New Class of Siloxane Polymers for Advanced LED Packaging

New devices and packaging schemes, such as flip-chip and chip and wafer scale packaging, emerge to address issues that limit market adoption of LED products. By that, new materials and integration challenges arise. The novel class of high stability siloxanes, based on proprietary monomers and polymers, is optimized to improve light efficiency individually or in combination and to address new packaging schemes. Proprietary siloxane chemistries provide the basis for stable encapsulation products with a refractive index of 1.62 (at 632.8 nm) increasing package light output efficacy compared to leading commercial encapsulation products.

By combining such high refractive index encapsulants with anti-reflection coatings, light extraction at the package level can be further enhanced. Such multi-layer encapsulation and light outcoupling schemes are particularly well suited for wafer level and chip scale packaging. Further, the proprietary siloxane polymers, in combination with novel nanomaterials, enable die attach products fully compatible with current process tools and flows but with superior heat dissipation and die shear strengths yielding significant improvements in LED device performance compared to commercial die attach adhesives.

Introduction

The efficiency and reliability of packaged LEDs are strongly linked to light extraction and heat dissipation, directly impacting cost per lumen as well as footprint of LED modules and systems. The external efficiency of the LED package is a result of several different factors. One part is the internal quantum efficiency quantifying the ratio of photons generated in the active semiconductor region from the number of electron-hole pairs injected into the LED chip. Internal quantum efficiency is a function of chip junction temperature that is controlled by heat dissipation from the chip with the layer attaching the die to the substrate often presenting the bottleneck for this process. Another factor is the photon extraction efficiency that stipulates the ratio of the photons extracted from the LED chip vs generated at the active semiconductor region. The final part is the light extraction and conversion efficiency at package level. The light extraction at the chip and package level is primarily related to the index of refraction of the encapsulant as well as the use of secondary light outcoupling optics, e.g. micro lenses, diffractive lenses or dome lenses. In white light LEDs, the encapsulant’s refractive index also impacts scattering losses within the phosphor wavelength conversion layer.

Recognizing the impact of optoelectronic packaging materials like encapsulants and die attach materials on the factors outlined above, they have advanced over time from organic epoxies to organosilicon polymers such as methyl and phenyl silicones. However, present commercial solutions are not sufficient to address all needs in high power LED and new packaging schemes such as flip chip packages in terms of optical properties, barrier properties, thermal stability and process temperature. Novel and proprietary siloxane polymers can address these requirements and challenges and are therefore potential candidates as advanced packaging materials. We report here a new class of proprietary siloxane polymers presenting a technology platform for high refractive index encapsulants and low sintering temperature die attach materials.

Siloxane Encapsulants

The optical power output efficacy of an LED device is, in part, limited by the amount of light extracted from the active semiconductor region of the chip as briefly mentioned above. A key factor in this regard is the interface between the high refractive index semiconductor substrate and the surrounding media, e.g. air or conventional methyl or phenyl silicones (e.g. refractive index of 1.4 - 1.55 at 633 nm). The smaller the difference in refractive index the less light is lost to internal reflection in the chip increasing the light output efficacy of the LED device. The new class of siloxane polymers described here yields a first generation LED encapsulation product, ILE-501, with a refractive index of 1.62 and 1.65 at 633 nm and 450 nm wavelength, respectively, and with superior barrier performance against environmental degradation offering significant opportunities to improve the light output efficacy as well as reliability of LED modules and systems.

Figure 1: Transmission curve (blue) and refractive index dispersion curve (red) for a film of 70 um thickness formed the ILE-501 LED encapsulation product. ILE-501 part A & B were mixed in 1:1 ratio in a Thinky Planetary Vacuum Mixer for 2 minutes at 1400 rpm at 1 kPa. Glass slides were coated and the sample cured at 150°C for 60 min.

The optical properties of this product were characterized by UV-VIS absorption spectrometry (Perkin Elmer LAMBDA 25 UV/VIS Spectrometer) and spectroscopic Ellipsometer (J.A. Woollam α-SE™) and are shown in the following Figure 1 (blue: transmission curve for a film of 70 um thickness; red: refractive index dispersion curve). As indicated the characterization reveals a refractive index of 1.62 at 633 nm wavelength that increases to about ~ 1.655 at 450 nm. Throughout the visible wavelength range, the film shows a very high transparency of over 99% that in combination with its high thermal stability makes this product well suited for mid- and high-power LED applications requiring long device lifetimes.

