The effect of reflow process on the contact resistance and reliability of anisotropic conductive film interconnection for flip chip on flex applications

The work presented in this paper focuses on the effect of reflow process on the contact resistance and reliability of anisotropic conductive film (ACF) interconnection. The contact resistance of ACF interconnection increases after reflow process due to the decrease in contact area of the conducting particles between the mating I/O pads. However, the relationship between the contact resistance and bonding parameters of the ACF interconnection with reflow treatment follows the similar trend to that of the as-bonded (i.e. without reflow) ACF interconnection. The contact resistance increases as the peak temperature of reflow profile increases. Nearly 40% of the joints were found to be open after reflow with 260 °C peak temperature. During the reflow process, the entrapped (between the chip and substrate) adhesive matrix tries to expand much more than the tiny conductive particles because of the higher coefficient of thermal expansion, the induced thermal stress will try to lift the bump from the pad and decrease the contact area of the conductive path and eventually, leading to a complete loss of electrical contact. In addition, the environmental effect on contact resistance such as high temperature/humidity aging test was also investigated. Compared with the ACF interconnections with Ni/Au bump, higher thermal stress in the Z-direction is accumulated in the ACF interconnections with Au bump during the reflow process owing to the higher bump height, thus greater loss of contact area between the particles and I/O pads leads to an increase of contact resistance and poorer reliability after reflow.     

Current-carrying capacity of anisotropic-conductive film joints for the flip chip on flex applications

The effect of the substrate-pad physical properties (surface roughness and hardness) on the current-carrying capacity of anisotropic-conductive film (ACF) joints is investigated in this work. Flip chips with Au bumps were bonded to the flexible substrates with Au/Cu and Au/Ni/Cu pads using different bonding pressure. It was found that the current-carrying capacity of ACF joints increased to a maximum value with the rise of the bonding pressure; then, it reduced if the bonding pressure continually increased. The maximum average value per unit area of Au/Ni/Cu pad and Au/Cu pad ACF joints is about 93 μA/μm² and 118 μA/μm², respectively, at 100-MPa bonding pressure. The variation trend of connection resistance is the opposite of current-carrying capacity. The variation of current-carrying capacity (or connection resistance) of Au/Cu pad joints is larger than that of Au/Ni/Cu pad joints. The current-carrying capacity is related to the variation of the resistance of ACF joints. The connection resistance of ACF joints depends primarily on the particle constriction resistance (R_coi), R_coi ∞ 1/a, where “a” is the radius of contact spot. A smaller contact area results in larger joule heat generation per unit volume (Q_g), Q_g ∞ 1/a⁴, which preferentially elevates the temperature of the constriction. The raised temperature increases the resistance because of the temperature-dependent coefficient of the metal resistivity. The theory of tribology is used to explain the difference between Au/Cu pad and Au/Ni/Cu pad ACF joints. For the Au/Cu pad ACF joints, the deformation of the particles’ upper and bottom sides is nearly symmetrical; the contact between conductive particles and pad has the character of “sliding contact,” especially under high pressure. For the Au/Ni/Cu pad ACF joint, the contact between particles and pad determined the conduction characteristics of ACF joints. It has the character of “static contact.” Thus, the current-carrying capacity (or connection resistance) of Au/Cu pad joints is more sensitive to the bonding pressure.     

Contact resistance and adhesion performance of ACF interconnections to aluminum metallization

Flip chip joining technology using anisotropically conductive films (ACFs) has become an attractive technique for electronic packaging. However, several factors have hindered the wide spread use of this technology. Along with the reliability issue, these factors also include the low availability and high cost of the bumped wafers. This paper introduces the feasibilities of using unbumped die with respect to ACF joints for flip-chip-on-flex (FCOF) assemblies. The unbumped dies contain only bare aluminum pads. Untill now the performance of ACF to Al metallization is a controversial issue from the published reports. In this study, two different test vehicles were used to study contact resistance and adhesion performance. Reliability of contact resistance for ACF joints with the unbumped dies was investigated in terms of varying the thickness of the Al pads. Adhesion performance of ACF to the Al metallization was compared with the adhesion performance of ACF to a glass substrate using the same ACF and the same bonding parameters.FCOF assemblies containing dies with thinner aluminum pads showed lower initial contact resistance and a lower rate of increment during accelerated aging tests. Three factors were considered as the potential causes for the above results: (1) lower concentration of aluminum oxide on the thin Al pad, (2) larger contact area per deformed particle with Au/Ni/Cu electrode for the interconnection of thin Al pad and (3) lower concentration of the defects in the thin Al pad. Contact resistance was found to increase during accelerated testing because of aluminum oxide formation on top of the pads.Contrary to the usual expectation, adhesion strength of ACF with the Al metallization was increased during 60 °C/95% RH testing. After 500 h of such moisture-soak testing, the adhesion strength becomes 3 times the initial value. The change in chemical state on the aluminum surface is considered to be responsible for higher adhesion strength. It is proposed that oxidation of Al surface due to diffused moisture and the new chemical bond formation at the adhesives/aluminum interface are the key reasons for good adhesion reliability.     

