Copper Pads Research Articles

Large‐Scale Synthesis of Silver Nanoparticles by Aqueous Reduction for Low‐Temperature Sintering Bonding

Silver nanoparticles with average diameter of 22.4 nm were prepared by aqueous reduction method for low‐temperature sintering bonding application. The reaction temperature and PVP concentration, which are the influential factors of nanoparticle characteristics, were investigated during reduction process. In our research, monodispersity of nanoparticles was remarkably improved while unfavorable agglomeration was avoided with the AgNO3/PVP mass ratio of 1 : 4 at the reaction temperature 30°C. Besides, copper pads were successfully bonded using sintering paste employing fresh silver nanoparticles with diameter of 20~35 nm at 200°C. In addition, after morphology of the bonding joint was analysed by scanning electron microscope (SEM), the porous sintering characteristics were confirmed.

Frequency-Dependent Low Cycle Fatigue of Sn1Ag0.1Cu(In/Ni) Solder Joints Subjected to High-Frequency Loading

The low-cycle-fatigue characteristics of solder joints, formed by reflowing Sn98.8/Ag1.0/Cu0.1/In0.05/Ni0.02 solder over electroless nickel immersion gold-plated copper pads, were investigated by dynamic cyclic bending of printed circuit boards (PCBs). The PCB strain amplitudes were varied from 1.2 × 10−3 to 2.4 × 10−3 and the flexural frequencies ranged from 30 Hz to 150 Hz, to simulate drop impact-induced PCB resonant frequencies. A trend of drastically decreasing fatigue life with cyclic frequency was observed, in contrast with previous reports indicating the reverse; this is attributed to the different failure mechanisms activated. A systematic procedure involving optimization followed by transformation was used to condense the strain–frequency–life data into a master curve expressed in strain–life space.

Impact of Ni concentration on the intermetallic compound formation and brittle fracture strength of Sn–Cu–Ni (SCN) lead-free solder joints

Cu6Sn5 and Cu3Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu6Sn5 and Cu3Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni)6Sn5/Cu6Sn5 and Cu3Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni)6Sn5 and Cu3Sn under thermal aging. The thicknesses of (Cu, Ni)6Sn5/Cu6Sn5 and Cu3Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.

Developments in the electrochemical noise method (ENM) to make it more practical for assessment of anti-corrosive coatings

Interest in the electrochemical noise method (ENM) as a technique for assessment of anti-corrosive properties of organic coating is constantly increasing. This is because it has advantages such as quickness, a non-intrusive nature and ease in gathering and interpretation of data. There are a number of available arrangements of the ENM that can be used to perform noise measurements. Two of them, called successively single substrate (“SS”) and no connection to the substrate (“NOCS”) have been examined and compared with DC resistance method in order to determine whether the ENM is attractive and accurate enough to be accepted as a convenient way to test organic coatings in site situations. Results have shown good correlation between these two ENM arrangements and the DC resistance method. As the most promising arrangement, the NOCS has been considered in more detail. NOCS involves taking measurements from three areas on the coated substrate without the need of electrical connection to the substrate. The aim of this part was to check if the individual values of resistance can be computed by varying the way the cells are connected to the measuring equipment. The results obtained allow the suggestion to be made that the result of a single measurement of NOCS is dominated by the cell which acts in this configuration as a working electrode 1 (WE1). In this work also an attempt has been made to deal with the practical considerations involved in making measurement in the field. Dismountable cells to act as temporary connectors to the substrate, in the form of “copper pads”, have been developed and are discussed. These “copper pads” have indicated their potential usefulness in conducting ENM measurements in site situations. To make ENM usable in the field, good reproducibility of obtained results is required. Therefore, experiments have been conducted to consider this aspect as well. It has been proven that ENM results are reproducible.

Frequency-dependent strain–life characteristics of Sn1.0Ag0.1Cu solder on cupper pad at high cyclic frequency

The low-cycle strain–life characteristic of solder joint formed by reflowing Sn1.0Ag0.1Cu solder onto copper pad has been investigated at the high cyclic frequency regime between 30Hz and 150Hz. Expressing the strain–frequency-life relation of the solder joint as εpcbfq=moNαλ, the frequency exponent q has been evaluated to be 0.34, implying decreasing fatigue life with increasing cyclic frequency, in opposite trend to creep–fatigue. Analysis of the test system has suggested that while the stresses experienced by the solder joints increases only marginally with increasing frequency, the fatigue strength of the solder joint decreases drastically with increasing frequency – due to reduction in the fracture toughness of solder joints with increasing strain rate – giving rise to a net reduction in fatigue life of the solder joint.

