PBGA封装的可靠性研究综述

通过传统BT类型的PWB材料与独特的PWB材料来PBGA封装的可靠性。相关的研究结果表明,后者同样具有相同的热循环稳定性和回流焊期间的疲劳强度,并具有较低的封装翘曲特点;模塑料的低吸湿性及粘片材料的高粘附强度和高断裂强度特性,有利于提高回流焊期间的疲劳强度和防止剥离现象的扩散。     

Reliability of plastic ball grid array package

The reliability of plastic ball grid array (PBGA) package is studied for different materials. The reliability of the PBGA packages using conventional Bismaleimide-Triazine type PWB and our original product PWB that is made of high Tg epoxy resin is evaluated. The PBGA package using our original PWB has a feature of lower warpage for the package, and has similar performance regarding the thermal cycling stability and the endurance during reflow soldering as compared with the PBGA using the conventional PWB. The endurance during reflow soldering for each PBGA is JEDEC STD Method A112 level 3. In order to improve the endurance during reflow soldering, not only PWB materials but also other factors are investigated. As a result, the molding compound with the property of low moisture absorption and the die attach material with the properties of high adhesion strength and fracture strength are effective to improve the endurance property during reflow soldering. The package crack mechanism during reflow soldering is briefly described as follows     

Thermal design of a broadband communication system with detailed modeling of TBGA packages

This paper presents a thermal modeling of a broadband network communication box partitioned into two stacked modules. A printed circuit board (PCB) is inside each module where an array of 16 tape ball grid array (TBGA) packages is surface mounted to the PCB. The TBGA package dissipates 6 W power each. In addition, 12 W of power is dissipated from four plastic ball grid array (PBGA) packages on the PCB. Pin-fin heat sinks are attached to the TBGA packages using silica-filled epoxy to enhance heat dissipation. Pin-fin heat sinks are also attached to the PBGA packages. Two exhaust fans are mounted at the flow exit to draw ambient air into the system at approximately 200 linear feet per minute (LFM) of velocity. The full Navier–Stokes equations for airflow are solved to simulate the forced convection cooling in the electronic module. Buoyancy effect was considered in the numerical model by incorporating Boussinesq-approximation. The TBGA packages are modeled in detail in order to obtain the package junction temperatures for system reliability evaluation and thermal design optimization. Detailed models of the attached pin-fin heat sinks and the epoxy interfaces are also utilized in this study. Compact heat sink model composed of a base plate and a resistance fluid volume is applied to model heat dissipation from the heat sinks attached to the four PBGA packages. System fan curve is used to simulate the fan operating conditions. The effect of changing system thermal design on the TBGA package junction temperatures as well as the hydraulic operating conditions of the system fans are examined and reported herein. The effect of radiation heat transfer is also examined. The importance of detailed modeling of the high power TBGA packages is demonstrated in this study. Simulation results were compared with JEDEC thermal test data under similar conditions of airflow.     

The Influence of Substrate Enhancement on Moisture Sensitivity Level (MSL) Performance for Green PBGA Packages

As the electronics industry migrates to the lead- and halogen-free (green) packages, many of the materials used in plastic ball-grid array (PBGA) substrates, in particular the molding compounds and die attaches, will have to be improved. The moisture sensitivity level (MSL) performance of the large nongreen PBGA packages are typically reduced by at least one JEDEC/IPC level at the lead-free reflow temperature of 260$^circ$C. Common failure mechanisms of traditional large size PBGA packages include popcorning, as well as delamination and cracks between the solder mask/copper interface in the multiple layer substrates. In this paper, the interfacial adhesion of traditional and advanced substrate materials and processing technologies are presented based on reliability tests of various PBGA packages subject to moisture soaking followed by reflow soldering at 260$^circ$C. It was found that substrate failures with delamination at the solder mask/copper interface were dramatically improved by introducing advanced materials and processes for multiple-layer substrates. However, the partial or full delamination at the mold compound/solder mask interface could still be observed after lead-free reflow soldering. There is an urgent need to improve the adhesion between mold compound/solder mask in order to achieve high MSL performance of large size and green PBGA packages.     

