Humidity Storage Test Research Articles

Nitrogen-doped amorphous carbon coating on copper pads for direct wire bonding with a long-term humidity reliability

To improve the long-term reliability of copper (Cu) pads used in LSIs, nitrogen-doped amorphous carbon (a-C:N) coating is applied to the Cu pads with direct wire-bonding to prevent Cu oxidation in humidity. To obtain a thin barrier, the thickness optimization of a-C:N layer was carried out under temperature humidity storage (THS) testing at the conditions of 85 °C/85% relative humidity. Cu pad chains coated with the optimized a-C:N film were fabricated by direct bonding with aluminum (Al) wires. The electrical connection up to 210 bonds was obtained with the 10 and 15 nm thick a-C:N coated Cu pads. The reliability test of the pad-chains under the THS was carried out, and a 15 nm thick a-C:N can preserve the electrical connection along 100 h of the THS test. The a-C:N coating is expected to improve the humidity reliability of Cu pads with direct wire-bonding for long-term data storage.

A novel graphene barrier against moisture by multiple stacking large-grain graphene

The moisture barrier properties of stacked graphene layers on Cu surfaces were investigated with the goal of improving the moisture barrier efficiency of single-layer graphene (SLG) for Cu metallization. SLG with large grain size were stacked on Cu surfaces coated with CVD-SLG to cover the grain-boundaries and defective areas of the underneath SLG film, which was confirmed to be oxidized by Raman spectroscopy measurements. To evaluate the humidity resistance of the graphene-coated Cu surfaces, temperature humidity storage (THS) testing was conducted under accelerated oxidation conditions (85 °C and 85% relative humidity) for 100 h. The color changes of the Cu surfaces during THS testing were observed by optical microscopy, while the oxidized Cu into Cu2O and CuO was detected by X-ray photoelectron spectroscopy (XPS). The experimental results were accord with the results of first-principle simulation for the energetic barrier against water diffusion through the stacked graphene layers with different overlap. The results demonstrate the efficiency of SLG stacking approach against moisture for Cu metallization.

Lifetime prediction model of Cu-based metallization against moisture under temperature and humidity accelerations

To establish the lifetime prediction model for Cu-based metallization against moisture, temperature humidity storage test under the various accelerated conditions is performed. The increase of Cu sheet resistance induced by Cu-oxidation is measured by a four-point probe method. X-ray photoelectron spectroscopy analysis is carried out to investigate the variation of oxidized Cu. The activation energy and the humidity acceleration factor for Cu-based metallization have been derived by the statistical analysis to predict the lifetime against moisture for the first time. The results indicate that Cu-based metallization is comparatively more sensitive to the temperature than humidity at around the practical use condition.

Effects of oxidation on reliability of screen-printed silver circuits for radio frequency applications

High reliability has become one of the crucial requirements for portable electronic devices, due to the high dependence of their radio frequency (RF) characteristics on the end-user's surroundings. The RF characteristics of screen-printed silver (Ag) circuits were investigated after a steady-state temperature and humidity storage test. A conductive paste containing Ag nanoparticles was screen-printed onto a silicon (Si) substrate and then sintered at 250°C for 30min in air. The printed Ag circuits were placed in a chamber at 85°C/85% relative humidity (RH) for various durations: 100, 300, 500, 1000h. The microstructural evolution and thickness profiles of the Ag circuits were observed with field emission scanning electron microscopy and α-step, respectively. The oxidation of the printed Ag circuit surface was analyzed with Auger electron spectroscopy. A network analyzer and Cascade's probe system in the frequency range of 40MHz to 40GHz were employed to measure the scattering parameters of the Ag circuits. The experimental results showed that the insertion losses at higher frequencies increased with increasing durations of exposure to the 85°C/85% RH environment, due to the thicker specific layer for oxidation on the circuit surfaces. The oxide layer was the dominant factor affecting the RF characteristics of the screen-printed Ag thin circuits. Therefore, it is essential to control the oxidation of printed circuits for versatile RF applications.

