Surface Mount Assembly Research Articles

Neurofuzzy modelling of the reflow thermal profile for surface mount assembly

Troubleshooting and production set-up time are often the major contributors for productivity loss in surface mount assembly. The solder reflow process is one of the least understood process segments. The research proposes a neurofuzzy reflow thermal profile control system for a convection oven. An incompatible temperature profile can result in a poor soldering quality and reduced system throughput. This paper proposes a neurofuzzy-based solder reflow thermal profile control system that responds to facilitate the production set-up process. The model is trained and constructed using data from both an experimental design and from historical production records. Customized computer code is used to generate a user-friendly human–machine interface and to link between neurofuzzy-reasoning rules and reflow oven set-up parameters for thermal profile planning and control. Empirical results illustrate the effectiveness and efficiency of the proposed system to solve a practical application.

Development of an integrated reflow soldering control system using incremental hybrid process knowledge

In this paper, we present an industrial application of intelligent knowledge-based reflow soldering control system to economically determine process parameters and assess the outputs for surface mount assembly. The system performance of surface mounting is significantly affected by three major interconnected process segments (including solder paste application, component placement, and solder reflow) and in combination. Many surface mount manufacturers have suffered both quality and productivity losses due to the lacks of effective line setup procedures, rapid process troubleshooting, and appropriate corrective action. This proposed system develops an integrated process control scheme, the framework of hybrid process knowledge extraction through neuro-fuzzy data training, and a knowledge-based system with a GUI man-machine interface for predicting and controlling the given system performance. The efficiency and effectiveness of the proposed system are illustrated using DPMO measurement and productivity analysis.

Characterization of the Solder Paste Release From Small Stencil Apertures in the Stencil Printing Process

Stencil printing is a critical first step in surface mount assembly. However, its robustness can be called into question because of the fact that about 50% or more of the defects found in the assembly of printed circuit boards (PCBs) are attributed to stencil printing 1. Manufacturing techniques for the assembly of certain flip chips, chip scale packages, 0201s, and fine pitch ball grid arrays are testing the limits of current stencil printing capabilities. This paper focuses on understanding the release of solder paste from the stencil, based on experimental and modeling approaches. The primary goal of the study is to characterize the performance of various aperture sizes and geometries based on release efficiencies and to compare them to predictions. The resulting model validation helps us better understand the print process for small features and offers options for increasing print yields. The study is divided into two phases. The first phase examines the release performance of various solder pastes from a variety of aperture sizes and geometries. The focus of this study is a comparison of square versus circular apertures when the nominal volume of paste to be deposited is kept constant. The second phase consists of developing a model that predicts paste-release efficiencies from small apertures and validating the model with experimental results.

A neural network-based prediction model for fine pitch stencil-printing quality in surface mount assembly

The soldering problems in surface mount assembly can represent considerable production cost increases and yield loss. About 60% of the soldering defect problems can be attributed to the solder paste stencil printing process. This paper proposes to solve a solder-paste stencil-printing quality problem by a neural network approach. Employment of a neuro-computing approach allows multiple inputs to the generation of multiple outputs. In this study, the inputs are composed of eight important factors in modeling the nonlinear behavior of the stencil-printing process for predicting deposited paste volumes. A 3 8-3 fractional factorial experimental design is conducted to efficiently collect structured data used for neural network training and testing. The results show that the proposed neural-network model is effective in solving a practical application.

A neurofuzzy-based quality-control system for fine pitch stencil printing process in surface mount assembly

Surface mount assembly defect problems can cause significant production-time losses. About 60% of surface mount assembly defects can be attributed to the solder paste stencil printing process. This paper proposes a neurofuzzy-based quality-control system for the fine pitch stencil printing process. The neurofuzzy approach is used to model the nonlinear behavior of the stencil printing process. Eight control variables are defined for process planning and control, including stencil thickness, component pitch, aperture area, snap-off height, squeegee speed, squeegee pressure, solder paste viscosity, and solder paste particle size. The response variables are the volume and height of solder paste deposited. The values of the response variables provide indicators for identifying potential quality problems. A 38–3 fractional factorial experimental design is conducted to collect structured data to augment those collected from the production line for neurofuzzy learning and modeling. Visual basic programming language is then used for both rule retrieval and graphical-user-interface modeling. The effectiveness of the proposed system is illustrated through a real-world application.

