{"id":190,"date":"2012-06-01T20:49:57","date_gmt":"2012-06-02T01:49:57","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=190"},"modified":"2012-06-01T20:49:57","modified_gmt":"2012-06-02T01:49:57","slug":"everything-you-know-is-wrong-january-2001","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=190","title":{"rendered":"Everything You Know Is Wrong January 2001"},"content":{"rendered":"

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

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

Copyright by Tony Kordyban 2001<\/p>\n

Dear Mr. Cooling Smart-Guy,<\/em><\/p>\n

Is being a Thermal Engineer in the electronics business a good career?\u00a0 I am in my junior year of engineering.\u00a0 I wanted to go with one of those dot-com outfits, but lately my e-mails just bounce back.\u00a0 I\u2019m looking for high pay, stock options, and not too many hours, and also a fast Internet link for downloading music at the office.\u00a0 What you do doesn\u2019t look too hard.\u00a0 Do you need to know anything to pick heat sinks out of a catalog?<\/em><\/p>\n

Buzz by Berkele<\/em>y<\/p>\n

 <\/p>\n

Dear Buzz,<\/p>\n

Good thing you have one more year of school.\u00a0 You still have a couple things to learn.<\/p>\n

A thermal engineer in electronics cooling needs some technical background \u2014 heat transfer, mechanical design, science of materials, and some laboratory skills.\u00a0 But probably just as important are communication skills, because half your job will be convincing marketing folks, digital circuit designers, and your own manager, that what you do is important.<\/p>\n

Being a thermal engineer in an electronics company is like being a chaplain in the army.\u00a0 You are outnumbered a thousand to one, and everybody thinks that because you don\u2019t carry a (soldering) gun, you don\u2019t do a real job.\u00a0 They don\u2019t listen to your sermons on Sunday, but when they find themselves on the brink of disaster, they run to you for salvation.<\/p>\n

I love it, but it\u2019s not for everybody.<\/p>\n

 <\/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<\/p>\n

 <\/p>\n

Dear Monsieur Know-It-All,<\/em><\/p>\n

These, as you say, doggone SMT (surface mount) components have shrunk so much that I cannot even consider putting a heat sink on them anymore.\u00a0 Is it possible to use the copper traces in the PCB (printed circuit board) as a heat sink?\u00a0 If so, how would I estimate the heat sink capability of the PCB, assuming that I know the pattern of copper traces and the thermal resistance of the component package?<\/em><\/p>\n

Regards,<\/em><\/p>\n

Quizzical in Quebec<\/em><\/p>\n

 <\/p>\n

Dear Quiz,<\/p>\n

The answer is yes and no.\u00a0 Yes, the PCB can act as a heat sink, and no, you won\u2019t be able to estimate how well it will work, using the package thermal resistance as a starting point.\u00a0 If you take a look at my December 2000 column<\/a>, there is a lovely drawing of a component on a PCB, with arrows showing heat spreading through the leads and into the board.<\/p>\n

You face four problems here to getting a simple answer to your question:<\/p>\n

1.\u00a0 I never give a simple answer.\u00a0 Sorry, just my nature.<\/p>\n

2.\u00a0 I assume the thermal resistance you are referring to is either Theta j-a<\/strong> (resistance between the junction and ambient) or Theta j-c<\/strong> (resistance between the junction and the case.\u00a0 By their very definition, they already include the heat sinking effect of a PCB in them.\u00a0 Unfortunately, it is probably nothing like the PCB you want to use.\u00a0 Here is how thermal resistance is measured, according to the JEDEC standard.\u00a0 Your component is mounted on a standard test board, about 4 inches square, as shown in Figure 1.<\/p>\n

\"\"<\/a><\/p>\n

The test board has copper traces on both sides, so it conducts heat away from the component.\u00a0 The PCB gives off heat to the surrounding air, plus it radiates to the walls of the test chamber, so it acts as a heat sink with from 10 to 100 times the surface area of the package alone.\u00a0 Junction temperature (Tj), case temperature (Tc) and the ambient air temperature (Ta) are measured, along with the electrical power (P) delivered to the part.\u00a0 Thermal resistances are then calculated from:<\/p>\n

 <\/p>\n

Theta j-a = P\/(Tj \u2013Ta)<\/strong><\/p>\n

Theta j-c = P\/(Tj-Tc)<\/strong><\/p>\n

Although it never says this in the component data sheet, the thermal resistance is not for the package, but for the total assembly of the package soldered onto a large PCB.\u00a0 What do you think would happen to the official value of Theta j-a<\/strong> if the test board was smaller?\u00a0 Theta j-a<\/strong> would increase, because the surface area of the assembly is less, just as if you had decreased the size of its heat sink.<\/p>\n

