Solder Reflow Profile Research Articles

The preparation and wettability of the Sn-9Zn-2.5Bi-1.5In solder paste for SMT process and high shear ball performance

Purpose This study aims to focus on the surface mount technology (SMT) mass production process of Sn-9Zn-2.5Bi-1.5In solder. It explores it with some components that will provide theoretical support for the industrial SMT application of Sn-Zn solder. Design/methodology/approach This study evaluates the properties of solder pastes and selects a more appropriate reflow parameter by comparing the microstructure of solder joints with different reflow soldering profile parameters. The aim is to provide an economical and reliable process for SMT production in the industry. Findings Solder paste wettability and solder ball testing in a nitrogen environment with an oxygen content of 3,000 ppm meet the requirements of industrial production. The printing performance of the solder paste is good and can achieve a printing rate of 100–160 mm/s. When soldering with a traditional stepped reflow soldering profile, air bubbles are generated on the surface of the solder joint, and there are many voids and defects in the solder joint. A linear reflow soldering profile reduces the residence time below the melting point of the solder paste (approximately 110 s). This reduces the time the zinc is oxidized, reducing solder joint defects. The joint strength of tin-zinc joints soldered with the optimized reflow parameters is close to that of Sn-58Bi and SAC305, with high joint strength. Originality/value This study attempts to industrialize the application of Sn-Zn solder and solves the problem that Sn-Zn solder paste is prone to be oxidized in the application and obtains the SMT process parameters suitable for Sn-9Zn-2.5Bi-1.5In solder.

Hermetic Package for MEMS Mirror

Applications for optical scanning and 3D sensing, mostly related to the consumer electronics and automotive markets, are becoming more and more demanding in terms of performance and power consumption. When using a MEMS based Laser Beam Scanning, enclosing the mirrors into a hermetic package helps to address these requirements. Two MEMS mirrors with different sizes and performance have been selected for this study. A unique package, suitable for both mirrors, has been properly designed. The package is based on a ceramic chip carrier with a soldering ring on the top surface. A metal cap with a sintered optical window is attached to the ceramic substrate. The optical window is tilted with respect to the mirror to avoid spurious reflections. The MEMS mirrors have been integrated onto the ceramic substrate by standard die attach and wirebonding technologies. Hermetic sealing has been developed under nitrogen atmosphere to guarantee the MEMS stable operation against humidity and contaminants. Flux-less SnAgCu solder preforms have been selected as sealing material. The solder reflow profile has been optimized in order to obtain a uniform solder fillet at the sealing region while minimizing the thermal stress to the mirrors. The hermeticity of the assemblies has been proved by fine and gross leak testing. The packages show a leak rate < 1.5 E-8 atm cc/s and pass the fine and gross leak test, defined accordingly to MIL-STD-883 TM 1014.18.

Selective Soldering on Printed Circuit Boards with Endogenous Induction Heat at Appropriate Susceptors

In this work, methods for the endogenous heating of printed circuit boards (PCBs) by means of inductive losses in built-in susceptors are presented. Two basic types of inductive heating were studied, the heating in the transversal field and the heating in the longitudinal field. Elementary test stands were constructed and characterized for both field geometries. These setups were then used to analyze various susceptor materials like copper and aluminum for the transversal field heating and nickel and iron for the longitudinal field heating. To demonstrate the soldering processes by means of inductive heating, exemplary processes were conducted on both test stands by emulating a standard solder reflow profile. The limitations of using induction heating on printed circuit boards are illustrated by component lead frames, which also heat up in the inductive field and can hence be damaged.In short, this paper presents a selective heating method, based on induction heating, for printed circuit boards. Furthermore possible setups for implementing this heating method are described.

Reflow soldering method with gradient energy band generated by optical system.