A new phase for vapour‐phase – an overview of the current status of vapour phase technology for soldering and related applications

This paper focuses on the renewed interest in the use of condensation, or vapour‐phase, soldering in lead‐free surface mount assembly soldering and notes also the corresponding legislation, which will preclude the use of lead‐based solders in most applications by 2006. The process is once again becoming popular, because of the absolute control of soldering temperature, and because high‐quality batch and in‐line equipment is now available. It notes that condensation heating with perfluorocarbon liquids and present well‐designed equipment can be used not only in the mass‐assembly of electronic circuit boards, but also for a large variety of other heating tasks in many industries.

A surface-mounted device assembly technique for small optics based on laser reflow soldering

The three-dimensional miniaturized optical surface-mounted device (TRIMO-SMD) is a new flexible and automated assembly technique for small optical components (maximum diameter for a lens is 2mm) based on laser reflow soldering technique. This technology can be compared to the electronic SMD technique but applied to micro-optical devices. We present some recent developments of TRIMO-SMD in its application to industrial products.

Application of numerical analysis to the optimisation of electronic component reliability screening and assembly processes

This study investigates the prediction of transient board-mounted electronic component heat transfer in environments representative of reliability screening and surface-mount assembly processes using computational fluid dynamics (CFD) analysis. The test cases are based on a board-mounted 160-lead Plastic Quad Flat Pack (PQFP) component, analysed in still-air and a range of forced airflows. For reliability screening, three types of transient operating conditions are considered, namely (i) component dynamic power dissipation in fixed ambient conditions, (ii) passive component operation in dynamic ambient conditions, and (iii) combined component dynamic power dissipation in varying ambient conditions. For surface-mount assembly, the test vehicle is exposed to a typical solder reflow thermal profile. Benchmark criteria are based on component junction temperature and component-printed circuit board (PCB) surface temperature, measured using thermal test dies and infrared thermography, respectively. Component/PCB numerical modelling is based on nominal geometry dimensions and material properties. Overall, component transient thermal behaviour is found to be accurately predicted, suggesting that CFD analysis could play an important role in providing critical boundary conditions to determine accelerated life testing parameters and optimise new assembly processes.

Critical Variables of Solder Paste Stencil Printing for Micro-BGA and Fine-Pitch QFP

Stencil printing continues to be the dominant method of solder deposition in high-volume surface-mount assembly. Control of the amount of solder paste deposited is critical in the case of fine-pitch and ultrafine-pitch surface-mount assembly. The process is still not well understood as indicated by the fact that industry reports 52-71% surface-mount technology (SMT) defects are related to the solder paste stencil printing process. The purpose of this paper is to identify the critical variables that influence the volume, area, and height of solder paste deposited. An experiment was conducted to investigate the effects of relevant process parameters on the amount of solder paste deposited for ball grid arrays (BGAs) and quad flat packages (QFPs) of five different pitches ranging from 0.76 mm (30 mil) to 0.3 mm (12 mil). The effects of aperture size, aperture shape, board finish, stencil thickness, solder type, and print speed were examined. The deposited solder paste was measured by an inline fully automatic laser-based three-dimensional (3-D) triangulation solder paste inspection system. Analysis of variance (ANOVA) shows that aperture size and stencil thickness are the two most critical variables. A linear relationship between transfer ratio (defined as the ratio of the deposited paste volume to the stencil aperture volume) and area ratio (defined as the ratio of the area of the aperture opening to the area of the aperture wall) is proposed. The analysis indicates that the selection of a proper stencil thickness is the key to controlling the amount of solder paste deposited, and that the selection of maximum stencil thickness should be based on the area ratio. The experimental results are shown to be consistent with a theoretical model, which are also described.

