Microelectronic Packaging Industry Research Articles

Strong grain size effect on the liquid/solid reactions between molten solder and electroplated Cu

Electrodeposition of Cu metallization and its soldering reaction with solder alloy are of technological significance in microelectronic packaging industry. This study aims to investigate the effect of Cu grain size on the liquid/solid reactions between molten Sn-3 wt% Ag-0.5 wt% Cu (SAC305) solder alloy and electroplated Cu. Two Cu electroplated films, denominated as fine-grained Cu (FG-Cu) and coarse-grained Cu (CG-Cu), are prepared and reacted with SAC305 at 260 °C for 5–20 min. Microstructural characterization shows that the Cu6Sn5 intermetallic compound (IMC) is formed in these two solder joints, while that in the FG-Cu solder joint grows at a faster rate than that in the CG-Cu solder joints. The Cu6Sn5 IMC in the FG-Cu solder joint exhibits a slender column-like grain morphology, unlike the common scallop-like grain morphology in the CG-Cu solder joints. The strong grain size effect is rationalized by comparing the contribution of two fluxes of ripening and interfacial reaction to the growth of Cu6Sn5.

Enhancement of the electrical and thermal conductivity of epoxy-based composite films through the construction of the multi-scale conductive bridge structure

Electrically conductive adhesive films (ECAFs), a novel electrically and thermally adhesive material, have been widely used in the large-area bonding of microelectronics packaging. However, it is still a challenge to optimize the silver micro-flakes (Ag-MFs) dispersion, and build a more effective electrically and thermally conductive network. In this study, we improved the dispersion of Ag-MFs in epoxy matrix through the intermolecular hydrogen bonding, and proposed a new multi-scale conductive bridge structure. Specifically, poly (3,4-ethylenedioxythiophene)/poly (styrene sulfonate) (PEDOT/PSS or P/P for short) can form the intermolecular hydrogen bonding with Ag-MFs based on the sulfonic groups of PSS chain and the carboxyl group located on the surface of Ag-MFs, resulting in improving dispersion of Ag-MFs. Besides, PEDOT can occur conformation change from benzoid conformation to quinone conformation and form PEDOT conductive crystalline nanofibril, and then multi-scale conductive bridge structure with Ag-MFs were constructed. Moreover, the initial viscosity of epoxy-based composite is maintained within 106 mPa s, which means Ag-MFs is uniformly dispersed after the addition of P/P. Scanning electron microscopy (SEM) and oscillating viscoelastic measurement were used to demonstrate the improved dispersion of Ag-MFs after the addition of P/P. In addition, atomic force microscope (AFM) and Raman spectroscopy were used to verify the conformation change of PEDOT. Compared with ECAF without P/P, the electrical and thermal conductivity of ECAF with 6 wt% P/P was enhanced by 73.1% and 166.3%, respectively. Meanwhile, scattering parameter (S-parameter) measurements demonstrated that ECAF with 6 wt% P/P had excellent radio-frequency behavior at 2–4 GHz, indicating high-performance in the microelectronics packaging industry.

Dependence of mechanical and thermal deformation behaviors on crystal size and direction of Cu3Sn intermetallic: A molecular dynamics study

The implementation of environment friendly technologies in microelectronic packaging industry has widened the essentialities of lead-free solder alloys. The Cu3Sn intermetallic compound is blameworthy for the adverse influences of the mechanical and thermal properties of the lead-free solder alloys. This study explored the effects of crystal size and direction on mechanical and thermal deformation behavior of Cu3Sn using the modified embedded atom method (MEAM) parameter in molecular dynamics simulation. The stress–strain characteristics were tracked down at 25 °C temperature and 1010 s−1 strain rate, and the thermal deformation manner was observed from −100 °C to 200 °C temperature, which came up with the thermal expansion coefficient (CTE). Investigations show that the tensile properties differ 43–63 % owing to anisotropy, whereas the CTE varies only less than 1 %. Meanwhile, the tensile properties and CTE plummet topmost 25 % and 58 % consecutively throughout the crystal size. However, the thermal expansion proclivity in different crystal size was quite indistinguishable at higher temperature.

