Low temperature curing of polyimide wafer coatings

A variable frequency microwave (VFM) was used for soldering the lead-free solders. Melting the Sn-3.5Ag, Sn-3.8Ag-0.7Cu and Sn46Pb solder pastes and their wetting on the metal pads were achieved by heating the organic flux vehicle through a radio frequency microwave. The lead-free solder interconnection between the component chips and the metal pads through VFM was first demonstrated. The microstructures of the lead-free solders/Cu and Sn surface joints formed by a conventional thermal reflow process and VFM were analyzed. From the intermetallic compound (IMC) formation study, it was found that more heat energy could be employed into the solder paste by the VFM heating process than by the thermal reflow process at the peak temperatures used in this study, because VFM provided more uniform heat than the reflow oven that may lead a temperature gradient in the test boards. It is believed that by VFM the soldering process at a lower peak temperature could be feasible than by a conventional reflow process.

This paper summarizes the effects of the variable frequency microwave (VFM) technique for rapid cure of polymeric encapsulants (such as flip-chip underfills) on the performance and reliability of electronic devices. Initial electrical performance of selected commercial IC packages following the application of VFM radiation stress has been recorded, and the performance has been compared to packages that were treated with comparable temperature cycles as applied in a convection oven. Failure analysis was performed on packages that showed electrical degradation to identify likely degradation and failure modes of the packaged ICs. Overall, our results show that VFM technology can safely be applied as an electronic packaging technology. However, proper control of VFM operating parameters is needed to ensure favorable performance of electronic devices.

The main objective of this study is to develop a stress-buffer-improved package that is subjected to a board level drop test under a specific-G impact level. In this regard, both the drop test experiment and the ANSYS/LS-DYNA simulations are carried out. Several studies have shown that the solder joints having the brittle intermetallic compound (IMC) layers within the wafer level chip scale packaging (WLCSP) are the weakest part. For the most part, this is due to the large relative motion occurring between the board and the chip. In addition, the stress buffer layer exhibiting a relatively large elongation which reduces the impact on the solder balls. Meanwhile, the novel stress-buffer-improve package's failure mode is different from the convention WLCSP structure which shifts to the trace damage of the chip side. The leading concern between the solder ball and trace damage is the critical region where failure occurred owing to the stress concentration effect. During the drop test experiment, the proposed stress-buffer-improved package is able to survive over 100 drops (most packages survived at above 200 drops). Hence, this drop performance very much surpasses the Joint Electron Device Engineering Council (JEDEC) criterion (drop number is 30 times). Nevertheless, the metal traces which are embedded in the stress buffer layer suffered relatively larger deformation. Generally, the stress concentration occurs at a single position, much like the trace/pad connecting junction in the analysis of detailed stress-buffer-improved package. Finally, the predict result in finite element (FE) analysis is similar to the broken metal trace's failure analysis in the drop test experiment.

As an excellent polymer material, polyimide (PI) plays an important role in practical applications. However, traditional filler doping modification is hard to simultaneously improve the dielectric properties and mechanical properties of the film. Here, PI–boron nitride (BN) nanosheets/PI–sodium titanate nanotubes (STNs)/PI–BN composite films with sandwich structure are prepared by in situ polymerization and multilayer coating processes, which significantly enhance the electrical and mechanical properties of dielectric films. With only 0.5 wt% fillers doping, the AC breakdown strength of the composite film can reach 183.7 kV mm−1, which is 16.41% higher than that of pure PI. When the filler doping is 1.5 wt%, the corona aging time of the composite film achieves 311.7% higher than that of pure PI film. In addition, the composite film exhibits outstanding elongation at break (58%) and tensile strength (138.92 MPa) performance. The 2D BN of the outer layer and 1D STNs of the middle layer effectively block the injection of charge carriers while improving the internal polarization response. A mechanism related to microstructure and interface is proposed to explain the improvement of electrical and mechanical properties in detail. This work provides a useful idea for the accurate design of high‐performance dielectric films.

Polyimide (PI) composite films with enhanced mechanical properties were prepared by incorporating modified fumed silica (FS) particles while preserving their optical and thermal characteristics. The PI matrix was synthesized using a fluorinated diamine, a fluorinated dianhydride, and a rigid biphenyl dianhydride via chemical imidization. Commercially available FS particles, including unmodified FS particles (0-FS) and particles modified with dimethyl (2-FS), trimethyl (3-FS), octyl (8-FS), octamethylcyclotetrasiloxane (D4-FS), and polydimethylsiloxane (PDMS-FS) were used. Scanning electron microscope images and nitrogen adsorption-desorption isotherms revealed well-defined porous structures in the FS particles. The water contact angles on the composite films increased compared to those of the pristine PI films, indicating improved water resistance. The PI/0-FS films exhibited a typical trade-off relationship between tensile modulus and elongation at break, as observed in conventional composites. Owing to the poor compatibility and agglomeration of the PDMS-FS particles, the PI/PDMS-FS composite films exhibited poor mechanical performance and diminished optical characteristics. Although the longer-chained FS particles (8- and D4-FS) improved the tensile modulus of the PI film by up to 12%, a reduction of more than 20% in toughness was observed. The PI composite films containing the methylated FS particles (2- and 3-FS) outperformed 8- and D4-FS in terms of mechanical properties, with PI/3-FS films showing an over 10% increased tensile modulus (from 4.07 to 4.42 GPa) and 15% improved toughness (from 6.97 to 8.04 MJ/m3) at 7 wt. % silica loading. Except for the PI/PDMS-FS composites, all composite film samples exhibited more than 86% transmittance at 550 nm. Regarding thermal properties, the glass transition temperature (Tg) and thermal stability remained stable for most composite films. In addition, PI/3-FS films demonstrated enhanced dimensional stability with lower coefficients of thermal expansion (from 47.3 to 34.5 ppm/°C). Overall, this study highlights the potential of incorporating specific modified FS particles to tailor the mechanical, optical, and thermal properties of PI composite films.