The impact of moisture in mold compound preforms on the warpage of PBGA packages

The moisture in the mold compound preforms influenced the resultant mechanical properties as well as the warpage of IC packages after the molding and post mold curing (PMC) process. Moisture will diffuse into the mold compound preforms (compound pallet) after thawing from cold room and exposing to clean room condition before molding process. The moisture will cause the package to swell, and combine with thermal stress, and finally result in the warpage of molded package after molding process. The main objective of this paper is to address the impact of moisture in the compound preforms on the warpage of the PBGA packages and explain the change in mechanical properties under different moisture conditions, e.g., the variations of the flexural modulus, T_g, and CTE with respect to moisture level. The compound preforms exposure to a clean room condition were simulated by a series of experiments. The warpage of PBGA packages were measured by the moiré test. The moisture control during the IC manufacturing process was highlighted in terms of the mechanical properties variation and warpage measurement due to the moisture effects on the mold compound preforms.     

Low-Cost Vacuum Molding Process for BGA Using a Large Area Substrate

We have developed a new ball grid array vacuum molding process using a large area substrate, called vacuum dip compression molding (VDCM), with the aim of reducing the substrate molding cost. The VDCM method imposes less of a burden on the environment because it enables efficient use of the substrate area for packaging, increasing the proportion of the effective area on the substrate. In contrast to the transfer molding method, VDCM does not generate cull or runner waste. VDCM also has the advantage of reducing the thickness of the mold resin. It can mold packages with various thicknesses using a single mold. Using VDCM equipment, we conducted a basic evaluation of molding performance. It was confirmed that no significant wire sweep occurred even when the wire length exceeded 3 mm, indicating that VDCM is less prone to wire sweep compared to transfer molding. The prototyped fine pitch ball grid arrays had a satisfactory level of moisture sensitivity.     

Microstructure and Orientation Evolution of the Sn Phase as a Function of Position in Ball Grid Arrays in Sn-Ag-Cu Solder Joints

The microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling is studied, with a focus on the Sn grain orientation in plastic ball grid array (PBGA) packages. Thermally cycled PBGA packages with a full array of 196 solder joints were examined after being subjected to various pre-conditions. Each PBGA package was polished to obtain plan-view cross- sections of each solder joint. Solder joints were characterized using both polarized optical microscopy and orientation imaging microscopy (OIM). The observations reveal that the distribution of single and multigrain Sn microstructure as a function of position in the package is dependent on the sample’s preconditions and thermal cycle history. Based on distribution maps from polarized optical microscopy observation, thermal aging has a relatively small impact on the overall fraction of single-grained solder joints. Thermal cycling, however, can cause many single-grained joints to transform into multigrained solder joints. The dependence of the grain structure distribution on different preconditions and evolution of the grain structures during thermal cycling are discussed.     

Integrated transient thermal and mechanical analysis of molded PBGApackages during thermal shock

During thermal shock, large thermal gradients exist within a molded plastic ball grid array (PBGA) package. The conventional assumption of uniform temperature distribution becomes invalid. In this paper, an integrated thermal-mechanical analysis was performed to evaluate the transient effect of thermal shock. For comparison, an isothermal analysis was also conducted. The computational fluid dynamics (CFD) method was used to obtain the thermal boundary conditions surrounding the package. The heat transfer coefficient obtained through CFD was compared to two analytical solutions. It was found that the analytical values were not acceptable in the time period of interest. Therefore, to obtain the actual maximum die stress, CFD solution has to be used instead of analytical solutions to derive the thermal boundary condition. This boundary condition was then applied to the package and a sequentially coupled heat transfer and thermal stress analysis was performed. The transient analysis has shown that high stresses occur in the die due to thermal shock, which can not be seen under the traditional isothermal assumption. The impact of plastic ball grid array (PBGA) package parameters on transient die stress was also studied, including mold thickness and substrate thickness. The results in this paper could be applied to either wire bond or flip-chip PBGA packages     

