Claire Ryan

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...

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.

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.

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.

A Simulated and Experimental Comparison of Lead-Free and Tin-Lead Solder Interconnect Failure Under Impact Stimuli

There is considerable reported evidence that a large percentage of failures which afflict portable electronic products are due to impact or shock during use. Failures of the external housing, internal electronic components, package-to-board interconnects, and liquid crystal display panels may occur as the result of accidental drops. Moreover, the introduction of lead-free solder to the electronics industry will bring additional design implications for future generations of mobile electronic systems. In this paper, drop tests performed on PCBs populated with ball grid arrays (BGAs) are reported. During testing, measurements from strain gages were recorded using a high-speed data acquisition system. Electrical continuity through each package was monitored during the impact event in order to detect failure of package-to-board interconnects. Life distributions were established for both a lead-free and a tin-lead solder for various drop heights. In addition, failure analysis was carried out using microsection techniques, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Resistance measurements throughout the drop event indicated that different failure mechanisms occurred for different drop heights. The explicit finite element (FE) method was employed to evaluate the peel stress at the critical solder joint and a stress-life model is then established for the lead-free solder. The maximum peel stress location was found to match the location of failure initiation revealed from the failure analysis. It was also discovered that, for board level drop testing, that there is a considerable difference between the lead-free solder characteristic life and the tin-lead solder characteristic life.Copyright

Experimental and Numerical Evaluation of SnAgCu and SnPb Solders Using a MicroBGA Under Accelerated Temperature Cycling Conditions

A comparative evaluation of the leading lead-free solder candidate (95.5Sn3.8Ag0.7Cu) and traditional tin-lead solder (63Sn37Pb) under thermal cycling conditions was carried out. A test vehicle consisting of four daisy chained 10×10 array 0.8mm pitch plastic micro ball grid arrays (microBGA) mounted on an 8-layer FR4 printed wiring board was designed. The board finish was organic solder preservative (OSP) for the lead-free devices and hot air solder levelled (HASL) in the case of the eutectic devices. An event detector was used to monitor the continuity of each daisy chain during accelerated temperature cycling, where the test vehicles were cycled with a ramp rate of approximately 3°C per minute from −40°C to 125°C, with 10-minute dwells and a total cycle time of 2 hours 10 minutes. Results to date plotted using a Weibull distribution indicate that the SnAgCu solder is more reliable under these conditions. Experiments were also carried out on large-scale lead-free solder specimens to determine the parameters required for the Anand viscoplasticity model. The Anand model was then implemented in finite element analysis using ANSYS® , where the submodelling technique was employed to determine the viscoplastic work per thermal cycle for each solder joint along the package diagonal. Schubert’s fatigue life model was used to predict the number of cycles to failure of each joint, although it should be noted that the necessary model parameters for the may need to be calibrated. Results indicate that the joint under the die edge is likely to fail first and that the SnAgCu solder is more fatigue resistant. The numerical predictions underestimate the fatigue life in both cases.© 2004 ASME

A stress-life methodology for ball grid array lead-free and tin-lead solder interconnects under impact conditions

Portable electronic products are often subject to impact or shock during use which leads to failures of the external housing, internal electronic components, package-to-board interconnects, and liquid crystal display panels. Moreover, the introduction of lead-free solder to the electronics industry will bring additional design implications for future generations of mobile information and communication technology (ICT) applications. In this paper, drop tests performed on printed circuit boards (PCBs) populated with ball grid arrays (BGAs) are reported. During testing, measurements from strain gauges were recorded using a high-speed data acquisition system. Electrical continuity through each package was monitored during the impact events in order to detect failure of package-to-board interconnects. Life distributions were established for both a lead-free and a tin-lead solder for various drop heights. The explicit finite element method (FEM) was employed to approximate the peel stress at the critical solder joint and a stress-life model was then established for both solders. Finally, failure analysis was carried out using microsection techniques, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). It was found that, for board level drop testing, different failure mechanisms can occur for different drop heights and that there is a considerable difference between the lead-free solder characteristic life and the tin-lead solder characteristic life.

