Reflow Soldering Process Research Articles

A Development of Welding Tips for the Reflow Soldering Process Based on Multiphysics

A reflow soldering process (RSP) is generally implemented in advanced manufacturing factories for welding small electronic components together to create a product using heat generated at the welding tip (WT). Improper WT design and operating conditions may lead to defects in some products; therefore, optimizing both is immensely significant in developing the RSP. Accordingly, this article proposes a successful RSP development based on multiphysics in a hard disk drive factory consisting of transient thermal-electric and structural simulations. First, a new shape series WT was designed, and a conventional shape, parallel WT, was considered as a case study. Then, they were assembled and experimented with the RSP actual operating conditions to collect essential data. Next, the heat transfer was determined using a transient thermal-electric simulation (TES). The simulation results showed uneven WT temperatures depending on applied voltages, time, and shapes, which were consistent with the experimental results. The higher the applied voltage, the greater the temperature generated at the WT. Finally, after using TES results as loads, the structural simulation showed WT total deformations, which could be consistent with actually occurring defects. The findings from this research are a new design of series WT and proper multiphysics methodology for developing the RSP.

Laser-Assisted Micro-Solder Bumping for Copper and Nickel-Gold Pad Finish.

Flip-chip bonding is a key packaging technology to achieve the smallest form factor possible. Using copper as a direct under-bump metal and performing bonding under little force and at a low temperature eliminates the processing step for the deposition of a suitable wetting metal and offers an economical solution for electronic chip packaging. In this paper, various samples with copper and nickel–gold surface finishes are used to apply an in-house solder bumping, flip-chip bonding and reflow process to exhibit the bump-bond feasibility. Native oxides are reduced using process gases, and copper surface protection and solder wetting are achieved using copper formate. Lead-free 40 µm solder balls were bumped on 80 µm copper pads and 120 µm copper pillars to demonstrate a full intermetallic Cu–Cu bond as a base study for stacking applications. Using a low-force bonding technique, various chips with different dimensions were bonded at 0.5–16 MPa, followed by a reflow step at a maximum temperature of 270 °C. Then, 30 µm solder balls are utilized to bump the samples with NiAu and Cu bond pads at 50 µm pitch. A mean shear strength of 44 MPa was obtained for the 30 µm Cu samples. To the best of our knowledge, 30 µm solder bumping directly on the copper pads by producing copper formate is a novel research contribution.

Reliability Sensibility Analysis of the PCB Assembly concerning Warpage during the Reflow Soldering Process

As a vital component of electronic products, the quality of the printed circuit board (PCB) assembly directly affects the applications and service life. During the reflow soldering process, the PCB assembly may suffer excessive warpage. Therefore, in order to avoid such a failure, this paper conducts the finite element simulation of the PCB assembly temperature change and the deformation process according to a theoretical model for heat transfer and then explores the causes of warpage and predicts the percentage of warpage that occurs during the reflow soldering. A reliability sensitivity analysis is creatively applied in the PCB assembly warpage study in order to uncover the quantitative influence of multiple factors with random errors on warpage. Moreover, the reliability calculation provides a theoretical basis for the allowable error range of geometric dimensions, material properties and process parameters of the PCB assembly. This research has a remarkable significance and is valuable to engineering in promoting quality and reliability, and increasing the yield of PCB assembly in mass production.

Toward reducing no-clean flux spatter during reflow soldering: Investigating the effect of flux type, solder mask, and solder pad design

This comprehensive study dealt with the reliability issue of no-clean flux spattering from the solder paste during the reflow soldering process. Several factors entering the process were investigated: the type of flux in the solder paste (ROL0/ROL1 according to the IPC J-STD-004B standard), the type of solder mask (liquid photo imageable/directly printed), and the type of solder pad design (copper-defined/solder mask-defined). A unique design of the test board was developed for a proper evaluation of flux spatter spots spattered from the solder paste. The testing board included real-sized soldering pads and a boron-silicate glass slide as a target area, ensuring the same evaluation process for all samples. The results showed a strong influence of flux chemistry on spattering. Changing the flux from ROL1 to ROL0 reduced the flux spot occurrence by 62 % on average. The type of solder pad design also slightly affected the flux spattering. No significant difference was found between the used solder masks; therefore, it is possible to use the novel, directly printed one without increasing the risk of flux spattering.