Drop Reliability of Epoxy-contained Sn-58 wt.%Bi Solder Joint with ENIG and ENEPIG Surface Finish Under Temperature and Humidity Test

The influence of two kinds of surface finish, namely electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG), on the interfacial reactions and drop reliability of epoxy-enhanced Sn-58 wt.%Bi solder has been investigated after temperature–humidity storage tests. The chemical composition and morphology of intermetallic compounds (IMCs) were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and electron probe microanalysis. Also, the mechanical reliability of solder joints was evaluated using board-level drop tests. The Sn-Bi epoxy solder/ENEPIG joint exhibited higher IMC growth rate than the Sn-Bi epoxy solder/ENIG joint. After 500 h at 85°C/85% RH storage condition, new IMCs were formed on the Ni3Sn4 layer in samples with both surface finishes. The results of board-level drop tests showed that the number of drops was higher for the ENIG than the ENEPIG surface finish. Solder joint fracture occurred along the interface between the solder and IMC layer for the ENIG surface finish. However, with the ENEPIG surface finish, the crack propagated between the IMCs.

Cost-Effective Optical Sub-Assembly Using Lens-Integrated Surface-Emitting Laser

A cost-effective optical sub-assembly (OSA) for high-speed optical interconnections using lens-integrated surface-emitting lasers (LISELs) is demonstrated. Two concepts of the OSA, one based on a direct-modulation (DM)-LISEL and the other based on a continuous-wave (CW)-LISEL for a silicon-photonics (SiP) platform, are proposed. The design parameters of the integrated lens that enable low fiber-coupling loss and the SiP platform are described. Coupling losses, as low as 4.9 dB for the single mode fiber and 4 dB for the SiP platform, were experimentally confirmed. Furthermore, theoretical and experimental evidence that demonstrates the OSA does not require an optical isolator (i.e., it is “isolator-free”) is presented. A DFB laser used for the LISEL had a low relative intensity noise (less than −140 dB/Hz), even with a back reflection of more than −20 dB. For non-Hermetic packaging, 500 h, 85°C/85% relative humidity storage tests experimentally demonstrated no degradation in the lasing characteristics.

Microstructural evaluation and failure analysis of Ag wire bonded to Al pads

We found the failure mechanisms in Ag wire bonded to Al pads during the high-temperature-storage lifetime test (HTST) and the unbiased highly-accelerated temperature and humidity storage test (uHAST). The native oxide layer on the Al pads caused a ball lift. The moisture and the thermal energy during uHAST along with the Cl− ion in epoxy molding compounds (EMCs) induced repetitive oxidation and reduction reactions of the Ag–Al intermetallic compounds (IMCs) with the Al pads. These repetitive reactions formed H2 gas as a by-product causing the formation of a micro-crack. In addition, the alumina layer acted as a resistive layer in the Ag–Al IMCs. The phases of the Ag–Al IMCs were identified as Ag2Al and Ag3Al, and the growth rates of those IMCs were measured at 150 and 175°C for 2000h.

ACF를 이용한 COG 접합 공정에서 도전볼의 음영비와 접촉 저항과의 관계

Chip on glass (COG) bonding using anisotropic conductive film (ACF) is a key technology to assemble a driver IC onto a LCD glass panel. In this paper, an experimental investigation was conducted to investigate the correlation between contact resistance and characteristics of image taken by machine vision based inspection system. The results show that the contact resistance was strongly influenced by the contrast ratio of conductive particle rather than the number of conductive particles. Also, number of conductive particles whose contrast ratio is below 0.75 is crucial for determining the quality of the assembled samples. On the other hand, in the result of high temperature high humidity storage test, the contrast ratio of samples was increased. However, in the case of open-circuit samples after temperature humidity storage test, the number of conductive particles whose contrast ratio is above 0.75 was more than that of the closed-circuit samples.