Thermomechanical Analysis of Plastic Ball Grid Arrays With Vapor Pressure Effects

This study adopts a mechanism-based computational approach to gain insights into the delamination and cracking of plastic ball grid array (PBGA) packages under moisture sensitivity test (MST) conditions. The possible crack paths in the molding compound are first examined by modeling the fully porous overmold with void-containing cell elements. These computational cells are governed by a Gurson constitutive relation, extended to account for vapor pressure effects. We show that the corner of the die/die-attach interface presents a likely site for crack initiation under MST conditions. Failure along this interface of interest is then examined by deploying a single row of computational cells along the die/die-attach interface. Under combined thermal and vapor pressure loading, delamination concurrently occurs at both the die corner and the die center; these competing damage sites lead to the rapid and complete delamination of the die/die-attach interfaces.      

热循环条件下空洞对PBGA焊点热疲劳寿命的影响

球栅阵列(ball grid array, BGA)封装器件的广泛应用使空洞对焊点可靠性的影响成为业界关注的焦点之一.采用非线性有限元分析方法和统一型粘塑性本构方程,以PBGA组装焊点为对象,建立了互连焊点热应变损伤的三维有限元模型,并基于修正的Coffin-Manson方程,分析了在热循环加裁条件下不同位置和大小的空洞对焊点疲劳寿命的影响.研究结果显示,位于原应力集中区的空洞将降低焊点疲劳寿命,基于应变失效机理,焊点裂纹易在该类空洞周围萌生和扩展;位于焊球中心和远离原应力集中区的空洞,在一定程度上可提高焊点的疲劳寿命.     

Impact of flip-chip packaging on copper/low-k structures

Copper/low-k structures are the desired choice for advanced integrated circuits (ICs). Nevertheless, the reliability might become a concern due to the considerably lower strength and greater coefficient of thermal expansion (CTE) of the low-k materials. To ensure successful integration of the new chips within advanced packaging products, it is essential to understand the impact of packaging on chips with copper/low k structures. In this study, flip-chip die attach process has been studied. Multilevel, multiscale modeling technique was used to bridge the large gap between the maximum and minimum dimensions. Interface fracture mechanics-based approach has been used to predict interface delamination. Both plastic ball grid array (PBGA) and ceramic ball grid array (CBGA) packages were evaluated. Critical failure locations and interfaces were identified for both packages. The impact of thin film residual stresses has been studied at both wafer level and package level. Both PBGA and CBGA packaging die-attach processes induce significantly higher crack driving force on the low-k interfaces than the wafer process. CBGA die-attach might be more critical than PBGA die-attach due to the higher temperature. During CBGA die-attach process, the crack driving force at the low-k/passivation interface may exceed the measured interfacial strength. Two solutions have been suggested to prevent catastrophic delamination in copper/low-k flip-chip packages, improving adhesion strength of low-k/barrier interface or adding tiles and slots in low-k structures to reduce possible area for crack growth.     

Sn-Ag-Cu Solder Joint Microstructure and Orientation Evolution as a Function of Position and Thermal Cycles in Ball Grid Arrays Using Orientation Imaging Microscopy

Thermally cycled plastic ball grid array (PBGA) packages with full arrays of 196 solder joints after various preconditions were examined to observe the microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling, focusing on Sn grain orientation. Each PBGA package was polished to obtain a plan-view cross-section of every solder joint, and characterized using both polarized optical microscopy and orientation imaging microscopy (OIM). By OIM observations, distribution maps were obtained based on Sn crystal c-axis orientations. Each precondition showed a characteristic distribution related to the combined thermal aging and thermal cycling history. This study on Sn grain orientation using OIM provides further understanding about deformation and microstructure evolution processes that occur during thermal cycling, and the impact of isothermal aging as a precondition.     

