thermattach thermally conductive adhesive tapes.

introduction

today's semiconductors, whether discrete power or logic ics, are smaller, run faster, do more and generate more heat. some microprocessors dissipate power levels that were once the exclusive domain of discrete power devices, namely 10 to 25 watts. these power levels require thermal management techniques involving large capacity heat sinks, good air flow and careful management of thermal interface resistances. a well designed thermal management program will keep operating temperatures within acceptable limits in order to optimize device performance and reliability.

semiconductors are kept within their operating temperature limits by transferring junction generated waste heat to the ambient environment, usually the surrounding room air. this is best accomplished by attaching a heat sink to the semiconductor package surface thus increasing the heat transfer between the hot case and the cooling air. s. lee [1] recently described how heat sinks may be selected to provide optimum thermal performance. once the correct heat sink has been selected, it must be carefully joined to the semiconductor package to ensure efficient heat transfer through this newly formed thermal interface.

cho-therm t/a 274 highly conformable thermally
conductive gap fillers.

the rate of conductive heat transfer, q, across the interface is given by

where k is the thermal conductivity of the interface, a is the heat transfer area, l is the interface thickness and tc and ts are the device case and heat sink temperatures. the thermal resistance of a joint, rc-s, is given by

and on rearrangement,

thus, the thermal resistance of the joint is directly proportional to the joint thickness and inversely proportional to the thermal conductivity of the medium making up the joint and to the size of the heat transfer area. thermal resistance is minimized by making the joint as thin as possible, increasing joint thermal conductivity by eliminating interstitial air and making certain that both surfaces are in intimate contact.

attaching a heat sink to a semiconductor package requires that two solid surfaces be brought together into intimate contact. unfortunately, no matter how well-prepared, solid surfaces are never really flat or smooth enough to permit intimate contact. all surfaces have a certain roughness due to microscopic hills and valleys as shown in figure 1a. superimposed on this surface roughness is a macroscopic non-planarity in the form of a concave, convex or twisted shape as shown in figure 1b. as two such surfaces are brought together, only the hills of the surfaces come into physical contact. the valleys are separated and form air-filled gaps.

figure 1. representation of surface irregularity.
a. surface roughness	b. poor surface flatness

when two typical electronic component surfaces are brought together, less than one percent of the surfaces make physical contact. as much as 99% of the surfaces are separated by a layer of interstitial air. some heat is conducted through the physical contact points, but much more has to transfer through the air gaps. since air is a poor conductor of heat, it should be replaced by a more conductive material to increase the joint conductivity and thus improve heat flow across the thermal interface.

several types of thermally conductive materials can be used to eliminate air gaps from a thermal interface, including greases, reactive compounds, elastomers and pressure sensitive adhesive films. all are designed to conform to surface irregularities, thereby eliminating air voids and improving heat flow through the thermal interface. some have secondary properties and functions, as well.

types of thermal interface materials

thermal greases are made by dispersing thermally conductive ceramic fillers in silicone or hydrocarbon oils to form a paste. sufficient grease is applied to one of the mating surfaces such that when pressed against the other surface, the grease flows into all voids to eliminate the interstitial air. excess grease flows out past the edges and the thinnest possible thermal joint is formed as the two surfaces come into contact at their high points. joint integrity has to be maintained with spring clips or mounting hardware.

thermal greases are notoriously "user unfriendly", but provide very low thermal resistance between reasonably flat surfaces. grease does not provide electrical insulation between the two surfaces¹, and excess grease that flows from the joint should be cleaned up to prevent contamination problems. greased joints can also dry out with time, resulting in increased thermal resistance.

the effect of thermal grease on heat transfer through an interface can be seen from the data in table 1.

table 1. case-sink and junction ambient thermal resistance for various interface materials. (pentium test chip operated at 6 watts under natural convection cooling with a wakefield pin fin heat sink)

this data was generated using a p54c pentium thermal test die powered to six watts in an amp socket 5. a wakefield 799-80ab pin fin heat sink was attached with a spring clip. without an interface material, rc-s was 2.9°c/w. the addition of thermal grease to the joint reduced the resistance to 0.9°c/w. inspection of the joint after testing showed that the grease covered over 90% of the area and the thickness was less than 0.07 mm.

thermally conductive compounds are an improvement on thermal grease as these compounds are converted to a cured rubber film after application at the thermal interface. initially, these compounds flow as freely as grease to eliminate the air voids and reduce the thermal resistance of the interface. after the interface is formed, the compounds cure with heat to a rubbery state and also develop secondary properties such as adhesion. formulations with adhesive properties do not require mechanical fasteners to maintain the integrity of the joint.