Reliability of adhesive interconnections for application in display module

The reliability of adhesive interconnections using anisotropic conductive film (ACF) and non-conductive film (NCF) was evaluated by measuring connection resistance during 500 cycles of thermal shock testing. The four-point probe method was used to measure the connection resistance of the adhesive joints constructed with Au bump on Si chip and Cu pad on flexible printed circuit (FPC). The connection resistance of the ACF joints was markedly higher than that of the NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in the ACF joints. The connection resistances of both interconnections decreased with increasing bonding force, and subsequently converged to about 10 and 1 mΩ at a bonding force of 70 and 80 N, for the ACF and NCF joints, respectively. During the thermal shock testing, two different conduction behaviors were observed: increased connection resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.     

Analysis of Failure Mechanism in Anisotropic Conductive and Non-Conductive Film Interconnections

Failure behaviors of anisotropic conductive film (ACF) and non-conductive film (NCF) interconnects were investigated by measuring the connection resistance. The four-point probe method was used to measure the connection resistance of the adhesive joints constructed with Au bump on Si chip and Cu pad on flexible printed circuit. The interconnection reliability was evaluated by multiple reflow process. The connection resistance of the ACF joints was markedly higher than that of NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in ACF joints. The connection resistances of both interconnections decreased with increasing bonding force, and subsequently converged to about 10 and 1 mOmega at a bonding force of 70 and 80 N, for the ACF and NCF joints, respectively. During the reflow process, two different conduction behaviors were observed: increased connection resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.     

Adhesion strength and contact resistance of flip chip on flex packages––effect of curing degree of anisotropic conductive film

The use of anisotropic conductive adhesives film (ACF) as an interconnect materials for flip-chip joining technique is increasingly becoming a vital part of the electronics industry. Therefore, the performances of the ACF joint turn into an important issue and depend mostly on the curing condition of the ACF matrix. ACF is a thermosetting epoxy matrix impregnated with small amount of electrically conductive particles. During component assembly, the epoxy resin is cured to provide mechanical connection and the conducting medium provides electrical connection in the z-direction. Therefore, the cure process is critical to develop the ultimate electrical and mechanical properties of ACF. The purpose of the present work is to investigate optimum curing conditions to achieve the best performance of ACF joints. Differential scanning calorimeter was used to measure the curing degree. Adhesion strength was evaluated by 90° peeling test. The contact resistance has also been studied as a function of bonding temperature and curing degree. Results show a strong dependence of curing condition on the electrical and mechanical performances. Adhesion strength increases exponentially with the curing degree. Whereas the contact resistance decreases with the curing degree and achieve the minimum value at 87% of curing. Co-relation of the curing degree of the ACF was also studied through the detailed investigation of the fracture surfaces under scanning electron microscopy.     

Reliability of Conductive Adhesives as a Pb-free Alternative in Flip-Chip Applications

The temperature-humidity reliability of anisotropic conductive film (ACF) and non-conductive film (NCF) interconnects is investigated by measuring the interconnect resistance during temperature-humidity testing (THT) at 85°C and 85% relative humidity. The four-point probe method was used to measure the interconnect resistance of the adhesive joints constructed with Au bumps on Si chips and Cu pads on flexible printed circuits (FPCs). The interconnect resistance of the ACF joints was markedly higher than that of the NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in the ACF joints. The interconnect resistances of both interconnects decreased with increasing bonding force, and subsequently converged to about 10 mΩ and 1 mΩ at a bonding force of 70 N and 80 N, for the ACF and NCF joints, respectively. During the THT, two different conduction behaviors were observed: increased interconnect resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.     

Study of short-circuiting between adjacent joints under electric field effects in fine pitch anisotropic conductive adhesive interconnects

Anisotropic conductive adhesive film (ACF) has been extensively used in the liquid crystal display (LCD) industry for decades on chip-on-glass (COG) applications. It offers the advantages in terms of fine-pitch capability and more environment compatibility. One of the very important performance requirements of using ACF in fine pitch interconnection is not to create leakage of electric current between adjacent joints as it may lead to abnormal display segments/pixels of the LCD. In this work, the possibility of short-circuiting between adjacent joints in fine pitch ACF interconnections under the effects of electric field was investigated. Insulation resistance measurements of the selected adjacent joints against electric field strength 1 V/μm for a 24 h testing duration were discussed and analyzed. The results showed a strong dependence of curing degree of the ACFs on the chance of short-circuiting between adjacent joints under field effects.     

