Mark A. Lamontia

The Effects and Non-Destructive Evaluation of Defects in Thermoplastic Compression-Loaded Composite Cylinders

Measurements and predictions of the effects of voids and ply waviness on compression and interlaminar shear properties of thermoplasticmatrix composites are presented. Correlations between ultrasonic Cscan loss to voids and to compression strain-to-failure are described. The data are drawn from a database of seventy-six cylinders and cylinder sections fabricated by Du Pont Advanced Material Systems since 1988 using an insitu consolidation filament winding and tape laydown process. Composites with graphite (AS-4 and IM-7) or S-2 glass fibers in polyetheretherketone (PEEK) or polyetherketone ketone (PEKK) thermoplastic matrices are included. Property measurements are from various mechanical and external hydrostatic compression pressure tests on cylindrical shells intended for marine submersible applications and on autoclave or compressionmolded flat panels. Performance data show the negative impact of voids and ply waviness on mechanical properties and cylinder performance. NDE data show a correlation between ultrasonic loss and void content only over a broad void content range; over the important 0% to 2.0% range, no correlation is observable. Data directly relating C-scan loss with compression strain-to-failure show a continuous loss of performance with increasing void content.

Performance of a Filament Wound Graphite/Thermoplastic Composite Ring-Stiffened Pressure Hull Model

The design, analysis, manufacture, and hydrostatic testing results of a 24- inchdiameter ringstiffened pressurehull model are presented. The AS-4 graphite/PEEK cylinder is manufactured using a nonautoclave insitu filament winding and tape laydown process. The cylinder incorporates 5 hoopwound rings and a shell (hoop/axial construction) that are integrally wound during manufacture. Ultrasonic inspection in conjunction with optical microscopy of end rings indicates that high quality is achieved with a void content of less than 1%. Translation of coupon data into the structure is quantified by subcomponent tests that measure insitu properties (A and Bbasis allowables) including shell axial compressive stiffness and strength and interlaminar shear strength. Analysis of the structure indicates that high interlaminar shear stress exists in the ring fillet areas. Subcomponent tests with supporting analysis to design the test method indicated that this failure mode was not critical. Design and analysis efforts focused on midbay and endbay performance. Axial compressive stress concentrations in the endbay were reduced by incorporating local increases in shell thickness and a steel insert ring that provided radial constraint of the shell to reduce the probability of an endbrooming failure mode. Steel hemispherical heads were designed and manufactured to further reduce axial compressive stress concentrations in the endbay. The test model was instrumented with strain gages and acoustic emission sensors and tested at Carderock Division, Naval Surface Warfare Center (CD-NSWC). Increments in pressure were followed by a 5-minute dwell time. Acoustic emission ceased for all pressures except the last two increments at 5250 and 5500 psi. The collapse pressure of 5500 psi was within 3% of our Bbasis prediction. The cylinder weight-to-displacement ratio, W/D, was 0.212 (unitless). Axial compression failure occurred in the cylinder midbay in agreement with our analysis.

Test Method Development to Quantify the In Situ Elastic and Plastic Behavior of 62%Sn–36%Pb–2%Ag Solder Ball Arrays in Commercial Area Array Packages at −40°C, 23 °C, and 125 °C

The objective of this study is to describe and evaluate test methods developed to experimentally characterize the in situ mechanical behavior of solder ball arrays connecting printed wiring boards to area array packages under tensile, compressive, and shear loading at −40, 23, and 125 °C. The solder ball arrays tested were composed of 62%Sn–36%Pb–2%Ag solder alloy. Finite element modeling was performed. The results indicated that the test fixture should be geometrically equivalent to the projected shape of the ball grid array to achieve uniform loading. Tension, compression, and shear tests were conducted. For tensile loading the interfaces and the solder balls are loaded in series resulting in a large apparent strain (13%). Various interfacial failure modes are observed. Under compression and shear loading the effect of the interfaces are negligible and therefore a significant deformation and a remarkable yielding behavior of solder ball arrays can be observed. Furthermore, the specimens tested under shear loading showed different failure modes such as cohesive or adhesive failure modes depending on the test temperature. From the overall results, it has been determined that shear loading is the most representative test to measure the actual mechanical behavior of solder in ball grid arrays.

Buckling Performance of Hydrostatic Compression-Loaded 7-Inch Diameter Thermoplastic Composite Monocoque Cylinders

The general instability, or buckling performance, of thick compression loaded composite cylinders is not well understood. Few cylinders have been hydrostatically tested that exhibit a well-controlled buckling failure. This work includes the design, fabrication, and test results for two monocoque cylinders tested in hydrostatic compression to measure buckling performance. The design focused on assuring that buckling failure would occur before a strength failure. The analysis accounted for the effects of end cap design and boundary conditions. The two cylinders were insitu thermoplastic filament wound using AS4 Graphite/PEKK and S-2 Glass/PEKK. Quality was excellent, with 1% void volume fraction and negligible waviness. Both cylinders buckled elastically in their respective tests. The test results include acoustic emissions (AE) and measured strains versus pressure. Experimental results show excellent correlation with analytical predictions.

Experimental In Situ Characterization and Creep Modeling of Tin-Based Solder Joints on Commercial Area Array Packages at −40°C, 23°C, and 125°C

The objective of this work was to experimentally determine the in situ creep behavior and constitutive model equations for a commercial area array package and printed wiring board assembly at −40, 23, and 125 °C through shear loading. The chip is connected to the printed circuit board by means of solder joints made of 62%Sn–36%Pb–2%Ag alloy. It was shown that the creep rate of solder ball arrays could be investigated using a stress relaxation method. Under the shear relaxation mode, the creep strain increases with temperature and can be described by a power law model with coefficients determined by finite element modeling (FEM). An analytical model was developed to describe the stress relaxation of an array with an arbitrary number of solder balls by defining an equivalent solder ball shear area as a fitting parameter. The resulting constitutive model is in excellent agreement with both FEM and experimental results at all test temperatures. A parametric study is conducted to investigate the creep response as a function of temperature for arrays consisting of a wide range of solder balls.

Effect of solder ball pitch and substrate material of printed wiring board on reliability under thermal cycling - a finite element analysis

A detailed Finite Element Model was created to study the effect of different parameters, structural and material, on the stresses/strains experienced by the different features in a composite containing chip package/solder ball/PWB, during thermal fatigue (-40/spl deg/C to 125/spl deg/C). The most prevalent failure mechanisms are the solder-ball failure due to cumulative creep-energy-density of the critical-solder-balls and the rupture of the surface dielectric (HDI Layer) of the PWB due to in-plane tensile strains. This analysis studied the effect of the solder ball pitch (800 /spl mu/m and 500 /spl mu/m) and the material properties of the HDI substrates, on the stresses at the solder balls. In general, a decrease in the BGA pitch increases the cumulative creep strain-energy density at the critical section of the solder ball, by about 50%. That, in turn, will increase the propensity of creep failure of the solder balls under thermal fatigue. Reduction of pitch-size also increased other stresses/strains of the solder balls (e.g. Von Misses stress). These stresses/strains at the solder balls were lower, when the HDI substrate in the PWB had low coefficient of thermal expansion (CTE). The adverse effects of decreasing pitch-size can be compensated by use of a reinforced surface dielectric with low CTE, and thus increase the reliability of the structure under thermal fatigue.