Comprehensive Thermo-Mechanical Stress Analyses and Underfill Selection of Large Die Flip Chip Ball Grid Array

The risk of pre-solder crack and bump crack after thermal cycling test is often observed in large die flip chip ball grid array (fcBGA) packages to affect the corresponding reliability. The reason always comes from the singularity of geometry as well as the mismatch of material properties, which cause the critical stress in intermetallic compounds on the copper pad layer and solder bump. Failure phenomena, such as polyimide and extreme low-k (ELK) delamination, solder crack and die crack have often been investigated by using incompatible and unsuitable underfill materials during the packaging process. Choosing the right underfill material for every package is essential in solving these potential failure risks. However, the resulting mechanical behaviors are more complicated to analyze because of the complex properties of under-fill materials. For the purpose of realizing the thermal-mechanical stress distributions and effects of under-fill materials in large die fcBGA, the reliability of the thermal cycling test with several commercial under-fill materials is evaluated. The statistical and simulation result achieved by adopting 3-D finite element analysis (FEA) is presented to validate the reliability result, which are well aligned with each other. To capture the most important mechanical properties in 28-nm ELK large die fcBGA, such as the parameters of packaging geometry, material properties of Young's modulus, coefficient of thermal expansion and glass transition temperature to comprehend the corresponding warpage and stress responses, a comprehensive simulation study is also utilized by FEA. Through these results, not only can the proper parameters to have better stress and warpage reduction be selected, but also the significant factors to provide the best reliability in 28-nm ELK large die fcBGA can be determined. This paper can be a good reference, and can effectively serve as design guidelines in cases where under-fill selection analyses and optimum mechanical solutions for 28-nm ELK large die fcBGA and other critical packages are required. It is helpful to prevent the critical failure issues that are caused by improper geometry and material selection with enabling reliable simulations and analyses.

LORD SolderBrace™ Wafer Applied Coating for Improved Reliability and Throughput in WLCSP

As more manufacturers look to increase the size of Wafer Level Chip Scale Package (WLCSP) dies and also look to decrease the ball pitch, the susceptibility of the die to fail during thermal cycling and drop shock testing increases. The stress conditions introduced during thermal cycling from the mismatches in Coefficients of Thermal Expansion (CTE) lead to solder fatigue. The failure of WLCSPs during drop shock is found at the solder/pad interface. The general solution to address solder fatigue during thermal cycling and solder joint stress at the copper pad interface has been capillary underfilling the chips after chip attachment. To address these issues, a new material from LORD Corporation – SolderBrace™ wafer applied coating, can be used to partially underfill the WLCSP die at the wafer level. This type of technology can be applied using existing equipment and processing techniques making these materials a more cost effective solution. This new material technology has enabled thermal cycling reliability improvements by replacing the final passivation layer with a new low CTE material as the partial underfill. This wafer applied partial underfill material technology has been successfully used to provide increased thermal cycling and drop shock reliability in WLCSPs using a number of different methods that have been previously described. The method to be discussed in this paper is a production process using a screen printed, photo defined polymer system that does not require any in-process post cure.

HTS and PCT Reliability of Chips and Flex Substrates Assembled Using a Thermosonic Flip-Chip Bonding Process

This study assesses the high-temperature storage (HTS) test and the pressure-cooker test (PCT) reliability of an assembly of chips and flexible substrates. After the chips were bonded onto the flexible substrates, specimens were utilized to assess the HTS test and PCT reliability. After the PCT and HTS tests, the die-shear test was applied to examine changes in die-shear forces. The microstructure of the test specimens was analyzed to evaluate reliability and to identify possible failure mechanisms. When the duration of the HTS test was increased, the percentage of gold bumps that peeled off from the surface of the copper pads on the chip side increased, and a crack was present at the bonding interface between the gold bumps and chip bond pads. This crack was due to thermal stress generated during the HTS test, and degraded the die-shear force of the assembly of chips and flexible substrates. After the PCT, the crack was present at the interface between deposited layers of copper electrodes after the specimens were subjected to the PCT for various durations. Moisture penetrated into the deposited layers of the copper electrodes, deposited layers lost their adhesion, and the crack progressed from the corner into the central bond area as the test duration increased. To improve the PCT reliability of assemblies of chips and flexible substrates using the thermosonic flip-chip bonding process, one must prevent moisture from penetrating into deposited layers of copper electrodes and prevent crack formation at the interface between nickel and copper layers. Underfill would be an effective approach to prevent moisture from penetrating into deposited layers during the PCT, thereby improving the reliability of the samples during the PCT.