Study of the self-alignment of no-flow underfill for micro-BGA assembly

As a concept to achieve high throughput low cost flip-chip assembly, a process development activity is underway, implementing next generation flip-chip processing based on large area underfill printing/dispensing, IC placement, and simultaneous solder interconnect reflow and underfill cure. The self-alignment of micro-BGA (ball grid array, BGA) package using flux and two types of no-flow underfill is discussed in this paper. A “rapid ramp” temperature profile is optimized for reflow of micro-BGA using no-flow underfill for self-aligning and soldering. The effect of bonding force on the self-alignment is also described. A SOFTEX real time X-ray inspection system was used to inspect samples to ensure the correct misalignment before reflow, and determine the residual displacement of solder joints after reflow. Cross-sections of the micro-BGA samples are taken using scanning electronic microscope. Our experimental results show that the self-alignment of micro-BGA using flux is very good even though the initial misalignment was greater than 50% from the pad center. When using no-flow underfill, the self-alignment is inferior to that of using flux. However, for a misalignment of no larger than 25% from the pad center, the package is also able to self-align with S1 no-flow underfill. However, when the misalignment is 37.5–50% from the pad center, there are 10–14% residual displacement after reflow. The reason is the underfill resistant force inhibiting the self-alignment of the package due to rapid increment of underfill viscosity during reflow. The self-alignment of micro-BGA package using no-flow underfill allows only <25% misalignment prior to the soldering. During assembling, although the bonding force does not influence on the self-alignment of no-flow underfill, a threshold bonding force is necessary to make all solder balls contact with PCB pads, for good soldering. The no-flow underfill is necessary to modify the fluxing/curing chemistry for overcoming the effect of tin metal salt produced during soldering on underfill curing, and for maintaining the low viscosity during soldering to help self-alignment.     

Reliability analysis of the fine pitch connection using anisotropic conductive film (ACF)

A novel method (the V-shaped curve) is presented to predict the failure probability of anisotropic conductive film (ACF) in IC/substrate assemblies. The Poisson function is used to calculate the probability of opening failure in the vertical gap between the pads, while the box and modified box models are used to estimate the probability of bridging failure between the pads in the pitch direction. The opening and bridging probabilities are combined using probability theory to establish four different failure prediction models. The results reveal that the model combining the Poisson function for fewer than six particles per pad with the modified box model provides the most accurate predictions of the failure probability of ACF in IC/substrate assemblies.     

Failure analysis of pad-height effects in the fine-pitch interconnection of the anisotropic conductive films

This study analyzes the effect of the upper-to-lower pad-height ratio on the global failure probability of IC/substrate assemblies packaged using Anisotropic Conductive Film (ACF). In modeling the failure of the IC/substrate package, the probability of an opening failure in the vertical gap between the pads is calculated using a Poisson function, while the probability of a bridging failure between the pads in the pitch direction is computed using a modified box model. The opening and bridging probabilities are then combined using probability theory to establish an overall failure prediction model for the IC/substrate assembly. The results show that the failure probability increases as the sum of the lower pad height and upper pad height increases, or as the ratio R_h of the upper pad height to the lower pad height increases. Furthermore, for a given gap size between the IC device and the substrate, the minimum failure probability is obtained when the ratio of the upper pad height to the lower pad height has a value of R_h = 1. Overall, the results suggest that the reliability of ACF-packaged IC/substrate assemblies can be improved by reducing the total height of the two pad arrays or by utilizing pad arrays with an equivalent height.     

An investigation into the effects of probing and wire bonding stress on the reliability of BOAC

This work describes two types of low stress bonding over active circuit (BOAC) structures applying a finite element analysis. The advantage of improving the chip area utility of the BOAC design is approximately 150–180 μm for each dimension. A 0.13 μm 2 Mb high-speed SRAM with fluorinated silicate glass (FSG) low-k dielectric was combined with these two BOAC structures as the test vehicles to evaluate the impact of the probing and wire bonding stress on the reliability. Initially, a cantilevered probe card was applied to probe the BOAC pads using the typical and the worse probing conditions. Before and after the circuits probing (CP1 and CP2) the experimental results were compared, including the 2 Mb high-speed SRAM yield and wafer bit map data. The difference between the CP1 and CP2 results were negligible for all probing split cells. Next, the cross-section of the BOAC pad under the probing area was investigated following the worst probing condition. In addition, the BOAC pads evaluate the bondability, including the use of ball shear, wire pull and cratering tests. Moreover, all BOAC packaging samples underwent reliability tests, including HTOL, TCT, TST, and HTST. All the bondability and reliability tests passed the criteria for both proposed BOAC structures. Finally, the immunity level of both proposed BOAC pads, for ESD-HBM (human body mode) and ESD-MM (machine mode), differed slightly from the normal pads. No performance degradation was detected. Accordingly, this work shows that both proposed BOAC structures can be used to improve the active chip area utility or save the chip area.     

