Mechanical Strength Of Joints Research Articles

Influence of Surface Treatments on the Mechanical Strength of Hotmelt Adhesive Joints Made of Cartonboards

The influence of surface treatments including pigment coating, surface sizing and calendering on the mechanical strength of hotmelt adhesive joints in pilot made cartonboards was studied. The mechanical strength of the joints was investigated using the Y-peel test device at 23°C and 50% relative humidity. Some of the samples were investigated with respect to the failure mode by scanning electron microscopy. The surfaces were characterized in terms of surface roughness, surface chemical composition, and adhesion behaviour. A strong adhesive bond displayed fibre tear. In addition to fibre tear, interfacial failure, i.e., failure between the cartonboard and the adhesive, was the main reason for fracture in the bonded assembly. The most important factor controlling the integrity of adhesive joints seemed to be the real contact area. The adhesive joints showed significantly higher strength when the hotmelt adhesive was first applied onto the rougher cartonboard of the assembly and then the smoother cartonboard was pressed on the adhesive than vice versa. The surface roughness of cartonboards mainly depended on whether the surface was pigment coated or not. Calendering displayed only a minor effect. No clear influence of surface chemical composition of the cartonboards on the adhesive joint strength was found due to the fact that changes in surface chemistry in this study also led to changes in surface roughness. The strongest adhesive joint was created between two medium-rough and surface-sized cartonboards.

Preparation and Properties of a Custom-Designed Nickel-Based Amorphous Brazing Foil

The custom-designed amorphous brazing filler metals in foil form has been developed for application in brazing stainless steel 0Cr18Ni9. The new alloys contains 10.5wt% chromium, 5.4wt% silicon, 1.5wt% boron, 3wt% cobalt and nickel as the balance. The melting range is within 971°C1070°C. The alloy exhibits good melting characteristics and wetting behavior. With a 25μm thick foil, the joint is free of brittle central eutectic phases inside the joint via diffusion of the alloying elements into adjoining base metal and results on a high mechanical joint strength.

Experimental investigations for mechanical joint strength following ultrasonically welded pin osteosynthesis

The purpose of this study was to determine whether fixation of cranial bone segments using ultrasonically welded pin osteosynthesis showed differences in mechanical stability as compared to fixation of cranial bone segments using screw osteosynthesis. Right and left cranial bone segments from each of 16 young sheep were obtained by craniotomy and re-fixed: on the right with a mesh plate and pins, and on the left with a mesh plate and screws. All osteosynthesis materials consisted of PDLLA, fully amorphous polyactid. A total of 167 cranial bone / mesh plate segments from 16 animals were investigated; 84 segments were pin-fixed and 83 segments were screw-fixed. The implantation time of the re-fixed segments ranged from 1 day to 196 days. The mechanical methods chosen for simulation of stress on the bone segment bonds were two bending tests (horizontal and vertical directions) and a tensile test. The values obtained in the mechanical tests indicate differences in the bond strength between the pin- and screw- fixation methods over the length of in vivo implantation time. The mechanical stability of the ultrasonically welded pin osteosynthesis bonds over the screw osteosynthesis bonds proved to be statistically significant. The implication of these findings should also be relevant in the field of medicine.

Strength of Soldered Joints Formed under Microgravity Conditions

Experiments have been carried out to compare the mechanical strength of joints soldered with Sn-Ag-Cu under different gravitational conditions. Joints soldered under microgravity (produced during a parabolic flight) have lower strength (by 32% in this case) than similar joints formed under normal gravity. Electron microscopy has shown that this is due to a larger volume of residual porosity (14%) in the joints formed in microgravity compared with <1% for joints formed in normal gravity. The residual porosity in joints formed in microgravity is mainly within the bulk of the solder, with some microporosity in the Cu6Sn5 intermetallic layer near the copper interfaces. The porosity not only weakens the joint, but also biases the failure path away from the intermetallic layer and into the bulk of the solder. These observations show that gravitational buoyancy is important for the expulsion of flux and flux residues from soldered joints.