Wafer-level chip scale packaging (WLCSP) using TSV technology has widely used in CMOS image sensor (CIS) products in mass production. With more IO and high reliability requirements, the new CIS-WLCSP structure using vertical TSV was developed. However, the isolation layer deposited by low-temperature PECVD suffer crack failure in strict reliability test. To solve this problem, finite element simulation tool was used for evaluation and the simulation results were verified by design of experiment. Both results showed the new design with passivation layer introduced as stress buffer layer was an effective and reliable solution for CIS-WLCSP based on vertical TSV.

Silicon nitride (SiNx) based films have been recognized as essential dielectric films in the microelectronics and optoelectronics industry due to their desirable properties, such as high electrical insulation, excellent thermal stability, and compatibility with integrated circuit fabrication processes. They are also a potential candidate for fabricating wavelength-selective reflective coatings and surface passivation layers in building-integrated photovoltaics technologies. SiNx-based films are one of the popular choices for antireflective coatings in photovoltaics as well. Recently, SiNx and oxynitride (SiOyNx) based thin film optical filters have been explored to provide distinct color rendering to solar-charged electrical vehicles. Since solar cells have a lifespan of many years and the coating surfaces are substantial, it is essential to produce films with controlled optical and mechanical properties and maintain mechanical integrity against corrosion and wear. This study aims to design a deposition process and optimize minimal parameters for stable plasma conditions during multilayer deposition of SiNx and SiOyNx films using an electron cyclotron resonance plasma-enhanced chemical vapor deposition reactor with SiH4/N2/O2/Ar precursor mixtures at 120 °C. The primary goal was to investigate the influence of gas flow adjustments on the optical and mechanical properties, specifically targeting the refractive index and mechanical properties of the films. We measured the refractive index and the absorption of the films using variable angle spectroscopic ellipsometry. Then, we evaluated the mechanical residual stress ex situ using the wafer curvature measurement method. We have determined the elastic modulus and the hardness of the films using nanoindentation. The experimental results have shown a significant dependence of the optical and mechanical properties on the deposition parameters. To investigate the factors contributing to the intrinsic mechanical stress and to better understand SiNx film degradation mechanisms, we have studied the effect of postdeposition thermal cycling on the properties of the films. Several thermal-cycling experiments from room temperature to 400 °C were performed on different SiNx films, and the results showed an irreversible variation of the mechanical stress toward the tensile stresses caused by delamination of the films, while the refractive index remained unchanged.

ABSTRACTTo investigate the effect of reactive end‐capping groups on film‐forming quality and processability, a series of molecular weight‐controlled aromatic poly(amic acid) (PAA) resins functionalized with phenylethynyl end groups were prepared via the polycondensation of 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA), para‐phenylenediamine (PDA), and 4‐phenylethynyl phthalic anhydride (PEPA) served as molecular‐weight‐controlling and reactive end capping agent. The PAA resins with relatively high concentrations endow enhanced wetting/spreading ability to form PAA gel films by solution‐cast method which were thermally converted to the fully‐cured polyimide (PI) films. The mechanical and thermal properties of PI films were investigated as a function of PAA molecular weights (Mn) and thermal‐curing parameters. Mechanical property, dimensional stability and heat resistance of the fully‐cured PI films with PAA Mn > 20 ×103 g mol−1 are found to be better than that of their unreactive phthalic end‐capped counterparts. The covalent incorporation of chain‐extension structures in the backbones, induced by thermal curing of phenylethynyl groups, might facilitate yielding a higher degree of polymer chain order and consequently improved resistance strength and elongation at break to tensile plastic deformation. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45168.