Laser reflow soldering is an important technology in electronic components processing. In this paper, we presented a simple but efficient method to achieve reflow soldering process with gradient energy band created by just two parallel mirrors. The detailed influence of the variety of optical parameters on the soldering process has been analyzed by using the finite element method. And the modulation of the optical parameters on reflow soldering parameters also has been demonstrated. In our experiment, one HR mirror and one-mirror with transmissivity of 10% have been used to create a gradient energy band with an incident laser power of 50W. In summary, both the simulations and the experiments show that the typical reflow soldering profile has been acquired by the optical system. The high quality joints on both the front and rear surface of the capacitor can be acquired by just one surface radiation of the optical system.

Effect of reflow soldering profile on intermetallic compound formation

Reflow soldering in a nitrogen atmosphere is a common process in surface mount technology assembly since it can increase solder joint reliability. The present study investigated the effect of different reflow soldering atmospheres, either air or nitrogen, on intermetallic compound IMC formation and growth. Several techniques of materials characterisation including optical, image analysis, scanning electron microscopy and energy dispersive X-ray analysis were used to characterise the intermetallics. Besides, the effects of cooling rate and isothermal ageing were also studied. In summary, reflowing under nitrogen atmosphere had better effect on IMC formation and growth compared to reflowing under air. Besides, the cooling rate of solder during reflow also appears to have an effect on the final structure of the solder joint, and controlling the growth behaviour of the IMC during subsequent isothermal ageing. However, further research can be carried out to determine the solder joint's strength produced by both reflow soldering profiles.

Energy Efficiency Investigation on High-Pressure Convection Reflow Soldering in Electronics Production

One of the key factors in efficient and defect-free electronics manufacturing is the soldering of electronic components to the printed circuit boards. The deciding factor in the reliability and lifetime of the product is the quality of the soldered interconnections. The setting up of the reflow soldering profile plays a crucial role in the electrical functionality and robustness of the product. Especially the miniaturization of the assemblies and the development of new materials make it inevitable for the definition of new processes, optimization and implementation. In this paper, the combination of over-pressure and convection reflow soldering to minimize the defect rate and the related energy analysis for energy efficiency will be discussed and presented. A statistical analysis with variations in solder time, cost, energy, quality trade-off in the over-pressure reflow soldering process for classical printed circuit boards (PCBs) has been demonstrated.

Thermal parameters optimization of a reflow soldering profile in printed circuit board assembly: A comparative study

This paper presents a comparative study for optimizing the thermal parameters of the reflow soldering process using traditional and artificial intelligence (AI) approaches. High yields in the reflow soldering process are essential to a profitable printed circuit board (PCB) assembly operation. A reflow thermal profile is a time-temperature graph which is used to properly control the thermal mass and heat distribution to form robust solder joints between the PCB and electronics components during reflow soldering. An inhomogeneous temperature distribution for a reflow thermal profile can cause various soldering defects, which can jeopardize product reliability and lead to significant productivity loss. In the multi-objective optimization problem, three alternative optimization methods are discussed and compared: response surface methodology (RSM), nonlinear programming (NLP), and a hybrid AI technique. A dataset was gathered using a 38-4 experimental design for the development of meta-models through response surface quadratic modeling. In the first method, RSM is used to acquire the optimal heating parameters, while in the second method NLP is used to derive a global solution based on the meta-models. The back-propagation neural network (BPN) is used in the third method to formulate the nonlinear relationship between the heating inputs and responses. A genetic algorithm (GA) is then used to elicit the optimal heating parameters from the established BPN model. The evaluation results show that all three methods provide satisfactory soldering performance in terms of the process capability, sigma level, and process window indices (PWIs). Particularly, the hybrid AI approach provides superior nonlinear formulation capability and optimization performance.

Resolving No Solder Joint Issue Thru Understanding of Basic Principle in Solder and Heat Transfer

Cu clip is one of the interconnect technology used in semiconductor packaging. It has a distinct advantage over Au and Cu wire as well as Al ribbon electrical performance. In a standard SO8 package the cu clip electrical resistance is less than 10% of eight 2.0mil Au wires. The better electrical performance coupled with the increase in Au prices, Cu clip has become more and more attractive for MOSFET discrete product like DC-DC converter. This paper discusses the one of the challenges faced during the Cu clip development for 8MLPD3.3x3.3, no solder joint. Thru the understanding of solder properties and basic principle of heat transfer the issued was resolved through solder reflow oven modification and profile optimization.