Tensile properties dependency on crystal size and direction of single crystal Ag3Sn intermetallic compound: a molecular dynamics study

The necessities of lead-free solder alloys have been expanded by the advent of eco-friendly technologies of microelectronic packaging industry. Mostly lead-free solder alloys are SAC, which is the alloy of tin, silver and copper metal. Ag3Sn, Cu3Sn and Cu6Sn5 inter-metallic compounds are formed during microelectronic packaging, which reduce the properties of SAC solder joints adversely. The present study explores direction dependent tensile properties of various sized single crystal anisotropic Ag3Sn. Molecular dynamics simulation platform LAMMPS is used to examine the elastic behavior of these crystals by modified embedded atom method (MEAM) potential parameters. The atomistic investigation is performed for four crystal sizes of 30×30×30, 60×60×60, 90×90×90 and 120×120×120 Å3 at a temperature of 298 K and a strain rate of 1010 s−1. A comprehensive insight from this work is that the tensile properties fluctuations due to direction are within 13–25% at any distinct crystal size, whereas fluctuations at any specific direction are within 5–17% for mentioned interest of crystal sizes. An optimum crystal size of 60×60×60 Å3, exhibits maximum tensile properties. However, the fracture initiation patterns are quite similar for all sizes and directions where the natures are investigated as ductile.

Size effects on IMC growth of Cu/Ni/Sn-3.5Ag microbump joints during isothermal aging and prediction of shear strength using ANN

Significant downsizing trend is currently directing the advanced microelectronic packaging industry, which poses new challenges to the manufacturing process and reliability of high-density chip interconnections. In this paper, to investigate the size effect of microbump joints on the interfacial structure and bonding strength, the chip-to-chip bonding joints fabricated by microbumps of same Cu/Ni/Sn-3.5Ag construction but of various sizes were experimentally examined. After thermocompression bonding process to fabricate the chip-to-chip microbump interconnections, the intermetallic compound (IMC) growth on the Ni/Sn-3.5Ag interface during 150 °C isothermal aging was compared for joints of 100, 40 and 25 μm in diameter as well as 35, 25 and 15 μm in solder layer height, using field emission electron microscopy and electron probe microanalysis. A clear size effect on the thickness and morphology of Ni3Sn4 IMC was observed, showing thicker IMC in joints of smaller diameter and lower height. The Ni concentration distribution in the solder layer was verified to be dependent on the joint size, with possible reasons being enhanced sidewall diffusion and grain boundary diffusion through IMC grains in small-diametered joints, as well as shorter diffusion path in lower joints. The shear strength of chip-to-chip bonding vehicles was also found to be size-dependent. In addition, artificial neural network (ANN) approach was applied to establish the numerical correlation of structural factors and bonding strength. It was found that a properly constructed ANN model can successfully predict the influence of the complex combinations of geometric and interfacial factors with highly accurate output, providing an effective tool for design and optimization of microbump interconnection structures and processes.

EFFECT OF GAMMA RADIATION ON MICROMECHANICAL BEHAVIOR OF SNPB SOLDER ALLOY

Tin-lead (SNPB) alloys are widely used in microelectronic packaging industry. It serves as a connector that provide the conductive path needed to achieve the connection from one circuit element to another circuit element. In this research, the effect of gamma irradiation on the micromechanical behaviour of tin-lead (SNPB) solder alloy has been investigated using the nano-indentation testing. Gamma radiation with a Cobalt-60 source were exposed to SNPB solders with different doses from 5 Gy to 500 Gy. In this study, the nano-indentation technique was used to understand the evolution of micromechanical properties (hardness and reduced modulus) of SNPB solder joints subjected to gamma irradiation. The results showed that the hardness of the SNPB alloys was enhanced with increasing of gamma radiation. The hardness was greatest at dose of 500 Gy of sample, 25.6 MPa and had the lowest value at un-irradiated sample. However, the reduced modulus was decreased by increasing the irradiation of gamma due to the intrinsic properties and the atomic bonding of the material.