Evolution of the interlayer microstructure and the fracture modes of the zirconia/Cu–Ag–Ti filler/Ti active brazing joints

Zirconia (3Y-PSZ) and titanium were joined by active brazing in vacuum with the 63 Ag–35.25 Cu–1.75 Ti (wt.%) filler at 840–870 °C under different contact loads. Brazing conditions did not affect noticeably the joint mechanical strength but a remarkable change of the interlayer microstructure was found. This microstructure evolution affected the fracture behavior: ductile fracture through the Ag-rich area changed to a brittle one through the intermetallic area in the titanium–filler interface after brazing with long soaking at maximum temperature.

Effects of Void Formation within the Pb-Free BGA Solder Balls on the Mechanical Joint Strength

The effects of void formation on the joint strength and failure mode of the solder joints were examined. A computational modeling technique, i.e., finite element modeling, was employed to investigate the effects. The effects of location, size and number of void were also investigated and compared with the case that has no void in the solder ball. When there was 1 or more voids in the solder ball, the joint strength decreased regardless of the location, size, and number of voids. Especially, the shear force of the joints decreased about 50% when 3 voids were formed in the solder ball. From the stress analyses, the possibility of preliminary brittle interfacial failure should be lower when voids were formed in the solder ball.

Reexamination of the solder ball shear test for evaluation of the mechanical joint strength

Ball shear tests were investigated in terms of the effects of test parameters, i.e., shear height and shear speed, with an experimental and non-linear finite element analysis for evaluating the solder joint integrity of area array packages. Two representative Pb-free solders were examined in this work: Sn–3.5Ag and Sn–3.5Ag–0.75Cu. The substrate was a common solder mask defined (SMD) type with solder bond pad openings of 460 μm in diameter. The microstructural investigations were carried out using scanning electron microscopy (SEM), and the intermetallic compounds (IMCs) were identified with energy dispersive spectrometry (EDS). Shear tests were conducted with the two varying test parameters. It was observed that increasing shear height at a fixed shear speed has the effect of decreasing shear force for both Sn–3.5Ag and Sn–3.5Ag–0.75Cu solder joints, while the shear force increased with increasing shear speed at fixed shear height. Shear heights that were too high had some negative effects on the test results such as unexpectedly high standard deviation values or shear tip sliding from the solder ball. Low shear height conditions were favorable for screening the type of brittle interfacial fractures or the degraded layers in the interfaces.

Correlation between the interfacial reaction and mechanical joint strength of the flip chip solder bump during isothermal aging

The interfacial microstructure of Sn-37Pb solder with immersion Au/electroless Ni-P under bump metallization (UBM) was studied using scanning electron microscopy (SEM) and electron probe micro analyzer (EPMA). A Ni3Sn4 intermetallic compound (IMC) layer was formed at the interface between the solder and Ni-P UBM upon reflow. However, after thermal aging, AuSn4 containing a certain amount of Ni dissolved in it, i.e. (Au,Ni)Sn4, appeared above the Ni3Sn4 layer. The shear force of the joints decreased with the aging period. The decrease of the shear force could be mainly caused by the coarsening effect of the microstructure within the solder, while the decrease after prolonged aging time should be due to the excessively grown IMC layer. A simple computational simulation was employed to interpret the failure mechanism of the shear tested specimens using finite element modeling (FEM). The FEM results explained well the ductile failure mode of the nearly whole tested joints.

Effect of moisture content of members on mechanical properties of timber joints

To investigate the effect of moisture content (MC) of members on the mechanical properties of timber joints, bending tests of precut joints and shear tests of dowel-type joints were carried out using timbers of Japanese cedar (Cryptomeria japonica D. Don) with three moisture conditions: green, kiln-dried with a MC target of 15%, and over-kiln-dried with a MC target of 5%. For the bending test, timbers were processed with a precut processing machine into “koshikake-ari” (a kind of dovetail joint) and “koshikake-kama” (a kind of mortise and tenon joint). A pair of members was jointed together without mechanical fasteners. Bolts (diameter = 12 mm) and nails (diameter = 2.45 mm) were used as dowels in the shear test. Bolted joints were constructed with one bolt and two metal side plates. Two nails and two metal side plates were used for the nailed joint. For precut joints, no clear effect of MC was recognized on maximum moment and initial stiffness. The maximum strength of mechanical joints assembled with kiln-dried wood was changed by the degree of drying. Stiffness of the joints assembled with kiln-dried specimens was larger than that of the joints assembled with green specimens.

