Underfill Adhesives Research Articles

Finite element analysis (FEA) modelling and experimental verification to optimise flexible electronic packaging for e-textiles

In this paper a three-dimensional model of a novel electronic package has been developed using Finite element analysis to evaluate the shear load, tensile, bending and thermal stresses. Simulations of a complete flexible flip chip electronic packaging method are performed to minimize stresses on the packaged electronic device to improve robustness and reliability. Three component under-fill adhesives (Loctite 4860, Loctite 480 and Loctite 4902) and three circuit substrate materials (Kapton, Mylar and PEEK) are compared and the optimal thickness of each is found by shear load, tensile load, bending test and thermal expansion simulations. A fixed die size of 3.5 mm × 8 mm × 0.53 mm has been simulated and evaluated experimentally under shear and bending load. The shear and bending experimental results show good agreement with the simulation results and verify the simulated optimal thickness of the adhesive layer. The Kapton substrate together with the Loctite 4902 adhesive were identified as the optimum in the simulation. The simulation of under-fill adhesive and substrate thickness identified an optimum configuration of a 0.045–0.052 mm thick substrate layer and a 0.042–0.045 mm thickness of the Loctite 4902 adhesive. The bending simulation has also been used to determine the neutral axis of the encapsulated electronic package in this paper, thus identifying the optimal material and thickness for the encapsulation layer of the package.

Structure-property relationships between microscopic filler surface chemistry and macroscopic rheological, thermo-mechanical, and adhesive performance of SiO2 filled nanocomposite underfills

SiO2 nanoparticles are attractive components for formulating highly filled underfill adhesives in the electronic packaging field. However, achieving uniform dispersion of inorganic SiO2 nanoparticles into an organic polymer matrix to obtain desired rheological and thermo-mechanical properties remains a significant challenge in nanocomposites engineering. To address the issue, previous studies mainly focused on tailoring the filler morphology and size, volume fraction and filler distribution. Here, in this work, in-situ modification of SiO2 nanoparticles by using organosilanes with different functional groups was conducted. We demonstrated the structure-property relationships between microscopic surface state of SiO2 nanofillers and macroscopic rheological, coefficient of thermal expansion (CTE) and adhesive properties of the resulting amine curing epoxy-based nanocomposite underfills. Our experimental results show that the modification of filler surface effectively enables the decrease of viscosity of underfill, and methacryl-terminated silane exhibits the highest efficiency among various organosilanes. Moreover, surface modification improves the adhesion strength between the underfill and substrate evidently, and nonpolar groups function better than their polar counterparts. In addition, modification with nonpolar methacryloxy and phenyl groups is capable of reducing CTE of nanocomposites effectively while other groups have no significant contribution to CTE reduction. These properties improvement can be attributed to the alteration of interfacial compatibility and adhesion in the SiO2 nanocomposites. Overall, our work provides a useful guideline for the rational surface chemistry design of SiO2 filler and optimizing the overall performance of underfill adhesives.

Stress Analysis and Optimization of a Flip Chip on Flex Electronic Packaging Method for Functional Electronic Textiles

A method for packaging integrated circuit silicon die in thin flexible circuits has been investigated that enables circuits to be subsequently integrated within textile yarns. This paper presents an investigation into the required materials and component dimensions in order to maximize the reliability of the packaging method. Two die sizes of $3.5 \,\,\text {mm} \times 8 \,\,\text {mm} \times 0.53\,\, \text {mm}$ and $2 \,\,\text {mm} \times 2\,\,\text {mm} \times 0.1 \,\,\text {mm}$ have been simulated and evaluated experimentally under shear load and during bending. The shear and bending experimental results show good agreement with the simulation results and verify the simulated optimal thickness of the adhesive layer. Three underfill adhesives (EP30AO, EP37-3FLF, and Epo-Tek 301 2fl), three highly flexible adhesives (Loctite 4860, Loctite 480, and Loctite 4902), and three substrates (Kapton, Mylar, and PEEK) have been evaluated, and the optimal thickness of each is found. The Kapton substrate, together with the EP37-3FLF adhesive, was identified as the best materials combination with the optimum underfill and substrate thickness identified as 0.05 mm.