Experimental investigation on stir casting of a metal matrix composite material

In this article, Al7075 matrix with SiC as reinforcement particle was developed and the mechanical properties such as tensile strength, hardness, and impact strength was investigated. Aluminum is preferred as a matrix phase because Al alloys have low density and good ductility. Silicon carbide is chosen as a reinforcement phase due to its brittle and hard properties to enhance the wear properties. Mechanical properties of aluminum metal matrix have been tested at different temperatures and holding time. It shows an ultimate tensile strength of 121 N/mm2 at 800°C processing temperature and 20 mins of holding time. At a processing temperature of 850°C, it shows maximum hardness and impact strength. Among all the fabrication processes, stir casting is chosen because stir casting process is the simplest and cheapest for fabricating metal matrix composites (MMCs). Microelectronic and aerospace packaging industry requires a material with optimum hardness and impact strength to prevent the material from wear and impact during material handling. These MMCs will be a replacement for traditionally used materials such as W-Cu, BeO, and Kovar in packaging application.

Novel low Ag-content Sn–Ag–Cu–Sb–Al solder alloys with enhanced elastic compliance and plastic energy dissipation ability by applying rotating magnetic field

The main scope of this research is to investigate the impact of rotating magnetic field (RMF) on the physical properties of the Sn–0.5Ag–0.5Cu–2.0Sb–0.1Al (SAC0505SbAl) solder alloy. The application of RMF during molten alloy cooling showed prominent effect on the morphology, thermal properties, and the consequent mechanical characteristics. It was shown that the dendritic morphology was changed from columnar dendrites to equiaxed grains with the application of RMF. In addition, RMF processing modified the eutectic phases in the alloy matrix. These notable modifications have resulted in a substantial increase in ductility (~ 30%) with a slight decrease in tensile parameters (UTS and YS). Such effects could enhance elastic compliance and plastic energy dissipation ability of solder alloys, which plays superbly fundamental function in drop-impact reliability. Differential scanning calorimetry (DSC) analysis reveals that significant decrease in pasty range (from 10.8 to 7.5 °C) and undercooling (from 13.1 to 9.0 °C) was attained after applying RMF. The stress exponent (n), the activation energy (Q), and deformation mechanism of the SAC0505SbAl alloy processed with and without RMF were discussed in detail. The obtained results are certainly expected to fill the knowledge gap about the behavior of these recently developed solder alloys under the effect of RMF that are potential alternatives as Pb-free interconnecting material in microelectronic packaging industry.

Failure mechanism analysis and experimental study of lead zirconate titanate actuators in piezoelectric micro jetting valve

Piezoelectric ceramics have recently become one of the most common materials in electromechanical industries, especially in the microelectronic packaging industry. In a jetting dispenser, the presence of imbalanced stress-induced defects in such materials prevents them from providing high stability during the jetting dispensing process. In this work, lead zirconate titanate piezoelectric ceramics (PZT ceramics) were used to actuate the amplifying mechanism of the jetting dispenser. However, during the cyclic dispensing, several ceramics were found to be damaged. In order to figure out what actually caused the damage, analytical and experimental studies were carried out to acquire characteristics of PZT ceramics with damages, and simulation studies of overstress were also performed. Details of the damage characteristics in PZT ceramics are discussed in this paper. This work will improve the stability and life of the piezoelectric dispenser and further promote its wide application in various fields.

