Thermal conductivity substrate-attach adhesives

During die attach, an adhesive creates a mechanical interface between the die and substrate in a semiconductor assembly. A typical dispense pattern has intersecting lines with strategically placed dots so that after die placement, the fluid is evenly distributed (Figure 10-12). Most die attach adhesives also provide thermal and/or electrical conductivity between the die and substrate. 

Thermally conductive adhesives may be filled with metal, ceramic, or inorganic particles. Silver-filled epoxies have high thermal conductivities, but may not be used where there is a risk of electrical shorting. In such cases, epoxies or other polymers filled with electrically resistive, but thermally conductive materials such as aluminum nitride, boron nitride, alumina, or beryllia must be used. Some applications for thermally conductive adhesives include attachment of power devices, heat sinks, large components such as capacitors and transformers, large ceramic substrates, and edge connectors. 

In general, film adhesives are best suited as insulation to isolate circuitry or to attach large devices or substrates. Insulative films have volume resistivities from 10 to 10 " ohm-cm, thermal conductivities from 0.8 to 1 W/mK, and lap shear strengths ranging from 1200 to 3000 psi. They are available in thicknesses from 2 to 8 mils. ° ... 

Besides high thermal conductivities, adhesives must combine low cost and relieve stresses from mismatches in the coefficients of expansion of the LED material and the substrates or packages to which they are attached. 

In a typical case where a silicon IC is attached to an alumina ceramic substrate that, in turn, is attached to the inside of a metal or ceramic package, the two epoxy interfaces can easily contribute 2.5 °C/watt to the total resistance. However, some silver-filled epoxies are reported to have high thermal conductivities, thus contributing 0.6 °C to 1 °C/watt. Actual measurements may differ considerably from calculated values because of reported thermal conductivities that differ from the actual, differences in the thicknesses of bond lines, voids in the adhesive, and incomplete mating of surfaces. Further in the analysis, the effects of lateral flow of heat and interactions of heat flow among adjacent components are often neglected. 

Lastly, a technique that has been used for many years involves first eutectic or solder attaching high-power devices to small heat spreaders such as molytabs (gold-plated molybdenum tabs that are the same size or slightly larger than the die). Thermally conductive adhesive is then used to attached the die-tab parts to a ceramic or laminate interconnect substrate. 

T0 assure optimum conductance, the adhesive must be applied as thinly and uniformly as possible. To control the thicknesses bondlines, thermally conductive paste adhesives have been formulated with collapsible spacers. The spacers are reported to control bondline thickness to 1.2 mils. These adhesives were developed for stacked die packages, but may also be used to attach ICs and other devices to substrates in plastic EGAs, CSPs, and array packages based on flexible tape or plastic laminates.Bondline thicknesses and uniformity may also be achieved by using film or preform tape adhesives and controlling the applied pressure and heat during cure. 

Curves of Figure 19 compare the data published for (a) boron nitride [37,40] (b) aluminium (c) diamond-[37-39] (d) aluminium nitride [37-42] (e) crystalline silica. It can be seen that, at 45 vol.%, the maximum thermal conductivity achieved with diamond powder is 1.5 W m K, while crystalline boron nitride at 35 vol.% affords 2.0Wm K. The thermal conductivity of silver-filled adhesives was studied by using silicon test chips attached to copper and molybdenum substrates [43]. The authors outline the importance of the shape factor A, related to the aspect ratio of the particles, to achieve the highest level of thermal conductivity. Another study reports the variation of the effective thermal resistance, between a test chip and the chip carrier, in relation to the volume fraction of silver and the thickness of the bond layer [44]. The ultimate value of bulk thermal conductivity is 2 W m at 25 vol.% silver. However, the effective thermal conductivity, calculated from the thermal resistance measurements, is only one-fifth of the bulk value when the silicon chip is bonded to a copper substrate. 

The three-dimensional models predict that the stress level can be reduced if the bond line thickness is increased from 25 to 75 or even 175 xm. However, the curves of Figure 50 show, e.g. that the maximum shearing stress decreases by a factor of two, from 33 to 17 MPa, when the thickness of the adhesive layer increases from 25 to 100 xm. A bond line thickness of 50-75 xm is generally recommended for the die attachment because of the negligible thermal impedance penalty. The experimental results indicate that, between 20 and 80 xm, the thickness of the adhesive joint does not greatly affect the thermal transfer capability. This behaviour has been explained by the fact that the interfacial thermal resistances between the adhesive and both the die and the substrate are much higher than that contributed by the bulk thermal conductivity of the adhesive materials. 

---