Thermal Tape Bridges Heat-Flow Gaps

Aug. 23, 2007
Advanced thermally conductive tape helps attach vital heat sinks needed to dial down the heat.

It's no surprise that as tightly packed semiconductor devices shrink and speed up, thermal management becomes one of the most important design considerations in modern electronics.

Efficient heat dissipation can be achieved by a variety of means, including natural airflow, fans, and heat sinks. But even the most careful thermal design relies on efficient conduction of heat from the component packaging itself. This, in turn, requires an effective thermal interface material (TIM) to couple heat from the component to its heat sink or heat plate, or the assembly chassis.

Even the best-machined surface can't be completely flat; thus, two components in contact always form a thermal interface that's partly made up of air gaps, which are inherently poor conductors of heat. The primary role of the TIM is to fill these air gaps and reduce the contact resistance between heat-generating components and heat-dissipating components.

The designer can choose between several types of TIM, depending on the application, short- and long-term performance requirements, assembly process and rework constraints, and cost.

All TIMs are designed to conform to surface irregularities, to eliminate air gaps, and to improve heat flow. However, the designer may have further application-specific requirements. These include providing physical protection to sensitive components, conformance around larger, inherently irregularly shaped objects, imparting electrical insulation or offering higher adhesive strength. A range of products is available to satisfy these differing requirements, ranging from thermally conductive grease, compounds, gels, pads, and phase change materials, to metallic solders, and thermal adhesive tapes.

Double-sided thermal tapes are typically deployed in applications that require strong attachment, ease of assembly, and clean workability. They generally use high bond strength, pressure-sensitive adhesive (PSA) films, filled with thermally conductive ceramic powder. An aluminium-foil or polyimide material, chosen for strength and ease of handling, provides support.

Polyimide tapes can provide electrical insulation, while aluminium enhances thermal conductivity. The nature of the carrier means that thermal tapes are almost invariably less conformal than greases and compounds. To combat this problem, a tape such as Chomerics’ THERMATTACH T413 or T418, which use a fibreglass carrier, can be used to provide better conformability.

Tapes not only enhance the thermal interface but more importantly they provide a higher adhesive strength than is available from any other type of TIM. As a consequence, they can be used as the attachment mechanism for the heat sink itself, eliminating mechanical fasteners and clamps. From an assembly point of view, they are convenient and easy to handle, require no special process steps such as curing, and are clean and reworkable.

The use model for thermal tape means that adhesive strength is just as important as thermal performance in selecting a suitable product. Moreover, it's worth bearing in mind that adhesive strength isn't a single quantity, it can be measured as peel strength, lap shear strength, die sheer strength, and holding power (sheer creep resistance).

It's vital to choose a tape that will provide the right combination of these characteristics, the major determining factor being selecting the correct type of adhesive for the substrate materials in question. Acrylic PSA is proven for use with either metallic or ceramic substrates, while silicone materials are more commonly used with plastic components. However, recent advances in surface preparation allow acrylic PSAs to be used with a much wider variety of materials.

Also, designers must be sure that the components themselves withstand the initial pressure required to establish the attachment in assembly. Typically, the tape must form a strong bond at application pressures of no more than 69 to 345kPa, and lasting only a few seconds, to protect the components. Research has shown that products like the THERMATTACH T418 tapes can achieve high die shear strengths in excess of 1200kPa with application pressures as low as 172kPa.

The designer must also carefully consider the physical load that may be placed upon the bond by a vertically attached heat sink. This loading must fall comfortably within the specified (and preferably tested) shear creep resistance of the tape, to avert an adhesive or cohesive failure that could prove catastrophic to the overall system. Moreover, any resultant creep in the adhesive layer can, in turn, cause a change in the thermal properties of the tape.

It's also worth bearing in mind that very few loading scenarios produce a constant perpendicular pulling force on the bonding area. Therefore, some element of peeling and delamination parallel to the joint is almost inevitable.

The physical dimensions of the tape have a strong bearing on performance. Thick tapes provide good conformability and hence better wetting, potentially stronger bonds, and improved thermal conduction at the interface. These results can be measured in terms of higher peel strength. Moreover, research has shown that a thicker tape substantially reduces the dependency of bond strength on application pressure.

The downside of using a thicker tape is two-fold. First, sheer strength (measured using a shear hang test) decreases as the thickness of the tape increases, largely due to the thicker bondline. Second, for given materials, the bulk thermal resistance of the tape increases with thickness.

Bond strength and creep behaviour both vary with thermal cycling and aging. Lap shear testing on Chomerics’ T418 tape, aged at various temperatures, revealed increasing lap shear strength as the tape ages, with no apparent thermal degradation. Conversely, strain decreases as the adhesive ages, probably due to hardening or toughening effects.

Whatever the conditions, use temperature and expected system lifetime are also key considerations. Selecting the correct tape therefore requires a balance of adhesion and thermal properties to survive throughout the product’s lifetime. Typical specifications would include an end-of-line lap shear strength of up to 862kPa, peel strength in excess of 8N/cm, and creep resistance up to 69kPa.

As component and system power densities increase, and space becomes ever more constrained, designers must pay greater attention to efficient, effective methods of thermal management. Successful attachment of heat-sinking components, enabled by advanced thermally conductive tape, is a vital part of that effort.

By paying careful attention to product selection, the designer can eliminate problems posed by the variety of materials encountered in modern electronic assemblies. Today’s tapes even offer the option of different materials on either side, to allow the right combination of adhesive and substrate.

Designers must also carefully consider the combination of loading and thermal properties required in their particular application, and preferably undertake their own testing that closely models the likely use scenario.

Y. Joon Lee is a senior scientist with Chomerics, a division of Parker-Hannifin.


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