components, and for one the thermal resistance I measured was about three times higher than the other.\u00a0 What gives?<\/em><\/p>\n

Missing Something in Cutters Gap<\/em><\/p>\n

 <\/p>\n

Dear Miss Ing,<\/p>\n

You bring up a very touchy subject.\u00a0 I once saw two guys from rival thermal interface companies almost get into a fist fight at a technical conference.\u00a0 They were arguing about the appropriateness of a particular ASTM test method for measuring the conductivity of a\u00a0 Phase Change interface material.\u00a0 One had grabbed the other by the lapels and was about to head butt him, but they were suddenly distracted when both of their cell phones rang simultaneously.<\/p>\n

\"\"<\/a>

Thermal Resistance for one-dimensional conduction: T2 - T1 = Q t \/ ( k A ) and by definition R = (T2 - T1 ) \/ Q so R = t \/ (k A) where k is the conductivity of the solid block<\/p><\/div>\n

The thermal resistance of interface materials is poorly understood, perhaps even by the scores of researchers working on it right now.\u00a0 I don’t think anybody (not even yours truly) has a good handle on it yet.\u00a0 At least not in a way that is useful to Electronics Cooling practitioners like you and me.<\/p>\n

Before I get into why interface resistance is so tricky, let’s review what we mean by thermal resistance in the first place.\u00a0 Let’s say we have a simple, rigid solid, like a block of copper, with heat flowing through it from one face to another, like this:<\/p>\n

So for a single, continuous, uniform solid you can calculate its thermal resistance from the thickness, the cross-sectional area, and the conductivity, which are all pretty easy to know.\u00a0 This is probably the method you used to estimate the resistance of your interface materials.\u00a0 It seems pretty simple.\u00a0 Why didn’t it work?<\/p>\n

What happens if you stack two blocks of copper?\u00a0 You can easily find the resistance of Block 1 and Block 2 from the equation above.\u00a0 But there is also a “contact resistance” at the joint between Blocks 1 and 2.\u00a0 I have shown<\/p>\n

\"\"<\/a>

The imperfect contact between two solids adds "contact resistance" to the total resistance of the assembly to the flow of heat.<\/p><\/div>\n

the adjoining surfaces as being rough and irregular for a reason.\u00a0 The contact between them is definitely not perfect, and is full of tiny gaps, no matter how smooth and flat the surfaces appear to the naked eye.\u00a0 The contact resistance is not easy to estimate.\u00a0 It depends on things like the two materials in contact, their flatness, surface quality, and the pressure squeezing them together. \u00a0It is the contact resistance you are trying to minimize by introducing an interface material.<\/p>\n

So what is the situation with your component and heat sink?<\/p>\n

Let’s say you put a squishy material between your component and heat sink.\u00a0 The resistance of the material layer itself is R = t\/kA.\u00a0 But there are two other resistances to worry about — Rc1, the contact resistance between the material and the bottom of the heat sink, and Rc2, the contact resistance between the material and the top surface of the component package.\u00a0 Note that these two contact resistances are probably not equal, and have no reason to be.<\/p>\n

Obviously, you are trying to get the total thermal resistance between the component and the air to be as low as possible.\u00a0 For that reason you would naturally choose the thinnest material that will still fill the gaps between the component and heat sink.\u00a0 The bulk resistance of the material increases with thickness.\u00a0 But if the thickness is very small, then the contact resistances (Rc1 and Rc2) dominate the total resistance of the joint.\u00a0 If that is true, then the conductivity of the material may not be very important at all.<\/p>\n

The joint resistance may be mostly dependent on the contact pressure (the force squeezing the heat sink against the component), and\/or the ability of the material to “wet” the surfaces.\u00a0 “Wetting” can be thought of as the material flowing into the microscopic cracks, crevices, nooks, crannies and pores in each surface, and displacing the air in those little \"\"<\/a>pockets.\u00a0 It doesn’t do any good to have a high conductivity material that traps a big air bubble in the center of the joint.\u00a0 The ability to wet a surface depends on what the two materials are, plus lots of other factors, like the presence of contaminants.<\/p>\n

Then you have the problem that when you use a “squishy” interface material, and then squeeze it, it changes in thickness, surface area, and maybe in density, or in the distribution of filler particles in the matrix, so its “properties” may not be the same as when they were measured for the vendor’s data sheet.<\/p>\n

