Flip chip underfill flow characteristics and prediction

This paper presents recent results on underfill flow characterization. The flow properties of a number of commercial and experimental underfills were recorded and analyzed using quartz test chips with specially designed bump patterns (e.g., peripheral, full array, and mixed designs). Each was bonded onto an organic laminate substrate to form a flip chip package. Underfill was then applied to the packages and flow time, filler settling, and air entrapment were evaluated. Good flow can be described in terms of three measurable parameters, namely, viscosity, contact angle, and more importantly, filler size and distribution. Viscosity and contact angle are commonly used in Hele Shaw and Washburn models. However, these models do not take filler properties into consideration. In general, underfills with particles less than 5 μm exhibited faster and more uniform flow fronts than materials with larger particles. The best flowing materials worked well with standoff heights between 50 and 75 μm, while the poorer flowing materials showed streaking, voiding, and fingering at these heights. At gaps of 25 μm, however, nearly all the materials exhibited pronounced and reproducible streaking     

Double bump flip-chip assembly

Capillary underfill remains the dominate process for underfilling Hip-chip die both in packages and for direct chip attach (DCA) on printed circuit board (PCB) assemblies. Capillary underfill requires a post reflow dispense and cure operation, and the underflow time increases with increasing die area and decreasing die-to-substrate spacing. Fluxing or no-How underfills are dispensed prior to die placement and cure during the solder reflow cycle. Since filler particles in the fluxing underfill can be trapped between the solder ball and the substrate pad during placement, the filler content of fluxing underfills is typically limited to <20% or assembly yield drops dramatically. At 20% filler concentration, the coefficient of thermal expansion (CTE) of the underfill is near that of the bulk resin (50-80 ppm//spl deg/C). In this paper, a double bump Hip-chip process is described. A filled capillary underfill is coated onto a wafer and cured. The wafer is then polished to expose the solder bumps. A second solder bump is formed over the original bump by stencil printing solder paste. After dicing, the die is assembled to the PCB using unfilled fluxing underfill. In the resulting structure, the low CTE underfill is near the low CTE Si die, and the higher CTE underfill is in contact with the PCB. In addition, the standoff height is increased compared to a conventional single bump assembly. In air-to-air thermal shock tests, the double bump assembly was /spl sim/ 1.5 X more reliable than the conventional single bump construction with fluxing underfill. Modeling results are also presented.     

Fill pattern and particle distribution of underfill material

Underfills can dramatically improve flip chip reliability. However, the fillers used in some underfills can be dispersed unevenly, causing less than optimal reliability. In this study, underfill dispensing was conducted using various fill patterns. Experimental results show that particle settling occurs during the curing process, rather than during dispensing, and is affected by the difference between filler and matrix densities and underfill viscosity. Particle migration is a secondary mechanism, which causes uneven filler distribution. A vertically oriented transfer molding machine could help to mitigate settling.     

Underfill of flip chip on organic substrate: viscosity, surface tension, and contact angle

In flip-chip packaging, an underfill is dispensed on one or two adjacent sides of the die. The underfill is driven by a capillary flow to fill the gap between the die and substrate. The application of an underfill reduces the stress to solder bumps and enhances the reliability of the solder joints. Underfill materials consist of epoxy or cyanate ester resins, catalyst, crosslinker, wetting agent, pigment, and fillers. Underfill materials are highly filled with the filler loading ranging from 40% to 70%. In terms of underfill material processing, fast flow and curing are desired for high throughput. The viscosity, surface tension, and contact angle are key material properties affecting the gap filling process. In order to achieve fast filling, it is required that an underfill material has low viscosity and low contact angle at dispensing temperatures. Due to curing of an underfill material at dispensing temperature, the viscosity increases with time, which complicates the underfill flow process. The rheological behavior of several underfill materials was experimentally studied. All the underfill materials showed strong temperature dependence in viscosity before the curing. The time dependent viscosity and curing of underfill materials were examined by a dynamic time sweep test. The effects of viscosity and curing behavior of underfill materials on underfill material processing were investigated. The material with a longer gel time had more stable viscosity at room temperature, and therefore longer pot life. Experimental methods were developed to measure the surface tension and the contact angle of underfills at temperatures over 100 °C. Results showed that the contact angle for underfill on a substrate was time dependent. The interaction between underfill and substrate affects not only gap filling, but also filleting. The effect of surface energies of flip-chip substrates on wetting angles was also studied. Experiment results showed that for the same underfill, the higher the surface energy of substrate, the better the filleting.     

