Michael Reid

Effect of Ag content on the microstructure of Sn‐Ag‐Cu based solder alloys

Purpose – The purpose of this paper is to examine the microstructure and evaluate the intermetallic compounds in the following lead‐free solder alloys: Sn98.5Ag1.0Cu0.5 (SAC105) Sn97.5Ag2.0Cu0.5 (SAC205) Sn96.5Ag3.0Cu0.5 (SAC305) and Sn95.5Ag4.0Cu0.5 (SAC405).Design/methodology/approach – X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to identify the main intermetallics formed during solidification. Differential scanning calorimetry (DSC) was used to investigate the undercooling properties of each of the alloys.Findings – By using XRD analysis in addition to energy dispersive spectroscopy (EDS) it was found that the main intermetallics were Cu6Sn5 and Ag3Sn in a Sn matrix. Plate‐like e‐Ag3Sn intermetallics were observed for all four alloys. Solder alloys SAC105, SAC205 and SAC305 showed a similar microstructure, while SAC405 displayed a fine microstructure with intermetallic phases dense within the Sn matrix.Originality/value – Currently, low‐silver content SAC alloys are bei...

A reliability model for SAC solder covering isothermal mechanical cycling and thermal cycling conditions

This paper presents an alternative to the use of energy-based methodologies for life cycle predictions of solder interconnects. Isothermal mechanical cycling testing has been conducted using joint-scale solder samples on a novel testing apparatus. The test data shows that work as a single parameter is insufficient in predicting failure; nor does the inclusion of cyclic frequency and mean temperature improve work-based methodologies. Here, a novel semi-empirical approach is presented in which stress, strain, strain rate and temperature are individually treated to create a model capable of predicting material behaviour under arbitrary cyclic loading conditions. The model constants are fitted to the results of the isothermal mechanical cycling tests, using load drop as a measure of damage. The calibrated model is then employed to predict the failure of a BGA device under thermal cycling. The modelling results show state-of-the-art agreement with the test data and superiority over Morrow model constants from literature that have been applied to this data set.

The influence of the Pb-free solder alloy composition and processing parameters on thermal fatigue performance of a ceramic chip resistor

This paper presents the results of a thermal fatigue study of a 2512 ceramic chip resistor assembled with various Pb free solders including SnCu, SAC105, SAC205, SAC305, and SAC405. The test matrix also includes some limited evaluations with other variables such as cooling rate (solidification rate), thermal preconditioning and nitrogen (inert) reflow atmosphere. The matrix also includes a SnPb eutectic control cells. The resistor test vehicle provides an expedient and self-consistent method for evaluating the relative fatigue performance of the various alloys. A study of the as-assembled solder joints was conducted to characterize the microstructure of the solder joints with varying silver content. Thermal fatigue was evaluated using an accelerated temperature cycle of 0/100 °C with dwell times of 10 and 60 minutes. The test results show a direct relationship between characteristic fatigue life and Ag content, with the higher Ag content alloys outperforming those with the lowest Ag content. As might be anticipated, there also was a consistent inverse relationship between fatigue life and dwell time for the Pb free solders. The failure analysis and microstructural evolution is characterized with optical metallography and scanning electron microscopy and the fatigue reliability of the Pb free solders is discussed in terms of the microstructures.

Phenomenological Study of the Effect of Microstructural Evolution on the Thermal Fatigue Resistance of Pb-Free Solder Joints

Unlike SnPb solders, the thermal fatigue reliability of the Sn-Ag-Cu (SAC) solders is believed to be influenced significantly by both the initial and evolving microstructures. This paper presents a phenomenological study of the relationship between the initial SAC solder joint microstructure, the evolving microstructure, and the thermal fatigue performance measured by accelerated temperature cycling (ATC). To reflect the board assemblies that are in field use, commercial surface mount components with multiple geometries and materials and from different package assemblers were joined to the board with different lead free SAC alloys. The initial microstructures of the board level solder joints were altered in a variety of ways including: 1) varying the solder joint cooling rate; 2) varying the number of solder reflow exposures; and 3) exposure to different isothermal temperature exposures. In all cases the solder joint microstructure was exposed to one or more of these treatments prior to exposure to temperature cycling. In addition, some of the test boards were exposed to different cycling dwell times to determine if the microstructural evolution that occurred during ATC testing effected the respective characteristic lifetimes of the joints. The microstructural evolution was tracked and characterized with optical metallography and scanning electron microscopy. These results could have practical implications in terms of limiting the ability to develop acceleration factors and effective strain-based models for predicting Pb-free solder joint life.

Study of Mixed Flowing Gas Exposure of Copper

This paper describes the results of copper coupons exposed to a class III mixed flowing gas environment MFG following the guidelines given by the Battelle Laboratory and the International Electrotechnical Commission for environmental testing. Corrosion products were studied in detail using scanning electron microscope, energy dispersive X-ray spectroscopy EDS, X-ray diffraction XRD, focused ion beam FIB, secondary ion mass spectroscopy SIMS, and transmission electron microscope. The weight gain measured after each exposure was compared with the weight gain calculated from the cathodic reduction of the corrosion layers and cross sectioning using an FIB. The result shows a relatively good correlation between the measured and the calculated experimental values of weight gain. As expected, within the first week, the different corrosion layers thickened until they formed a thick layer that became the determining step for further growth. After several days of exposure the Cu coupons developed a complex multilayered structure consisting of cuprous oxide Cu2S, cupric oxide CuO, copper sulfide Cu2S, covellite CuS, and evidence of antlerite 3CuO SO3 2H2O. No Cl-containing corrosion products were identified using XRD. However, EDS and SIMS analysis showed that Cl was distributed throughout the corrosion products, indicating that although Cl is inside the corrosion products, it is not part of the crystalline structure. Also, this suggests that Cl plays an important role in accelerating the corrosion of Cu during exposure to the MFG class III test.