CAPP for Electronics Manufacturing Case Study: Fine Pitch SMT Laser Soldering

The trend to high-density packaging in surface mount assembly has highlighted inherent difficulties in the assembly line of these very fine pitch devices. A variety of defects is still common in printed circuit board assembly (PCBA) technology despite all the improvements made, and rework of PCBA is inevitable and performed manually in PCBA manufacturing floors since the cost of each component and PCB itself may be hundreds of dollars. In the last few years, the number of rework stations available on the market has grown considerably including the automated ones, but there has still been no significant reduction in the number of the defects. In electronics manufacturing, rework is defined as the activity that replaces defective components with those that are acceptable such that the populated board performs to the specifications. Increasing product complexity, decreasing component size, and using double sided boards have made rework more difficult and the economic reworking of PCBAs is one of the main problems facing PCB manufacturers. PCBA manufacturing has been relatively improved with fully automated, accurate assembly machines and the use of robots. Although significant improvement in automated rework has also been made by the author, it has been shown that the outcomes of the automated rework line have not produced a high enough level of the reliable yield percentage. The objective of this research project is to make a contribution towards this surface mount rework by creating a Computer Aided Process Planning (CAPP) system of circuit board defects so that the defects that necessitate rework operation can be detected through the electronics manufacturing assembly line. With the development of CAPP tool, the rework will be removed from the PCBA line and PCBA process parameters which cause joint level reliability problems will be troubleshot with the help of the developed system.

Package to Board Interconnection Shear Strength (PBISS): Effect of Surface Finish, PWB Build-Up Layer and Chip Scale Package Structure

The quality of the interconnection in a fine pitch, area array chip scale package (CSP) is evaluated at the system level using a new test method, the package to board interconnection shear strength (PBISS) technique. The influence of printed wiring board (PWB) sample finish, build-up layer and package structure are quantified after surface mount assembly. Clear differences were evident in the shear strength and fracture location data indicating that the PBISS method is sensitive to the presence of black pad, weak build-up layer and package stiffness. The PBISS value of the CSPs with OSP/RCC-FR4 pad/build-up layer combination were measured to be 37 /spl plusmn/ 6 MPa and 38 /spl plusmn/ 2 MPa for the two different structures investigated. The PBISS method is demonstrated to be a viable candidate technique to quantify interconnection quality at the system level due to issues such as black pad, etc. and this method can help identify weaknesses in the interconnection chain for effective assessment of supplier or product quality.

A novel measurement technique for stencil printed solder paste

The printing process is the most critical step in surface mount assembly. To control and monitor printing, complex instrumentation is available for measuring the print consistency. This paper describes a novel robust method, which introduces critical variables defining print quality. Conventional measurement methods include only the volume and average height of solder paste. The limitations of using just these parameters are discussed and a new method is introduced. In our novel approach, the matrix is levelled first to take account of any tilt in the plane of the substrate surface. Then pseudo‐virtual object parameters are calculated. Major attributes of the proposed measurement method is its insensitivity to both the specific location of the printed deposit and the location of a specific defect. This algorithm, coupled with variables defined in the analysis package, delivers a new and flexible approach to the printed media.

Application of Fuzzy C-means Algorithm in SMT Assembly Grouping Problem

The work in this paper is mainly on the grouping of printed circuit boards (PCBs) for Surface Mount Assembly in the electronics industry. The arrangement of PCBs among several surface mount machine lines is a typical kind of group technology (GT) problem. From literature, there are various clustering techniques developed to solve the clustering problems. In this paper, fuzzy c-means clustering (FCM) is used to solve the PCB grouping problem. The FCM is applied in a real problem and the result shows that there should be a systematic method to arrange the scheduling of PCB assemblies in electronic industry to improve the operations planning process.