Novel reworkable fluxing underfill for board-level assembly

Underfills are traditionally applied for flip-chip applications. Recently, there has been increasing use of underfill for board-level assembly including ball grid arrays (BGAs) and chip scale packages (CSPs) to enhance reliability in harsh environments and impact resistance to mechanical shocks. The no-flow underfill process eliminates the need for capillary flow and combines fluxing and underfilling into one process step, which simplifies the assembly of underfilled BGAs and CSPs for SMT applications. However, the lack of reworkability decreases the final yield of assembled systems. In this paper, no-flow underfill formulations are developed to provide fluxing capability, reworkability, high impact resistance, and good reliability for the board-level components. The designed underfill materials are characterized with the differential scanning calorimeter (DSC), the thermal mechanical analyzer (TMA), and the dynamic mechanical analyzer (DMA). The potential reworkability of the underfills is evaluated using the die shear test at elevated temperatures. The 3-point bending test and the DMA frequency sweep indicate that the developed materials have high fracture toughness and good damping properties. CSP components are assembled on the board using developed underfill. High interconnect yield is achieved. Reworkability of the underfills is demonstrated. The reliability of the components is evaluated in air-to-air thermal shock (AATS). The developed formulations have potentially high reliability for board-level components.     

Dependence of Flip Chip Solder Reliability on Filler Settling

Thermomechanical reliability of solder joints in flip-chip packages is usually analyzed by assuming a homogeneous underfill ignoring the settling of filler particles. However, filler settling does impact flip chip reliability. This paper reports a numerical study of the influence of filler settling on the fatigue estimation of flip-chip solder joints. In total, nine underfill materials ( 35 vol% silica filler in three epoxies with three filler settling profiles for each epoxy) are individually introduced in a 2-D finite element (FE) model to compare the thermal response of flip chip solder joints that are surrounded by the underfill. The results show that the fatigue indicators for the solder joints (inelastic shear strain increments and inelastic shear strain energy density) corresponding to a gradual, nonuniform filler profile studied in this paper can be smaller than those associated with the uniform filler profile, suggesting that certain gradual filler settling profiles in conjunction with certain resin grades may favor a longer solder fatigue lifetime. The origin of this intriguing observation is in the fact that the solder fatigue indicators are a function of the thermal mismatch among the die, substrate, solder, and underfill materials. The thermal mechanics interplayed among these materials along with a gradual filler profile may allow for minimizing thermal mismatch; and thus lead to lower fatigue indicators.      

Nano-Underfills for High-Reliability Applications in Extreme Environments

Silica particles are used as a filler material in electronic underfills to reduce coefficient of thermal expansion of the underfill-epoxy matrix. In traditional underfills, the size of silica particles is in the micrometer range. Reduction in particle sizes into the nanometer range has the potential of attaining higher volume fraction particle loading in the underfills and greater control over underfill properties for higher reliability applications. Presently, no-flow underfills have very low or no filler content because micron-size filler particles hinder solder joint formation. Nano-silica underfills have the potential of attaining higher filler loading in no-flow underfills without hindering solder interconnect formation. In this paper, property prediction models based on representative volume element (RVE) and modified random spatial adsortion have been developed. The models can be used for development of nano-silica underfills with desirable thermo-mechanical properties. Temperature dependent thermo-mechanical properties of nano-underfills have been evaluated and correlated with models in a temperature range of -175degC to 150degC. Properties investigated include, temperature dependent stress-strain, creep and stress relaxation behavior. Nano-underfills on 63Sn37Pb eutectic and 95.5Sn3.5Ag1.0Cu leadfree flip-chip devices have been subjected to thermal shock tests in the range of -55degC to 125degC and -55degC to 150degC, respectively. The trade-offs between using nano-fillers instead of micron-fillers on thermo-mechanical properties and reliability has been benchmarked.     

