{"id":209,"date":"2012-06-01T20:50:49","date_gmt":"2012-06-02T01:50:49","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=209"},"modified":"2014-04-13T20:22:14","modified_gmt":"2014-04-14T01:22:14","slug":"everything-you-know-is-wrong-april-2001","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=209","title":{"rendered":"Everything You Know Is Wrong April 2001"},"content":{"rendered":"

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

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

\n

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

Dear Doggone Thermal Guy,<\/em><\/p>\n

I have an RF amplifier device which operates fairly inefficiently. Only about 1\/3 of the power it takes in goes out as RF . The rest is dissipated as heat (5 watts).\u00a0\u00a0 The figure below shows an arrangement recommended by the RF amplifier vendor to insure that the chip is well-connected to the heat sink. The heat sink is mounted on the side of the board opposite the device. To get an adequate thermal path, the vendor recommends a “pedestal” or pin, which is press fit into the sink, and soldered to the backside of the device (the device has a metal backside plate for this purpose).\u00a0<\/em><\/p>\n

\"\"<\/a>Some of our circuit designers have tried this approach, but using a “solder plug” instead of a pin\/pedestal.\u00a0 This seems to work, but our manufacturing group says it is hard to manufacture. I can see why, since it must take a bit of work to fill a 1\/8 inch diameter hole full of solder.\u00a0 Other designers say that simply stitching the front and back planes of the circuit with a dense pack of vias will be enough.\u00a0<\/em><\/p>\n

Do you have any recommendations? We want to make sure that the heat sink is well-connected, but also that the board design is manufacturable.\u00a0<\/em><\/p>\n

Murky in Albuquerque\u00a0<\/em><\/p>\n

 <\/p>\n

Dear Murky,<\/p>\n

I am shocked that you think your RF device is relatively inefficient.\u00a0 At least 1\/3 of its input power isn’t completely wasted, as it is in almost any other electronic component.\u00a0 How much useful energy comes out of a typical microprocessor?\u00a0 Dribs and drabs of a few milliamps once in a while on the signal leads.\u00a0 It is so small that I usually assume that exactly 100% of the electrical power of any component is converted to heat.\u00a0 But I digress.<\/p>\n

It is easy to see why your manufacturing folks hate this heat sink arrangement.\u00a0 Did the vendor who recommended it ever try to build such a thing on a production line?\u00a0 No doubt the thermal performance is great.\u00a0 There is metal-to-metal contact all the way from the heat spreader on the RF device to the large heat sink.<\/p>\n

But they must be twisting their safety glasses into pretzels trying to solder the heat sink to the backside of the printed circuit board.\u00a0 My opinion is that it will be very difficult to get that solder paste to re-flow underneath a large heat sink.\u00a0 The thermal mass of the aluminum extrusion will prevent the solder from melting in the re-flow oven for a long time, probably much longer than all the rest of the components on the board can stand.\u00a0 Either you cook the rest of the board, or you get a bad solder joint under the heat sink.<\/p>\n

I am also afraid that the same problem (the high thermal capacitance of the heat sink) will prevent a good solder joint from forming between the pin and the heat spreader plate on the bottom of the RF device.\u00a0 The heat sink will “suck away” the energy needed to re-flow the solder unless you use a very long soldering time.\u00a0 And a long soldering time could damage the die inside your RF device.<\/p>\n

Those complainers in manufacturing probably have a similar problem with that “solder plug” idea.\u00a0 Thermally it is nearly as good (although solder is not as good a conductor as aluminum.)\u00a0 With the heat sink in contact with the solder, it will be difficult to make it melt and flow where you want it to go.<\/p>\n

The “whole bunch of vias” idea sounds like the most manufacturable approach to me. Let’s see if we can estimate its thermal resistance, compared to the pin and solder plug methods.<\/p>\n

\"\"<\/a>Let’s say we will solder the RF device to a copper pad on the top side of the board, packing as many vias as possible under that pad to connect it to the back side of the board.\u00a0 (A via is a small hole in a circuit board, whose sides are plated with copper.\u00a0 The normal purpose of a via is to make an electrical connection between the copper layers in a multi-layer circuit board.\u00a0 Just by accident, it can also be a conductive path for heat.)<\/p>\n