Effect of aging conditions on interfacial reaction and mechanical joint strength between Sn–3.0Ag–0.5Cu solder and Ni–P UBM

The interfacial microstructure of Sn–3.0Ag–0.5Cu solder with electroless Ni/immersion Au (ENIG) was studied using SEM, EPMA and TEM. (Cu,Ni) 6Sn 5 intermetallic compound layer was formed at the interface between the solder and Ni–P UBM upon reflow. However, after isothermal aging the AuSn 4, with a certain amount of Ni dissolved in it, i.e. (Au,Ni)Sn 4 appeared above the (Cu,Ni) 6Sn 5 layer. Two distinctive layers, P-rich and Ni–Sn–P, were additionally found from the TEM observation. The analytical studies using EDS in TEM revealed that the averaged composition of the P-rich layer is close to that of a mixture of Ni 3P and Ni, while that of the Ni–Sn–P layer is analogous to the P-rich layer but containing a small amount of Sn in it. Shear force of the flip chip solder joints decreased during aging at various temperatures. The decrease in shear force was mainly caused by the coarsening of microstructure within the solder and increase of the interfacial intermetallic compound thickness. Nearly all of the test specimens showed ductile failure mode and this could be well explained with the results of FEM analyses.

Reliability study of the electroless Ni–P layer against solder alloy

Copper (Cu) has been widely used in the under bump metallurgy of chip and substrate metallization for chip packaging. However, due to the rapid formation of Cu–Sn intermetallic compound (IMC) at the tin-based solder/Cu interface during solder reaction, the reliability of this type of solder joint is a serious concern. In this work, electroless nickel–phosphorous (Ni–P) layer was deposited on the Cu pad of the flexible substrate as a diffusion barrier between Cu and the solder materials. The deposition was carried out in a commercial acidic sodium hypophosphite bath at 85 °C for different pH values. It was found that for the same deposition time period, higher pH bath composition (mild acidic) yields thicker Ni–P layer with lower phosphorous content. Solder balls having composition 62%Sn–36%Pb–2%Ag were reflowed at 240 °C for 1 to 180 min on three types of electroless Ni–P layers deposited at the pH value of 4, 4.8 and 6, respectively. Thermal stability of the electroless Ni–P barrier layer against the Sn–36%Pb–2%Ag solder reflowed for different time periods was examined by scanning electron microscopy equipped with energy dispersed X-ray. Solder ball shear test was performed in order to find out the relationship between the mechanical strength of solder joints and the characteristics of the electroless Ni–P layer deposited. The layer deposited in the pH 4 acidic bath showed the weak barrier against reflow soldering whereas layer deposited in pH 6 acidic bath showed better barrier against reflow soldering. Mechanical strength of the joints were deteriorated quickly in the layer deposited at pH 4 acidic bath, which was found to be thin and has a high phosphorous content. From the cross-sectional studies and fracture surface analyses, it was found that the appearance of the dark crystalline phosphorous-rich Ni layer weakened the interface and hence lower solder ball shear strength. Ni–Sn IMC formed at the interfaces was found to be more stable at the low phosphorous content (∼14 at.%) layer. Electroless Ni–P deposited at mild acidic bath resulting phosphorous content of around 14 at.% is suggested as the best barrier layer for Sn–36%Pb–2%Ag solder.

