Thermal Conductivity Of Underfill Research Articles

The Effect of Metallic Interconnect Spacing on the Thermal Resistance of Flip-Chip Light-Emitting Diodes With Underfill Encapsulation

While highly thermally conductive underfill material has been proposed as a solution for reducing the thermal resistance of flip-chip light-emitting diode (FCLED) packages, studies up until this point have not taken into account the layout of the underlying metallic interconnects and its impact on thermal resistance. This paper provides a deeper investigation into how interconnect spacing affects thermal resistance in conjunction over a range of underfill thermal conductivities. Test vehicles utilizing FCLEDs and copper–tin interconnects were fabricated, tested for thermal resistance, and simulated with ANSYS finite-element analysis software. A follow-up parametric study was performed, in which the effectiveness of increasing underfill thermal conductivity was demonstrated to be highly dependent on the distance between conductive metal interconnects. It was found that when the gap between interconnects is sufficiently small, the thermal conductivity of the underfill plays a negligible role in the overall thermal resistance of the FCLED package. For instance, when the gap is $200~\mu \text{m}$ , an underfill with a relatively high thermal conductivity of 1.0 $\text{W}\cdot \text{m}^{-1}\cdot \text{K}^{-1}$ underfill only reduces thermal resistance ( $\Theta _{\mathrm {JC}}$ ) versus a no-underfill scenario by 2.6% more than does an underfill with a thermal resistance of 0.25 $\text{W}\cdot \text{m}^{-1}\cdot \text{K}^{-1}$ . This effect can be leveraged during package design to allow for unfilled underfill materials to be used, reducing cost and process time.

Bump and Underfill Effects on Thermal Behaviors of Flip-Chip LED Packages: Measurement and Modeling

The goal of this paper is to experimentally and numerically study the thermal behaviors of flip-chip (FC) light-emitting diode (LED) packages with and without underfills and, furthermore, to compare it with conventional wire-bonding LED packages in order to understand its thermal dissipation mechanism. In experimental analyses, the junction temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> and surface temperatures are measured by a junction temperature tester, thermal couples, and an infrared thermal imager, whereas the numerical analysis is carried out by an ANSYS simulation. After the validation of the simulation model with experimental results, the effects of FC bump number and underfill thermal conductivity on both T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> and thermal resistance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> of the packages are investigated by this validated model. Furthermore, a simple model of effective thermal conductivity for a composite material of bumps and underfills is proposed for thermal analysis of the FCLED packages. Thermal results for the packages are presented and discussed in terms of volume fraction and thermal conductivity for bumps and underfills in this paper.

Comparative study of thermally conductive fillers in underfill for the electronic components

General underfill for the flip-chip package had a low thermal conductivity of about 0.2 W/mK. Thermal properties of underfill were measured with various fillers, such as silica, alumina, boron nitride, (BN) and diamond. Coefficient of thermal expansion (CTE) was changed by filler content and CTE of silica 60 wt.% was 28 ppm; BN 30 wt.%, 25 ppm; alumina 60 wt.%, 39 ppm; and diamond 60 wt.%, 24 ppm. The viscosity of underfill was measured with the cone and plate rheometer. Thermal diffusivity was measured with the laser flash method. Diamond filler loaded underfill showed the highest thermal conductivity 60 wt.%; 1.17 W/mK at 55 °C. Thermal conductivity of underfill was changed with a transition of heat capacity by the temperature increment in same filler content. In case of different filler content, thermal conductivity was changed with a transition of the thermal diffusivity.

Evaluation of Thermal Enhancements to Flip-Chip-Plastic Ball Grid Array Packages

Enhancements to thermal performance of FC-PBGA packages due to underfill thermal conductivity, controlled collapse chip connection (C4) pitch, package to printed wiring board (PWB) interconnection through thermal balls, a heat spreader on the backside of the die, and an overmolded die with and without a heat spreader have been studied by solving a conjugate heat transfer problem. These enhancements have been investigated under natural and forced convection conditions for freestream velocities up to 2 m/s. The following ranges of parameters have been covered in this study: substrate size: 25–35 mm, die size: 6.19×7.81 mm (48 mm2 area) and 9.13×12.95 mm (118 mm2 area), underfill thermal conductivity: 0.6–3.0 W/(m K), C4 pitch: 250 μm and below, no thermal balls to 9×9 array of thermal balls on 1.27 mm square pitch, and with copper heat spreader on the back of a bare and an overmolded die. Based on our previous work, predictions in this study are expected to be within ±10% of measured data. The conclusions of the study are: (i) Thermal conductivity of the underfill in the range 0.6 to 10 W/(m K) has negligible effect on thermal performance of FC-PBGA packages investigated here. (ii) Thermal resistances decrease 12–15% as C4 pitch decreases below 250 μm. This enhancement is smaller with increase in die area. (iii) Thermal balls connected to the PTHs in the PWB decrease thermal resistance of the package by 10–15% with 9×9 array of thermal balls and PTHs compared to no thermal balls. The effect of die size on this enhancement is more noticeable on junction to board thermal resistance, Ψjb, than the other two package thermal metrics. (iv) Heat spreader on the back of the die decreases junction-to-ambient thermal resistance, Θja, by 6% in natural convection and by 25% in forced convection. (v) An overmolded die with a heat spreader provides better a thermal enhancement than a heat spreader on a bare die for freestream velocities up to about 1 m/s. Beyond 1 m/s, a heat spreader on bare die has better thermal performance.