Then I would attach the heat sink to the bottom of the board with adhesive<\/em>, not solder paste.\u00a0 You might be able to use thermally-conductive, double-sided tape, if the two surfaces are flat enough.\u00a0 The heat sink can be attached after the re-flow process, so it will not interfere with the solder joint formation under your RF device.<\/p>\n

Let’s see if we can estimate how much copper we add to a printed circuit board when we add vias.\u00a0 What we need to do is estimate the percent of cross-sectional area under the component that is copper, and how much is epoxy (or air or other material).\u00a0 That is highly dependent on the geometry of the vias.\u00a0 Scope out this sketch.<\/p>\n

You can’t drill holes in a board right next to each other.\u00a0 There is a minimum distance they must be spaced, that depends on the hole diameter and the thickness of the board.\u00a0 Let’s call the edge-to-edge spacing of the holes “s”.<\/p>\n

Then there is the diameter of the hole itself, “d”, and the thickness of the copper plating inside the hole, “t”.<\/p>\n

The area of the board taken up by one via is given by:<\/p>\n

Area = (d + s)2<\/sup><\/strong><\/p>\n

The cross-sectional area of the copper barrel in the hole is given by:<\/p>\n

Area cu<\/sub> = “Pi” d t<\/strong><\/p>\n

So the percent of copper due to adding the via is<\/p>\n

Percent Cu =<\/strong>\u00a0 “Pi” d t \/ (d + s)2<\/sup><\/strong><\/p>\n

And from that you can figure out the effective thermal conductivity of the board in the region filled with vias.\u00a0 You multiply the conductivity of copper (about 380 W\/mC) by the Percent Cu.\u00a0 You can neglect the tiny amount contributed by the epoxy.\u00a0 For example, if we throw in some numbers for diameter and spacing and thickness and come out with 10% copper, we can say the conductivity of the board in that region is 38 W\/mC.\u00a0 We’ll need this little calculation in a minute.<\/p>\n

Now I’ll give you the super-simplistic equation for calculating the thermal resistance of a heat conduction path.\u00a0 Let’s assume everything is one-dimensional.\u00a0 (For two or more dimensions, I have to start charging money.)<\/p>\n

 <\/p>\n

R = L \/ (k X A)<\/strong><\/p>\n

where<\/p>\n

R<\/strong> is thermal resistance<\/p>\n

L<\/strong> is the length of the path<\/p>\n

k<\/strong> is the thermal conductivity of the material<\/p>\n

A<\/strong> is the cross-sectional area of the path<\/p>\n

We can use this to estimate the thermal resistance of the three proposed thermal paths from the RF device to the heat sink.\u00a0 We know, or can make a good guess about most of the variables in this equation.\u00a0 The cross-sectional area of the path is known because you told me the hole for the solder plug is 1\/8 inch diameter.\u00a0 One guess I need to make is the thickness of the board (which is L, the length of the path from the component to the heat sink).\u00a0 I will assume a typical board thickness of 1\/16 inch.<\/p>\n

Then we have to make some initial design decisions.\u00a0 How big should the vias be?\u00a0 They can vary in size from 0.010 to 0.100 inches, depending on their purpose and manufacturing standards.\u00a0 I will pick for starters a diameter of 0.012 inch (0.3 mm).\u00a0 And with no real justification, I will assume that the I can pack them so close together that the spacing from edge of one via to the next is the same as the diameter — 0.012 inch.\u00a0 I believe the plating is about 0.0007 inch thick for 1\/2 ounce copper.\u00a0 Those dimensions give me a Percent Cu of 4.6%, and an effective conductivity for the board of 17 W\/mC.<\/p>\n

Another number I need is the thermal resistance of the adhesive layer.\u00a0 Without getting too sticky about it, let’s just say it’s 1 C\/W for now.\u00a0 That’s in the realistic range.\u00a0 You can always put in a better number when you select your favorite adhesive.<\/p>\n