To illustrate the impact of the optical properties of the new Siloxane material on the performance of LED packages, plastic leaded chip carrier (PLCC) multi chip packages were encapsulated with ILE-501 and phenyl (Reference 1) and methyl (Reference 2) silicones with a refractive index of 1.54 and 1.41, respectively. In addition, a multi-layer encapsulation scheme was tested, wherein an ILE-501 encapsulated LED package was coated with a proprietary siloxane based anti-reflection coating (ARC) with a refractive index of 1.4. In all cases, a commercial YAG phosphor was mixed with encapsulant material, the color temperature was adjusted to 5,000 K and the packages were identically processed and operated at 2 W electrical power.

Table 1: Comparison of three different LED encapsulation products regarding their refractive indices and its impact on the relative lumen efficacy of 2 W PLCC multi chip packages as well as their relative permeability against water, air and sulfur vapors

The results of the evaluation are shown in table 1. Water vapor and oxygen transfer rates were determined following ASTM F-1249 and ASTM D-3985 measurement protocols. The permeability to sulfur vapor diffusion was determined by measuring the reflectance of encapsulated silver films before and after exposing the samples to sulfur vapors for 24 hours at 70°C (samples enclosed in 100 ml bottle with 0.2 g of sulfur) with a Perkin Elmer UV-VIS Lambda 950 spectrometer with integrating sphere.

As can be seen in Table 1, the new encapsulation product with a refractive index of ~ 1.655 at 450 nm resulted in 6% and 10% higher lumen output efficacy compared to commercial phenyl (Reference 1) and methyl (Reference 2) silicones. The sample utilizing a multilayer encapsulation scheme “ILE-501 +ARC” showed an even higher output efficacy illustrating the importance and potential of carefully engineering the optical interface between the LED chip and the environment for the performance of LED modules and systems.

Further characterization of the new siloxane polymers revealed that they form improved barriers against environmental influences (e.g. air, water and sulfur) compared to conventional methyl and phenyl silicone products. Table 1 summarizes the results of measurements of the water vapor and oxygen transfer rates as well as corrosion resistance of encapsulated Ag films upon exposure to sulfur vapors for Ithe new and two commercially available and commonly used reference products. The results are normalized for ILE-501 and show that both commercial products have higher permeability for oxygen as well as water and sulfur vapors than ILE-501. The superior barrier performance may, in particular, be of commercial significance for novel narrow bandwidth red phosphors and quantum dots with high environmental sensitivity. But also conventional phosphor applications may benefit from this aspect as illustrated by exposing silver plated leadframe packages using the three encapsulant products to sulfur vapors in an accelerated lifetime test. Within 24 hours at 70°C, packages encapsulated with methyl and phenyl silicones turn black due to silver corrosion whereas packages encapsulated with ILE-501 remain silvery and shiny (data not shown). This superior performance illustrates the potential impact this new class of siloxane polymers can have for the long term stability and reliability for a wide variety of LED modules and systems.

Siloxane Die Attach Materials

In order to maximize the efficiency of a LED chip, it is key to lower the chip junction temperature by utilizing better interconnect materials at packaging level or by reducing thermal barriers between the chip junction and substrate in other ways. A significant portion of the junction-to-substrate thermal resistance comes from the joint between chip and substrate, the die attach layer. The thermal resistance of this interface layer is determined by its thermal bulk conductivity and boundary resistance as well as thickness of the die attach layer.

Leveraging the same novel proprietary siloxane polymer platform as for optical encapsulation products described above, several versions of novel siloxane-based die-attach materials have been developed. Addressing various LED packaging schemes, the die attach product line includes reflective electrically conductive, reflective electrically insulating and transparent electrically insulating materials. The novel siloxane polymers are used as a matrix in these products due to their excellent thermal and light stability over the conventional epoxy binders but also due to their great adhesion via covalent bonding to various metal and oxide surfaces. Improved adhesion has benefits for device reliability but also increases heat transfer between the chip to die attach layer interface as well as the die attach layer to package interface.

Here we present a comparison of two novel siloxane-based die attach products against two market leading reference materials. The first case study includes IDA-125, an electrically conductive die attach material comprising siloxane polymer as a binder and in-house synthesized silver particles as a filler. This product can be cured at 150°C in 30 minutes by a pressure less method making it attractive for applications requiring fast processing. As a reference, a commercially leading silver sintering die attach product was selected requiring processing at 200°C for 60 minutes. Bulk thermal conductivities were measured using a Netzsch LFA 467 laser flash analysis tool and the junction-to-substrate thermal resistance was determined with a T3Ster thermal tester (Mentor Graphics) from a 75 W UV chip-on-board (COB) LED module (Figure 2). The total heat dissipation of the tested module is about 50 W at a heat dissipation density of about 20 W/cm2. Furthermore, both materials were tested for die shear strength according to Mil-Std-883 Method 2019.7 standard.