A review on numerical approach of reflow soldering process for copper pillar technology

A review on numerical approach of reflow soldering process for copper pillar technology

Micromechanical response of SAC305 solder alloy under gamma radiation via nanoindentation approach

PurposeThis paper aims to investigate the effect of different doses of gamma radiation on the micromechanical response (hardness properties and creep behaviour) of 96.5Sn-3.0Ag-0.5Cu (SAC305) solder alloys.Design/methodology/approachSAC305 solder pastes deposited on printed circuit boards (PCBs) were subjected to a reflow soldering process to form soldered samples. The soldered samples were irradiated with a gamma source at different doses (5–50 Gy). Nanoindentation testing was used to determine the hardness properties and creep behaviour after gamma irradiation.FindingsThe results showed that the hardness of SAC305 solder alloys gradually increased up to 15 Gy and then gradually decreased to 50 Gy of gamma irradiation. The highest hardness value (0.37 GPa) was observed on SAC305 solder alloys exposed to 15 Gy irradiation. Hardening of SAC305 solder alloy was suggested to be due to the high defect density induced by the gamma irradiation. Meanwhile, exposure to 50 Gy irradiation resulted in the lowest hardness value, 0.13 GPa. The softening behaviour of SAC305 solder alloy was probably due to the evolution of defect size in the solder joint. In addition, the creep behaviour of the SAC305 solder alloys changed significantly with different gamma irradiation doses. The creep rates were higher at a dose of 10 Gy up to a dose of 50 Gy. Gamma irradiation caused the SAC305 solder alloy to become more ductile compared to the non-irradiated alloy. The stress exponent also showed different deformation mechanisms with varying gamma doses.Originality/valueResearch into the micromechanical properties of solder alloys subjected to gamma irradiation has rarely been reported, especially for Sn-Ag-Cu lead-free solder. Thus, this research provides a fundamental understanding of the micromechanical response (hardness and creep behaviour) of solder, especially lead-free solder alloy, to gamma irradiation.

Effect of alloy particle size and stencil aperture shape on solder printing quality

PurposeReflow solder joint quality is significantly affected by the ability of the solder to perfectly fill pad space and retain good solder joint shape. This study aims to investigate solder joint quality by quantitatively analyzing the stencil printing-deposited solder volume, solder height and solder coverage area.Design/methodology/approachThe dispensability of different solder paste types on printed circuit board (PCB) pads using different stencil aperture shapes was evaluated. Lead-free Type 4 (20–38 µm particle size) and Type 5 (15–25 µm particle size) solder pastes were used to create solder joints according to standard reflow soldering.FindingsThe results show that the stencil aperture shape greatly affects the solder joint quality as compared with the type of solder paste. These investigations allow the development of new strategies for solving solder paste stencil printing issues and evaluating the quality of solder joints.Originality/valueThe reflow soldering process requires the appropriate selection of the stencil aperture shape according to the PCB and the solder paste according to the particle-size distribution of the solder alloy powder. However, there are scarce studies on the effects of stencil aperture shape and the solder alloy particle size on the solder paste space-filling ability.

Compact numerical modelling of transient condensate layer formation on small components during vapour phase soldering

In this paper transient condensation and heat transfer been investigated on a printed circuit board (PCB) and a surface mounted (SMD) capacitor during a vapour phase reflow soldering (VPS) process. Classic VPS processes provide rapid heating with high heat transfer coefficient rates due to condensation that forms a continuous liquid film on the surfaces. Our aim was to develop a simple and fast computable method that, instead of relying on static descriptions, can reproduce the transient dynamics of the process. It was defined as an inverse heat conduction problem (IHCP) and proposed a solution based on finite difference modelling (FDM) with an alternating direction implicit (ADI) scheme. The layer dynamic was described with a parametric differential equation, and it was coupled to the concentrated and distributed thermal models of the PCB and the capacitor. The parameter values were determined by fitting to measured data. It was found that the heat transfer coefficient is inversely proportional to the layer thickness, and it changes over time according to the dynamic properties of layer growth. It was corroborated that the heat transfer depends on the geometry and thermal properties of the heated surfaces in VPS ovens. The presented method shows good agreement with the validation data. The new method provides a significant update over previous modelling approaches describing the condensation-based heating of the VPS process.