Prediction of pad cratering fracture at the copper pad – Printed circuit board interface

Pad-crater fracture was characterized in terms of the critical strain energy release rate, Jci, measured at various mode ratios, ψ. Specimens were prepared from lead-free chip scale package-PCB assemblies and fractured at low and high loading rates in various bending configurations to generate a range of mode ratios. The specimens tested at low loading rates all failed by pad cratering, while the ones tested at higher loading rates fractured in the brittle intermetallic layer of the solder. The Jci of pad cratering increased with the phase angle, ψ, but was independent of surface finish (OSP and ENIG) and reflow profile (time above liquidius 60s and 120s). The generality of the J=Jci(ψ) failure criterion to predict pad-cratering fracture was then demonstrated by predicting the fracture loads of single lap-shear specimens made from the same lead-free assemblies.

Wetting of Cu and Al by Sn-Zn and Zn-Al Eutectic Alloys

Wetting properties of Sn-Zn and Zn-Al alloys on Cu and Al substrates were studied. Spreading tests were carried out for 3 min, in air and under protective atmosphere of nitrogen, with the use of fluxes. In the case of Zn-Al eutectic, spreading tests were carried out at 460, 480, 500, and 520 °C, and in the case of Sn-Zn eutectic at 250, 300, 350, 400, 450, and 500 °C, respectively. Solidified solder/substrate couples were cross-sectioned and subjected to microstructure examination. The spreading tests indicated that the wetting properties of eutectic Sn-Zn alloys, on copper pads do not depend on temperature (up to 400 °C), but in the lack of protective atmosphere, the solder does not wet the pads. Wettability studies of Zn-Al eutectic on aluminum and copper substrates have shown a negative effect of the protective nitrogen atmosphere on the wetting properties, especially for the copper pads. Furthermore, it was noted that with increasing temperature the solder wettability is improved. In addition, densities of liquid solders were studied by means of dilatometric technique.

Drop-Shock Failure Prediction in Electronic Packages by Using Peridynamic Theory

Peridynamic (PD) theory is used to investigate the dynamic responses of electronic packages subjected to impact loading arising from drop-shock. The capability of the PD theory to predict failure is demonstrated by simulating a drop test experiment of a laboratory-type package. The failure predictions and observations are exceptionally similar. For the drop test simulation of a production-type package, the finite element method (FEM) and PD theory are coupled via a submodeling approach. The global modeling is performed using the FEM while the PD theory is employed for the submodeling and failure prediction. The analysis yielded the outermost solder joint as the critical joint, with failure at the interface between the solder and copper pad on the printed circuit board side.

Numerical Investigation of Thermo-Mechanical Behaviour of Ball Grid Array Solder Joint at High Temperature Excursion

The solder joints of surface mount components (SMCs) experience thermal degradation culminating in creep and plastic shear strain deformation when subjected to cyclic temperature load over time. Degradation at the joints is due to thermal stress induced by the incompatible, differential and nonlinear expansion mismatch of the different bonded materials in the assembly. The stress magnitude influences the strain behaviour. Plastic strain response of solder joint is critical at the materials interface at the lower part of the joint due to the occurrence of wider variation in the coefficient of thermal expansion of the bonded materials and this may lead to static structural failure. The life expectancy of electronic components reduces exponentially as the operating temperature increases thus making reliability a key concern for electronic systems operating at high temperatures and in harsh environments. This paper reports on the numerical investigation of thermo-mechanical response of a critical BGA joint especially the character of plastic deformation of SnPb solder used in forming the joint as well as the joint’s high temperature reliability. The analysis uses a 3-D models to predict the effect of the transient thermal load on the static structural integrity of a single BGA joint. In this study, the base diameter of solder ball (interface between the PCB, copper pad and the solder) experienced higher damage than the top diameter interconnects. The paper provides a simplified methodology to study the reliability of BGA solder joint at high temperatures excursion.

Thermal Study on High-Power White LED down Light

An actual packaging structural of high power three-chip white LED lamp is designed. The highest temperature of LED chip is 111.2°C, and the temperature range of heat sink is 72.6°C～73.8°C by finite element method (FEM). The respective temperature of lead frame and heat sink are 76.0°C and 71.0°C, which are measured by the thermocouples experiment. Based on the consistency between simulation results and experiment data, considering the steady temperature distribution of the lamp and heat transfer theory, several optimization methods are proposed. The effects of copper pad dimension, solder paste area and natural convection coefficient on the temperature field distribution are calculated.