Bendable 임베디드 전자모듈의 손상 메커니즘

A bendable electronic module has been developed for a mobile application by using a low-cost roll-to-roll manufacturing process. In flexible embedded electronic module, a thin silicon chip was embedded in a polymer-based encapsulating adhesive between flexible copper clad polyimide layers. To confirm reliability and durability of prototype bendable module, the following tests were conducted: Moisture sensitivity level, thermal shock test, high temperature & high humidity storage test, and pressure cooker tester. Those experiments to induce failure of the module due to temperature variations and moisture are the experiment to verify the reliability. Failure criterion was 20% increase in bump resistance from the initial value. The mechanism of the increase of the bump resistance was analyzed by using non-destructive X-ray analysis and scanning acoustic microscopy. During the pressure cooker test (PCT), delamination occurred at the various interfaces of the bendable embedded modules. To investigate the failure mechanism, moisture diffusion analysis was conducted to the pressure cooker's test. The hygroscopic characteristics of the encapsulating polymeric materials were experimentally determined. Analysis results have shown moisture saturation process of flexible module under high temperature/high humidity and high atmosphere conditions. Based on these results, stress factor and failure mechanism/mode of bendable embedded electronic module were obtained.

Adhesion Characteristics of Silver Tracks Screen-Printed on Polyimide with an Environmental Reliability Test

Printable and flexible electronics are increasingly being used in numerous applications that are miniaturized, multi-functional and lightweight. Simultaneously, reliability issues of the printed and flexible electronic devices are getting more attention. The adhesion of screen-printed silver (Ag) tracks on a polyimide (PI) film was investigated after two kinds of the environmental reliability test: a constant-temperature storage test, and a steady-state temperature and humidity storage test. Atmospheric-pressure plasma (APP) was adopted on the PI film surface to improve the poor adhesion derived from the inherent hydrophobicity. The Ag tracks constructed via screen printing were sintered at 250 degrees C for 30 min in air using a box-type muffle furnace. Some samples were exposed under 85 degrees C and 85% relative humidity (RH) for various durations (24, 72, 168 and 500 h), and others were aged at 85 degrees C with same durations to compare the influence of moisture on the adhesion. The adhesion of the screen-printed Ag tracks was evaluated by a roll-type 90 degrees peel test. The peel strength of the screen-printed Ag tracks decreased by 76.74% and 69.88% after 500 h run of the 85 degrees C/85% RH test, and the aging test, respectively. The weakest adhesion was 4.98 gf/mm after the 500 h run of the 85 degrees C/85% RH test. To demonstrate these experimental results, the microstructural evolution and chemical bonding states of the interfacial surfaces were characterized using a field emission scanning electron microscope (FE-SEM), and X-ray photoelectron spectroscope (XPS), respectively.

Development of a Novel Compliant-Bump Structure for ACA-Bonded Chip-on-Flex (COF) Interconnects With Ultra-Fine Pitch

The demand for high-density electronic applications is growing. This work develops a novel chip-on-flex (COF) package with sidewall-insulated Au-coated polyimide (PI) compliant-bumps. A double-layer anisotropic conductive adhesive (ACA) material that meets the assembly requirement is adopted for the ultra-fine pitch interconnects. A process for manufacturing 20- μm pitch compliant-bumps is proposed for ACA-bonded COF packages. The double-layer ACA consists of an ACA layer with a diameter of 2.8 μm conductive particles and an NCA layer as an interlayer to bind a silicon chip with a flexible substrate. The bonding accuracy for ultra-fine pitch is determined using X-rays. To evaluate the quality of bonding, the electrical insulation is tested and the contact resistance of the daisy chain with 606 input/output (I/O) around the periphery of the chip is measured. The double-layer ACA material is assembled at different bonding temperatures to study the effects of bonding temperature on the interface adhesion using differential scanning calorimetry (DSC) and a 90° peeling test. The reliability of the fabricated COF interconnects is also evaluated by performing an 85°C/85% relative humidity thermal humidity storage test (RH THST) for 1000 h and a -55°C ~ 125°C thermal cycling test (TCT) for 1000 cycles. The interfaces between the silicon chip and the substrate of the failed samples in the reliability tests are then observed using the cross-sectional scanning electron microscopy (SEM). The compliant-bump-bonded samples with the double-layer ACA provide their excellent electrical insulation performance even at a joint space of 5 μm whereas the Au-bump samples have a short-circuiting rate of more than 50%. Notably, the contact resistance also remains stable and varies by under 3% in both the RH THST for 1000 h and the TCT for 1000 cycles. The presented results show the reliable bonding quality and stable contact resistance of the COF package that is bonded with the compliant-bump structure using the double-layer ACA, indicating its great potential for use in ultra-fine pitch applications.