Interfacial reactions and mechanical properties of Sn–3.0Ag–0.5Cu solder with pure Pd or Pd(P) layers containing thin-Au/Pd/Ni(P) surface-finished PCBs during aging

The microelectronics packaging industry, although rapidly growing, faces several challenges including 3-D integration, issues with multifunctional capability, and fluctuating input/output (I/O) density, among others. Better-performing microelectronics assemblies for mitigating these challenges require alloys with superior solderability and minimal metallization layer thickness. To this end, in this study, we investigated two kinds of electroless-nickel electroless-palladium immersion gold (ENEPIG) with 0.3 μm Ni, 0.1 μm pure Pd or Pd-phosphorous (Pd(P)), and 0.1 μm Au layers plated on a printed circuit board (PCB) substrate. To analyze the effects of the pure Pd and Pd(P) layers in the thin ENEPIG, we evaluated the interfacial reactions and mechanical properties of the SAC305 solder with a pure Pd or Pd(P) layer in the thin ENEPIG joints after aging at 150 °C. Needle-type and chunky-type (Cu,Ni)6Sn5 IMCs were formed at the interfaces of the pure Pd and Pd(P) joints, respectively. The (Cu,Ni)6Sn5 IMC of the pure Pd joint was thinner than that of the Pd(P) joint after reflowing and aging for 100 h. However, the total IMC of the Pd(P) joint was thinner than that of the pure Pd joint from 250 to 1000 h. In a low-speed shear test, the shear strength of the Pd(P) joint was higher than that of the pure Pd joint for the entire aging time. Most fractures occurred at the Sn-rich surface with a ductile mode, regardless of the different substrates and aging times. After high-speed shear testing, the shear strength of the pure Pd joint was higher than that of the Pd(P) joint until aging for 100 h. After aging for 250 h, the shear strength of the Pd(P) joint was higher than that of the pure Pd joint. The results for brittle fracture rate were similar to those for high-speed shear strength. Hence, Pd(P) joints are expected to demonstrate higher reliabilities than pure Pd joints after long aging treatment.

Development of a Piezo-Driven Liquid Jet Dispenser with Hinge-Lever Amplification Mechanism.

Owing to the quick response, compact structure, high precision, huge blocking force generation, and ease of operation, piezoelectric actuators are urgently being adopted in the field of advanced dispensing for jetting performance improvement and fulfillment of precision requirements in microelectronics packaging, adhesive bonding, and miniaturization industry. This research focuses on the fundamental design and development of a piezo-electrically driven compact fluid dispenser using the principle of a class-one lever for amplification of needle displacement, and enhancement of application areas of the developed jet dispenser. Using fundamental lever principle, geometry-based modelling is carried out to fabricate a working prototype of a normally closed hinge-lever type dispenser. Preliminary experiments are carried out to witness the workability of the fabricated dispenser to deliver 100 dots of working fluid per second that will provide a novel device for dispensing of various fluids, and the proposed amplification mechanism suits various other piezoelectric applications as well.

Advancement solidification microstructure and mechanical properties of Sn–2.0Ag–0.5Cu alloy by applying a rotary magnetic field

In the present study, for the first time, rotating magnetic field (RMF) was used to improve the solidification microstructure and mechanical properties of Sn–2.0Ag–0.5Cu (SAC205) solder alloy. Results revealed that the microstructure and tensile behaviour were improved. As observed, after applying RMF, grain size of β-Sn reduced to be ~ 10 μm which was decreased by ~ 60%. As well, an average size of IMCs formed in SAC205-B alloy were ~ 10–30 μm which were reduced by ~ 40–66%. Therefore, the growth rate of IMCs has been successfully suppressed with RMF. Interestingly, a decrease in grain size and IMCs thickness indicated the beneficial effect of applying RMF on the solder alloy. Consequently, in terms of tensile tests, SAC205 with RMF showed the highest strength over the entire temperatures and strain rates range. Moreover, UTS, YS, YM and El. % at room temperature (25 °C) of SAC205 alloy with RMF were ~ 9.0%, ~ 26.0%, ~ 8.0% and ~ 9.0% greater than that of RMF-free SAC205 alloy. Also, results showed that tensile strength of SAC205 alloy with and without RMF are remarkably sensitive to changes in both temperature and strain rate. Furthermore, the average stress exponent (n) and activation energy (Q) for RMF-free SAC205 and SAC205 with RMF solder alloys have been discussed. The obtained results should prove beneficial in the microelectronic packaging industry.

