Unique two-beam laser ultrasonic inspection (LUI) probes were developed for the inspection of the quality of all types of chip packages. Microelectronic assembly houses demand reliable quality inspection tools that can identify all interconnect defects across the wide range of chip packages in order to allow for cost-effective rework (when applicable) and to prevent field failures. LUI is a noncontact, nondestructive, and cost-effective technique that can be used as both an online and offline tool for evaluating the quality of solder ball interconnections. In this paper, the authors present the development and validation of unique two-beam LUI probes. The developed probes were validated by using them to evaluate the quality of large flip-chip ball grid array (FCBGA) packages. These two-beam laser probes produce high-amplitude signal in multilayer packages. The high-amplitude signal, in turn, improves the signal-to-noise ratio, which facilitates the detection of very small defects as well as large defects. FCBGA packages that underwent four-point bending tests were successfully evaluated for defects such as laminate separations, pad cratering, and cracks. Finally, the packages were cross-sectioned for scanning electron microscopy (SEM) imaging to validate the LUI results. Finite-element (FE) simulation of the package with four-point bending loading conditions was also carried out to further validate the LUI results.

The implementation of surface mount ball grid array (BGA) technologies in microelectronics industry has given way to the manufacturing of even more compact systems with reduced size and pitch of solder bumps that are increasingly challenging to inspect for their quality assurance. Current non-destructive inspection methods such as X-ray, and Acoustic Microscopy techniques have limitations in identifying defects. The low cost, non-destructive Laser Beam Probe Ultrasonic Inspection (LBPUI) technique has the potential to overcome these limitations and enable rapid assessment of chip packages on and off the assembly line. In this study, Flip Chip BGA packages subjected to bend testing are inspected using the LBPUI technique. Two optical fiber probes are used to carry pulsed Nd:YAG laser energy to the surface of the FCBGA package to induce ultrasound in the package. The quality of solder balls are assessed by analyzing the out-of-plane displacement responses on the surface of the package as measured by a laser interferometer. The results from LBPUI technique are validated with the results of destructive cross-sectional analysis.

Flip chip packaging has a higher performance, and I/O density when compared with wire bonded packaging. However, wire bonding is extensively used in the electronic packaging industry. One of the difficulties with flip chip technology is testing its reliability. Advancements in Scanning Acoustic Microscopy (SAM) can provide insight into 1st level joint interconnects. However, these traditional solder joint inspection methods have been very unsatisfactory in assessing 2nd level interconnects because of the location and physical configuration of these solder joints. Non-destructive methods like SAM and X-Ray have their own limitations in assessment of 2nd level interconnects. The Laser Ultrasonic Inspection Technique uses laser pulses to generate bulk ultrasonic waves in the package, and the reflected waves from the 2nd level interconnect can give us information about the quality of the joint. A fiber-coupled laser interferometer is used to measure the transient out of plane displacements on the surface of the package. This is a direct measure of the reflected ultrasonic wave strength. Laser power can be adjusted depending on the size of the package to generate bulk waves of sufficient strength to reach the interconnections. In this project, Flip Chip Ball Grid Array (FCBGA) packages (foot print size 52.5mm x 52.5mm) from Cisco Systems were subjected to mechanical shock tests and analyzed using a Laser Ultrasonic Inspection (LUI) system. The results are validated using cross sectioning method. The samples are also tested using X-Ray and C-SAM, and the results are compared with those of the LUI system. The LUI system test results have demonstrated that the LUI method holds a great promise for assessing the presence of defects in 2nd level interconnects.

