J. P. Lucas

Hygrothermal effects of epoxy resin. Part II: variations of glass transition temperature

Abstract Three epoxy systems (DGEBA+mPDA, TGDDM+DDS, and Fiberite 934TM) were used to investigate glass transition temperature (Tg) variation of epoxy under hygrothermal environment exposure. Materials were immersed in distilled water at constant temperatures of 45oC, 60oC, 75oC, and 90oC for water absorption and then desorbed at different temperatures. Thermomechanical analysis (TMA) and differential scanning calorimetry (DSC) were employed to determine Tg changes at different hygrothermal stages. The investigations revealed the following results: i) the change of Tg does not depend solely on the water content absorbed in epoxy resins, ii) Tg depends on the hygrothermal history of the materials, iii) for a given epoxy system, higher values of Tg resulted for longer immersion time and higher exposure temperature, and iv) the water/resin interaction characteristics (Type I and Type II bound water) have quite different influence on Tg variation. A sorption model and collateral evidence introduced in Part I of the series were used to interpret and explain Tg variation in epoxy resin systems. Both Type I and Type II bound water influence Tg variations, albeit in different ways. Type I bound water disrupts the initial interchain Van der Waals force and hydrogen bonds resulting in increased chain segment mobility. So Type I bound water acts as a plasticizer and decreases Tg. In contrast, Type II bound water contributes, comparatively, to an increase in Tg in water saturated epoxy resin by forming a secondary crosslink network. The experimental Tg values encompass the combined effect of the two water-resin interaction mechanisms described briefly in the preceding text and in detail in Part I of this paper series. The often-cited polymer-diluent model used to predict Tg variation of polymers exposed diluent media is lacking when a dual sorption mechanism is involved during hygrothermal exposure process.

The effects of a water environment on anomalous absorption behavior in graphite/epoxy composites

Abstract The effects of a water environment on moisture (H2O) absorption characteristics of a unidirectional T300/934 graphite/epoxy composite material have been investigated by the measurement and analysis of weight change, hygrothermal induced expansion, surface crack formation, and surface mass loss. Specimens were immersed in distilled water at 45, 60, 75, and 90 °C for more than 8000 h. Weight-change profiles for the composite exhibited divergence from the theoretical Fickian diffusion law. Scanning electron microscopy (SEM) and dimension measurements revealed clear evidence of surface peeling and dissolution of the specimen. A crack/mass-loss model has been used in order to explain the behavior of the composite material during the water absorption process. The general characteristics of water-induced weight gain in graphite/epoxy composites are discussed with respect to the moisture diffusion phenomenon and deviations in the weight-change profiles.

Creep behavior in Cu and Ag particle-reinforced composite and eutectic Sn-3.5Ag and Sn-4.0Ag-0.5Cu non-composite solder joints

Creep properties were determined for small, geometrically realistic Pb-free solder joints. Solder joints were prepared with eutectic Sn-3.5Ag and Sn-4.0Ag-0.5Cu solder alloys. Composite solder joints were made using the eutectic Sn-3.5Ag alloy as the matrix with 15 vol % of mechanically added ∼6 μm size Cu and ∼4 μm size Ag reinforcing particles. Creep tests were conducted on these joints at 25 °C, 65 °C and 105 °C representing homologous temperatures ranging from 0.61 to 0.78. Qualitative and quantitative evaluations of creep behavior were obtained from the distortion of excimer laser-induced surface ablation markings on the solder joint. Various creep parameters, such as global and localized creep strain, variation of creep strain and strain-rate, activation energy for creep, and the onset of tertiary creep were determined. General findings in this study revealed that the creep resistance in composite solder joints is significantly improved with Cu particle reinforcements. In contrast, the improvement in the creep properties of Ag particle-reinforced composite solder joints was far less even though highly uniform deformation in the joint was observed. The strain noted at the onset of tertiary creep for Cu and Ag reinforced composite solder joints was typically lower compared to non-composite solder joints. The activation energies for creep were similar for all the solder materials investigated in this study.

Microstructural characterisation of reflowed and isothermally‐aged Cu and Ag particulate reinforced Sn‐3.5Ag composite solders

Composite solders were prepared by mechanically dispersing 15v% of Cu or Ag particles into the eutectic Sn‐3.5Ag solder. The average sizes for the nominally spherical Cu and Ag particles were 6 and 4 microns, respectively. Two different processing methods were used to prepare the composite solders: blending the powdered particles with solder paste, and adding particles to the molten solder at 2808C. The composite solders were characterised by studying the morphology, size and distribution of the reinforcing phase. Particular interest and emphasis are given towards the modifications of the reinforcements during the reflow process. Microstructural features and chemical analysis of the composite solders were studied using optical and scanning electron microscopy (SEM), and energy dispersive x‐ray (EDX) analysis. The effect of reflow and isothermal ageing on the microstructure as well as the morphological changes in the interfacial IM layer of the composite solders were extensively analysed. A mechanism for IM layer growth is proposed for solid state isothermal ageing.