How does that apply to your situation?\u00a0 Most likely you are not designing a board with all the components 8 inches apart from each other in every direction.\u00a0 My guess is that your parts are packed so tightly that the leads nearly touch.\u00a0 So how much of the PCB would you say is dedicated to the component you are interested in?\u00a0 Much less than a 4 inch by 4 inch square, I bet.\u00a0 That means the Theta j-a<\/strong> got off a data sheet is, at best, an understatement of the thermal resistance.\u00a0 Just forget about Theta j-a<\/strong>.\u00a0 For a real board, in a real system, it is as useful as a fireplace on the International Space Station.<\/p>\n

3.\u00a0 It is possible to calculate the conduction heat transfer within the PCB from a component.\u00a0 But not by hand.\u00a0 All you have to do is make a 3-D finite element model of the PCB, with all the copper traces and epoxy layers shown in complete detail.\u00a0 You will also need a finite element model of the component package, because, as I hinted in #2, you don\u2019t know its true thermal resistance.\u00a0 And maybe models of all the other parts on the board, too.\u00a0 If you are going to do all that, then you should have a Computational Fluid Dynamics model of the air flow over both sides of the board, too.\u00a0 Once you have that, who cares about Theta j-a<\/strong>.\u00a0 You are going to solve for the entire temperature field and flow field around your board.\u00a0 You get all the component temperatures directly without having to estimate heat sinking capability.\u00a0 In fact, using a CFD modeling program like FLOTHERM or ICEPAK is the only rigorous way of solving a conjugate heat transfer problem like this.\u00a0 It\u2019s not simple, but it\u2019s not impossible.\u00a0 I do it every day as part of my job.\u00a0 Most of the time I don\u2019t bother to model all the copper and epoxy separately.\u00a0 I just use the lumped value of 10 W\/mK for the thermal conductivity of a typical PCB.<\/p>\n

4.\u00a0 Let\u2019s say you don\u2019t care what the exact value of the conductivity of the PCB is.\u00a0 You decide to cram as much copper around your device as you can to maximize the heat spreading.\u00a0 You will now be at odds with the poor folks who solder the parts on your PCB.\u00a0 They HATE big copper planes near solder pads.\u00a0 During soldering, fat copper traces suck away the heat that is supposed to melt the solder to form the electrical joints.\u00a0 So you get a board with lots of opens, partially-formed, or intermittent solder joints.\u00a0 Turning up the heat in the soldering machine to overcome this effect can damage the PCB or components.\u00a0 Your manufacturing engineer will come around looking for \u201cthermal relief,\u201d which is a phrase that has the opposite meaning Thermal Engineers expect.\u00a0 Thermal relief means disconnecting the solder pads from the copper in the PCB as much as possible, using the thinnest possible traces that will carry the electric current.\u00a0 So if you insist on lots of copper around your part to get the heat out, you will end up causing soldering problems.<\/p>\n

Maybe I should sum up.\u00a0 Heat does spread into the PCB from components, so the PCB can be considered a heat sink.\u00a0 You will never be able to estimate how good a heat sink the PCB can be with simple hand calculations.\u00a0 Thermal resistance values already assume lots of heat spreading into the PCB, which may not be true, or even ethical.\u00a0 And lastly, if you add lots of copper, crossing your fingers that it will reduce temperature for your trouble-spot components, you may screw up the soldering process.<\/p>\n

See what I mean about being pessimistic?<\/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<\/p>\n

Dear Hot-Shot,<\/em><\/p>\n

The Quality Department insists we need a full time thermal analyst for our Engineering Department.\u00a0 I\u2019m stuck hiring one.\u00a0 What kind of minimum qualifications should I be looking for?\u00a0 I figure once this thermal panic dies down, I can switch him over to something productive, like mechanical drafting, or hi-pot testing in the factory.\u00a0 Can a thermal analyst do both?<\/em><\/p>\n

H. F. Potter, BedfordFalls<\/em><\/p>\n

 <\/p>\n

Dear Mr. Potter,<\/p>\n

You are wise not to jump into this thermal racket with both feet.\u00a0 My advice is to start small and work you way up.\u00a0 Hire a college intern for the job and see how it works out.\u00a0 By the way, I have a resume from Berkeley \u00a0I am forwarding to you.<\/p>\n

——————————————————————————<\/p>\n

\n

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

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

 <\/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 cool electronics.\u00a0 He does have some book-learnin\u2019, with a BS in Mechanical Engineering from theUniversityofDetroit(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 http:\/\/www.asme.org\/pubs\/asmepress<\/a>,\u00a0 <\/em><\/strong>Order Number 800741.<\/p>\n

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

Answers to those Doggone Thermal Design Questions By Tony Kordyban Copyright by Tony Kordyban 2001 Dear Mr. Cooling Smart-Guy, Is being a Thermal Engineer in the electronics business a good career?\u00a0 I am in my junior year of engineering.\u00a0 I wanted to go with one of those dot-com outfits, but lately my e-mails just bounce […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":{"footnotes":""},"class_list":["post-190","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/190","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=190"}],"version-history":[{"count":4,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/190\/revisions"}],"predecessor-version":[{"id":226,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/190\/revisions\/226"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=190"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}