Let’s see how the performance of the three paths compares:<\/p>\n\n\n\n\n\n\n\n
\n

Thermal path<\/strong><\/p>\n<\/td>\n

\n

k<\/strong><\/p>\n<\/td>\n

\n

Resistance<\/strong><\/p>\n<\/td>\n

\n

Temperature rise at 5W<\/strong><\/p>\n<\/td>\n<\/tr>\n

\n

(W\/m<\/strong>\u00b0<\/strong>C)<\/strong><\/p>\n<\/td>\n

\n

(<\/strong>\u00b0<\/strong>C\/W)<\/strong><\/p>\n<\/td>\n

\n

(<\/strong>\u00b0<\/strong>C)\u00a0<\/strong><\/p>\n<\/td>\n<\/tr>\n

\n

Aluminum pin<\/p>\n<\/td>\n

\n

180<\/p>\n<\/td>\n

\n

1.12<\/p>\n<\/td>\n

\n

5.60<\/p>\n<\/td>\n<\/tr>\n

\n

Solder plug<\/p>\n<\/td>\n

\n

50<\/p>\n<\/td>\n

\n

4.02<\/p>\n<\/td>\n

\n

20.1<\/p>\n<\/td>\n<\/tr>\n

\n

Vias and adhesive<\/p>\n<\/td>\n

\n

17<\/p>\n<\/td>\n

\n

11.8 (vias) + 1 (adhesive) = 12.8<\/p>\n<\/td>\n

\n

64.0<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The vias and adhesive ARE the worst thermal path.\u00a0 But how bad is that path?\u00a0 If all 5 W from your RF device go through this path, the temperature rise due to its resistance will be about 64 degrees C.\u00a0 Whether that is bad or not depends on how close you are to the temperature limit of the RF device when it is operating in its worst case environment.\u00a0 Maybe you have 64 degrees to spare (unlikely).\u00a0 If so, it might be worth spending them to get a manufacturable design.<\/p>\n

But I have not optimized the via dimensions at all.\u00a0 You could increase the area under the chip filled with vias, for example.\u00a0 I bet if you play around with the dimensions, sticking to your manufacturing standards for vias, of course, you could find a combination that works better.\u00a0 (It also makes sense to use the real thickness of your board, since you know it.)<\/p>\n

A circuit board vendor gave me a design tip that you might want to consider, too.\u00a0 He suggested filling the vias with solder.\u00a0 He claimed that a 0.012 inch via would wick up solder paste during the re-flow process and be completely filled with solder if you put paste on both sides of the board.\u00a0 Filling the hole with solder would increase the via conductivity in my example to 27 W\/mC.\u00a0 That would reduce the resistance to 8.4 C\/W, and the temperature rise would only be 37 deg C.<\/p>\n

Talk to your circuit board vendor.\u00a0 Talk to an adhesives vendor.\u00a0 Don’t be afraid to question their claims.\u00a0 Because now you have a way to think about the problem that will help you find your own optimal solution.<\/p>\n

———————————–<\/p>\n

Dear Lord of the Pants-of-Smartness,\u00a0<\/em><\/p>\n

I love your articles, and I am finding lots of good stuff on electronics cooling here on CoolingZone, but it is all in dribs and drabs, to coin a phrase.\u00a0 A beginner like me struggles to put it all together.\u00a0 Is there a basic textbook on this subject that can walk me through the thermal design and analysis process from beginning to end?\u00a0 Some of the books you have recommended in the past are out of print.\u00a0<\/em><\/p>\n

Mike in Mechanicsville<\/em><\/p>\n

 <\/p>\n

 <\/p>\n

Dear Mike,<\/p>\n

Out of print?\u00a0 You never heard of a library?<\/p>\n

Seriously, there are some good books on electronics cooling that you can’t buy through amazon.com, because they are out of print, or just seem that way.\u00a0 But university libraries all over the globe are bulging with these tomes, and the librarians are begging people to check them out to get the dust off of them once in a while.\u00a0 Check you local university library.\u00a0 If you are not a registered student, they may not lend them to you directly.\u00a0 Sometimes you can get them on inter-library loan through your local public library.<\/p>\n