The major focus of this work is to examine the dynamics of velocity amplification through pair-wise collisions between multiple masses in a chain, in order to develop useful machines. For instance low-cost machines based on this principle could be used for detailed, very-high acceleration shock-testing of MEMS devices. A theoretical basis for determining the number and mass of intermediate stages in such a velocity amplifier, based on simple rigid body mechanics, is proposed. The influence of mass ratios and the coefficient of restitution on the optimisation of the system is identified and investigated. In particular, two cases are examined: in the first, the velocity of the final mass in the chain (that would have the object under test mounted on it) is maximised by defining the ratio of adjacent masses according to a power law relationship; in the second, the energy transfer efficiency of the system is maximised by choosing the mass ratios such that all masses except the final mass come to rest following impact. Comparisons are drawn between both cases and the results are used in proposing design guidelines for optimal shock amplifiers. It is shown that for most practical systems, a shock amplifier with mass ratios based on a power law relationship is optimal and can easily yield velocity amplifications of a factor 5–8 times. A prototype shock testing machine that was made using above principles is briefly introduced.

We examine the dynamics of velocity amplification through pair-wise collisions between multiple masses in a chain, in order to develop very-high acceleration shock-testing machines. A theoretical basis for determining the number and mass of intermediate stages in such a shock amplifier, based on simple rigid body mechanics, is proposed. The influence of mass ratios and the coefficient of restitution on the optimization of the system is identified and investigated. In particular, two cases are examined: in the first, the velocity of the final mass in the chain is maximized by defining the ratio of adjacent masses according to a power law relationship; in the second, the energy transfer efficiency of the system is maximized by choosing the mass ratios such that all masses except the final mass come to rest following impact. Comparisons are drawn between both cases and the results are used in proposing design guidelines. It is shown that for most practical systems, a shock amplifier with mass ratios based on a power law relationship is optimal and can easily yield velocity amplifications of a factor 5-8 times.

Many contemporary innovations in MEMS devices result from the miniaturization of existing macro-scale systems, exploiting changes in physical phenomena with scale. In terms of a system subject to mechanical stimuli, the response of the system changes significantly as scale decreases: in particular, natural frequencies increase and the system can sustain higher acceleration levels without damage. The objective of this paper is to investigate the response of a miniature scale cantilever beam to high-G impact stimuli in order to gain an understanding of its response. The test model is a machined aluminium cantilever-beam simply supported at one end. The beam cross section is 400 × 200 microns and has lengths of 4mm, 10mm and 20mm. The test bed is an Instron Dynatup 9250HV drop table. The beam response under impact is monitored using an IDT X-Stream XS-4 high-speed camera fitted with a telecentric 10x lenses. Theoretical and computer simulated models using ANSYS and LS-DYNA software are developed and compared with experimentally measured data to verify the accuracy of the techniques used to analyse the structural behaviour of the cantilever beams. The use of high-speed imaging when testing devices for short duration events proves to be beneficial for obtaining several data sets not achievable by post processing techniques.

Ionic migration on printed wiring boards (PWBs) involves the transport of metal ions between two copper tracks under bias through an aqueous electrolyte, which results in deposition at the cathode. Dendrite-like deposits are then formed, leading to a short circuit and subsequent failure. In order to assess the effects of moisture condensation, this work compares two extreme tests for ionic migration on PWBs. The two tests are water droplet (WD) and cyclic temperature-relative humidity (20/spl deg/C to 35/spl deg/C, 95%RH maximum, with 21 hr cycle) conditions under a bias of 5 V DC. The cyclic test was conducted over a 21 day period with continuous in-situ monitoring of dendritic growth. Water drop tests were performed in order to draw correlation with cyclic tests. Investigative techniques were conducted to evaluate the migration development on the PWBs after testing using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS).

Elevated heat dissipation and simultaneous reductions in package sizes are well documented for a range of electronics systems. The problem is heightened in portable systems where the space available for the implementation of an active cooling methodology is limited and conventional cooling products are too large. Using micro scale radial flow fans is a potential solution. However, little is known about the aerodynamic effects of reducing the fan scale and therefore Reynolds number to the extent required for typical portable electronic applications. This paper investigates this issue, by quantifying the reduction in aerodynamic performance which accompanies the reductions in scale. To do this, geometrically similar radial flow fans were fabricated with diameters ranging from 80 to 10mm. Measurements of the rotors' geometries are presented, showing a high degree of geometric similarity between the fans. The aerodynamic performance of each of the fans was measured. Non-dimensional performance of each of the larger fans were almost identical, while the performance plot of the smallest fan differed significantly from the others. The paper tentatively concludes that a fundamental change in flow phenomena has emerged in the smallest scale fan which has altered its aerodynamic characteristics.

