High Thermal Impedance

Overheating of devices resulting in electrical malfunctioning or reduction in the life expectancy of the device and circuit can be the direct result of poor conductivity and high thermal impedance of the adhesive. Thermal conductivity of adhesives becomes critical as devices are operated at higher speeds, consume more power, and dissipate more heat. Semiconductor junction temperatures must remain within normal limits, particularly for 

Thermal-transfer adhesives that are electrically insulating also exhibit wide ranges of thermal conductivities, depending on the filler type and amount. The thermal conductivities of epoxies filled with boron nitride or diamond are approximately 4 W/m K and 12 W/m K, respectively, while those of the more common aluminum-oxide-filled adhesives range from 1-2 W/m K. 

Thermal conductivity requirements are specified in both military and industry specifications generally as 1.5 W/m K or greater for electrically conductive adhesives and 0.15 W/m K or greater for electrically insulative adhesives. 

The selection of highly conductive adhesives is a necessary factor, but not the only one in minimizing thermal impedance. Applying as thin abond-line as possible and avoiding the introduction or creation of voids are also essential in reducing thermal impedance. 

Bondline thickness. The control of bondline thickness is essential for high-power devices. A thin bondline is required since thickness is directly proportional to the jimction-to-case thermal resistance, djc (see Ch. 2). Specially formulated solvent-based hybrid adhesives can produce imiform bondline thicknesses of one mil or less. Small hard spacers, acting as shims, added to paste adhesive formulations, have also been used to control thickness. The use of preforms instead of paste adhesives also assures 

---

The major failure modes of adhesives include loss of adhesion, high thermal impedance, loss of electrical contact, and corrosion. To a lesser extent, failures may occur because of metal migration causing high leakage currents and even electrical shorting. Sloughing of particles from the... 

Thermal isolation. If the detector is used in a hot environment or is cooled, then it must have some thermal isolation from the environment. The required thermal impedance depends on the temperature difference between the cold-sink and the environment and, if cooled, the available cooling power. To achieve that isolation may require the use of special high-thermal-impedance materials (see Chapter 12, Cryogenics) in some cases, the package may have to be evacuated or backfilled with a low-thermal-conductivity gas. Both of these impose special cleaning ( outgassing ) requirements, and necessitate an evacuation ( pump-out ) port, and hermetic seals for any windows and electrical leads. These vacuum issues are described in Chapter 13 (Vacuum). 

Figure 12.11 Temperature sensor lead attachment, (a) Poor technique short leads, poor anchoring, (b) Good technique high thermal impedance and good anchoring.

Secondly, heat is transferred towards the surface, within the 0.4 mm mixed con-duction/convection layer, via a very large temperature gradient of the order of 5000-10,000 K/m, by a relatively weak thermal process. With a high thermal impedance, the process consists of a static thermal conductance enhanced about 1.5-2.5 times by penetration of some of the intermittent convection from the Rayleigh/Benard convection below. 

When the disturbance ceases, the surface sub-layer regions re-estabUsh themselves, the high thermal impedance reappears and the evaporation rate falls. Since region 2, the conduction/convection layer, is so thin, the self-repair takes place in a few seconds or minutes and the evaporation rate consequently recovers its previous normal value in the same time. 

The literature of polyimines is extensive [164-173]. A number of researchers have tried to synthesize high molecular weight polymers but failed due to poor solubility in organic solvents. Polyimines are of great interest because of their high thermal stability [174-176], ability to form metal chelates [174-177], and their semiconducting properties [178-181]. Due to insolubility and infusibility, which impeded characterization of the molecular structure, the application of these polymers is very limited and of little commercial importance. 

When constituted of metals, thermopiles exhibit a very low noise, in particular only thermal noise if the voltage amplifier used for signal amplification has a very high input impedance. 

The main factors controlling a sandstone compaction are shales diagenesis and subsequent overpressures, the presence of evaporite formations which permit heat loss due to their high thermal conductivity, and early quartz overgrowths which resulted in a solid sandstone skeleton before Mesozoic subsidence. These overgrowths have impeded further reservoir compaction. Mapping such early quartz overgrowth devel-... 

M. Fleckenstdn, S. Fischer, O. Bohlen, et al, Thermal Impedance Spectroscopy—A method for the thermal characterization of high power battery cells, Journal of Power Sources, vol. 223, pp. 259-267, 2013. 

---