If you just have to have your own copy of an out-of-print classic, you can often find them through one of the out-of-print booksellers on the Internet.\u00a0 My wife suggests half.com<\/a> and Powells<\/a>, which have found some obscure items (such as “The Best of Women’s Science Fiction”) for her.<\/p>\n

So which ones should you look for?\u00a0 Here’s a list of books I have actually read and\/or used:<\/p>\n

IN PRINT\u00a0<\/strong><\/p>\n

Dave S. Steinberg, Cooling Techniques for Electronic Equipment,\u00a0<\/em> John Wiley & Sons<\/a><\/p>\n

 <\/p>\n

OUT OF PRINT<\/strong><\/p>\n

Gordon Ellison, Thermal Computations for Electronic Equipment<\/em>, Van Nostrand Reinhold or Krieger<\/p>\n

(This book has been extensively revised and re-issued under a new titile:
\n Thermal Computations for Electronics: Conductive, Radiative, and Convective Air Cooling<\/a><\/p>\n

Allan D. Kraus and Avram Bar-Cohen, Thermal Analysis and Control of Electronic Equipment<\/em>, McGraw Hill<\/p>\n

Allan W. Scott, Cooling of Electronic Equipment,<\/em> John Wiley & Sons<\/p>\n

Here are a couple of more IN PRINT books that I found on amazon.com.\u00a0 I have not read them, but they look promising from their Tables of Contents.\u00a0 I offer them for your investigation.<\/p>\n

Al Krum, Thermal Management Handbook : For Electronic Assemblies<\/em><\/a><\/p>\n

Air Cooling Technology for Electronic Equipment<\/em> by Sung-Jin Kim (Editor), Sang Woo Lee (Editor)<\/a><\/p>\n

That should be enough to get you started.\u00a0 If any doggone loyal readers have other suggestions, please let me know.<\/p>\n

P.S.\u00a0 I almost forgot another book by an obscure author.\u00a0 It’s not exactly what you asked for.\u00a0 It’s more of a book of what NOT to do in cooling electronics:\u00a0 Hot Air Rises and Heat Sinks<\/a><\/em>.\u00a0 According to the publisher, they have to clear out a lot more boxes of it before it can be called out of print.<\/p>\n

———————————–
\nHi, T.K.,<\/em><\/p>\n

It’s Mike again.\u00a0 While you were surfing the net looking for cooling books, did you happen to notice any fascinating books about how to drive your car more safely?\u00a0 My teenage son is getting his license soon, and I wish there was a book written by a curmudgeon with literally a million driving miles under his belt who could share some of that experience, in an honest and engaging tone that my son would actually want to read.<\/em><\/p>\n

M in M<\/em><\/p>\n

 <\/p>\n

Dear M.,<\/p>\n

Funny you should mention!\u00a0 There’s a book called How to Drive into Accidents – And How Not to<\/a><\/em> by Robert A. Pease, that sounds right up your alley.\u00a0 (Yes, it’s that same Robert Pease analog circuits guy.)\u00a0 I never thought I needed to know how a differential works to drive a car, but after reading this, I can’t imagine driving any other way.<\/p>\n<\/div>\n

\n

\u00a0<\/strong><\/p>\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 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 http:\/\/www.asme.org\/pubs\/asmepress<\/a>,\u00a0 <\/em><\/strong>Order Number 800741.<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Answers to those Doggone Thermal Design Questions By Tony Kordyban Copyright by Tony Kordyban 2001 Dear Doggone Thermal Guy, I have an RF amplifier device which operates fairly inefficiently. Only about 1\/3 of the power it takes in goes out as RF . The rest is dissipated as heat (5 watts).\u00a0\u00a0 The figure below shows […]<\/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-209","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/209","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=209"}],"version-history":[{"count":3,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/209\/revisions"}],"predecessor-version":[{"id":229,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/209\/revisions\/229"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=209"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}