An approach to curing thermosetting polymers is the use of Variable Frequency Microwave (VFM) systems, which have been shown to cure encapsulant materials in substantially shorter times than conventional methods [1]. The microwave curing of thermosetting polymer material is a complex process, with interactions between electromagnetic field intensity, temperature, polymer cure kinetics and resulting development of potential thermo-mechanical stresses. As the real-time in-situ experimental analysis of a number of these parameters is extremely challenging, a numerical model has been developed to assess this process. The integration of microwave curing into an existing assembly machine imposes the requirement of a compact microwave system, based on a solid state source. Such a system has been designed and installed into our prototype demonstrator. A series of shear tests have been performed on an electrically conductive adhesive and an encapsulation material. No major deviation between microwave-cured material and material cured in a convection oven has been witnessed. Fourier Transform Infra-Red (FTIR) experiments were carried out on uncured, conventionally and microwave cured samples. The spectra of microwave cured and conventionally cured samples were practically identical. A series of LM-2940 dies have been die- and wirebonded to a Mirror Semiconductor open cavity package. Tests have demonstrated that the encapsulation and subsequent microwave curing can be performed without evident detrimental effects.

To solve the mystery of early thermal cycling failures on a relatively small size (1.6 mm × 2.4 mm) wafer level chip scale package (WLCSP) with bump on re-passivation (BOR) structure, details of bump geometry parameters are looked into. While the under bump metal layer stack is first confirmed within specifications, UBM offset from center point is found greater than typical on the failed units. To fully understand the impact of this bump parameter shift that is defined by lithographic process, additional geometric configurations of the bump structure and materials, as well as bump parameter deviations are studied on the stress in solder, UBM and on chip SiN passivations. The study takes into consideration the thickness/size of UBM, aluminum metal pad, SiN passivation layer and PI re-passivation layer. UBM materials, such as nickel vs copper, are also included in the study. Board-level thermal cycling test (TMCL) is conducted on a WLCSP with 8×8 bump array at 0.4mm pitch. Simulation reveals that size ratio and bump offset indeed change the stress level in the critical passivation layer, and the results well explain the early failures recorded in a device qualification reliability test and follow up experimental verifications. Conclusion of this work should be applied when defining design rules for robust reliability performance and manufacturability of this traditional WLCSP bumping technology.

In the case of embedded chip type CSPs (Chip Scale Packages) made by embedding chips in PCBs (Printed Circuit Boards), problems occur due to differences in the CTE (Coefficient of Thermal Expansion) between the PCBs and the chips. This study tested a method to solve this problem by inserting thermal SBLs (Stress Absorbing Layers) into the embedded chips, thereby improving the reliability of the connection between the chip and the PCB. This study focused on a production method used to form a SBL on a Silicon Active Layer in order to make embedded PCBs and the material used as SBLs was absorbent in the polyimide family, which was composed of a material containing at least 30% polysiloxanes. For the experiment, two types of samples were used, including: 20mm × 20mm sized silicon active layers without any SBL formed on them; those with an SBL formed only on the top surface. The samples were produced and placed on the PCBs. To examine the effects of the physical damage, 3 point bending tests were conducted and the results were analyzed.

The wafer level package (WLP) is a cost-effective solution for the electronic package, and has been increasingly applied during recent years. In this study, a new packaging technology developed based on the concepts of the WLP, the panel base package (PBP) technology, is proposed in order to further obtain the capability of signals fan-out for the fine-pitched integrated circuit (IC). In the PBP, the filler material is selected to fill the trench around the chip and provide a smooth surface for the redistribution lines. Therefore, the solder bumps could be located on both the filler and the chip surface, and the pitch of the chip side is fanned-out. The design concept and the manufacturing process of the PBP would first be described in this study. The three-dimensional finite element (FE) model is established based on the real testing sample, and the thermo-mechanical behavior of the PBP is simulated. It is found that the solder joint reliability of the PBP can be highly improved because of the applied stress buffer layer (SBL). However, the accumulated stress/strain from the coefficient of thermal expansion (CTE) mismatch may transfer to the metal lines in package. In order to enhance the robustness of the redistribution lines, the bypassed type interconnect is suggested. Moreover, the trace/pad connecting junction and the conductive via which have smooth outline are preferred to avoid the stress concentration effect.

Effect of nitrogen flow rate on microstructures and mechanical properties of metallic coatings by warm spray deposition

The effects of crosslinking agents (crosslinkers) on polyimide (PI)/graphene oxide (GO) hybrid films were extensively investigated. The surface of GO was modified with amino groups using 4‐aminobenzylamine to improve compatibility with pyromellitic dianhydride/4,4′‐oxydianiline PI, and two kinds of crosslinkers were used: tris(4‐aminophenyl)amine and 1,3,5‐triazine‐2,4,6‐triamine (melamine). The mechanical, thermal and optical properties of the PI hybrid films were investigated. In particular, the transparency and physical properties of the PI hybrid films containing amino‐functionalized GO with homogeneous dispersion were improved. As the content of the crosslinker increased, a crosslinking network was formed between the PI chains, and the stiffness of the hybrid films was increased. The glass transition temperature, heat resistance and mechanical properties were also enhanced. The PI hybrids prepared with a rigid crosslinker exhibited higher optical transparency due to the reduction of the intermolecular charge transfer interactions with increasing interchain spacing between the PI chains. © 2018 Society of Chemical Industry

Investigation of degradation mechanisms in mechanical properties of polyimide films exposed to a low earth orbit environment