Thermally conductive high heat electronic adhesion
Last update on Jul 29, 2025
This article was originally published in 2008 and revised in 2025.
Various types of thermal interface materials (TIM) can be used to facilitate conductive high heat transfer across two adjacent plastic surfaces. This depends on the components on either side of the thermal joint, as well as the nature of the surfaces and thermal gaps involved.
Many other factors are also considered in the choice of TIM, including:
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Thermally conductive adhesives include epoxies, acrylics, and silicones available in thermally conductive formulations. They eliminate the need for mechanical fasteners that are often otherwise required.
In this article, we will explore the role of TIM adhesives, their types, benefits, and how new innovations are improving thermal management in electronic assemblies.
Overview of TIM adhesives
Thermally conductive adhesives are another TIM alternative for joining electronic microprocessors with heat sinks. They offer mechanical adhesion and electrical performance (both conductive and insulative) in addition to thermal conduction.
Their major advantage is the ability to hold the heat sink in place without the need for bulky clamps or other mechanical fasteners. These TIM adhesives must strike a careful balance between physical properties to protect packaged chips from stresses. The stress can be caused by differences in the coefficients of thermal expansion (CTE) of the chip, package, and heat sink.
Epoxy, silicone, and acrylic adhesives, as one or two-part materials ranging from free-flowing liquids to non-flowing pastes, are available in thermally conductive formulas. These adhesives wet the surfaces of the parts like thermal grease and phase change TIM.
However, they usually do not perform as effectively thermally. They also have a disadvantage that they often require curing with heat or ultraviolet radiation. This adds to the assembly process time and cost and may also damage sensitive components.
Bead-on-bead thermally conductive adhesive
Henkel's electronic group last year introduced LOCTITE® bead-on-bead thermally conductive adhesive. It is a two-part filled acrylic adhesive designed for ease of use. Based on proprietary cure technology, these adhesives represent a new model in the bonding of heat-dissipating electrical and electronic devices to conventional heat sinks and spreaders.
It has a unique two-part, no-mix, no-measure acrylic adhesive chemistry. This allows manufacturers to apply each part independently to the electronic components. Thus, replacing the traditional methods of pre-mixing or solvent activation and eliminating issues associated with pot life management.
No precise meter-mix dispense systems are needed, and manufacturers are provided an extremely wide process window. The curing occurs once the two components come into contact during assembly. At room temperature, the adhesive bond reaches fixture strength in 3-5 minutes and is fully cured in 48-72 hours.
Application methods for bead-on-bead adhesive
(Source: Henkel Corporation)
The adhesive part A is colored yellow and part B blue so that when the parts are assembled, the resulting bond line becomes green. This color-coded system has the following benefits:
- minimizes the potential for production errors,
- permits ready visual inspection of the final assembly
The two adhesive parts can be applied with automatic dispensers or screen printers. It can be either as beads on top of each other or on separate component surfaces. The adhesive parts do not have to be dispensed in equal quantities.
While optimum thermal and adhesive performance is obtained by using a part A to B ratio as close to 50:50 as possible, acceptable adhesion is achieved with ratios as much as 60:40 and 40:60. The thermal conductivity of the adhesive is 1.75 W/m-K (watts per meter, degrees Kelvin).
The adhesive is supplied in two versions, i.e., the base material LOCTITE® 3875 and one that incorporates 7 mil (0.007 inches) glass spacer beads LOCTITE® 3876. The glass beads are used to ensure that bond lines are a consistent thickness and provide electrical isolation from the heat sink.
Two-part (yellow and blue) LOCTITE® bead-on-bead adhesive
(Source: Henkel Corporation)
High thermal conductivity adhesive
Dow Corning Electronics launched its new high thermal conductivity, silver-filled silicone adhesive DA-6534. The high-performance thermal adhesive was developed to deal with overheating in advanced flip-chip, ball grid array (FC-BGA) devices, and other advanced components.
The unique heat-cured one-part adhesive combines silver filler with the proven reliability/flexibility of silicone-based chemistries. The resulting adhesive provides exceptional thermal conductivity and long-term reliability. This includes the ability to maintain elasticity at high and low temperatures.
As a thixotropic material, this high-performance thermal adhesive offers good dispensability. Its bond line thickness (BLT) can be managed through pressure and time. Additionally, the DA-6534 has been designed to be inert and not produce any byproducts. This has been achieved as a result of the hydrosilylation reaction of vinyl polymers and the use of a hydrogen crosslinker.
Dow Corning's new adhesive has demonstrated a thermal resistance of 0.09 cm2 C/W at 24 microns and surpasses the performance of traditional TIMs. The traditional TIMs may require thermal filler loadings above 70% or the use of ceramic fillers. Both of these can compromise adhesion and thermal reliability.
DA-6534 adhesive thermal impedance vs. bond line thickness (BLT)
(Source: Dow Corning)