The Corrosion of Electronic Resistors

Precision thick chip resistors are used in a variety of different industries, from telecommunications to automotive electronics, and as such can be exposed to mild and aggressive corrosive environments. This paper investigates the corrosion performance of two generic precision thick chip resistors in a controlled corrosive atmosphere consisting of 60degC, 4 ppm H2S and water vapor in purified air. The resistors were exposed in an environmental chamber for periods of 5, 10, 15, 30, and 60 days. Following exposure, the samples were cross sectioned and subjected to surface analysis using microscopy and microanalysis. After the initial stages of exposure, corrosion was observed on only one of the two types of resistors. The corrosion developed because H2S gas and water vapor diffuses through the thin protective organic layer on the resistor, and subsequently reacts with the silver conductor layer. Corrosion was facilitated by poor overlapping of the solder and nickel layer and, in particular the glass binder over the glass overcoat, which allowed silver and sulphur to diffuse along the interface. In addition, this poor overlapping allowed contact between the nickel layer and the silver layer resulting in the development of an electrochemical corrosion cell. The main corrosion products that developed were silver sulfide (Ag2S) and nickel sulphur residue.

Corrosion Resistance of Copper-Coated Contacts

Corrosion of electronic components can produce a wide range of failure signatures, from intermittent electrical faults to complete functional breakdown. This paper presents an investigation on the exposure of a simple connector-coating system. The system consists of a copper contact coated with a nickel layer underneath a gold finish layer. The system was characterized using the following techniques: optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), secondary ion mass spectroscopy (SIMS) and focused ion beam (FIB). After initial characterization, the connector was exposed to 2, 4, 7, 15, and 30 days in an aggressive environment consisting of 90% relative humidity, 40°C, and 4 ppm H 2 S. Digital images of the corrosion products that developed on the contacts after exposure clearly demonstrated localized corrosion by-products present on the connector surface. SEM, EDAX, and SIMS analysis of the corrosion sites demonstrated the presence of copper sulfide and nickel sulfur corrosion product, which suggest a two-step mechanism: first, the Ni layer is attacked by the aggressive environment at the sites where the gold layer is not available, followed by the diffusion of copper through the nickel layer. FIB cross-sectional analysis revealed that surface defects in the gold layer resulted in sites for corrosion initiation and subsequent development of a thick copper sulfide layer of approximately 5 μm. It is concluded that this copper connector coating system does not prevent the formation of insulating corrosion products on the surface of the connector in a very aggressive environment.

The failure mechanisms of micro-scale cantilevers in shock and vibration stimuli

Contemporary shock testing of micro-devices is carried out in controlled test environments where test parameters can be monitored with current metrology techniques. Due to demanding environments and limited scope of design rules, the reliability of micro devices has become a concern. A modified Hopkinson pressure bar (HPB) is used to investigate failure mechanisms of single crystal silicon (SCS) micro-cantilever devices under high-g accelerations. Response upon impact is monitored using high speed imaging (HSI) to ascertain the cause of failure. White light interferometry (WLI) and scanning electron microscopy (SEM) are used as post analysis techniques to investigate cause of failure and fracture topography. The modified HPB method in conjunction with high speed imaging allowed valid prediction of modal and temporal failure information of the micro cantilevers. WLI investigated the effects of deep reactive ion etching (DRIE) etching on crack instigation. SEM identified octahedral cleavage of SCS as the dominant failure mechanism of the micro-cantilevers.

In-Situ Optical Creep Observation and Constitutive Modelling of Joint-Scale SAC Solder Shear Samples

In this paper the creep behaviour of lead-free hypo-eutectic Sn96.5Ag3.0Cu0.5 solder is evaluated. A series of creep tests at different stress/temperature and strain rate/temperature pairs have been conducted. The tests were observed in-situ with a high magnification camera system. The quantitative data gained from the tests was used to fit an Anand viscoplastic model. Optical observation results are presented from selected tests, showing the occurrence of surface effects such as shear bands, voiding and rumpling. From these observations the main deformation mechanisms were derived and compiled in terms of their dependence of the test conditions.

Microstructural Development of Copper Sulfide on Copper Exposed to Humid H2S

Pure copper samples were exposed in an environmental chamber for 2, 4, 7, 15, and 30 days at 90% relative humidity, 40°C, and 4 ppm hydrogen sulfide (H 2 S). Samples were subsequently subjected to microscopy and microanalysis using different techniques: scanning electron microscopy, energy analysis dispersive X-ray spectroscopy, X-ray diffraction, focused ion beam (FIB), and secondary ion mass spectroscopy. The corrosion samples were cross sectioned and the different corrosion layers were imaged using FIB. After 30 days exposure the predominant corrosion products were copper sulfide (Cu 2 S) and cuprite (Cu 2 O). Once the Cu 2 S reached a minimum thickness, the rate of growth of the layer became parabolic due to the limiting Cu + diffusion through a thickening film. As the layers reach a critical thickness (∼ 1000 nm) internal stresses and defects in the corrosion layer allow virtually free access of H 2 S and O to the underlying layers, consequently accelerating the film growth. ©2007 The Electrochemical Society. [DOI: 10.1149/1.2436612] All rights reserved.