软抛光垫条件下非晶态Ge2Sb2Te5的化学机械抛光

Chemical mechanical planarization（CMP） of amorphous Ge₂Sb₂Te₅（a-GST） is investigated using two typical soft pads（politex REG and AT） in acidic slurry.After CMP,it is found that the removal rate（RR） of a-GST increases with an increase of runs number for both pads.However,it achieves the higher RR and better surface quality of a-GST for an AT pad.The in-situ sheet resistance（R_s） measure shows the higher R_s of a-GST polishing can be gained after CMP using both pads and the high R_s is beneficial to lower the reset current for the PCM cells. In order to find the root cause of the different RR of a-GST polishing with different pads,the surface morphology and characteristics of both new and used pads are analyzed,it shows that the AT pad has smaller porosity size and more pore counts than that of the REG pad,and thus the AT pad can transport more fresh slurry to the reaction interface between the pad and a-GST,which results in the high RR of a-GST due to enhanced chemical reaction.     

Influence of pad shape on self-alignment in electronic packaging

Anisotropic self-alignment of the noncircular pads is investigated to reduce the misalignment in electronic packaging, and the effects of the direction and length ratio of the noncircular pads are analyzed. The restoring forces of circular and noncircular pads are calculated numerically using the surface evolver and are compared with the experimental data. The restoring force in the minor-axis direction of the noncircular pad becomes largest followed by the circular pad and the major-axis direction of the noncircular pad. Directionality increases with the length ratio, which implies that more accurate alignment can be achieved in the specific direction.     

Effects of solder reflow on the reliability of flip-chip on flex interconnections using anisotropic conductive adhesives

This work describes the work of an investigation of the effects of solder reflow process on the reliability of anisotropic conductive film (ACF) interconnection for flip-chip on flex (FCOF) applications. Experiments as well as computer modeling methods have been used. The results show that the contact resistance of ACF interconnections increases after the reflow and the magnitude of the increase is strongly correlated to the peak reflow temperature. In fact, nearly 40 percent of the joints are open when the peak reflow temperature is 260/spl deg/C, while there is no opening when the peak temperature is 210/spl deg/C. It is believed that the coefficient of thermal expansion (CTE) mismatch between the polymer particle and the adhesive matrix is the main cause of this contact degradation. To understand this phenomenon better, a three-dimensional (3-D) finite element (FE) model of an ACF joint has been analyzed in order to predict the stress distribution in the conductive particles, adhesive matrix and metal pads during the reflow process. The stress level at the interface between the particle and its surrounding materials is significant and it is the highest at the interface between the particle and the adhesive matrix.     

Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection

The effects of different bonding parameters, such as temperature, pressure, curing time, bonding temperature ramp and post-processing, on the electrical performance and the adhesive strengths of anisotropic conductive film (ACF) interconnection are investigated. The test results show that the contact resistances change slightly, but the adhesive strengths increase with the bonding temperature increased. The curing time has great influence on the adhesive strength of ACF joints. The contact resistance and adhesive strength both are improved with the bonding pressure increased, but the adhesive strengths decrease if the bonding pressure is over 0.25 MPa. The optimum temperature, pressure, and curing time ranges for ACF bonding are concluded to be at 180–200 °C, 0.15–0.2 MPa, and 18–25 s, respectively. The effects of different Teflon thickness and post-processing on the contact resistance and adhesive strength of anisotropic conductive film (ACF) joints are studied. It is shown that the contact resistance and the adhesive strength both become deteriorated with the Teflon thickness increased. The tests of different post-processing conditions show that the specimens kept in 120 °C chamber for 30 min present the best performance of the ACF joints. The thermal cycling (−40 to 125 °C) and the high temperature/humidity (85 °C, 85% RH) aging test are conducted to evaluate the reliability of the specimens with different bonding parameters. It is shown that the high temperature/humidity is the worst condition to the ACF interconnection.     

