Tsung-Yu Pan

Friction Stir Spot Welding of Advanced High-Strength Steels - A Feasibility Study

An exploratory study was conducted to investigate the feasibility of friction stir spot welding advanced high-strength steel sheet metals. The fixed pin approach was used to weld 600MPa dual phase steel and 1310MPa martensitic steel. A single tool, made of polycrystalline cubic boron nitride, survived over one hundred welding trials without noticeable degradation and wear. Solid-state metallurgical bonding was produced with welding time in the range of 2 to 3 seconds, although the bonding ligament width was relatively small. The microstructures and hardness variations in the weld regions are discussed. The results from tensile-shear and cross-tensile tests are also presented.

Microstructures and failure mechanisms of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets

Microstructures and failure mechanisms of spot friction welds in aluminum 6111-T4 lap-shear specimens are investigated based on experimental observations. Two types of tools, a Type I tool with a flat tool shoulder and a Type II tool with a concave tool shoulder, were used to join the aluminum sheets with different processing parameters. Optical micrographs of the cross sections of spot friction welds made by the two types of tools in lap-shear specimens before and after failure are examined. These spot friction welds show the failure mode of nugget pullout under lap-shear loading conditions. However, the micrographs show different microstructures and failure mechanisms for spot friction welds made by the two types of tools with different processing parameters. The experimental observations suggest that under lap-shear loading conditions, the failure is initiated near the stir zone in the middle part of the nugget and the failure propagates along the circumference of the nugget to final fracture.

Intermetallic compound growth on Ni, Au/Ni, and Pd/Ni substrates with Sn/Pb, Sn/Ag, and Sn solders [PWBs]

The growth mechanism of the Ni/sub 3/Sn/sub 4/ intermetallic compound (IMC) during aging was studied with three different solders (100Sn, Sn-3.5Ag, and Sn-37Pb) on three different substrates (Ni, Ni/Au, and Ni/Pd), at the temperatures of 75, 100, 125, and 160/spl deg/C from 1 to 36 days. The growth rates of Ni/sub 3/Sn/sub 4/ with Sn on Ni and Ni/Au substrates were similar, growing to about 6 /spl mu/m after 36 days at 160/spl deg/C, but only to about 1-2 /spl mu/m after 36 days at a temperature below 100/spl deg/C. The growth rate of Ni/sub 3/Sn/sub 4/ with Sn-37Pb on Ni/Au substrate was close to that with Sn for the same substrates. However, the Sn-3.5Ag solder showed a slower growth rate of Ni/sub 3/Sn/sub 4/ on both Ni and Ni/Au substrates, resulting in only about half the thicknesses when compared to Sn on the same substrates. In addition to the Ni/sub 3/Sn/sub 4/ compound, a PdSn/sub 4/ compound was observed on the NiPd substrates. The growth rate of Ni/sub 3/Sn/sub 4/ on the Ni/Pd substrate is much slower than that on either the Ni or the Ni/Au substrate, possibly due to the existence of the PdSn/sub 4/ layer between Ni and the solder. At temperatures lower than 100/spl deg/C, there is hardly any Ni/sub 3/Sn/sub 4/ detected for Sn-3.5Ag and Sn-37Pb solders for up to 36 days. The apparent activation energies, Q, are in the range of 3 to 12.8 Kcal/mole, and Q for Ni/sub 3/Sn/sub 4/ with Sn is the highest for the three solders on both the Ni and Ni/Pd substrates, and those for Sn-3.5Ag the lowest. However, Q for Ni/sub 3/Sn/sub 4/ growth with Sn-3.5Ag is the highest on the Ni/Au substrate. A thick Ni/sub 3/Sn/sub 4/ layer may pose potential reliability issues as evidenced by the fractured morphology of the intermetallics due to a 10.7% volume shrinkage during the transformation from solid phase Sn and Ni to the Ni/sub 3/Sn/sub 4/ compound.