Surface and Buried Interfacial Structures of Epoxy Resins Used as Underfills Studied by Sum Frequency Generation Vibrational Spectroscopy

Flip chip technology has greatly improved the performance of semiconductor devices, but relies heavily on the performance of epoxy underfill adhesives. Because epoxy underfills are cured in situ in flip chip semiconductor devices, understanding their surface and interfacial structures is critical for understanding their adhesion to various substrates. Here, sum frequency generation (SFG) vibrational spectroscopy was used to study surface and buried interfacial structures of two model epoxy resins used as underfills in flip chip devices, bisphenol A digylcidyl ether (BADGE) and 1,4-butanediol diglycidyl ether (BDDGE). The surface structures of these epoxies were compared before and after cure, and the orientations of their surface functional groups were deduced to understand how surface structural changes during cure may affect adhesion properties. Further, the effect of moisture exposure, a known cause of adhesion failure, on surface structures was studied. It was found that the BADGE surface significantly restructured upon moisture exposure while the BDDGE surface did not, showing that BADGE adhesives may be more prone to moisture-induced delamination. Lastly, although surface structure can give some insight into adhesion, buried interfacial structures more directly correspond to adhesion properties of polymers. SFG was used to study buried interfaces between deuterated polystyrene (d-PS) and the epoxies before and after moisture exposure. It was shown that moisture exposure acted to disorder the buried interfaces, most likely due to swelling. These results correlated with lap shear adhesion testing showing a decrease in adhesion strength after moisture exposure. The presented work showed that surface and interfacial structures can be correlated to adhesive strength and may be helpful in understanding and designing optimized epoxy underfill adhesives.

Effects of Under-Fill Curing on Substrate Warpage

The formulation of flip-chip under-fill adhesives has become more critical to avoid stress-induced cracking at the solder ball interfaces farthest from the center of the die (neutral point) and of the low-k dielectric layers on the die surface. Single die packages, multiple die packages, and thin core-less substrates have now become so warped as to make the ball attach and board attach processes difficult. Recommended under-fill supplier cure profiles are generally accepted without much modification. This work compares the stress-induced substrate warpage developed in flip-chip assemblies using seven different under-fills from three different suppliers. Both low and high Tg materials from each supplier are included. Standard isothermal convection oven cures and variable frequency microwave (VFM) cures were compared with programmed multi-step cure profiles to determine the optimum conditions for lowest die stress and substrate warpage. Due to the complex nature of modern under-fill mixtures and the significant differences in the heating mechanisms, the thermo-mechanical properties of the cured resins are not easily interpreted with respect to cause and effect, however some of the results are surprising. The differences in warpage between “full cure” and “cure to Tg” profiles for both heating methods are demonstrated and discussed with respect to potential adhesive network structures from model studies and actual thermal data. Recommendations are made for dispense temperatures, ramp rates, soak temperatures, and extended curing. These effects are important when assemblies face additional heat processing such as subsequent solder reflow.

Influence of temperature and humidity on adhesion of underfills for flip chip packaging

This paper systematically discusses the influence of temperature and humidity on the adhesion performance of underfill material (epoxy cured with acid anhydride), which was evaluated by die shear test after exposure to various conditions. The inherent adhesion strength between the underfill and passivation is not affected significantly by thermal cycling between -55/spl deg/C and 125/spl deg/C for 1000 cycles. The adhesion strength of underfill material decreases with the increase of test temperature in the investigated range, due to the decrease of modulus of the underfill with the increase of temperature. A sharp decrease in adhesion strength occurs as temperature increases toward the glass transition temperature of the underfill material. Adhesion strength of underfill with different passivation materials decreases after aging in a high temperature and high humidity environment. The extent of the decrease depends on underfill formulation and the hydrophilicity of the passivation material. Hydrophilic passivation such silicon oxide (SiO/sub 2/) and silicon nitride (Si/sub 3/N/sub 4/) shows much more severe adhesion degradation than hydrophobic passivation such as benzocyclobutene (BCB) and polyimide (PI). Adhesion degradation is slower than moisture diffusion. The adhesion stability for hydrophilic passivation can be successfully improved by use of a coupling agent such as silane that introduces stable chemical bonds at interface.