Diffusion bonding of TI-6AL-4V to AISI 316L stainless steel: mechanical resistance and interface microstructure

The interface microstructure and mechanical strength of joints obtained by diffusion bonding of alloy Ti-6A1-4V to AISI 316L stainless steel are presented. Bonding took place at 850, 900 and 950°C, maintained for different lengths of time (60 to 180 min). The highest recorded shear strength was equal to 382 MPa and was displayed by the specimen bonded at 950°C/180 min. Three different fracture paths were identified, each correlated to a characteristic strength range. From microprobe measurements and x-ray diffraction studies the following interface reaction products were identified: σ phase, α-Ti, Fe2Ti, β-Ti and Fe-Ti. The interface growth kinetics obeyed a quadratic law, indicating diffusion controlled growth of the β-Ti layer. Finally, from correlations identified between β-Ti layer thickness and shear strength variations, some hypotheses on the role of both pores and interface reaction product on mechanical strength were presented.

Dynamic tensile strength of composite laminate joints fastened mechanically

The tensile strength of composite joints is determined under dynamic loading conditions. The composites are laminates made from hybrid fiber reinforced plastic (HFRP) and carbon fiber reinforced plastic (CFRP). Three different mechanically fastened joint configurations are considered: they are the pin-connected, single-lap and double-lap type. The joint strength under dynamic load is found to be lower than that under quasi-static load. The pin-connected joint was the weakest. Investigated also are the influence of geometric parameters for pin-connected HFRP laminate joints. The results shed light on how to improve the bearing strength of mechanical joints when encountering dynamic loads.

炭素繊維複合材構造継ぎ手の疲労強度設計について

A study for fatigue strength of mechanical joints used in composite structures was made on the acquisition and interpretation of S-N data, the evaluation of fatigue life estimation methods and the presentation of fatigue design principle. Through this study the followings came to our knowledge. 1) The ratio of yield life to failure life becomes smaller as loading increases . 2) The effect of loading frequency on S-N data is little for circular hole specimens (no load transfer) but is very large for double shear joints. 3) The optimum clamping torque seems to exist for fatigue life to be longest. 4) The life prediction by use of Linear Residual Strength Reduction Fatigue Model (LRSRFM) gives better estimation than Miner's rule. 5) The scatter factor of composite structures must be 1.5 times larger than that of aluminum structures.

Procedures for Metallographic Examination and Mechanical Strength Testing of Fine Pitch Surface Mount IC Package Solder Joints

The reduction in IC package lead pitches in surface mount solder assembly and the current high emphasis on quality and reliability of printed circuit assemblies have created a requirement for microanalysis of fine pitch solder joints in manufacturing situations. Of particular interest are metallographic analysis, detection of solder joint defects and mechanical strength testing of solder joints. Much has been published in the literature on the results of such evaluations in specific applications but little has been available on procedures for use in the microanalysis itself, particularly for fine pitch solder joints. Detailed procedures for fine pitch solder joint microanalysis, which the authors have verified down to 0.5 mm (0.02 in.) lead pitches, are presented. In particular, the authors present procedures for metallographic examination of tin‐lead and tin‐lead‐silver solder joints. In addition, test parameters are given for a repeatable technique of fine pitch solder joint mechanical strength testing that allows mechanical strength measurements to be obtained from almost every lead on a fine pitch surface mount IC package.

Laser soldering of surface-mounted devices for high-reliability applications

Laser soldering processes are characterized by the temperature during soldering within the joint as a function of time and as a function of the soldering parameters. These parameters are optimized by a metallurgical assessment of the quality of the joints. The thermal-fatigue properties of soldered joints important for space applications were investigated for different solder alloys by the thermal-cycling test according to ESA PSS-01-704. In addition, measurements of the shear strength were carried out for a small number of specimens in order to check if the mechanical strength of joints is altered by thermal cycling.