Comparative Study of Phenolic-Based and Amine-Based Underfill Materials in Flip Chip Plastic Ball Grid Array Package

Phenolic and amine epoxy systems are widely used as hardeners in underfill materials for flip chip packaging. A comparison was made between these two systems in order to evaluate the reliability performance of a flip chip plastic ball grid array (FC-PBGA). The coefficient of thermal expansion, glass transition temperature (Tg), Young’s modulus (E), and fracture toughness were revealed by using a thermal mechanical analyzer, a dynamic mechanical analyzer, and a single-edge notch three-point bending test, while moisture absorption study was performed using an 85°C/85% relative humidity chamber. The adhesion strength with different conditions of temperature and humidity was performed using a die shear test. The series of standard reliability tests such as accelerated temperature cycle test, pressure cooker test, thermal humidity storage test, and high temperature storage test were executed upon the FC-PBGA, which was filled by phenolic and amine epoxy systems of underfill materials. It was found that the adhesion strength of phenolic-based underfills is better than that of amine-based underfills in almost all test conditions. Phenolic-based underfills also demonstrated better reliability compared with amine-based underfills.

Experimental/Numerical Analysis of Thermally Induced Warpage of Ultrathin Chip-on-Flex (UTCOF) Interconnects

Future applications of flexible displays and wearable electronics will need 3-D stacked flexible interconnects. The interconnection of an ultrathin chip-on-flex (UTCOF) that can provide flexibility is one approach that meets this requirement. Therefore, thermally induced warpage of UTCOF interconnects using anisotropic conductive adhesive (ACA) is investigated. In this paper, the effects of the ACA joint material properties, bonding temperature, and chip thickness on warpage of ACA-bonded UTCOF interconnects are examined experimentally and numerically. Two film types of ACA materials, ACA-P and ACA-F, are assembled under different bonding temperatures to study the effects of bonding temperature on warpage via out-of-plane deformation measurements using a micro figure measurement instrument. Micro Au-bump and compliant-bump assemblies in 80-μm-pitch dummy test vehicles are evaluated. Moreover, ultrathin chips with 25-50 μm thickness were assembled onto polyimide flex substrates to study the effects of chip thickness on thermally induced warpage. The 85°C/85% relative humidity thermal humidity storage test (RH THST) was also conducted for 1000 h for the UTCOF assembled with the selected process parameters. The interfaces between the ultrathin silicon chip and substrate are inspected in cross-sectional scanning electron microscopy (SEM) images. To validate the results of the experiments, a rigorous 3-D finite element (FE) analysis model integrating both thermal and thermal-mechanical behaviors of the UTCOF is established and performed using the ANSYS program. Experimental and numerical results indicate that the warpage of the micro compliant-bump assembly is less than that of the micro Au bump. Furthermore, the averaged warpage of the ACA-P-bonded samples with the Au bump using a 50- μm-thick chip is around 50.3 μm at a bonding temperature of 160°C whereas that of the ACA-F-bonded samples is 64.4 μm at 190°C. Additionally, both thermal expansion mismatch and the thermal gradient between the ultrathin silicon chip and substrate strongly affect the thermal-mechanical behaviors of the UTCOF interconnects. As expected, warpage increases as thickness of an ultrathin silicon chip decreases. A strong correlation exists between FE analysis results and experimental results. The manufacturing technology for high-density and flexible UTCOF interconnects with ACA joints is thus established.