Effects of substrate design on underfill voiding using the low cost, high throughput flip chip assembly process and no-flow underfill materials

The formation of underfill voids is an area of concern in the low cost, high throughput, or "no-flow" flip chip assembly process. This assembly process involves placement of a flip chip device directly onto the substrate pad site covered with pre-dispensed no-flow underfill. The forced motion of chip placement causes a convex flow front to pass over pad and solder mask-opening features promoting void capture. This paper determines the effects of substrate design on the phenomena of underfill voiding using the no-flow process. A full-factorial design experiment analyzes several empirically determined factors that can affect void capture in no-flow processing. The substrate design parameters included pad height, solder mask opening height, pad/solder mask opening separation, and pad pitch. The process parameters include chip placement velocity and underfill viscosity. The process robustness is measured in terms of the number of voids created during chip placement, and is further analyzed for the location and any visible modes of void formation. The goal of the work is to determine improved substrate designs to minimize voiding in flip chip processing using no flow underfills.     

Studies of latent catalyst systems for pot-life enhancement of flipchip underfills

Underfill encapsulant is the material used in flip-chip devices that fills the gap between the integrated circuit (IC) chip and the organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of flip-chip devices as compared to nonunderfilled devices. Current underfill encapsulants generally consist of epoxy resin, anhydride hardener, catalyst, silica filler, and other additives to enhance the adhesion, flow, etc. Catalyst determines underfill properties including pot-life, cure speed, and cure temperature. However, long pot-life and fast cure at relatively low temperature (~150°C) are desirable, as such, it requires a room temperature latent catalyst which would be able to catalyze the epoxy curing efficiently at desirable temperature. Currently, the pot-life of commercial underfills at room temperature is normally less than one day. The underfills have to be stored in the freezer at -40°C and in the dry ice for shipping. The objective of this work was to test various catalyst systems that have the potential to enhance the pot-life of the underfill without adversely affecting its curing. The pot-lives of the underfill with various catalysts were obtained from their viscosity versus time relationships, which were established by measuring the viscosities of the underfill with these catalysts periodically using a stress-controlled rheometer. The curing of the underfills was studied using a differential scanning calorimetry (DSC). The pot-life and curing data of the underfill pre-mixed with each of these catalysts are presented in this paper     

Flow time measurements for underfills in flip-chip packaging

Flow time is a key material property for underfill materials in flip-chip applications. In this paper, we will discuss how to use flow time testing for underfill flow evaluation and material screening. The flow time of several underfills was measured at elevated temperatures using test pieces made from glass microscope slides. The material properties impacting underfill flow, such as viscosity, contact angle, and surface tension, were also experimentally measured and used to calculate estimated flow times using the Washburn equation. Empirical and calculated flow times were compared. The effects of channel width and flow distance on flow time were also studied. Additionally, the effect of a tilted stage on flow time, epoxy tongue, and void formation was evaluated.     

Processing and reliability of CSPs with underfill

The use of chip scale packages (CSPs) is rapidly expanding, particularly in portable electronic products. Many CSP designs will meet the thermal cycle or thermal shock requirements for these applications. However, mechanical shock and bending requirements often necessitate the use of underfills to increase the mechanical strength of the CSP-to-board connection. This paper examines the assembly process with capillary and fluxing underfills. Issues of solder paste versus flux only, solder flux residue cleaning and reworkability are investigated with the capillary flow underfills. Fluxing underfills eliminate the issues of flux-underfill compatibility, but require placement into a predispensed underfill. Voiding during placement is discussed. To evaluate the relative performance of the underfills, a drop test was performed and the results are presented. All of the underfills significantly (5-6x) improved the reliability in the drop test compared to nonunderfilled parts. Test vehicles were also subjected to liquid-to-liquid thermal shock testing. The use of underfill improved the thermal shock performance by /spl ges/5x.     