Case Studies in Quality and Reliability Analysis of Fine Pitch Solder Joints

Rapid feedback on the effect of changes in materials or assembly process parameters on solder joint quality is essential in process optimisation for fine pitch surface mount solder assembly. This is particularly the case where the demands of high volume production do not allow lengthy experimentation to be carried out on the production line. The greater precision and care required in analysis of fine pitch solder joints places a further constraint on the speed at which the analysis can be carried out. The techniques of microstructural analysis, solder joint mechanical strength testing and short duration environmental stress testing must be used to meet a rapid turnaround quality and reliability analysis requirement and to maximise the amount of information obtained from each stage of the analysis. Three case studies are detailed which demonstrate the use of these techniques in a high volume production context to provide rapid feedback on joint quality. The case studies have been selected for presentation not only to demonstrate the analysis techniques but also because they address issues which are of current interest due to the increased usage of fine pitch packages in production and the constraints on the use of solvents for cleaning of circuit boards. The studies are:

Freeting fatigue strength of mechanical joints

An analytical approach to the calculation of fretting fatigue strength of mechanical joints based on a stress model of the fretting fatigue damage mechanism has been presented. The fretting fatigue limit has been defined as a stress (expressed by the threshold stress intensity factor) required for further propagation of the non-propagating main fatigue crack appearing in a cyclically loaded member. Some practical recommendations resulting from the theory have been suggested.

The Role of Chemical Bonding in the Adhesion of Elastomers

Abstract A review is given of several studies of the effect of interfacial bonding upon the mechanical strength of an adhesive joint. In the first, polybutadiene layers were crosslinked by a free radical process while in contact with silane-treated glass. A direct proportionality was found between the minimum peel strength of the joint, at high temperatures and low rates of peeling, and the vinyl content of the silane treatment liquid. Covalent bonding between the diene polymer and vinyl groups on the treated glass was inferred. When radioactively tagged silanes were employed, extensive combination with the glass substrates was demonstrated. Again, the greater the amount of vinyl silane found on the treated glass surface, the greater the mechanical strength of adhesion between the treated glass and a polybutadiene overlayer. In another series of experiments, two partially crosslinked sheets of polybutadiene were pressed together before the crosslinking was taken to completion. The additional crosslinking was determined from measurements of the elastic properties and of the degree of equilibrium swelling by a compatible liquid. Again, the mechanical strength of adhesion between the two sheets under threshold conditions was found to be directly proportional to the inferred degree of interfacial interlinking. Thus, at least at high temperatures and low rates of peel, there is substantial evidence for a direct correlation between the mechanical strength of a joint and the degree of interfacial chemical bonding. Moreover, the relationships established in these studies allow other bonding systems to be diagnosed as chemical or physical in nature. For example, a dramatic increase in the strength of adhesion between two crosslinked polybutadiene layers was observed if they were exposed to air or oxygen for periods of an hour or two before being pressed into contact. It is inferred that interfacial chemical bonds are formed as a consequence of rapid surface oxidation reactions.

The role of chemical bonding in the adhesion of elastomers

A review is given of several studies of the effect of interfacial bonding upon the mechanical strength of an adhesive joint. In the first, polybutadiene layers were crosslinked by a free radical process whilst in contact with silane-treated glass. A direct proportionality was found between the minimum peel strength of the joint, at high temperatures and low rates of peeling, and the vinyl content of the silane treatment liquid. Covalent bonding between the diene polymer and vinyl groups on the treated glass was inferred. When radioactively tagged silanes were employed, extensive combination with the glass substrates was demonstrated. Again, the greater the amount of vinyl silane found on the treated glass surface, the greater the mechanical strength of adhesion between the treated glass and a polybutadiene overlayer. In another series of experiments two partially-crosslinked sheets of polybutadiene were pressed together before the crosslinking was taken to completion. The additional crosslinking was determined from measurements of the elastic properties and of the degree of equilibrium swelling by a compatible liquid. Again, the mechanical strength of adhesion between the two sheets under threshold conditions was found to be directly proportional to the inferred degree of interfacial interlinking. Thus, at least at high temperatures and low rates of peel, there is substantial evidence for a direct correlation between the mechanical strength of a joint and the degree of interfacial chemical bonding. Moreover, the relationships established in these studies allow other bonding systems to be diagnosed as chemical or physical in nature. For example, a dramatic increase in the strength of adhesion between two crosslinked polybutadiene layers was observed if they were exposed to air or oxygen for periods of an hour or two before being pressed into contact. It is inferred that interfacial chemical bonds are formed as a consequence of rapid surface oxidation reactions.