Reliability and Flexibility of Ultra-Thin Chip-on-Flex (UTCOF) Interconnects With Anisotropic Conductive Adhesive (ACA) Joints

The need for flexible interconnects in advanced applications in consumer electronic products is increasing rapidly. The reliability and flexibility of ultra-thin chip-on-flex (UTCOF) interconnects formed using anisotropic conductive adhesive (ACA) are thus investigated. Two films of ACA materials, namely ACA-P and ACA-F, are assembled at different bonding temperatures to study the effect of temperature on the adhesion at the substrate-adhesive and adhesive-chip interfaces using differential scanning calorimetry (DSC) and a 90° peeling test. The contact resistance of a daisy chain with 188 input/output (I/O) is measured to examine the quality of bonding through dummy test samples with an 80-μm pitch. The reliability of the fabricated UTCOF interconnects bonded via selected ACA joints is evaluated by performing an 85°C/85% RH thermal humidity storage test (THST) for 1000 h, and their flexibility is evaluated in static bending and four-point bending tests. The interfaces between the ultra-thin silicon chip and the substrate of failed samples in the THST and four-point bending testing are then investigated by scanning electron microscopy (SEM), which is utilized to obtain cross-sectional images. Finite element analysis is also conducted to elucidate the failure mechanism of the UTCOF interconnects in the four-point bending test. The averaged maximum allowable deflections of the fabricated UTCOF interconnects with ACA-P and ACA-F materials are 26% and 168%, respectively, higher than those of the COF interconnects with a chip thickness of 670 μm. Moreover, the contact resistance remains stable, varying by less than 10%, in the static bending test with a bending radius of 30 mm. According to the results thus obtained, give the appropriate choice of an ACA material and the optimal curing conditions, the UTCOF interconnects with ACA joints reliably serve as flexible interconnects for use in consumer electronic products.

Feasibility study of non-conductive film (NCF) for plasma display panel (PDP) application

In this study, attempts were made to develop an electrically reliable and low cost bonding process for plasma display panels (PDPs) by using non-conductive film (NCF). The adhesive joints were constructed with Ag electrodes on a glass substrate and Au finished Cu electrodes on a flexible printed circuit (FPC). To clarify the feasibility of NCF application, all experimental results of NCF were compared with those of anisotropic conductive film (ACF) which is currently used for PDP application. The reliability of adhesive interconnections using NCF and ACF was evaluated by measuring daisy resistance during high temperature and humidity storage test (85 °C, 85% RH for 1000 h). Ag electrochemical migration, which leads to the degradation of surface insulation resistance, was also investigated by measuring insulation resistance during the storage test (1000 h at 85 °C and 85% RH). The initial resistance of NCF joints was lower than that of ACF joints due to larger contact area between Ag and Au finished Cu electrodes in the NCF joints. During the reliability test, the daisy resistances of both joints increased slightly due to Z-directional swelling of the adhesives, but NCF joints showed more stable resistance than ACF joints. In addition, upon application of high voltage of 100 V (DC current), severe degradation of insulation resistance due to the Ag migration was observed in ACF joints, whereas no remarkable decrease of insulation resistance was observed in NCF joints. Conclusively, the application of NCF to PDP interconnection process has good potential from a practical point of view.

Influence of underfill materials on the reliability of coreless flip chip package

A flip chip package was assembled by using 6-layer laminated polyimide coreless substrate, eutectic Sn37Pb solder bump, two kinds of underfill materials and Sn 3.0Ag 0.5Cu solder balls. Regarding to the yield, the peripheral solder joints were often found not to connect with the substrate due to the warpage at high temperature, modification of reflow profile was benefit to improve this issue. All the samples passed the moisture sensitive level test with a peak temperature of 260 °C and no delamination at the interface of underfill and substrate was found. In order to know the reliability of coreless flip chip package, five test items including temperature cycle test (TCT), thermal shock test (TST), highly accelerated stress test (HAST), high temperature storage test (HTST) and thermal humidity storage test (THST) were done. Both of the two underfill materials could make the samples pass the HTST and THST, however, in the case of TCT, TST and HAST, the reliability of coreless flip chip package was dominated by underfill material. A higher Young’s modules of underfill, the more die crack failures were found. Choosing a correct underfill material was the key factor for volume production of coreless flip chip package.