Modeling of a non-Newtonian flow between parallel plates in a flip chip encapsulation

The viscosity of the underfill encapsulant may be different under the conditions of different shear rate, filler content, and temperature. Most of the encapsulant is epoxy containing silica fillers. It exhibits non-Newtonian behavior in the underfill flow. The effect of the viscosity variations on the underfill filling flow was investigated in this study. An analytical model of the filling flow is proposed to accomplish the shear-rate depending viscosity. Due to the addition of fillers in the encapsulant, the viscosity may exhibit both shear thinning and thickening behaviors depending on the temperature and filler content. This study proposes a model of the viscosity considering both effects. In the situations demonstrated in the results, the shear thinning and thickening effects may have major influence on the velocity profile and the filling speed.     

Visco-elastic-plastic properties and constitutive modeling ofunderfills

The thermo-mechanical testing of HYSOL PP4526 underfill is reported, including the details of sample preparation and test procedures. It is found that the Young's modulus of the underfill depends on both temperature and applied strain rate. The constitutive framework proposed for solder alloys has been applied successfully to model the thermo-mechanical properties of the underfill in this paper. Excellent agreement between model predictions and experimental data is achieved, The test data and calibrated constitutive model can be used for the analysis and design of advanced electronic packages with underfills such as flip-chip packages     

Effect of underfill filler settling on thermo-mechanical fatigue analysis of flip-chip eutectic solders

Underfills containing filler particles exhibit filler settling during the (capillary-based) wicking and curing processes, thus causing the reliability estimation to deviate from that of the presumed base of no filler settling. This paper examines the thermo-mechanical responses of the solder joints in flip-chip packaging to various conditions of filler settling. We built five y-dependent profiles for describing the uniform, bilayered, and gradual settling of filler spheres in the through-depth direction of the underfill and used the Mori-Tanaka method to calculate the effective material properties of the filler-resin underfill compound by considering a linearly elastic, temperature-dependent resin with a glass transition temperature range of 70-130 °C. For each settling profile we analyzed the fatigue indicators, referred to as the inelastic shear strain ranges and the inelastic shear strain energy densities of the solder joints, and compared their magnitudes against the extent of filler settling. The results show that the fatigue indicators depend on the extent of filler settling. A greater extent of bilayered filler settling produced larger (in magnitude) fatigue indicators. The fatigue indicators associated with gradual filler settling, however, were almost always smaller, on average, than those associated with no filler settling, indicating that some types of filler settling might favor a longer solder fatigue life. This preliminary but intriguing finding may be partially explained by considering the asymmetric thermal mismatch in the through-depth direction of the underfill; a comparatively good thermal match near the bottom side of the solder joints may compensate for the thermal mismatch at the top side, thus contributing to an overall better thermal match in the solder joint.     

Effects of Underfill Materials and Thermal Cycling on Mechanical Reliability of Chip Scale Package

The thermomechanical reliability of chip-scale packages (CSPs) with various underfills was evaluated by measure the electrical resistance under thermal shock and four-point bending fatigue tests. The underfill containing cycloaliphatic-type epoxy resin had lower resistance than without cycloaliphatic-type epoxy resin under thermomechanical fatigue test because the cycloaliphatic-type epoxy resin was able to mechanically relax more than the other types. The lifetimes of the CSPs under thermomechanical fatigues were strongly dependent on the properties of the underfill.     