Figure 2: LED chip-on-board (COB) module (75 W) comprising 63 385 nm UV chips in a 7x9 configuration on an aluminum ceramic substrate utilized for thermal resistance testing of IDA-125 and a commercial reference product

The bulk thermal conductivities of the IDA-125 and reference die attach products as determined by laser flash measurements was 25-30 W/mK and 40-60 W/mK, respectively. Obviously, the reference material showed substantially higher bulk thermal conductivity, presumably due to higher silver content and more aggressive curing conditions. A comparison of the bulk thermal conductivities may lead to the conclusion that this reference product may be a superior solution for efficient heat dissipation in power LED applications but it fails to take into account the role thermal contact resistance plays in the thermal performance of the overall LED package. In fact, measurements of the thermal impedance of the chip and die attach layer for the two products were close to identical and below 0.2 K/W at comparable bond line thickness (BLT) of 15um (Figure 3). In correspondence with this characterization, the optical evaluation of the LED COB modules using the two die attach products showed identical results with optical power output efficacies of 34% and light output of over 25 W at 385 nm wavelength. However, illustrating the superior interface quality using the IDA-125 die attach product the die shear strength was determined to be 2 times higher than with the reference product. The higher die shear strength at lower processing temperatures of the IDA-125 product compared to the reference product while providing similar overall heat dissipation and hence optical performance despite the lower bulk thermal conductivities is a significant advantage and illustrative of the potential this class of new siloxane polymers has for die attach or for thermal interface materials in general. The low processing temperature of the new Siloxane based material compared to conventional silver sintering products also opens the door to applying such product to conventional plastic lead frame packages providing superior heat dissipation characteristics previously not attainable.

The comparison in figure 3 shows a very similar thermal (and optical) performance despite IDA-125 being cured at significantly lower process conditions (30 min. at 150°C) than the reference product (60 min at 200°C). In contrast, IDA-125 shows higher die shear strength (2x) than the reference product illustrating the superior interface quality of the new material.

Figure 3: Thermal resistance comparisons as cumulative structure function for 75 W LED COB modules with IDA- 125 and reference die attach products

In a second case study, a new transparent electrically insulating die attach product, IDA-313, comprising novel proprietary siloxane polymers and ceramic filler, is compared to a leading transparent silicone die attach adhesive. Both products utilize the same pressureless curing conditions (60 min. at 150°C) yielding films with thermal conductivities of 0.5 W/mK and 0.2 W/mK as well as transparencies of >90% (2.5 um film) and >99% (5 um film) for IDA-313 and the reference product, respectively.

LED packages were formed using both products for attaching sapphire LED chips to copper-nickel-silver contact pads on alumina substrates with bond line thicknesses of 1.5μm and 4 um for IDA-313 and the reference product, respectively. Relative lumen output at 1 W electrical power and junction temperatures at environmental temperatures of 25°C and 85°C were measured for both types of LED packages. In addition, the die shear strength of the LED packages was determined at room temperature. As seen in Table 2, the IDA-313 product showed two times higher die shear strength than the reference product even though the bond line thickness was one third of the reference. This thin bond line thickness in combination with the >2x higher thermal conductivity of the material itself resulted in higher lumen output (3%) and lower junction temperatures (Δ -8°C) for IDA-313 compared to the reference die attach product. Therefore, the new siloxane polymer based die attach products provide superior optical performance and higher reliability due to better adherence to the LED die and interconnect substrate when compared to commercial reference products.

From the above two case studies, it can be concluded that the quality of the interface between the die attach layer and the substrate as well as the LED chip has a profound impact on the thermal and mechanical performance of the LED stack and that these aspects need to considered in addition to bulk thermal conductivities when designing and manufacturing mid and high power LED and COB modules.

Table 2: Comparison of the relative lumen output, die shear strength and junction temperature of sapphire LED chips attached to alumina substrates using transparent electrically insulating IDA-313 and a leading commercial silicone die attach product

Conclusions

Novel and proprietary siloxane polymers provide a technology platform for a range of improved products for conventional LED packaging and addresses challenges and opportunities in emerging packaging schemes. New LED encapsulant and die attach products significantly increase the efficiency and reliability of LED devices advancing the market adoption and penetration of LED applications by lowering the overall cost of modules and systems while meeting or exceeding current performance requirements.