A reliability evaluation of lead-free ball grid array (BGA) solder joints through mechanical fatigue testing

Lead is a hazardous substance which, when ingested can be toxic to humans; therefore it has been banned by a European Union directive in an aim to reduce its harmful effects on health and the environment. The ban, which comes into force on July 1/sup st/ 2006, means that electronic manufacturers must transfer from a tin-lead soldering process to a lead-free process. In this paper a reliability evaluation of a tin-silver-copper (SnAgCu) solder is presented with a baseline of tin-lead (SnPb). An experiment was carried out to optimize the surface mount reflow process and the reliability of the resulting solder joints was investigated using a torsional mechanical fatigue test method. The test vehicle comprised of an 8-layer FR4 printed circuit board (PCB) mounted with four ball grid array (BGA) components - each package comprising four daisy-chains. The basic principle of the torsion test was to stress the BGA solder joints repetitively in order to determine the number of cycles to failure. Graphs of the cycle number versus resistance were created and the numbers of cycles to failure were determined. The failure mechanisms were examined using cross-section and scanning electron microscope (SEM) techniques which showed cracking that initiated at the upper corner of the solder joint and propagated in the solder along the upper copper pad. This failure mechanism was observed for both SnAgCu and Snb solder joints. From a comparison of number of cycles to failure, the reliability of SnAgCu BGA solder joints was found to be superior to that of SnPb joints in torsion tests.

Optimising lead-free screen-printing and reflow process parameters

Introducing a lead-free solder replacement requires studies to be conducted by electronic assembly manufacturers in order to determine process alteration requirements and suitability of current equipment. This paper presents the results of an investigation of the screen-printing and reflow steps of the surface mount technology (SMT) manufacturing process. Experiments were conducted to investigate these two processes using a tin-silver-copper (95.5Sn3.8Ag0.7Cu) solder and a baseline of standard tin-lead (63Sn37Pb). 10/spl times/10 array micro Ball Grid Arrays (BGAs) mounted on 8-layer FR4 printed wiring boards (PWBs) were used with an organic solderability preservative (OSP) finish for use with lead-free components, and a hot air solder level (HASL) finish for use with tin-lead components. The screen-printing experiment investigated the deposition of the solder paste on the board. The parameters used in the investigation were print speed, squeegee pressure, snap-off distance, separation speed and cleaning interval with the responses being measurements of paste height and volume. Optimum screen-printer settings were determined which give adequate paste volume and height and a good print definition. The reflow experiment investigated the following parameters of the temperature profile; preheat, soak and reflow temperatures, and conveyor speed. The solder joints were examined using cross-section analysis and X-ray techniques in order to determine the presence of defects. The outcome of the investigation is a set of optimum settings for the screen-printer and reflow oven for use with SnAgCu lead-free solder.

SnAgCu Micro-Ball Grid Array (BGA) Solder Joint Evaluation Using a Torsion Mechanical Fatigue Test Method

Due to the hazard which lead poses to health and the environment the EU is banning its use in electrical and electronic equipment from July 2006. This ban along with the market drive to more environmentally friendly products means that tin-lead solders must be replaced with lead-free alternatives. This paper presents the results of an experimental investigation of the mechanical fatigue properties of tin-silver-copper (SnAgCu) solder joints with a baseline of tin-lead (SnPb). The test vehicle comprised of an 8-layer FR4 printed circuit board (PCB) mounted with four micro-ball grid array (BGA) components — each with a total of 100 solder balls in a 10×10 array. The solder joints were formed using surface mount reflow processes optimised for both solder types. A torsion mechanical fatigue test was employed to evaluate the solder joints — the principle of which was to stress the solder joints repetitively in order to determine the number of cycles to failure. The BGA components were daisy-chained — the resistance across each daisy-chain was monitored continuously during the cyclic defection of the test board. A profile of the increase in resistance with cycle number was established and the number of cycles to failure determined. The failure mechanism induced by the cycling was examined using cross-section and scanning electron microscopy (SEM) techniques. The results for SnAgCu joints show a superior performance during torsion mechanical fatigue testing than SnPb joints; giving a greater number of cycles to failure. The results from the tests presented in this paper show that the torsion test method provides a viable alternative to ATC as a qualification method for solder joints, while also providing substantial time savings — taking weeks rather than months to complete.© 2005 ASME