Study on warpage of the AlSiC substrate in reflow soldering process of the IGBT module

Using the pre-warped substrate to pack IGBT modules can increase the contact area between the substrate and the radiator after the reflow soldering. There are many factors affecting the shape of the substrate’s back after soldering, and determining the effect of each factor by experiments would cost significantly. In this paper, a FEA model of the IGBT module’s reflow soldering process is established by ANSYS to predict the back surface of the pre-warped AlSiC substrate after soldering. Initially, in order to improve the accuracy of the FEA model, the material properties of AlSiC were determined at different temperatures. Subsequently, based on the result of DSC, the solder’s phase transition was simulated in the model. The calculated results showed that the proposed FEA model could predict the shape feature of the AlSiC pre-warped substrate’s back surface after the reflow soldering, and the prediction error of substrate’s residual warpage was 46.05%. The model provides a reliable reference for the design of pre-warped substrate in IGBT module, reducing the test cost and shortening the design period substantially.

Smarter Temperature Setup for Reflow Oven to Minimize Temperature Variation Among Components

The thermomechanical reliability of surface mount components (SMCs) is closely related to the reflow soldering process. With the rise of board complexity, an optimal reflow profile must consider the mix of large and small components, the density of the components on the board, and the board dimensions. It has been a significant challenge to achieve uniform temperature on SMCs with different sizes and different thermal masses, such as capacitors, transistors, quad-flat no-leads (QFN), and ball grid array (BGA) packages. To reflow large components such as BGA packages, the minimum reflow temperature should be reached, meanwhile, the temperature should not exceed the damaging temperature of the smaller or heat-sensitive components. The recommended reflow profile is defined as a band or process window. To achieve good quality soldering, the temperature applied to the solder under varied sizes of components should be within the recommended band. This article introduces a numerical method based on a compact model to predict the reflow profile for bulky BGA packages. The impinging gas temperature is used as the boundary condition to predict the temperature distribution throughout a board with different sized components. The proper oven temperature setting to achieve the ideal profiles is obtained finally.

An intelligent system for reflow oven temperature settings based on hybrid physics-machine learning model

PurposeThis paper aims to provide the proper preset temperatures of the convection reflow oven when reflowing a printed circuit board (PCB) assembly with varied sizes of components simultaneously.Design/methodology/approachIn this study, computational fluid dynamics modeling is used to simulate the reflow soldering process. The training data provided to the machine learning (ML) model is generated from a programmed system based on the physics model. Support vector regression and an artificial neural network are used to validate the accuracy of ML models.FindingsIntegrated physical and ML models synergistically can accurately predict reflow profiles of solder joints and alleviate the expense of repeated trials. Using this system, the reflow oven temperature settings to achieve the desired reflow profile can be obtained at substantially reduced computation cost.Practical implicationsThe prediction of the reflow profile subjected to varied temperature settings of the reflow oven is beneficial to process engineers when reflowing bulky components. The study of reflowing a new PCB assembly can be started at the early stage of board design with no need for a physical profiling board prototype.Originality/valueThis study provides a smart solution to determine the optimal preset temperatures of the reflow oven, which is usually relied on experience. The hybrid physics–ML model providing accurate prediction with the significantly reduced expense is used in this application for the first time.

Interfacial Reaction between SAC3807 Lead-Free Solders and Different Copper Substrate via Reflow Soldering Process

The different composition material of copper substrate significantly affects the intermetallic compound (IMC) formation and the solder joints durability. This study was conducted on the interfacial reaction between lead-free solder and the different copper substrates via reflow soldering. The selected substrate is copper (Cu) and copper-beryllium (Cu-Be). The lead-free solder involved is Sn-3.8Ag-0.7Cu (SAC3807) solder ball with a diameter of 700 μm. All the samples were subjected to the isothermal aging process. The material characterization and analysis on the IMC formation were examined by scanning electron microscopy (SEM), optical microscope (OM), and energy dispersive X-ray analysis (EDX). After the reflow process, the result revealed that Cu3Sn, Cu6Sn5 IMC layer formed at SAC3870/Cu and SAC3870/Cu-Be interface. The changes to a rod-like shape Cu6Sn5 and irregular needle-shaped Cu3Sn4 occur after the aging treatment on SAC3870/ Cu. Meanwhile, the IMC layer for SAC3870/Cu-Be shows a rod-like shape transformed into a blocky-like shape Cu6Sn5 and Cu3Sn4 diamond-shape. This result indicates that Ag3Sn nanosized was formed on the intermetallic surface during the aging process for both SAC3807/Cu and SAC3807/Cu-Be. The Ag3Sn nanosized element at SAC3807/Cu-Be is many compared to SAC3807/Cu. In addition, IMC thickness for SAC3807/Cu-Be shows a thicker layer than SAC3807/Cu. Lastly, in this research, the element of Be in SAC3807/Cu-Be cannot be defined because the beryllium element is not easily detected as the percentage was very low.