Formation and growth of interfacial intermetallic layers in eutectic Sn-Ag solder and its composite solder joints

Single shear lap joints were made by soldering two Cu substrates with eutectic Sn-Ag solder, and its composite solders containing FeSn/FeSn2 or Ni3Sn2 intermetallic particles introduced by an in-situ method. Ageing of solder joints was performed at 70, 100, 120, 150, 180 °C for 1400 h. The growth of the interfacial intermetallic compound (IMC) layers was characterized assuming diffusion-controlled growth kinetics. Effects of such FeSn/FeSn2 and Ni3Sn4 particulates on the IMC layer growth rate were extensively characterized. Composite solder joints in the fabricated condition formed thinner IMC layers compared to the corresponding non-composite solder joints. The Cu6Sn5 IMC layer grew faster at temperatures above 120 °C (T/Tm=0.8), while growing slower at temperatures below 120 °C in composite solder joints. In-situ introduced FeSn/FeSn2 and Ni3Sn4 particle reinforcements in composite solder joints proved effective in reducing the overall growth of the interfacial Cu6Sn5 IMC layer only at lower temperatures. Composite solder joints exhibited slower growth of the Cu3Sn layer during ageing at all temperatures used in this study. Two different regions having different activation energies depending on the temperature were identified for the growth of Cu6Sn5 and Cu3Sn IMC layers.

Quantification of creep strain distribution in small crept lead-free in-situ composite and non composite solder joints

Abstract Single shear lap creep specimens with a 1 mm 2 cross sectional area (similar in size to solder joints used in electronic packaging) were developed using lead free solders to examine the effect of in situ composite microstructures on the creep resistance and damage accumulation processes at temperatures between 25 and 150°C. Local strain measurements were made optically by following the change in shape of a scratch. Average creep strain measurements on non-composite solder joints were similar to comparable data from solder joint specimens in the literature. The distribution of strain across the specimen was measured in detail to determine how strain instabilities develop near interfaces, or near the center of the solder joint. The evolution of strain is quantified across the joint and related to microstructural features such as voids and reinforcements. Room temperature creep resistance is far superior in the composite, but at elevated temperatures, both types of joints have similar creep resistance. By analysis of the local strain history, a criteria for damage initiation is identified in terms of the local strain where the onset of tertiary creep is first observed, and subsequently accelerates heterogeneous straining in the joint. The in situ composite solders cause a more homogeneous strain evolution compared to the non-composite solder, but local tertiary creep commences at a lower strain than in the non-composite joint, but only, at lower temperatures and higher strain-rates. The impact of these different constitutive behavior and damage accumulation conditions is discussed as it pertains to improving the long term reliability of a solder joint.

Intermetallic morphology around Ni particles in Sn‐3.5Ag solder

The influence of the thermal reflow profile on the formation and resultant morphology of the intermetallic layer that developed at the Ni particle reinforcements within an eutectic Sn‐Ag composite solder matrix was investigated. The composite solder was fabricated by mechanically dispersing 15 vol% Ni particles into eutectic Sn‐3.5Ag solder paste. Two distinct intermetallic compound (IMC) morphological microstructures were observed around the Ni reinforcements. IMC morphological microstructure apparently varied depending on the amount of heat input and differences in heating rates used in the reflow profile. A “sunflower” IMC morphology was typically noted when the total amount of heat input was small. However, with sufficient heat input, a faceted “blocky” IMC morphology was consistently achieved. Multiple‐reflow thermal profiling experiments were conducted to measure and compare the amount of heat input necessary to change the sunflower IMC morphology around Ni particle reinforcements to the blocky morphology.

Effects of Pb contamination on the eutectic Sn‐Ag solder joint

Eutectic Sn‐Ag solder is being considered as a potential replacement for Sn‐Pb solders. A potential drawback to using the eutectic Sn‐Ag solder is its higher melting point, 221°C, compared with the eutectic Pb‐Sn solder. Owing to its higher melting temperature, the eutectic Sn‐Ag solder is also being considered for automotive under‐the‐hood applications, which experience high temperature environments. Electronic components and/or circuit boards are often coated with Pb‐bearing solder to facilitate soldering operations. Soldering Pb‐bearing solder coated components and/or boards with eutectic Sn‐Ag solder will result in joints contaminated with Pb. In this study, the effects of Pb contamination on eutectic Sn‐Ag solder joints were investigated using three ternary alloys made by incorporating some Pb into eutectic Sn‐Ag solder. These ternary alloys all showed a peak at 178°C in heating curves obtained using Differential Scanning Calorimetry (DSC), which resulted from the ternary eutectic composition in the Sn‐Ag‐Pb system. The Pb phases in the ternary alloys were found to be dispersed throughout the microstructure. A practical implication of Pb contamination in eutectic Sn‐Ag solder joints is that the service temperature of such joints would be limited by the lower melting temperature of the ternary eutectic phase.