The effects of underfill on the reliability of flip chip solder joints

Thermal fatigue damage of flip chip solder joints is a serious reliability concern, although it usually remains tolerable with the flip chip connections (of smaller chips) to ceramic boards as practiced by IBM for over a quarter century. However, the recent trend in microelectronics packaging towards bonding large chips or ceramic modules to organic boards means a larger differential thermal expansion mismatch between the board and the chip or ceramic module. To reduce the thermal stresses and strains at solder joints, a polymer underfill is customarily added to fill the cavity between the chip or module and the organic board. This procedure has typically at least resulted in an increase of the thermal fatigue life by a factor of 10, as compared to the non-underfilled case. In this contribution, we first discuss the effects of the underfill to reduce solder joint stresses and strains, as well as underfill effects on fatigue crack propagation based on a finite element analysis. Secondly, we probe the question of the importance of the effects of underfill defects, particularly that of its delamination from the chip side, on the effectiveness of the underfill to increase thermal fatigue life. Finally, we review recent experimental evidence from thermal cycling of actual flip chip modules which appears to support the predictions of our model.     

Comparative study between prepared electrical grease and the imported one

Grease lubrication is a complex mixture of science and engineering, requiring an interdisciplinary approach, and is applied to the majority of bearings worldwide. Grease can be more than a lubricant; it is often expected to perform as a seal, corrosion inhibitor in electrical joints, electrical insulator and electrical connection improver. This work is concerned, therefore, with the comparative study between prepared electrical grease and the imported one. Physicochemical and electrical properties for the prepared grease and the imported one were evaluated. The results of dropping point, penetration, dynamic viscosity, corrosion inhibition, evaporation loss, total acid number and oil separation for the prepared grease under investigation are mostly the same compared with the imported grease. In addition, the results interpreted in terms of the physical and chemical properties of both greases revealed that no remarkable differences. In this respect, calorimetric study shows that the prepared grease, like the imported one, is thermally stable up to about 200 °C then decomposition and degradation started slightly faster and higher than that of the imported one. The electrical and dielectric parameters are very close at and around room temperature then the increase of charge carriers' mobility at higher temperatures explains the deviation from stability in case of the prepared grease. One can conclude that the prepared electrical grease could replace efficiently the imported electrical grease especially in isothermal application at and around room temperature.     

Experimental investigation of propagation mechanisms and fracture morphology for coalbed methane reservoirs

Fracture propagation mechanisms in coalbed methane (CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fractures in coal specimens at different angles between the face cleat and the maximum horizontal principal stress were investigated with hydraulic fracturing tests. The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. “Step-like” fractures were formed in most experiments due to the existence of discontinuous butt cleats. The hydraulic fractures were more likely to divert or propagate along the butt cleat with an increase in the angles and a decrease in the horizontal principal stress difference. An increase in the injection rate and a decrease in the fracturing fluid viscosity were more conducive to fracture networks. In addition, the influence on fracture propagation of the residual coal fines in the wellbore was also studied. The existence of coal fines was an obstacle in fracturing, and no effective connection can be formed between fractures. The experimental investigation revealed the fracture propagation mechanisms and can provide guidance for hydraulic fracturing design of CBM reservoirs.     

Mechanical strength of Sn-3.5Ag-based solders and related bondings

The tensile strengths of bulk solders and joint couples of Sn-3.5Ag-0.5Cu, Sn-3.5Ag-0.07Ni, and Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge solders and the shear strengths of ball grid array (BGA) specimens, solder-ball-attached Cu/Ni/Au metallized substrates were investigated. The tensile strength of the bulk is degraded by thermal aging. The Ni-containing solder exhibits lower tensile strength than Sn-3.5Ag-0.5Cu after thermal aging. However, the Ni-containing solder joints show greater tensile strength than the Cu/Sn-3.5Ag-0.5Cu/Cu joint. Fracture of the solder joint occurs between the intermetallic compound (IMC) and the solder. The shear strength and fracture mechanism of BGA specimens are the same regardless of solder composition.     

球形触点阵列与四边扁平封装的比较

球形触点阵列（ＢＧＡ）封装技术作为一种新的封装技术越来越受到人们的重视。从安装率和成本、表面安装印制板设计以及焊点可靠性三个方面对球形触点阵列与四边扁平封装（ＱＦＰ）进行了比较。分析认为，ＢＧＡ和ＱＦＰ各有其优势和劣势，各有其应用重点。     

Effects of cooling rate on mechanical properties of near-eutectic tin-lead solder joints

This paper reports the results of a study on the effect of the cooling rate during solidification on the shear creep and low cycle shear fatigue behavior of 60 Sn/40 Pb solder joints, and on bulk solder tensile properties. Solder joints were made with three different initial microstructures by quenching, air-cooling and furnace-cooling. They have similar steady-state strain rates under creep at relatively high shear stresses (i.e. in the matrix creep region) but creep at quite different strain rates at lower shear stresses (i.e. in the grain boundary creep region). These results are ascribed to the refined grain size and less lamellar phase morphology that results on increasing the cooling rate. Tensile tests on bulk solders that were cold-worked, quenched and furnace-cooled show that a faster cooling rate decreases the ultimate strength and increases the ductility at low strain rates. The fatigue life of quenched solder joints is shown to be longer than that of the furnace-cooled joints.     