In Situ Observation of Liquid Solder Alloys and Solid Substrate Reactions Using High-Voltage Transmission Electron Microscopy.

The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilizing creative setups to “freeze” the reactions at different stages during the reflow soldering process. However, full understanding of the dynamics of the process is difficult due to the lack of direct observation at micro- and nano-meter resolutions. In this study, high voltage transmission electron microscopy (HV-TEM) is employed to observe the morphological changes that occur in Cu6Sn5 between a Sn-3.0 wt%Ag-0.5 wt%Cu (SAC305) solder alloy and a Cu substrate in situ at temperatures above the solidus of the alloy. This enables the continuous surveillance of rapid grain boundary movements of Cu6Sn5 during soldering and increases the fundamental understanding of reaction mechanisms in solder solid/liquid interfaces.

Understanding the Effect of Reflow Profile on the Metallurgical Properties of Tin–Bismuth Solders

Sn–Bi alloys are desirable candidates for soldering components on printed circuit boards (PCBs) because of their low melting point and reduced cost. While certain tin–bismuth solders are well characterized many new alloys in this family have been developed which need proper characterization. The following study looks at the behavior of four different Sn–Bi alloys—traditional 42Sn58Bi and 42Sn57Bi1Ag and two new tin–bismuth alloys—in solder paste during the reflow soldering process. Each alloy was processed using different reflow profiles that had varying times above liquidus (TALs) and peak temperatures. The PCBs were then analyzed to see how the processing variables influenced wetting, voiding, microstructure, intermetallic layer composition, and thickness. After analysis, the PCBs were then subjected to thermal cycling experiments to see how reflow profile impacted microstructure evolution. The results demonstrated that reflow profile affects properties such as metal wetting and voiding. It does not however, greatly impact key metallurgical properties such as intermetallic layer thickness.

Investigation of solder beading phenomenon under surface-mounted electrolytic capacitors

PurposeThe purpose of this paper is to study the solder beading phenomenon (referring to larger-sized solder balls) of surface-mounted electrolytic capacitors. Solder beading could induce failures by violating the minimal electrical clearance on the printed circuit board (PCB). In modern lead-free reflow soldering, especially in high-reliability industries, such as automotive, aeroplane and aerospace, detecting and preventing such defects is essential in reliable and cost-effective manufacturing.Design/methodology/approachThe large size of the involved components may block the view of automatic optical inspection; therefore, X-ray inspection is necessary. To detect the failure mode, X-ray imaging, cross-section grinding, optical microscopy and Fourier transformed infrared spectroscopy were used. High-resolution noncontact profilometry and optical microscopy were used to analyse component designs. The surface mounting process steps were also analysed to reveal their dependence on the issue. Test methods were designed and performed to reveal the behaviour of the solder paste (SP) during the reflow soldering process and to emphasise the component design relevance.FindingsIt was found that the reduction of SP volume only reduces the failure rate but does not solve the problem. Results show that excessive component placement pressure could induce solder beading. Statistical analysis revealed that differences between distinct components had the highest effect on the solder beading rate. Design aspects of solder beading-prone components were identified and discussed as the primary source of the problem.Practical implicationsThe findings can be applied in surface-mount technology production, where the total failure count and resulting failure costs could be reduced according to the findings.Originality/valueThis paper shows that component design aspects such as the low distance between the underside of the component and the PCB and blocked proper outgassing of volatile compounds of the SP can be root causes of solder beading under surface-mounted electrolytic capacitors.

Fully Connected Convolutional Neural Network in PCB Soldering Point Inspection

In Electronics Manufacturing Services (EMS) industry, Printed Circuit Board (PCB) inspection is tricky and hard, especially for soldering point inspection due to the extremely tiny size and inconsistent appearance for uneven heating in reflow soldering process. Conventional computer vision technique based on OpenCV or Halcon usually cause false positive call for originally good soldering point on PCB because OpenCV or Halcon use the pre-defined threshold in color proportion for deciding whether the specific soldering point is OK or NG (not good). However, soldering point forms are various after heating in reflow soldering process. This paper puts forward a VGG structure deep convolutional neural network, which is named SolderNet for processing soldering point after reflow heating process to effectively inspect soldering point status, reduce omission rate and error rate, and increase first pass rate. SolderNet consists of 11 hidden convolution layers and 3 densely connected layers. Accuracy reports are divided into OK point recognition and NG point recognition. For OK soldering point recognition, 92% is achieved. For NG soldering point recognition, 99% is achieved. The dataset is collected from KAGA Co. Ltd Plant in Suzhou. First pass rate at KAGA plant is increased from 25% to 80% in general.

