HWP-Modified Polyimide Films for Thermal Stability and Heat Transfer

Polyimide films are often used where electrical insulation must be retained through fabrication and subsequent thermal cycling. Their dimensional response becomes more important as electronic assemblies become denser. A small change in the dimensions of a dielectric layer can be enough to disturb alignment between adjacent layers, weaken adhesion at an interface, or contribute to laminate warpage. In severe cases, these changes may also affect electrical reliability. Inorganic fillers with low thermal expansion are commonly introduced to limit this movement. As filler content rises, the film may become less flexible during repeated bending; its dielectric response and particle-polymer adhesion can also change. Poor dispersion is equally problematic because it interrupts uniform stress transfer and can hinder heat flow through the composite. For this reason, reducing the coefficient of thermal expansion is only part of the materials problem. The film must still bend without damage, conduct heat more readily than the unfilled polymer, and continue to function as a dielectric layer. Negative-thermal-expansion fillers are of interest because their contraction upon heating can compensate for expansion in the polymer matrix. Their usefulness depends on more than that intrinsic response, however. At the filler contents needed to influence dimensional stability, thermal behavior, dielectric properties, and the quality of the particle-polymer interface must remain compatible with the intended film.

Negative-thermal-expansion fillers can reduce the positive thermal expansion of the surrounding polymer because they contract as temperature rises and the difficulty is to identify a filler whose thermal, dielectric, and interfacial properties remain suitable when added at substantial loading. The chemistry at the particle surface strongly influences this behavior. Even a filler with suitable intrinsic expansion behavior cannot contribute fully if it aggregates in the polymer, is not adequately wetted, or forms interfaces that weaken the film during tensile or flexural deformation. In a recently published research paper in Polymer Composites Dr. Xueyi Yu, Dr. Kaijin Chen, Mr. Yanwei He, Dr. Siwei Liu, Prof. Zhengping Qiao, and Professor Yi Zhang from Sun Yat-Sen University developed Hf₂(WO₄)(PO4)₂ nanoparticles with negative thermal expansion and incorporated them into PMDA/ODA polyimide films. They compared six surface-modification chemistries and identified KH-550-modified HWP as the most effective filler for reducing thermal expansion through improved compatibility with the polyimide precursor. The resulting 550/PI composite series combined filler-mediated dimensional control with enhanced thermal conductivity while retaining useful flexibility and nearly unchanged dielectric properties.

The research team prepared HWP by solid-state reaction from HfO₂, WO₃, and ammonium phosphate, followed by extended dry ball milling to obtain nanoparticles. Structural characterization established an orthorhombic Pbcn phase composed of interconnected HfO₆ octahedra together with WO₄ and PO₄ tetrahedra in a corner-sharing arrangement. The measured composition corresponded to the intended Hf:W:P ratio of 2:1:2, while diffraction results agreed closely with the calculated relaxed crystal structure. Particle-size analysis placed the median size near 248 nm, with the upper end of the distribution near 332 nm.

The thermal behavior of the powder gave the material its importance to polyimide dimensional control. The authors found HWP had negative thermal expansion from 50 to 400°C, although the magnitude varied across the measured temperature intervals. It also showed limited mass loss over the examined temperature range. These observations established HWP as an inorganic phase capable of contributing a compensating expansion response within the thermal window used to assess the composite films. The authors used six coupling agents to modify the HWP surface: D4, KH-570, KH-560, KH-550, ND-42, and NDZ-102. They dispersed each modified powder into the polyamic-acid precursor, cast, and thermally imidized to form composite films. The comparison was broad enough to separate the contribution of the negative-expansion filler from the contribution of surface functionality. HWP reduced the coefficient of thermal expansion in all cases, but KH-550 produced the lowest values at equivalent filler contents.

That outcome was linked to the chemistry of the modified interface. Hydrolyzed KH-550 formed silanol groups that bonded with hydroxyl groups on the HWP surface, creating an organic-inorganic boundary layer through Si-O-Si linkages. Its amino functionality also interacted with the polar carboxyl and amide groups of the polyamic-acid precursor. These interactions improved particle wetting and dispersion in the polymer matrix, allowing HWP to reduce thermal expansion more effectively.

The team found the composition that provided the most useful balance contained 25 vol% KH-550-modified HWP and the resulting 550/PI-25 film had a coefficient of thermal expansion of 27.8 ppm/K and retained excellent flexibility, enduring more than 3 × 10⁵ bending cycles at a bending radius of 0.3 mm. Mechanical properties initially improved at lower filler contents, then declined as the HWP fraction increased and the growing inorganic phase altered matrix-filler interactions. Thermal conductivity increased steadily with HWP loading and reached 0.896 W/m·K for 550/PI-25, more than six times that of pure PI. This increase was associated with the formation of heat-transfer pathways between neighboring particles and lower interfacial thermal resistance after surface modification. Infrared thermal measurements also showed faster heat transfer through the filled films. In copper-laminated structures, increasing HWP content improved peel strength and reduced warpage, while the laminate containing 25 vol% filler showed almost no visible deformation.

The 550-mHWP/polyimide films are relevant to flexible electronic assemblies in which a dielectric layer must manage heat while remaining dimensionally stable during thermal processing and repeated operation. In such structures, polyimide serves as an insulating layer between components that expand differently with temperature and a reduction in the film’s coefficient of thermal expansion can therefore reduce the internal mismatch between polyimide and copper layers, limiting the warpage that develops when a laminated structure is heated and cooled.

The copper-laminate experiments illustrate the material’s use in flexible copper-clad laminates. The researchers coated the polyamic-acid precursor directly onto copper foil and completed imidization without an added resin adhesive layer. As the content of KH-550-modified HWP increased, peel strength rose and laminate warpage decreased. At 25 vol% filler, the laminate showed almost no warping. This behavior supports flexible circuits in which adhesion to copper and dimensional control are managed within the dielectric layer itself. Thermal management represents a second application area. The composite containing 25 vol% modified HWP reached a thermal conductivity of 0.896 W/m·K, substantially higher than that of the unfilled polyimide film. Infrared thermal imaging showed that filled films transferred heat more rapidly from a heated surface. This behavior is useful where a thin insulating layer must distribute heat generated by chips, LEDs, or radio-frequency power components rather than act as a thermal barrier. Mechanical flexibility remains important in these settings because thin electronic films are often folded, bent, or repeatedly deformed during handling and use. Composite films with up to 25 vol% filler sustained more than 300,000 U-bending cycles at a 0.3 mm bending radius. The findings therefore identify a compositional range in which increased thermal conductivity and lower thermal expansion do not require the immediate loss of repeated-bending capability. The 25 vol% material is especially relevant because it retained this flexibility while displaying low thermal expansion and improved heat transfer.

The relatively limited changes in dielectric constant and dielectric loss also matter for insulating components operating at high frequency. Within the reported measurements, incorporation of modified HWP improved thermal transport and dimensional stability without substantially altering dielectric behavior. This makes the material relevant to flexible dielectric films, copper-clad laminates, and thermally demanding electronic packaging structures where heat dissipation, adhesion, bending durability, and thermal-expansion matching must be addressed together.