Plastic Ball Grid Array Solder Joint Reliability for Avionics Applications

The solder-joint reliability of plastic ball grid array (PBGA) packages for an avionics application was assessed. The fatigue life under combined temperature cycling and random vibration conditions was initially simulated using analytical models. Further simulation was used to plan an accelerated test and to estimate an acceleration factor between field conditions and test conditions, and experimentally verified. The combination of simulation and experimental results was then used to reassess the solder-joint reliability under field conditions. Based on the simulation and test results, it was estimated that PBGA packages without underfill would not satisfy the long-term life expectation for the avionics application under study. In addition, the experimental results also show that the simple application of linear damage superposition (Miner's rule), which neglects the interactions between loads, will underestimate the damage and overpredict the product life. Therefore, calculating the acceleration factor is necessary and cost-efficient for providing the basis of product field reliability assessment     

Ribbed package geometry for reducing thermal warpage and wire sweepduring PBGA encapsulation

The effects of changing package shape to a ribbed geometry on thermal warpage and wire sweep of a plastic BGA (PBGA) are investigated in this paper. Three rib geometries (border, diagonal, and cross) with a variation of rib widths and thicknesses are compared with the original plane geometry. Finite element analyses of thermal warpage during the reflow process of PBGA molding with and without ribbed geometry are carried out. Numerical modeling shows that the border rib has the least thermal warpage at the reflow condition. Flow visualization was performed to study the effect of rib geometry on wire sweep, and demonstrates that the wire sweep in ribbed packages is significantly less than that in the original nonribbed package     

Plasma cleaning of plastic ball grid array package

An investigation of O₂, Ar and Ar/H₂ plasma cleaning was carried out on plastic ball grid array (PBGA) substrates to study its effects on surface cleanliness, wire bondability and molding compound/solder mask adhesion. Optimization of the plasma cleaning process parameters was achieved using the contact angle method and verified by auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and wedge pull tests. It was found that both the wedge bond quality and moisture sensitivity of a 225 I/O PBGA package were improved after plasma cleaning. Furthermore, atomic force microscopy (AFM) characterization and XPS analysis revealed that the solder mask has undergone plasma-induced surface modification. Cross-contamination of Au and F traces on the solder mask that has occurred during plasma cleaning was identified by XPS. This study has demonstrated the benefits and consequences of plasma cleaning for a PBGA package.     

激光重熔在钎料凸点成形中的应用

激光由于具有高能量输入密度以及可局部加热的优点而在面阵列电子封装钎料凸点成形中具有潜在的优势，介绍了激光重熔在面阵列封装钎料凸点成形中的研究进展，并且对PBGA共晶钎料球激光重熔进行了工艺研究，研究结果表明；采用合适的激光输入能量可以在非常短的时间内获得表面质量光滑的钎料凸点。     

多层芯片应用中的封装挑战和解决方案

The continuous growth of stacked die packages is resulting from the technology‘s ability to effectively increase the functionality and capacity of electronic devices within the same footprint as a single chip.The increased utilization of stacked die packages in cell phone and other consumer products drives technologies that enable multiple die stacks within a given package dimension.This paper reviews t6he technology requirements and challenges for stacked die packages.Foremost among these is meeting package height is 1.2mm for a single die package.For stacked die packages,two or more die need to fit in the same area.That means every dimension in the package has to decrease,including the die thickness.the mold cap thickness,the bond line thickness and the wire bond loop profile.The technology enablers for stacked die packages include wafer thinning,thin die attachment,low profile wire bonding,bonding to unsupported edges and low sweep molding.     