Study on underfill/solder adhesion in flip-chip encapsulation

Underfill materials are employed in flip-chip assemblies to enhance solder joint reliability performance. We have studied the adhesion strength of two underfill samples with tin/lead (Sn/Pb) eutectic solder and tin/copper (Sn/Cu) lead-free solder, benchmarked with a copper surface. It was found that the adhesion of underfills and both solder materials was about 1/3 of the adhesion between underfills and copper. The effect of temperature and humidity aging as well as flux residue on adhesion strength was also investigated. A loss of adhesion was observed after the pressure cooker test, but 85/spl deg/C/85% RH aging and flux residue revealed only a slight influence on adhesion strength. Surface analysis was performed on solid surfaces including copper, Sn/Pb eutectic solder, Sn/Cu lead-free solder and cured underfills by using the three-liquid-probe three-component surface tension method with a goniometer. The surface tension of liquid underfills was measured by the pendent drop method, and their contact angles on copper, Sn/Pb eutectic solder and Sn/Cu lead-free solder were also measured with a goniometer. The thermodynamic work of adhesion for underfills with copper and solder surfaces of different conditions was then calculated following these two surface analysis approaches. It was found that the thermodynamic work of adhesion was not correlated with the lap shear strength of underfills with copper and solder materials. Thus, the wetting property of an underfill on a substrate is not the determining factor for its practical adhesion strength. Various possible techniques for improving the adhesion of underfills and solder materials were then considered, and the use of additives in underfill formulations was experimented. However, we have not observed any significant effect of adhesion strength enhancement from any of these additives. Further tests of these additives with the base underfill formulation seemed to reveal a slight possibility to enhance adhesion of underfills and solders by proper manipulation of the underfill and/or flux formulation.

STUDY ON PROPERTY OF UNDERFILL BASED ON EPOXY CURED WITH ACID ANHYDRIDE FOR FLIP CHIP APPLICATION

The adhesion and thermo-mechanical properties of epoxy underfills depend on the epoxy resin, the hardener, and the catalyst. In this study, three different epoxy resins, ERL4221 (cycloaliphatic type), EPON862 (bisphenol F type), and EPON 8281 (bisphenol A type), were cured with 4-methylhexahydrophthalic anhydride (MHHPA) as the hardener using different catalysts: cobalt acetylacetonate (CAA), imidazole derivatives, and tertiary amines. The flow behavior of the epoxy systems was studied with a rheometer. The curing profiles were recorded using a differential scanning calorimeter (DSC), revealing varying catalytic effect for the different catalysts. The curing peak temperature increased in the following order: tertiary amine < imidazole derivatives < cobalt acetylacetonate. The bulk properties of the systems were studied through a thermo-mechanical analyzer (TMA). Epoxy resins cured with different catalysts showed different glass transition temperatures (Tg) and coefficients of thermal expansion (CTE). Among them, the CAA catalyzed systems showed the highest Tg and low CTE. Due to the cycloaliphatic structure of the ERL4221 resin, the cured ERL4221 systems showed the highest Tg compared to the other systems with the same catalysts. EPON8281 systems generally had lower moisture absorption than the other epoxy systems. The surface tension of the underfills was measured. The adhesion strength was evaluated by a die shear test with SiO2 and Si3N4 (SiN) passivated silicon dies as substrates. Although there was no significant difference among the surface tension and contact angles of the underfill materials on the SiO2 or SiN, there were significant differences in the adhesion. For the ERL4221+MHHPA systems, the CAA catalyzed system exhibited excellent adhesion with SiO2 and SiN passivation. However, poor adhesion resulted with the catalysis by the imidazole derivatives and the tertiary amines. The bisphenol A and bisphenol F type epoxy resins showed excellent adhesion. Although the CAA catalyzed systems had higher Tg compared to the other systems, their adhesion was worse after aging in a high humidity environment at high temperatures.

Underfill adhesion to BCB (Cyclotene/sup TM/) bumping and redistribution dielectrics

The adhesion of several commercial underfills (Dexter Hysol FP4511, FP4527, and CNB775-34) to BCB (CYCLOTENE 4022, 4024) has been studied through die shear testing and subsequent failure analysis. Die shear values range between 69-30 MPa. Failure analysis by optical microscopy, profilometry and XPS spectroscopy indicates mixed mode failure at various interfaces. From the die shear data collected before and after 24 h water boil for Cyclotene 4022 and 4024 (with AP3000 adhesion promoter) and underfillers FP 4511 and 4527 we find the die shear strength decreases an average of 11% for the four comparisons. Adhesion promoters based on vinyltriacetoxysilane or 3-amino-propyltriethoxysilane show equivalent die shear performance. Substrate surface cleaning based on UV ozone treatment reveals oxidation of the BCB surface which by SIMS analysis remains <0.1 /spl mu/m deep after 10 min of treatment.