Effects of Pd Surface Coating on the Strength and Fracture Behavior of Cu Micro Bonding Wires

Because of their high strength and oxidation resistance as well as low cost, Cu micro bonding wires have received increased interests in microelectronic packaging industry. The present work revealed that the coating boundary provided additional strength improvement by blocking dislocation gliding. Furthermore, the greater strain concentration along slip bands introduced larger cracks within the Pd coating layer with coarser grains. The present study provides insights for reliable Pd-coated Cu micro bonding wire processing and bonding applications.

Detection of Micro Solder Balls Using Active Thermography Technology and <italic>K</italic>-Means Algorithm

Solder bump/ball technology has been extensively applied in microelectronic packaging industry. However, the size of solder balls/bumps as well as the pitch are getting smaller and smaller, conventional inspection techniques are insufficient for diagnosis of the defect. It is indispensable to explore new methods for solder joint inspection. In this paper, a nondestructive diagnosis system based on active thermography was proposed. The test vehicles, named as SFA1 and SFA2, were excited by the laser pulse, and the consequent thermal response of the packages was captured by a thermal imager. In order to improve the signal-to-noise ratio, the polynomial fit and differential absolute contrast techniques were utilized to reconstruct the thermal images. Then, the statistical features corresponding to each solder ball were extracted from the reconstructed thermal images, and used for clustering analysis with K -means algorithm. The results show that all the solder balls were recognized accurately, which demonstrates that the intelligent system using active thermography and K -means algorithm is effective for defects inspection in microelectronic packaging industry.

(Invited) Additive Impact on Cu Pillar Electrodeposition Process for Packaging Applications

The micro-electronics packaging industry is seeking new Cu electroplating products to achieve improved I/O density in chips and to enable a variety of packaging architectures. Solder-capped copper pillars are currently used as interconnects in advanced chip packaging. The technology trend is to utilize finer pitch size and smaller Cu pillar interconnects, thereby categorizing electroplated Cu pillars as standard pillars (20-75 µm feature sizes) and µ-pillars (<20 µm feature sizes). A new technology in chip packaging that allows multiple chip stacking to improve I/O density is the Fan-Out Wafer Level Packaging (FO-WLP) technology which also utilizes electroplated Cu pillars with dimensions in the order of 200 x 200 µm, also called mega pillars. Key performance metrics for µ-, standard, and mega Cu pillar products are their ability to plate Cu pillars with exceptional height uniformity along with the ability to maintain a void-free integration with the lead-free solder. For some Cu pillar applications, especially mega pillars, the industry is seeking Cu pillar products that can support very high electrodeposition rates in order to enable high wafer throughput. R&D efforts to develop new products that meet industry requirements in the Cu pillar technology space will be shared with a focus on the design and impact of additives in achieving these design goals.

A review on the mechanisms of ultrasonic wedge-wedge bonding

Although ultrasonic (US) wire bonding has been widely applied in microelectronic packaging industry for decades, the bonding mechanisms are not yet well understood. This article reviews the state-of-the-art understanding of the wedge–wedge bonding mechanisms and its four phases 1) Pre-deformation and activation of US vibration, 2) Friction, 3) US softening and 4) Interdiffusion. The aim of this review is to provide fundamental insight into the bonding process and guidance for future research.By surveying existing work in the field, several gaps in current knowledge of the bonding mechanisms can be identified: 1) the relative motion behaviors between wire, substrate and bonding tool are uncertain; 2) the exact local temperature increments at the wire/substrate interface and the role of these temperature increments are not known; 3) the oxides removal mechanism is not well understood, mainly due to the nano-size of oxides and the very short duration of the self-cleaning process; 4) the understanding of the softening mechanism is incomplete and the influence of the softening on the substrate has not been examined; 5) the microwelds formation and breakage rates have been analyzed, neither experimentally nor numerically. Filling these gaps has the potential to further enhance the US wire bonding process.

A Study on Electrical Reliability Criterion on Through Silicon Via Packaging.