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.

The major focus of this work was the determination of the nine constants required for Anand's viscoplastic constitutive model for a lead-free solder alloy, 95.5Sn3.8Ag0.7Cu and to compare them with those for SnPb. The test specimen was a cast dog bone shape based on ASTM E 8M-01, with a diameter of 4mm and a gauge length of 20mm. A series of tensile experiments were carried out: constant displacement tests ranging from 6.5 x 10(-5)/S to 1.0 X 10(-3)/s at temperatures of 20 degrees C, 75 degrees C, and 125 degrees C; constant load tests at a range of loads from 10MPa to 65MPa, also at temperatures of 20 degrees C, 75 degrees C, and 125 degrees C. A series of non-linear fitting processes was used to determine the model constants. Comparisons were then made with experimental measurements of the stress-plastic strain curves from constant displacement rate tests: it was found that the model matched the experimental data at low strain rates but did not capture the strain hardening effect, especially at high strain rates. A finite element model of the test was also constructed using ANSYS software. This software includes the Anand model as an option for its range of viscoplastic elements, requiring that the nine constants be input. In this case, an 8-noded axisymmetric element (VISCO108) was used to model the test specimen under constant displacement rate loading. The model was then used to predict the stress-plastic strain curve and this was compared to both the experimental measurements and the fitted Anand model. Reasonable agreement was found between the Anand model and the FE predictions at small strain rates. Finally, a BGA device was simulated under accelerated temperature cycling conditions using ANSYS with the fitted Anand for the SnAgCu solder joints. A Morrow-type fatigue life model was applied using empirical constants from two published sources and good agreement was found between experiment and predicted fatigue life.

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.

The major focus of this work was the experimental determination of the nine constants required for Anand's viscoplastic constitutive model for a lead-free solder alloy, 95.5Sn3.8AgO.7Cu. A series of tensile experiments were carried out: constant displacement rate tests with strain rates ranging from 6.0E-5/s to 1.0E-3/s at temperatures of 20/spl deg/C, 75/spl deg/C, and 125/spl deg/C; constant load tests at a range of loads from 8MPa to 64MPa for the same temperature range. The test specimen was a cast dog bone shape based on the ASTM E 8M-01 standard, with a diameter of 4mm and a gauge length of 20mm. Nonlinear least-squares fitting was used to determine the model constants. Comparisons were then made with experimental measurements of the stress-inelastic strain curves: excellent agreement was found. A finite element model of the test was also constructed using ANSYS 8.1 software. This software includes the Anand model as an option for its range of viscoplastic elements, requiring that the nine constants be input. In this case, an 8-noded axisymmetric element (VISCO108) was used to model the test specimen under constant displacement rate loading. The model was then used to predict the stress-inelastic strain curve under constant displacement rate conditions and this was compared to both the experimental measurements and the fitted Anand model. It was found that the Anand model and finite element predictions matched the experimental stress-inelastic strain curves for small strain rates, but that at higher strain rates the strain hardening behaviour of the solder was not captured accurately. The benchmarking of the ANSYS software showed that the Anand model was being implemented as expected. Using the fitted parameters in an FE model of an electronic component undergoing thermal cycling is likely to give acceptable results as the strain rates in this case are comparatively small.

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.

Ionic migration has been the subject of intensive study, both theoretical and experimental, over the past 40 years. It is known as a reliability concern for printed wiring boards (PWBs) in high density microelectronic packaging and power electronic packaging. Ionic migration is an electrochemical phenomena that occurs primarily under normal ambient conditions: i.e. when the local temperatures and current densities are low enough to allow moisture on the surface. Standardised test 85°C/85%RH is typically used for accelerating and predicting ionic migration failure, however, the possibility of moisture condensation — a prerequisite for ionic migration — at a relatively high temperature and low relative humidity is unlikely. In order to assess more realistic and less thermally severe environments, this work examines prolonged steady state exposure of PWBs. Steady-state conditions of 90%RH at 30°C under a bias of 5V DC were tested over a 210 day period with continuous in-situ monitoring of dendritic growth. Investigative techniques were conducted to evaluate the migration development on the PWBs after testing using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). This paper will demonstrate that steady-state thermal humidity bias (THB) tests appear to provide ionic migration behaviour similar in service conditions, however, do not demonstrate the dramatic failure associated with ionic migration.