Optimization for simulation of WL-CSP subjected to drop-test with plasticity behavior

A simulation of the board-level drop-test is performed to evaluate some WL-CSP (wafer-level chip-scale packages) performances. An elasto-plastic model is applied on both solder bump and copper pad materials. It intends to demonstrate that copper plasticity is mandatory due to the large plastic strain occuring in these materials. A statistical analysis discusses the required accuracy for the modeling analysis. This analysis is combined with a components’ lifetime prediction based on a representative plastic strain. Finally, it is possible to do a relatively fast calculation while meeting the determined accuracy.

Fracture mechanics study on the intermetallic compound cracks for the solder joints of electronic packages

The crack behavior of the intermetallic compounds (IMCs) for the solder joints of electronic packages under the thermal cycling test conditions was analyzed using the finite element method in this research. A flip-chip plastic ball grid array package was selected as the analysis target. The two-dimensional in-elastic finite element method was employed to calculate the fracture mechanics parameters at the crack tip; such as the modes I and II stress intensity factors. Two locations were considered herein to initiate the crack: the first one was on the interface between the IMC and the copper pad (interfacial cracks); and the other one was at the inner part of IMC (IMC cracks). Furthermore, the effects of crack length and thickness of IMC on the crack tip parameters were also studied in this investigation. In the numerical simulation, the properties of the solder joint and the underfill were assumed to be elastic–plastic–creep and viscoelastic, respectively. The sub-modeling technique was used in the finite element analysis to reduce the computational time and the crack tip parameters were obtained using the simulated results of the displacement field near the crack tips. The results show that for both interfacial cracks and the IMC cracks, the modes I and II SIFs decrease as the length of crack increases, and the decreasing trend becomes more stable when the cracks propagate longer except for the variation of the mode II SIF for IMC cracks. Furthermore, the thickness of the IMC has slight effect on the magnitudes of SIFs under the conditions of constant crack lengths.

Damage Mechanics Model for Solder/Intermetallics Interface Fracture Process in Solder Joints

The relatively brittle solder/IMC interface fracture process in reflowed solder joints is examined using finite element (FE) method. The interface decohesion is described using a traction-separation quadratic failure criterion along with a mixed-mode displacement formulation for the interface fracture event. Reflowed Sn-4Ag-0.5Cu (SAC405) solder ball on OSP copper pad and orthotropic FR4 substrate under ball shear push test condition at 3000 mm/sec is simulated. Unified inelastic strain constitutive model describes the strain rate-response of the SAC405 solder. Comparable simulated and measured load-displacement values during solder ball shear push test serve as validation of the damage-based FE model. Results indicate a nonlinear damage evolution at each material point of the solder/IMC interface during the ball shear push test. The normal-to-shear traction ratio at the onset of the interface fracture is 1.59 indicating significant induced bending effect due to shear tool clearance. Rapid interface crack propagation is predicted following crack initiation event with the average crack speed up to 24.6 times the applied shear tool speed. The high stress concentration along the edge of the solder/IMC interface facilitates local crack initiation and dictates the shape of the predicted dynamic crack front.

Thermal Resistance and Reliability of High-Power LED Packages Under WHTOL and Thermal Shock Tests

The light emitting diode (LED) packaging problems associated with high cost, high junction temperature, low luminous efficiency, and low reliability have to be resolved before the LED gaining more market acceptance. In this paper, chip-on-plate (CoP) LED packages with and without phosphors are evaluated in terms of thermal resistance and reliability under wet and high-temperature operation life (WHTOL) and thermal shock tests. The WHTOL test is with the condition of 85°C/85%RH and 350 mA of forward current for 1008 h, while thermal shock test is with 200 cycles at temperature ranging from -40°C to 125°C. The thermal behavior of the CoP packages was analyzed by 1-D thermal resistance circuit (1-D TRC) with and without spreading angle, 3-D TRC method, and 2-D axisymmetric finite element method. The feasibility of these analyses was evaluated and discussed in detail by comparing those results with experimental measurements. The reliability results indicated that all CoP packages with phosphors in the silicone encapsulant failed after 309 h in the WHTOL test, but all those without phosphors still survived after 1008 h. The failure modes were found to be the debonding of the aluminum wire from the chip or copper pad of the substrate. However, after the aluminum wire was replaced by gold wire, all the packages with and without phosphors passed after 1008 h. For these survival packages in the WHTOL test, their thermal resistances of junction-to-air and junction-to-aluminum substrate increased by about 12 and 9°C/W, respectively. Moreover, it was also found that there is a difference of 38°C/W in the junction-to-air thermal resistances for the packages between under natural and forced convections in the chamber during the WHTOL test. This might yield the different reliability data, unless the flow conditions in the test chamber are specified in this standard test. Furthermore, all the packages with and without phosphors could pass 200 cycles in thermal shock test, with minor changes in the thermal resistances. However, the degradation of luminous flux in the packages with phosphors was found to be greater than those without phosphors by 14% vs. 9%.