Process monitoring strategies for surface mount manufacturing processes

Establishing reliable surface mount assemblies requires robust design and assembly practices, including stringent process control schemes for achieving high yield processes and high quality solder interconnects. Conventional Shewhart-based process control charts prevalent in today's complex surface mount manufacturing processes are found to be inadequate as a result of autocorrelation, high false alarm probability, and inability to detect process deterioration. Hence, new strategies are needed to circumvent the shortcomings of traditional process control techniques. In this article, the adequacy of Shewhart models in a surface mount manufacturing environment is examined and some alternative solutions and strategies for process monitoring are discussed. For modeling solder paste deposition process data, a time series analysis based on neural network models is highly desirable for both controllability and predictability. In particular, neural networks can be trained to model the autocorrelated time series, learn historical process behavior, and forecast future process performance with low prediction errors. This forecasting ability is especially useful for early detection of solder paste deterioration, so that timely remedial actions can be taken, minimizing the impact on subsequent yields of downstream processes. As for the automated component placement process where very low fraction nonconforming frequently occurs, control-charting schemes based on cumulative counts of conforming items produced prior to detection of nonconforming items is more sensitive in flagging process deterioration. For the reflow soldering and wave-soldering processes, the use of demerit control charts is appealing as it provides not only better control when various defects with a different degree of severity are encountered, but also leads to an improved ARL performance. Illustrative examples of actual process data are presented to demonstrate these approaches.

Microstructural modifications and properties of Sn-Ag-Cu solder joints induced by alloying

Slow cooling (1–3°C/sec) of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens, made by hand soldering, simulated reflow in a surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shear-strength testing, using Sn-3.5Ag (wt.%) as a baseline. Consistent with predictions from a recent Sn-Ag-Cu ternary phase-diagram study, either Sn dendrites, Ag3Sn primary phase, or Cu6Sn5 primary phase were formed during solidification of joint samples made from the selected near-eutectic Sn-Ag-Cu alloys. Minor substitution of Co for Cu in Sn-3.7Ag-0.9Cu refined the joint-matrix microstructure by an apparent catalysis effect on the Cu6Sn5 phase, whereas Fe substitution promoted extreme refinement of the Sn-dendritic phase. Ambient-temperature shear strength was reduced by Sn dendrites in the joint microstructure, especially coarse dendrites in solute poor Sn-Ag-Cu, e.g., Sn-3.0Ag-0.5Cu, while Sn-3.7Ag-0.9Cu with Co and Fe additions have increased shear strength. At elevated (150°C) temperature, no significant difference exists between the maximum shear-strength values of all of the alloys studied.

Modeling and implementation of a neurofuzzy system for surface mount assembly defect prediction and control

A high-speed surface mount assembly can reduce both production cost and time; however, it could allow an enormous number of boards to be built before a problem is detected. Therefore, early detection and assessment of a surface mount assembly problem is critical for cost-effective manufacturing. This paper proposes a neurofuzzy system for surface mount assembly defect prediction and control. Hybrid data from both in-process quality control database and from a fractional factorial experimental design are collected for neurofuzzy learning and modeling. Customized programming codes are generated for rule retrieval and for graphical user interface modeling. The proposed system is successfully implemented at a surface mount assembly plant, ll significantly improves plant throughput by the downtime reduction that is a result of a better defect prediction and control.

Sn-Ag-Cu solders and solder joints: Alloy development, microstructure, and properties

Slow cooling of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens made by hand soldering simulated reflow in surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shearstrength testing, using Sn-3.5Ag (wt.%) as a baseline. Minor substitutions of either cobalt or iron for copper in Sn-3.7Ag-0.9Cu refined the joint matrix microstructure, modified the Cu6Sn5 intermetallic phase at the copper substrate/solder interface, and increased the shear strength. At elevated (150°C) temperature, no significant difference in shear strength was found in all of the alloys studied. Ambient temperature shear strength was reduced by largescale tin dendrites in the joint microstructure, especially by the coarse dendrites in solute poor Sn-Ag-Cu.