Accurate RF electrical characterization of CSPs using MCM-D thin film technology

The thin film multilayer multichip module-deposited (MCM-D) technology of IMEC is used for characterising the RF electrical performance of two types of chip scale packages (CSPs). The measurement technique called MCM-on-package-on-MCM (MoPoM) enables accurate measurements and de-embedding in the gigahertz (GHz) range of frequencies. Wafer processing of the MCM-D technology allows for several design structures to be integrated on a single mask. The packages chosen are a 120-pin plastic ball grid array (PBGA) and an 80-pin polymer stud grid array (PSGA). Lumped element models extracted from measurements and three-dimensional simulations show good agreement with the measurements up to 6 GHz for the BGA and the PSGA. The electrical performance of the packages is compared at 1.8 GHz (GSM), 2.4 GHz (Bluetooth), and 5.2 GHz (HiperLAN) and at 5.2 GHz both the packages exhibit a return loss of lower than -10 dB and hence cannot be used in most cases without design improvement. We also show that the influence of encapsulant is significant while transmission line detuning due to the package is not significant at microwave frequencies. We also briefly mention about the crosstalk effects. We demonstrate the significant degradation in the performance of a 5.2 GHz MCM-D low noise amplifier (LNA) after packaging. A significant improvement in package performance is observed by conjugate matching the package interconnects.     

The accuracy of structural approximations employed in analysis ofarea array packages

At the present time, area-array packages are a very common electronics packaging approach. One of the major concerns in designing such packages is the reliability of solder joints, die, and the various material interfaces present in the package. Currently, analytical, numerical, and experimental methods are employed in the analysis of thermo-mechanical stresses/strains in area array packages. The sources of error in these analytical and numerical models may be broadly characterized as being due to geometry representation, material behavior, solution procedure, and due to the accuracy in representing the load history. In this paper we assess the errors in package models due to geometry representation and material behavior using a representative area-array package, namely the 225 input/output (I/O) plastic ball grid array (PBGA). The package deformation due to a fixed temperature change is experimentally characterized using Moire interferometry and numerically simulated using both two- and three-dimensional finite element models. The difference in behavior between the finite element prediction and experimental results is explained using solder material behavior data available in the literature. A comparison of accuracy as well as efficiency is made between the different finite element models. Finally, conclusions are drawn on the aspects of package construction and material that influence behavior, and on the most efficient finite element model to accurately capture this behavior     

Determination of residual strains of the EMC in PBGA during manufacturing and IR solder reflow processes

Even though recently published results indicated that residual strains of the epoxy molding compound (EMC) play a key role on the warpage values and shapes of the plastic ball grid array (PBGA) packages, it is still unknown about how these residual strains build up and change during the manufacturing and infrared (IR) solder reflow processes. The purpose of this study is to quantify the residual strains of the EMC in the PBGA packages during the aforementioned processes using a combination of experimental, theoretical and numerical approaches. In the experiments, a full-field shadow moiré is used for measuring their real-time out-of-plane deformation (warpage), during heating and cooling conditions, of two types of the PBGA specimens (without a silicon chip inside) with the same EMC but different substrates (with glass transition temperature T_g = 172 and 202 °C). Furthermore, Timoshenko’s bi-material theory associated with the measured and temperature-dependent elastic moduli and coefficients of thermal expansion for the EMC and substrates is applied for extracting residual strains of the EMC from shadow moiré results. In the analysis, the finite element method cooperating with those determined residual strains is employed to numerically simulate the thermal-induced deformations of the PBGA specimens, in order to verify mechanics. The full-field warpage of the specimens from shadow moiré is documented before and after post-mold curing, solder reflow and during the temperature cycling (from room temperature to 260 °C). The residual strains of the EMC for the specimens with low-T_g and high-T_g substrates after post-mold curing are found to be 0.059% and 0.134%, respectively, which double those before post-mold curing, and further down to 0.035% and 0.08% after the first thermal cycling. After the first cycling, the residual strains remain almost constant during heating and cooling processes. This phenomenon is also observed at lead-free solder reflow processes. Therefore, the residual strains of the EMC induced by the chemical shrinkage of the EMC curing and possibly mold flow pressure are different between the specimens with low-T_g and high-T_g substrates, and these residual strains could change during post-mold curing and the first solder reflow processes.