Corner bonding of CSPs: processing and reliability

The use of chip-scale packages (CSPs) has expanded rapidly, particularly in portable electronic products. Many CSP designs will meet the thermal cycle or thermal shock requirements for these applications. However, mechanical shock (drop) and bending requirements often necessitate the use of underfills to increase the mechanical strength of the CSP-to-board connection. Capillary flow underfills processed after reflow provide the most common solution to improving mechanical reliability. However, capillary underfill dispense, flow, and cure steps and the associated equipment add cost and complexity to the assembly process. Corner bonding provides an alternate approach. Dots of underfill are dispensed at the four corners of the CSP site after solder paste print but before CSP placement. During reflow, the underfill cures, providing mechanical coupling between the CSP and the board at the corners of the CSP. Since only small areas of underfill are used, board dehydration is not required. This paper examines the manufacturing process for corner bonding including dispense volume, CSP placement, and reflow. Drop test results are then presented. A conventional, capillary process was used for comparison of drop test results. Test results with corner bonding were intermediate between complete capillary underfill and nonunderfilled CSPs. Finite-element modeling results for the drop test are also included.     

Effects of Different Kinds of Underfills and Temperature–Humidity Treatments on Drop Reliability of Board-Level Packages

The hygrothermal and mechanical reliability of board-level packages with various underfills under sequential temperature and humidity (TH) testing and drop testing were investigated. Board-level packages with underfill had greater resistance to drop shock than that without underfill, indicating that underfill protects the package from failure by absorption of the applied drop shock. The underfill, which was composed of polypropylene glycol epoxy resin and silane, exhibited good reliability for drop shock because of the improved adhesion of the underfill compared with that without the polypropylene glycol epoxy resin and silane. In addition, the drop reliability of board-level packages with underfill decreased with increasing TH test duration. Adhesion between the substrate and underfill or between the solder and underfill was decreased by moisture absorption. Components positioned at the board center were more susceptible to failure by drop shock than were corner components.     

Adhesion characterization of no-flow underfills used in flip chipassemblies and correlation with reliability

Adhesion is one of the key properties of underfills used in flip chip assemblies. This paper characterizes the adhesion strengths of no-flow underfill materials to various die passivations using the shear test techniques. A novel shear test vehicle with planner underfill layers between the die and substrate is presented. The adhesion strengths and failure modes of the no-flow underfill materials during shear testing correlate well with their thermal shock reliability test results. Underfill adhesion related failures such as delamination and crack are investigated and correlated between flip chip assemblies and shear test vehicle assemblies without solder joint interconnects     

Adhesion issues in flip-chip on organic modules

Flip chip attach on organic carriers is a novel electronic packaging assembly method which provides advantages of high input/output (I/O) counts, electrical performance and thermal dissipation. In this structure, the flip chip device is attached to organic laminate with predeposited eutectic solder. Mechanical coupling of the chip and the laminate is done via underfill encapsulant materials. As the chip size increases, the thermal mismatch between silicon and its organic carrier becomes greater. Adhesion becomes an important factor since the C4 joints fail quickly if delamination of the underfill from either chip or the solder mask interface occurs. Newly developed underfills have been studied to examine their properties, including interfacial adhesion strength, flow characteristics, void formation and cure kinetics. This paper will describe basic investigations into the properties of these underfills and also how these properties related to the overall development process. In addition, experiments were performed to determine the effects on adhesion degradation of flip chip assembly processes and materials such as IR reflow profile, flux quantity and residues. Surface treatment of both the chip and the laminate prior to encapsulation were studied to enhance underfill adhesion. Accelerated thermal cycling and highly accelerated stress testing (HAST) were conducted to compare various underfill properties and reliability responses     

Thermal fracture toughness measurement for underfill during temperature change

The failure of organic packages during thermal cycling is often associated with failure of the underfill by fracture. The fracture toughness of underfills measured by applying a mechanical stresses to the material at a constant temperature is used as a measure of the propensity of underfill fracture. However, this fracture toughness does not take into account transient temperature effects during thermal cycling. To include temperature effects a fracture toughness induced by applying thermal stresses is defined and a method to measure this thermally induced fracture toughness is described. Results on two commercial underfills are presented. Comparison of the conventional, mechanically induced fracture toughness and the new, thermally induced fracture toughness shows that underfill fracture toughness including thermal effects is significantly smaller than the conventional values. This indicates that the mechanical toughness method overestimates the underfill/package reliability that becomes subject to temperature change. The difference is explained using fracture energy concept.