Experimental Investigation of Epoxy Bonded Polymethylmethacrylate Joints

This paper presents an experimental investigation into various aspects of epoxy-bonded polymethylmethacrylate (PMMA) and PMMA-to-aluminium joints. The effects of adhesive thickness, overlap area, surface roughness, and environmental exposure on the joint strength were studied. Results indicated that the joint strength was not directly proportional to the overlap area, while sanding had a positive effect on the joint strength. A negative effect was observed when adhesive thickness was increased. The fatigue behaviour of adhesively-bonded joints under dynamic loading was found to be independent of frequency, for the range of values tested; however, it was dependent on the test temperature with greater reduction in fatigue life observed in PMMA-to-aluminium joints at higher temperature. Empirical equations from which the fatigue life of joints can be predicted were obtained by regression analysis. Intermittent fatigue testing of the joints was also performed. The epoxy adhesive tested proved to be a satisfactory choice for outdoor exposure. The rate of degradation of the adhesive was slow with the adherend itself degrading at a faster rate than the adhesive or the bondline.     

Skeleton-Based Human Action Recognition via Screw Matrices

With the recent advent of low-cost acqui-sition depth cameras, extracting 3D body skeleton has be-come relatively easier, which significantly lighten many dif-ficulties in 2D videos including occlusions, shadows and background extraction, etc. Directly perceived features, for example points, lines and planes, can be easily ex-tracted from 3D videos such that we can employ rigid motions to represent skeletal motions in a geometric way. We apply screw matrices, acquired by using rotations and translations in 3D space, to model single and multi-body rigid motion. Since screw matrices are members of the special Euclidean group SE(3), an action can be repre-sented as a point on a Lie group, which is a differen-tiable manifold. Using Lie-algebraic properties of screw al-gebra, isomorphic to se(3), the classical algorithms of ma-chine learning in vector space can be expanded to man-ifold space. We evaluate our approached on three public 3D action datasets: MSR Action3D dataset, UCF Kinect dataset and Florence3D-Action Dataset. The experimental results show that our approaches either match or exceed state-of-the-art skeleton-based human action recognition approaches.     

Recent Observations on Tin Pest Formation in Solder Alloys

The most recent observations of the response of bulk samples of several lead-free solder alloys, exposed to temperatures below the allotropic transition for tin for extended periods, are reported. Tin pest has been observed in Sn-0.5Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and Sn-3.0Ag-0.5Cu alloys at both −18°C and −40°C. The process is slow and inconsistent, usually requiring several years, but may eventually result in complete disintegration of the sample. No tin pest was detected in Sn-Zn-3Bi or in the traditional Sn-37Pb solder alloy after exposure for up to 4 and 10 years, respectively. It is suggested that nucleation is affected by local composition and that extremely small amounts of either intentional solute or impurity are influential. Growth of tin pest is accompanied by a large volume change, and it is likely that stress relaxation ahead of the expanding grey tin front is a controlling factor. A stronger matrix would be more resistant in this case, and at the temperatures of exposure Sn-37Pb is stronger than either Sn-3.5Ag or Sn-0.5Cu. The absence of tin pest, to date, on actual joints is attributed to their restricted free surface area and the greater strength associated with very small samples.     

Damage accumulation under repeated reverse stressing of Sn-Ag solder joints

To better understand the effect of repeated reverse stress in solder joints, a new testing method was developed. Tin-silver solder joints were fabricated, constrained between Cu blocks, and then subjected to repeated shear loading in a tensile tester. Constant strain amplitudes were applied to simulate service conditions. However, large loads were used to accelerate the damage accumulation. Microstructural features of the damage were very similar to those found with studies on thermomechanical fatigue (TMF) of small, single shear lap samples. Concentrated-shear banding or striations were observed to form along Sn dendrites. The load behavior of the solder with each cycle and during hold times at the extreme strain amplitude was consistent with damage accumulating with each successive cycle. Effects of strain amplitude, hold times at the stress extremes, number of cycles, and solder-joint thickness were found to play significant roles on the stress-strain behavior and surface damage.