since the binder cures to a rubber, these compounds do not have the migration or the dry joint problems associated with thermal greases. compounds can be used to fill large gaps where greases would bleed from the joint on account of their migratory nature. clean-up is also simple as excess material is easily removed after it has been cured to a rubber.

the thermal performance of a typical compound is shown in table 1. since the compound behaves like a grease, the joint is nearly void free and the improvement in thermal resistance over a dry joint is similar to that of grease, 0.8 vs. 2.9°c/w.

thermally conductive elastomers are silicone elastomer pads filled with thermally conductive ceramic particles, often reinforced with woven glass fiber or dielectric film for added strength. these elastomers are available in thickness from about 0.1 - 5 mm and hardness from 5 to 85 shore a. unlike compounds and greases, elastomer pads provide electrical insulation and can be used between surfaces that are at different electrical potential. they are typically used under discrete power devices where electrical isolation is required.

elastomers do not flow freely like the greases or compounds, but will deform if sufficient compressive load is applied to conform to surface irregularities. figure 2 shows the effect of applied pressure on the thermal impedance of high durometer elastomer materials typically used to insulate power devices. at low pressures, the elastomer cannot fill the voids between the surfaces and the thermal interface resistance is high. as pressure is increased, more of the microscopic voids are filled by the elastomer and the thermal resistance decreases. for most high durometer materials, mounting pressures around 300 to 500 psi eliminate the interstitial voids and reduce interface resistance to a minimum. mounting pressure must be permanently maintained by using fasteners or springs to hold the two surfaces together.

figure 2. applied pressure vs. thermal impedance.

table 1 shows the thermal performance of a highly thermally conductive elastomer at low pressure. despite a 5 w/m-k thermal conductivity, rc-s is 1.8°c/w, twice that of grease. inspection of the disassembled interface after the test showed that there was less than 30% contact between the material and the two surfaces.

thermally conductive adhesive tapes are double-sided pressure sensitive adhesive films filled with sufficient ceramic powder to balance their thermal and adhesive properties. the adhesive tape is usually supported either with an aluminum foil or a polyimide film for strength and ease of handling. polyimide support also provides electrical insulation. adhesive tapes perform much like the elastomeric films, in that they also require some initial mating pressure to conform to irregularities in the mating surfaces. they are also unable to fill large gaps between non-flat surfaces. however, once the joint is formed, the adhesive tapes require no mechanical support to maintain the mechanical or thermal integrity of the interface.

adhesive tapes provide convenience in attaching a heat sink to a semiconductor package because, unlike liquid adhesives, no cure time is required. the film is applied to one of the surfaces, usually to the heat sink, and it is then forced into contact with the semiconductor package to complete the thermal joint. the application pressure is typically 10 to 50 psi for a few seconds duration. the bond thus formed can be considered permanent and the heat sink is reliably attached to the semiconductor.

t642/643 form-in-place thermally conductive
gap fillers.

however, this convenience comes at a price. as table 1 shows, rc-s for tapes is only slightly better than a dry joint. this is because the thermal tapes do not fill gaps as well as liquids, and thermal joints made with tapes will normally include considerable interstitial air gaps. for the most part, the quality of the two joining surfaces will determine the amount of contact that can be achieved and the thermal performance that can be expected. the high shear strength of these thermal tapes means that reliable joints between heat sinks and semiconductors can be achieved, even with poor surfaces and no mechanical fasteners.

conclusion

in summary, a variety of materials and approaches are available to manage or minimize the thermal resistance of semiconductor package-to-heat sink interfaces. thermal greases and compounds will provide the lowest interface resistance, but they are pastes and require care in handling. elastomers eliminate handling problems but they sometimes require high compressive loads even with well prepared surfaces. thermal tapes offer great convenience but their gap filling properties are limited. the success of any particular combination of heat sink, interface material and heat sink will depend on the thoroughness of the design, the quality of the interface material and its proper installation.

¹ thermal greases and compounds generally are not electrically conductive. however, they do flow from a thermal joint and allow the joining surfaces to make electrical contact.

dr. miksa de sorgo
chomerics division of parker hannifin
77 dragon court, woburn, ma 01888-4014 usa
tel: +1 (617) 939 4643
fax: +1 (617) 939 4155
email: [email protected]

references

1. s. lee, how to select a heat sink, electronics cooling, vol. 1, no. 1 pp.10-14, june 1995