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 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 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 lead-free and eutectic solders for various drop heights. Microsections of the failed interconnects were obtained to determine the failure mechanisms for a range of drop heights. The life test data presented in this paper suggests that for board level drop testing different failure mechanisms can occur at different stress levels and that there is a considerable difference between lead-free solder characteristic life and tin-lead (SAC) solder characteristic life.

Micro-scale polymerase chain reaction (micro-PCR) systems offer substantial advantages over macro-scale systems. Smaller sample volumes are required, and faster process times are feasible. Thermal control of micro-PCR systems is a substantial technical challenge, however. The PCR process requires the fluid sample to be cycled through three temperature ranges — typically 90–95°C, 50–65°C and 72–77°C for denaturation, hybridisation and replication respectively. Durations of the three steps are required to be in the ratio of 4:4:9. In this paper, the thermal analysis of a continuous flow micro-PCR device is reported. The objective of the analysis is to optimize the thermal performance of the device for fast amplification cycles with high efficiency - an efficient PCR features rapid heating and cooling between steps, and good temperature uniformity within each step. The device comprises an array of parallel microchannels formed within a polypropylene substrate to carry fluid, with the base of the substrate mounted on an aluminium carrier. Substrate depth is 500 micron, and each channel is 60 micron wide by 40 micron deep. Thermoelectric cells (TECs) are bonded to the carrier, and powered by a thermoelectric controller with feedback from sensors embedded in the carrier. A Pyrex Glass slide is bonded to the substrate to form closed channels. Arrays of film heaters mounted on the slide adjacent to the channel are used to establish the required temperature regions along the channel. By pumping the fluid at a fixed flow rate, temperature cycling of specific period is achieved. Thermal analysis of the substrate is performed using an approximate closed-form solution, in conjunction with Finite Element (FE) and Computational Fluid Dynamics (CFD) simulations. The analysis is used to conduct a parametric study in order to determine the optimum configurations of substrate materials, cooling conditions, heaters and flow rates required to impose specific temperature cycles. The use of thermoelectric cells is shown to increase the rate of change of temperature between the various regions, improving the efficiency and decreasing the cycle time of the PCR process. Cycle times of 6s or less are shown to be feasible, yielding benefits in time saved for multiple amplifications. Finally, the analysis is also used to identify the dimensionless parameters which govern the thermal characteristics of the device, illustrating the importance of the Biot number.

A European Union ban on lead in most electrical and electronic equipment will be imposed as of July 1st 2006. The ban, along with market pressures, means that manufacturers must transfer from a tin-lead soldering process to a lead-free process. In this paper the implications on the surface mount (SMT) soldering process are presented. A set of experiments was conducted to investigate the screen-printing and reflow steps of the SMT process using a tin-silver-copper (95.5Sn3.8Ag0.7Cu) solder and a baseline of standard tin-lead (63Sn37Pb). 10×10 arrays of micro Ball Grid Array (micro-BGA) components mounted on 8-layer FR4 printed wiring boards (PWBs) were used. The screen-printing experiment addressed 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, peak and cool down temperatures, and conveyor speed. The resulting solder joints were evaluated using cross-section analysis and x-ray techniques in order to determine the presence of defects. A mechanical fatigue test was also carried out in order to compare the strength of the solder joints. The overall quality of the lead-free solder joints was determined from these tests and compared to that of tin-lead. The outcome is a set of manufacturing guidelines for transferring to lead-free solder including optimum screen-printer and reflow oven settings for use with an SnAgCu solder.

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.

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.