Difference between volume or bulk resistivity and surface or sheet resistivity

Bulk Thermal Conductivity vs. Thermal Impedence

With this entry I’d like to tack­le the issue of ther­mal con­duc­tiv­i­ty vs. ther­mal impe­dence in adhe­sive bond­lines. Although some adhe­sive man­u­fac­tur­ers may claim to have bulk ther­mal con­duc­tiv­i­ty val­ues high­er than 30 W/m°K, device man­u­fac­tur­ers need to appre­ci­ate that the bulk val­ue is only an indi­ca­tor of poten­tial for heat trans­fer through the adhe­sive bond­line. The mate­r­i­al with the low­est ther­mal imped­ance should actu­al­ly be the goal, as it is with this that the best heat trans­fer can actu­al­ly be obtained.

The mate­r­i­al with the low­est ther­mal imped­ance should actu­al­ly be the goal

The fac­tors that affect ther­mal imped­ance include: adhe­sion at the inter­face, sur­face wet­ting, thin adhe­sive bond­lines and void-free bond­lines. If the inter­fa­cial adhe­sion is weak or impact­ed by filler align­ment, then the ther­mal resis­tance across the inter­face will be so great that the bulk resis­tiv­i­ty becomes mean­ing­less. If the bond­line can be kept thin, for exam­ple by using a sol­vent­less adhe­sive, then heat trans­fer will be very effi­cient regard­less of the dif­fer­ence in the bulk ther­mal conductivity.

What is the thermal conductivity of air?

Con­sid­er for a moment that air has a ther­mal con­duc­tiv­i­ty of 0.0261 W/m°K (@25°C). Even a mod­er­ate­ly-filled sil­i­ca epoxy has a ther­mal con­duc­tiv­i­ty of 0.3 W/m°K which is already more than 10 times more con­duc­tive than air. By the same token, con­sid­er a high­ly sol­vent-filled adhe­sive. Should the least amount of air be present, whether through a void or delam­i­na­tion, then the bulk ther­mal con­duc­tiv­i­ty is imme­di­ate­ly no longer the same.

With this in mind, I believe that the high­est sol­vent-free, organ­ic-filled pastes we will see will nev­er be high­er than 10 W/m°K, but that these pastes are still good can­di­dates for appli­ca­tions requir­ing high­er ther­mal con­duc­tiv­i­ty — even those appli­ca­tions request­ing val­ues greater than 10 W/m°K. I remain open to the pos­si­bil­i­ty that high­er ther­mal-con­duc­tiv­i­ty prod­ucts may appear on the mar­ket, but I would look first to the test method used to deter­mine the ther­mal con­duc­tiv­i­ty and then to the void-free bond­lines that these adhe­sives can achieve before being con­vinced that they make supe­ri­or can­di­dates for high ther­mal­ly con­duc­tive applications.

CAPLINQ is a spe­cial­ty plas­tics sup­pli­er offer­ing a range of con­duc­tive mate­ri­als includ­ing our broad range of elec­tri­cal­ly con­duc­tive plas­tics and anti­sta­t­ic tapes and films. If you have any ques­tions, please don’t hes­i­tate to con­tact us to find out how we can help you with oth­er ques­tions relat­ed to bulk ther­mal con­duc­tiv­i­ty or ther­mal impedence.

About Chris Perabo

Chris is an energetic and enthusiastic engineer and entrepreneur. He is always interested in taking highly technical subjects and distilling these to their essence so that even the layman can understand. He loves to get into the technical details of an issue and then understand how it can be useful for specific customers and applications. Chris is currently the Director of Business Development at CAPLINQ.

16 thoughts on “Bulk Thermal Conductivity vs. Thermal Impedence

  1. dear sir,
    i am more con­cerned about how to increase the ther­mal con­duc­tiv­i­ty so that i can get less tim­ing for cool­ing the reac­tor & ther­by increas­ing my production.

    prod­uct- styropore
    preav­i­ous k val­ue-490 w/mk
    resent k val­ue- 465 w/mk
    k val­ue decreased to 25 units
    am get­tin more time to cool the reac­tion mix­ture, which affect my production.
    am more con­cerned about how to increase the k val­ue, there is no dipo­si­tion on jack­et side.

  2. Sor­ry, our spe­cial­ty is work­ing with ther­mal­ly con­duc­tive adhe­sives, or oth­er spe­cial­ty chem­i­cals. Since the sty­ro­pore falls out­side this cat­e­go­ry, our com­ments may not be of much use to you.

  3. Where the Sty­ro­pore avail­able? We require the list of sup­pli­ers. Bet­ter, if get the Indian/ Asian Supplier.

  4. CAPLINQ can sup­ply the Sty­ro­pore or equiv­a­lent mate­r­i­al. What we require with these inquiries are the size of prod­uct required and the quan­ti­ty request­ed. Once this is received, CAPLINQ can pro­vide a quo­ta­tion for you.

  5. i have a ques­tion about heat transfer
    let say: one face of alu­minum plat 1.5cm thick is main­tained 700 degree cel­sius and the oth­er face is main­tained at 20 degree cel­sius (air outside).
    ther­ma con­duc­tiv­i­ty for alu­minum is 202w/mxk
    area is 0.0491 m square

    so the answer is approx­i­mate 449.63Kw/m square

    so… it take how long to bal­ance the both temperature????

    thank you

  6. Though this isn’t the right forum to dis­cuss ther­mo­dy­nam­ics equa­tions, this is indeed a sim­ple for­mu­la that fol­lows New­ton’s law of cool­ing which states that the rate of heat loss of a body is pro­por­tion­al to the dif­fer­ence in tem­per­a­tures between the body and its sur­round­ings. The law is:

    Q = h · A(To — Ta)
    Q = Heat trans­fer in Watts
    h = Heat trans­fer coefficient
    A = Sur­face area of the heat being transferred
    To = Tem­per­a­ture of the objec­t’s surface
    Ta = Tem­per­a­ture of the surroundings

    Grab a paper and pen­cil and you’ll get your answer in no time!

  7. please i need table for as many ther­mal con­duc­tiv­i­ty of sub­stances as possible.

  8. i need your high lev­el elec­tri­cal­ly con­duc­tive film for my project. may i know high volt­age it can con­duct dur­ing elec­tro sta­t­i­cal­ly seper­at­ed as two elec­trodes and we only sup­ply the dc high volt­age. and i need only 4meters of film only. there is any chance to get in india .please send me the reply as soon as possible .

  9. i need your high lev­el elec­tri­cal­ly con­duc­tive film for my
    project. may i know high volt­age it can con­duct dur­ing electro
    sta­t­i­cal­ly seper­at­ed as two elec­trodes and we only sup­ply the dc
    high volt­age. and i need only 4meters of film only. there is any
    chance to get in india .please send me the reply as soon as
    pos­si­ble . it should bear (1–5kv) and high inducing

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