Reliability of unencapsulated SMD plastic film capacitors

Small and low cost unencapsulated SMD plastic film capacitors were manufactured with different terminal metal compositions and dielectric materials. Capacitors made with a polyethylene naphthalate film dielectric were produced using a winding method. The terminals were metallized using the flame spraying process. The terminals of the test capacitors consisted of three different metal layers. The base metal layer, which was aluminum, was coated with brass or copper. The top layer was a sprayed lead‐free, tin‐based solder to ensure the solderability of the terminals. The reliability of the unencapsulated test capacitors was evaluated using standard temperature cycling, humidity storage, and high temperature environmental tests. Solderability and resistance to soldering heat were tested by mounting the test capacitors using the reflow soldering technique. The electrical properties including capacitance, insulation resistance, and dissipation factors at 1kHz and 100kHz were verified.

Plastic-molded LTS DC-SQUIDs for multichannel biomagnetic measurement systems

A plastic molding technique for SQUIDs has been developed to protect the chip from moisture damage. A 1.5-mm-thick layer of molding compound (epoxy resin, elastic resin, and silica powder) was poured over Ketchen-type DC-SQUID that has Nb/AlO/sub x//Nb Josephson junctions. The moisture resistance of these plastic-molded SQUIDs was examined by subjecting them to temperature and humidity storage tests (60/spl deg/C, 90% relative humidity, 1,000 hours), and then measuring their performance characteristics. The results showed that the plastic packaging protected the chip from moisture damage, while not affecting the noise performance of the SQUIDs. The plastic molding therefore improved reliability and ease in handling of such SQUIDs.

Laser trimming of thick film metal resistors on aluminum nitride substrates

Problems and their countermeasures in laser trimming of thick-film metal resistors on aluminum nitride (AIN) substrates are studied. Trimming was made possible by selection and combination of suitable Q-switching frequency, laser power, and number of irradiations. For example, under the conditions of 1 kHz, 2.0 W, and double irradiation, resistance values could be controlled to within +or-0.3% of target values. Resistance changes of trimmed metal resistors were less than +5.5% and comparable to those on alumina after thermal cycle tests (-55-150 degrees C, 1000 times), high-temperature storage tests (150 degrees C, 1000 h), and high-temperature and humidity storage tests (80 degrees C, 90% RH, 1000 h). Since the insulation characteristics of an irradiated portion strongly depended on the irradiation conditions, suitable conditions were selected for practical applications. The degree of insulation degradation after the laser irradiation was different for the two types of AlN substrates studied. This is attributed to the different amounts of free Al formed on the AlN surfaces. Further, AlN has a higher absorption factor and smaller reflectivity than alumina for a YAG laser beam. Thermal and electrical damage of AlN are affected by these properties. >

The analysis of chemical and metallurgical changes in microcircuit metalization systems

Chemical and metallurgical changes in IC metalization systems have been studied during a program of device reliability testing and evaluation. This paper discusses the four reactions responsible for the majority of the observed metalization failures: 1) migration of aluminum under silicon dioxide layers, 2) movement of silicon through both gold and aluminum wires resulting in loss of mechanical strength, 3) structural changes in the molybdenum-gold system occurring during temperature storage testing, and 4) the electrochemical attack of the molybdenum barrier layer resulting from humidity storage testing with applied bias. Using standard optical techniques and a commercially available electron beam microanalyzer/scanning electron microscope, the following results were obtained. 1) At temperatures near the silicon-aluminum eutectic point, aluminum migrated under the thermally grown oxide in the form of triangles oriented with respect to the underlying silicon. Aluminum migration was observed at lower temperatures when bias was applied to the device. This occurred at the interface of the thermal silicon dioxide and a vapor deposited glass. 2) Silicon precipitation at wire grain boundaries was observed in both aluminum and gold lead wires. The mechanisms of solid-state diffusion and high-current density electromigration of silicon resulted in wire fracture or slip at grain boundaries in the wires. 3) Silicon penetration into gold conductor stripes through defects in the molybdenum barrier layer resulted in severe reordering of the gold at temperatures in excess of 300°C. The silicon-gold eutectic was formed at temperatures greater than 370°C causing complete dewetting of the molybdenum layer. At lower temperatures, grain boundary flaking due to loss of gold cohension and adhesion was observed. 4) Whiskers which formed on molybdenum barrier layers during 85-85 humidity storage testing with applied bias contained high concentrations of sodium. Several known compounds of sodium and molybdenum are discussed in light of the X-ray data taken on the electron beam microprobe.