Understanding the process window for printing lead-free solder pastes

Solder paste is primarily used as a bonding medium for surface mount assemblies (SMA) in the electronics industry, and is typically deposited using the stencil printing process. Stencil printing is a very important and critical stage in the reflow soldering of surface mount devices, and a high proportion of all SMA defects are related to this process. This is likely to continue with the drive toward the introduction of lead-free solder pastes. Work is continuing on the metallurgical properties of these lead-free solders, including solder joint strength and material compatibility. However, the initial challenge for the new Pb-free formulations is in achieving repeatable solder deposit from print to print and from pad to pad. To meet this challenge, new flux formulations are being developed. For a smooth transition to Pb-free soldering formulations, a proper understanding of the solder paste printing performance is necessary. The key parameters that affect solder paste printing have been identified and are the subject of numerous studies. In lead-free solder paste, the replacement of lead with other elements (including Bi, Cu) changes the density of this dense suspension. In this paper, we investigate the effects of printer parameters, i.e. squeegee speed and pressure (defined as the process window) on the printing performance of a variety of lead-free solder pastes. A three-level design of experiment on these factors was used. Comparisons are presented with lead-rich solder pastes. The metal content of the lead-free solders had a significant effect on the process window.

Characterization of the melting and wetting of Sn-Ag-X solders

There is tremendous interest at present with Pb-free solder assembly in the surface mount assembly industry in response to recent Japanese and European initiatives and proposed governmental restrictions regarding Pb usage and disposal. Many different solder alloys have been proposed as potential Pb-free solder replacements and the most promising of these fall into the general alloy families of tin-silver (Sn-Ag), tin-silver-copper (Sn-Ag-Cu) and tin-silver-bismuth (Sn-Ag-Bi). Published melting point data on some of these alloys indicates that they should be capable of reduced reflow temperatures relative to the commonly available Sn-3.5Ag alloy, which melts at 221/spl deg/C. Differential scanning calorimetry (DSC) and reflow visualization was used to characterize the melting and wetting of the Pb-free alloys and generate the practical reflow temperature requirements. This was compared to the DSC data to gain insight on the meaning of the DSC melting data for surface mount applications. The results show that, in general, the wetting performance of the Sn-Ag-Bi alloys are more similar to Sn-Ag and Sn-Ag-Cu than would be predicted by the major onset melting temperature data as measured by the DSC.

The effect of power cycling on the reliability of lead-free surface mount assemblies

A power cycling in-test monitoring system has been constructed to test the reliability of eight different lead-free surface mount assemblies. The assemblies included SnAg3.8Cu0.7 and SnAg3.8Cu0.7X solder joints on OSP-Cu (organic solderability preservative on Cu), electroless NiAu, immersion Ag and immersion Sn board metallizations. One Pb containing assembly, Sn62PbAg2 on OSP-Cu board metallization, was included for comparison. The components on the assemblies were 1206 resistors (Sn100 metallization on the end terminals) and 100 lead QFP with gullwing leads (SnPb15 metallization). All assemblies experienced up to 5000 power cycles of ambient to 100/spl deg/C with a 15 min dwell at each temperature. Solder joint reliability was evaluated by monitoring electrical resistance after each power cycle and examining mechanical strength and microstructure with number of power cycles. The 1206 resistors on four of the assemblies (one of which was the Pb containing assembly) exhibited electrical resistance increases after 4000 power cycles. All resistor samples decreased in strength by more than 70% at 5000 cycles and cracks appeared after 1000 power cycles. All gullwing lead solder joints exhibited good reliability over 5000 power cycles, with no resistance increase and no strength reduction. Small cracks appeared after 3000 power cycles.