少量データにおける畳み込みニューラルネットワークを用いた基板そり量変位の予測

A convolutional neural network, which reproduce a function by data, was used to predict the amount of warp distortion of a four-layer circuit board in a reflow soldering process. The data used for training are material properties such as Young's modulus, board thickness, and residual copper content as input data, while the warpage strain data to be predicted is the amount of warp of the circuit board obtained from the measurement. Since several distortion data to be predicted was insufficient to be used for training, a newly proposed data augmentation method used to increase a total amount of data. Proposed data augmentation is evaluated through the result of the predictions and discussed.

The intelligent design of reflow soldering based on an improved high-dimensional surrogate assisted optimization method

The Efficient Global Optimization (EGO) optimizes the temperature curve of the reflow soldering process by constructing the Kriging model. Compared with the traditional Surrogate Assisted Optimization (SAO), it can meet two different requirements: exploitation of optima and exploration of uncertain areas. However, due to the inversion of the covariance correlation matrix and the solving of Kriging-related parameters, the Kriging approximation process for high-dimensional problems is time consuming and even impossible to construct. For this reason, a High-Dimensional Kriging Modeling method through Linear Discriminant Analysis (HDKRG-LDA) method is proposed by considering the correlation parameters and the design variables of a Kriging model. It uses LDA to transform a high-dimensional correlation parameter vector in Kriging into low-dimensional one, which is used to reconstruct a new correlation function. In this study, the HDKRG-LDA method assisted EGO algorithm is applied to optimize the temperature curve of the reflow soldering process. In the optimization results of the reflow soldering, the method can effectively predict the temperature curve, and provide guidance for the production process of the reflow soldering.

Noncontact Reflow Oven Thermal Profile Prediction Based on Artificial Neural Network

This research proposes a thermal profile prediction model in the soldering reflow process (SRP). The thermal profile is the temperature of the solder paste during SRP and is affected by multiple factors [i.e., temperature setting for the oven chamber (thermal recipe) and conveyor belt speed]. The quality of the printed circuit board (PCB) is determined mainly by the thermal profile, because the components are attached to the board by the solder joint, which is generated during SRP. The thermal profile shape decides the performance of the solder joint, such as its strength. Solder paste manufacturers usually provide a target profile based on the physical properties of the solder paste composition. Comparing the experimental profile and target profile is the most simple, direct, and effective way to judge the PCB quality. In general, the thermal profile is collected from the thermocouples attached to the solder paste. To overcome drawbacks such as low testing efficiency, a noncontact machine learning-based thermal profile prediction model is developed in this research to monitor thermal performance. The noncontact prediction is of great significance to the PCB production line and meets the requirements of Industry 4.0 with a high degree of automation. Specifically, the prediction model is developed based on an artificial neural network (ANN). Several environmental factors are utilized as input data to forecast the board temperature. Then, a pooling model is devised to address the thermal profile. The proposed model is mainly for the passive components, such as 1005. The prediction model shows the promising ability to predict the thermal profile by showing a 97.2% accuracy with the testing data.

Automotive optoelectronic components submitted to thermal shock: Impact of component architecture on mechanical reliability

New LED technologies & modules allow glare-free automotive lighting. Valeo solutions include the first high-definition LED lighting system. These LEDs technology endure extreme tests conditions in form of thermal shocks or mechanical vibrations to insure reliability under the lifetime of automotive usage. Strict regulations require lead-free solders. Advanced optimization of LEDs for automotive lighting led to multilayered and miniaturized stack-ups. The first lead-free solders were revealed as a weak part of the electronic components and LED assembly. Enormous efforts have been spent on optimizing the lead-free solder material, design and reflow process. However, component miniaturization can also lead to high thermal gradients. Present work analyses the relation between component architectures and thermal shock induced fatigue damage. First, damage after N1 and N2 thermal shock cycles was characterized experimentally. Genuine sample preparations by a Cross polisher followed by high-resolution SEM observations reveal void growth in the solder material and cracking in the current circuit Layer. A two-scale finite element model reproduces both damage types. Simulation results can help to provide design improvements on the component architectures and modules to avoid (minimize) both damage types.