\"\"<\/a>

Interface material can have a different thermal resistance when used with a plastic package than with a metal component package, just because the effective area of heat flow is different.<\/p><\/div>\n

The total thermal resistance depends on so many factors that the only reliable way to know how a material will act in your application is to try it.<\/p>\n

There is another reason why the performance of an interface would differ widely from one type of component package to another, even if contact resistance is negligible.\u00a0 The joint thermal resistance can depend on the conductivity of the component package.\u00a0 No, I’m not kidding.<\/p>\n

Compare two 40 mm square Ball Grid Array (BGA) packages.\u00a0 Both have a 15 mm square die.\u00a0 One is overmolded in plastic ( k = 0.2 to 0.5 W\/mK ) which does not spread heat well.\u00a0 The other is a flip-chip BGA that has a copper (k = 400 W\/mK) heat spreader plate bonded to the upper surface of the die.<\/p>\n

Let’s assume the interface material wets all the surfaces perfectly and contact resistance is negligible.\u00a0 Then the thermal resistance of the interface is R = t\/(kA).\u00a0 The joint resistance is inversely proportional to A, the area through which heat flows (assuming the material thickness is the same).\u00a0 But in the case of the plastic BGA, heat does not spread sideways from the die because of its low conductivity.\u00a0 The effective area through the interface is only (15 mm)2<\/sup>, the area of the die.\u00a0 On the other hand, the copper heat spreader easily spreads the heat to the full (40 mm)2 <\/sup>area.\u00a0 The effective area is 7 times greater and the thermal resistance about 1\/7 what you get with the plastic BGA.<\/p>\n

So there doesn’t need to be any “exaggeration” by interface vendors for there to be huge variation in how their materials perform in your application.\u00a0 Get the vendors’ best advice for each application, and then proceed by trial and error.\u00a0 If that’s not fun enough for you, then invite a few representatives to visit on the same day, and have them duke it out in back by the loading dock.<\/p>\n

\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014<\/p>\n

Isn\u2019t Everything He Knows Wrong, Too?<\/strong><\/p>\n

The straight dope on Tony Kordyban<\/strong><\/em><\/p>\n

Tony Kordyban has been an engineer in the field of electronics cooling for different telecom and power supply companies (who can keep track when they change names so frequently?) for the last twenty years.\u00a0 Maybe that doesn\u2019t make him an expert in heat transfer theory, but it has certainly gained him a lot of experience in the ways NOT to\u00a0cool electronics.\u00a0 He does have some book-learnin\u2019, with a BS in Mechanical Engineering from the University of Detroit (motto:Detroit\u2014 no place for wimps) and a Masters in Mechanical Engineering from Stanford (motto: shouldn\u2019t Nobels count more than Rose Bowls?)<\/p>\n

\"\"In those twenty years Tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney.\u00a0 He has run into so much nonsense in the field that he has found it easier to just assume \u201ceverything you know is wrong\u201d (from the comedy album by Firesign Theatre), and to question everything against the basic principles of heat transfer theory.<\/p>\n

Tony has been collecting case studies of the wrong way to cool electronics, using them to educate the cooling masses, applying humor as the sugar to help the medicine go down.\u00a0 These have been published recently by the ASME Press in a book called, \u201cHot Air Rises and Heat Sinks:\u00a0 Everything You Know About Cooling Electronics Is Wrong.\u201d\u00a0 It is available direct from ASME Press at 1-800-843-2763 or at their web site at\u00a0http:\/\/www.asme.org\/pubs\/asmepress<\/a>,\u00a0\u00a0<\/em><\/strong>Order Number 800741.<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

Answers to those Doggone Thermal Design Questions By Tony Kordyban Copyright by Tony Kordyban 2002 To the Thermal Explaining Guy, What’s the deal with thermal interface materials?\u00a0 I’m referring to the whole collection of goops, greases, silicone pads and gap fillers intended to be stuck between a component and a heat sink.\u00a0 They are supposed […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-394","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/394","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=394"}],"version-history":[{"count":2,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/394\/revisions"}],"predecessor-version":[{"id":400,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/394\/revisions\/400"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=394"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}