Thermal cycling induced plastic deformation in solder joints. II. Accumulated deformation in through hole joints

A thermal-cycled through-hole solder joint is studied. Nonlinear elastic/plastic solder properties at different temperatures and a steady-state creep law are used to characterize the deformation of the eutectic Pb-Sn solder. Large geometry changes in the solder and pin structure are observed experimentally. The deformation observed is much larger than expected from a simple thermal expansion mismatch calculation. This phenomenon is explained as a combination of plastic and creep deformations which accumulate during the thermal cycling. Because of the complexity of multiaxial stresses in the joint due to thermal expansion mismatch, finite-element analysis is required to characterize stress and strain in the solder joint. The large plastic deformation observed in the solder fillet is quantitatively simulated by the analysis. The majority of the deformation is a result of the time-dependent creep, while deformation occurring during the time-independent temperature change is minor. The displacement of the IC pins is recorded after each cycle and approaches the value observed in an actual joint after 1000 cycles. Thermal cycling fatigue life prediction based on uniform shear and the Coffin-Manson equation is found to be insufficient in dealing with the complex deformation mechanism of solder.<<ETX>>

Investigation of fatigue lives of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets based on fracture mechanics

The fatigue lives of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets are investigated here. The paths of fatigue cracks near spot friction welds are first discussed. A fatigue crack growth model based on the Paris law for crack propagation and the local stress intensity factors for kinked cracks is then adopted to predict the fatigue lives of spot friction welds. The global and local stress intensity factors based on a recent work of Wang and Pan for resistance spot welds in lap-shear specimens are used to estimate the local stress intensity factors of kinked cracks with experimentally determined kink angles. The results indicate that the fatigue life predictions based on the Paris law and the local stress intensity factors as functions of the kink length agree well with the experimental results.

Current carrying capacity of copper conductors in printed wiring boards

The effect of thermal management of the copper conductors on PWBs (printed wiring boards) having different dimensions and arrangements is discussed. Design charts have been generated to include the parameters of conductor thickness from 1 to 3 oz, width from 5 to 20 mils, spacing at 8 mils, board thickness from 31 to 62 mils, input current, and temperature rise up to 50/spl deg/C. The analysis is based on finite element modeling with a heat transfer film coefficient obtained from infrared thermal imaging analysis of a test board. Of all the geometric parameters considered, conductor width and spacing are the primary parameters influencing thermal resistance. Conductor thickness is next, and board thickness proves to be the least sensitive parameter.<<ETX>>

Experimental Analysis of Thermal Cycling Fatigue of Four-Layered FR4 Printed Wiring Boards

Long-term reliability of electronic packaging has become a greater challenge as a result of ever increasing power requirements and the decreasing size of electronic packages. In this study, the effects of three variables on plated-through hole (PTH) design have been investigated on the thermal cycling fatigue lives in four-layered printed wiring boards (PWB’s). These three variables were evaluated at two levels each: (a) hole size (0.030 and 0.040 in.), (b) internal pad (presence or absence), and (c) epoxy-plugged holes (plugged or unplugged). The electrical resistance was measured on 40 test boards with 23 design of 8 daisy-chain PTH nets each. Full factorial analysis and analysis of variance indicate that all three factors had significant influence on PTH fatigue life, but no two-factor or three-factor interactions were found. Metallurgical analysis reveals that the failure mechanism is barrel cracking near the internal pad. This mechanism has been illustrated by a finite element analysis in this study and correlated by a SEM stereoimaging analysis in the literature. The increase of electrical resistance with thermal cycles correlates well with an analytical barrel crack model. The crack length in each net at specific cycles is calculated, but fails to match predictions from a fracture mechanics model.

Failure Mechanisms of Sandwich Specimens With Epoxy Foam Cores Under Bending Conditions

Sandwich specimens with DP590 steel face sheets and structural epoxy foam cores are investigated under three-point bending conditions. Experimental results indicate that the maximum loads correspond to extensive cracking in the foam cores. Finite element simulations of the bending tests are also performed to understand the failure mechanisms of the epoxy foams. In these simulations the plastic behavior of the steel face sheets is modeled by the Mises yield criterion with consideration of plastic strain hardening. A pressure sensitive yield criterion is used to model the plastic behavior of the epoxy foam cores. The epoxy foams are idealized to follow an elastic perfectly plastic behavior. The simulation results indicate that the load-displacement responses of some sandwich specimens agree with the experimental results. Based on the results of finite element simulations, for the given geometry and the combination of materials, cracking within the foam cores is due to large shear deformation which comes from the large difference in the load carrying capacities of the steel face sheets and the epoxy foam cores.