Failure mechanisms of ACF joints under isothermal ageing

The possible failure mechanisms of anisotropic conductive film (ACF) joints under isothermal ageing conditions have been identified through experiments. It has been found that ACF joints formed at higher bonding temperatures can prevent increases in the contact resistance for any ageing temperature. The higher the ageing temperature the higher the electrical failure rate is. The formation of conduction gaps between the conductive particles and the pads and damages to the metal coatings of the particle have been identified as the reasons behind the electrical failures during ageing. In order to understand the mechanism for the formation of the conduction gap and damages in metal coatings during the isothermal ageing, computer modelling has been carried out and the results are discussed extensively. The computer analysis shows that stresses concentrate at the edges of the particle-pad interface, where the adhesive matrix meets the particle. This could lead to subsequent damages and reductions in the adhesion strength in that region and it is possible for the conductive particle to be detached from the pad and the adhesive matrix. It is believed that because of this a conduction gap appears. Furthermore, under thermal loading the thermal expansion of the adhesive matrix squeezes the conductive particle and damages the metal coatings. Experimental evidences support this computational finding. It is, therefore, postulated that if an ACF-based electronic component operates in a high temperature aging condition, its electrical and mechanical functionalities will be at risk.     

Investigation of defect on copper bond pad surface in copper/low k process integration

In this work, inspection tools and surface analysis instruments were used to inspect and to analyze the defects at copper bond pads fabricated with copper/low k dual damascene deep submicron interconnect process integration. The defects at level are believed to be responsible for metal peeling at the Ta + Al and copper interface observed during chip wire bonding operation. The analysis results of the trace defects’ chemical composition show that the trace defects are the remainder of dielectric materials of passivation layer that is deposited on the top of the chip for protection. Copper oxide is also found to be present at the copper bond pads surface. A clear copper bond pad surface could be obtained using optimized dielectric pad window opening plasma etching conditions with suitable level plasma etching power and some overetch, improved photoresist stripping with oxygen and wet clean recipe with some chemicals. A clear copper bond pad surface will contribute to obtainment higher adhesion and lower contact resistance at Ta + Al and copper pad interface.     

Electroless Ni/Au Bump on a Copper Patterned Wafer for the CMOS Image Sensor Package in Mobile Phones

Wafer bumping technology using an electroless Ni/Au bump on a Cu patterned wafer is studied for the flip chip type CMOS image sensor (CIS) package for the camera module in mobile phones. The effect of different pretreatment steps on surface roughness and etching of Cu pads is investigated to improve the adherence between the Cu pad and the Ni/Au bump. This study measures the shear forces on Ni/Au bumps prepared in different ways, showing that the suitable pretreatment protocol for electroless Ni plating on Cu pads is “acid dip followed by Pd activation” rather than the conventional progression of “acid-dip, microetching, and Pd activation.” The interface between the Cu pad and the Ni/Au bump is studied using various surface analysis methods. The homogeneous distribution of catalytic Pd on the Cu pad is first validated. The flip chip package structure is designed, assembled, and tested for reliability. The successful flip chip bonding in the CIS package is characterized in terms of the cross-sectional structure in which the anisotropic conductive film (ACF) particles are deformed to about 1.5 μm in diameter. The experimental results suggest that electroless Ni/Au can be applied to the flip chip type CIS package using Cu patterned wafers for high mega pixel applications.     

Uniformity of an Electroless Plated Ni on a Pad Connected to Different Size Pads or a Pn Junction for Under Bump Metallurgy in a Flip-Chip Assembly

We investigated electroless Ni uniformity on Al metal pads connected to different size pads or a pn junction for under bump metallurgy in flip-chip assemblies. In an electrically isolated pad, Ni thickness decreased as the pad size decreased. Because of nonlinear diffusion of Pb²⁺ stabilizer in the plating solution, fewer electrons were supplied to the smaller pad than to the larger pad by an anodic oxidation reaction on the pad surface. In pads smaller than 50 mum square, the Ni thickness increased when connected to a 100 mum square pad. This increase might be caused by electrons flowing from the 100 mum square pad to the smaller pad to produce an equipotential for the connected pads. In addition, the Ni thickness was increased by electrical connection to an n-type Si in the presence of fluorescent light illumination for a pn junction area larger than 100 mum². For a pad connected to a p-type Si, however, Ni thickness decreased in comparison to that of an electrically-isolated pad, regardless of the light illumination or pn junction area. The change of Ni height on pads connected to the pn junction is attributable to photoelectrons injected into the n-type Si, or to electron-hole recombination in the p-type Si. These results indicate that the pads should be of the same size within a chip for better Ni uniformity. Moreover, blocking light during Ni electroless plating can eliminate Ni thickness differences due to an n-type Si connection.