Three-dimensional (3D) structure with through silicon via (TSV) technology is emerging as a key issue in microelectronic packaging industry, and electrical reliability has become one of the main technical subjects for the TSV designs. However, criteria used for TSV reliability tests have not been consistent in the literature, so that the criterion itself becomes a technical argument. To this end, this paper first performed several different reliability tests on the testing packaging with TSV chains, then statistically analyzed the experimental data with different failure criteria on resistance increasing, and finally constructed the Weibull failure curves with parameter extractions. After comparing the results, it is suggested that using different criteria may lead to the same failure mode on Weibull analyses, and 65% of failed devices are recommended as a suitable termination for reliability tests.

Using RBF networks for detection and prediction of flip chip with missing bumps

Flip chip has been extensively used in microelectronic packaging industry. With the trend of solder bumps towards small volume and ultra-fine pitch, defect inspection of flip chips has become more difficult. In this work, we introduce radial basis function networks for detecting and predicting missing solder bumps, a typical defect in flip chips. Eight time and frequency domain features extracted from the flip chip vibration data are inputted to the networks, and flip chips with missing bumps distributed adjacently or randomly are detected and predicted. For the PAC2.1 flip chips with missing bumps distributed adjacently, we distinguish the defective flip chips from the reference one with a 100% accuracy. The flip chips with 1 to 2 missing solder bumps are then trained and detected accurately by the network, and the chip with 3 missing bumps can be predicted exactly as well. After that, we train the network with the data of flip chips with 1 to 4 missing bumps. The network can recognize the number of the missing bumps in these flip chips with a 100% accuracy, and predict 5 missing bumps in the flip chip with the accuracy of 91.7%. For further validation, we use the PB08 flip chips with missing bumps distributed adjacently or randomly for training, testing and prediction, and also obtain high accuracies. These prove the feasibility of using RBF networks for detection and prediction of flip chips with missing bumps in engineering.

Managing Voids in Adhesives for Medical Devices

Void content within the adhesive layer dispensed over the connector module in neuromodulation devices is critical. Voids can affect the thermal transfer, bond strength, and stress decoupling of the adhesive, which can eventually lead to degraded device reliability. The effects of voids are investigated for a silicone based adhesive used in these medical devices. There are many challenges associated with the curing process that this adhesive undergoes which drive the need for unique and specific testing to determine optimal process settings. The parameters evaluated included curing time, temperature, thickness, and relative humidity. Reduction in void content was linked with mechanical properties including hardness and tensile strength. Achievement of void free medical adhesives will encourage today's microelectronic packaging industry to meet the challenges of smaller and denser medical devices.

Solder Joint Reliability Assessment for a High Performance RF Ceramic Package

The prediction of long term solder joint reliability, (SJR), of microelectronic devices and packaging solutions continues to challenge the microelectronic packaging industry, particularly with the introduction of lead-free materials, the push for higher performance (frequency/speed/thermal) and lower unit cost. High performance packages are generally custom designed and therefore have minimal industry data on configuration specific reliability performance. In this application, the package substrate coefficient of thermal expansion, (CTE), was closely matched to the die resulting in a relatively large CTE mismatch between the package and organic PCB. In addition, the package RF and thermal performance requirements required this particular solution to be configured as a “cavity down” perimeter ball array with a large central ground pad to electrically couple the package to the PCB. Given the package's unique design requirements and CTE mismatch, even modest daily temperature swings of 20°C usually found in a controlled or “Central Office” environment could have an adverse impact on the interconnect reliability. This study provides an overview of the solder joint reliability assessment methodologies performed for a custom design lead-free, high performance RF package as part of the requirements to demonstrate compliance to product specifications. SJR life predictions were made for varying package BGA configurations using a multi-tiered approach using constitutive material models, thermo-mechanical finite element simulations, and material specific fatigue models. Empirical accelerated life testing was performed and a life prediction obtained through modeling was validated. Finally, statistical failure distributions were fit to empirical data and discussed in the context of absolute solder life predictions of small fractions unit failures, (100ppm).