Wafer Non-Uniformity Analysis during Chemical Mechanical Polishing Based on Finite Element Method

A contact-mechanics-based finite model for Cu/ low-k chemical mechanical polishing is presented. 2D axisymmetric quasi-static model for chemical mechanical polishing which includes four-layer structure: Si, low-k, Copper and polishing pad is established. The mechanical response at the interface between the silicon, low-k, copper, and pad is simulated under the loading of the chemical mechanical polishing. The effect of slurry is simplified as the friction force reacted onto both the copper and the polishing pad in the finite element model. Down pressure, status of slurry and the elastic modulus of polishing pad are treated as the parameter in the simulation. Using the model, the effects of applied down pressure, pad properties, status of slurry on the non-uniformity of the wafer surface can be readily evaluated. Simulation results show that the distribution of the Von Mises stresses across a wafer’s surface correlates with experimental removal rate profiles.

Effects of multiple BGA rework on strength of solder joints

PurposeThe purpose of this paper is to investigate effects of the multiple rework of ball grid array (BGA) components on mechanical strength of BGA balls, as well as any possible intermetallic (IMC) embrittlement, and obtain data correlated with possible estimation on the maximum permitted limits of BGA rework.Design/methodology/approachIn this paper, mechanical strength of BGA components assemblies with multiple numbers of rework operations was evaluated. Mechanical evaluation was conducted using BGA ball shear tests and four‐point bending tests of BGA assemblies. Test samples were prepared under the following conditions: virgin, one, two, three and five BGA reworks. Failure mechanism was evaluated using cross‐section and SEM analysis.FindingsThe results show that both ball shearing tests and four‐point bending tests indicates that strength of BGA solder ball itself was not reduced significantly after repair/rework operation from one to five cycles. The IMC structure layer after rework is a mixture of IMC, Sn‐rich and Pb‐rich phases. This mixture layers with thickness even more than 10 μm in thickness does not show reduction of strength of BGA solder balls and do not cause premature embrittlement. However, the bonding strength of the copper pads to the laminates is reduced with rework/repair operation, with the great reduction coming from the first and second rework operation.Practical implicationsIn general, the industry recommends two rework cycles for BGA components on the same spot. This study indicates that further rework (up to five) causes little degradation, therefore there is room to increase the total rework cycle limit beyond recommended two for plastic BGA components.Originality/valuePresented test results shows that in most cases industry overestimates risks associated with increased embritlement of the BGA solder joints due to the intermetallics growth after multiple BGA rework operations. Strength reduction of BGA assemblies is mostly associated with reduction of bonding strength of the copper pads to the laminates is reduced with rework/repair operation and number of reworks could be increased in most cases.

Hybrid Experimental-Computational Approach for Solder/IMC Interface Shear Strength Determination in Solder Joints

Damage-based models for solder/intermetallics (IMC) interface often require the interface properties such as tensile and shear strengths. The minute size of the solder joint renders direct experimental determination of these properties impractical. This paper presents a hybrid experimental-computational approach to determine the shear strength of solder/IMC interface. Displacement-controlled ball shear tests are performed on as-reflowed and thermally-aged solder specimens. The observed sudden load drop in the load-displacement curve corresponds to the crack initiation event and the load is indicative of the shear strength of the solder/IMC interface. Finite element simulation of the ball shear test is then employed to establish the complex stress states at the interface corresponding to the onset of fracture. The finite element model consists of Sn40Pb solder, Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> intermetallic and Ni layers, copper pad and a rigid shear tool. Unified inelastic strain theory describes the strain rate-dependent response of the solder while other materials are assumed to behave elastically. Quasi-static ball shear test is simulated at 30°C with a prescribed displacement rate of 0.5mm/min. Results show that steep stress gradients develop in the shear tool-solder contact and solder/IMC interface regions indicating effective load transfer to the interface. The bending (normal) stress is found to be of the same order of magnitude as the maximum shear stress. Higher stress values are predicted near the leading edge of the solder/IMC interface. The equivalent shear stress condition to the triaxial stress state at the interface, represented by the absolute maximum shear stress, τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max,abs</sub> should have reached the shear strength of the interface at fracture. The resulting shear strength of Sn40Pb/Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> interface is determined to be 87.5 MPa.