Effects of Solder Paste Volume and Reflow Profiles on the Thermal Cycling Performance of Mixed SnAgCu/SnPb Solder Joints

Manufacturers of most high-reliability microelectronics products are still exempt from the requirement to eliminate Pb from their soldering process, and neither their components nor their printed circuit boards can always survive a lead-free reflow profile. When faced with the need to incorporate a lead-free ball grid array or chip-scale package into one of their products, they are thus faced with three alternatives: attaching it separately using a rework station to establish a localized lead free profile; reballing the component with eutectic SnPb solder; or soldering it to the board with a eutectic SnPb solder paste to produce high-Sn joints with a few percent of Pb. Any of these approaches may affect the final reliability of the assembly in its own way, and comparisons between them by accelerated testing are confounded by very different acceleration factors. Optimization of solder paste volumes and reflow profiles for mixed soldering is of course less affected by this, but common experiences have been that optimized mixing requires a typical lead-free reflow profile. This clearly defeats the purpose of mixing in many cases. Two different sets of experiments were conducted to assess the consequences of soldering SnAgCu (SAC) balls with a eutectic SnPb paste for the thermal fatigue resistance of the resulting solder joints. The first experiment addressed effects of solder paste volume and common reflow profile alternatives. Based on the results of this, the second set of experiments involved the development of an improved low-temperature profile and the comparison of this with common SnPb and lead-free profiles in terms of thermal cycling performance. Five different high and moderate strain area array components were tested with SnPb, SAC, and mixed SAC/SnPb solder joints, and the results are discussed on the basis of known test parameter dependencies and trends. The improved profile was shown to optimize the performance of the mixed joints, but indications are that these may still compare poorly to pure eutectic ones for some components. All five components may have shorter lives with lead-free than with SnPb joints in service.

Vapour Pressure Modelling for Plastic Encapsulated Microelectronics Subjected to Lead‐Free Solder Reflow Profile

Abstract: A universal, practical vapour pressure modelling scheme for the plastic encapsulated microelectronics (PEMs) subjected to lead‐free solder reflow profile and other high temperature conditions is presented. The proposed technique employs finite‐element moisture diffusion and structural analysis to calculate the dynamic vapour pressure buildups at the interfacial delaminations in the PEMs. The modelling scheme uses a non‐iterative approach, and it is capable of accurately and efficiently determining the dynamic vapour pressure information required for the reliability‐related design and failure analysis of various kinds of PEMs. The modelling scheme has considerable value to the development of new materials, innovative structures, and effective protection methods for enhancing the performance and reliability of plastic packages.

Use of TG/FT-IR in material characterization

The use of thermogravimetry (TG) in series with Fourier transform infrared (FT-IR) spectroscopy enhances some of the capabilities of both analytical techniques. TG continues to be an useful method for understanding how the mass of a sample changes as a function of time and temperature, but TG cannot tell us exactly what is happening during transitions. FT-IR is useful for understanding the qualitative nature of molecules and mixtures. However, FT-IR often cannot tell us what happened during a thermal transition if all we can do is examine the sample before and after it is heated. By combining the two methods, an FT-IR spectrometer can monitor the effluent of a TG and allow us to analyze the evolved gas. TG/FT-IR can be used to monitor many types of thermal experiments including cure profiles, solder reflow profiles, and material decomposition. In simple experiments, it can be used to discern subtle differences between what should be identical materials. A summary of recent analyses will be presented.

Forecasting and Parameters Optimization of Reflow Soldering Profile Based on BPNN and GA

This paper proposes an effective method to determine the optimal parameters settings of reflow soldering profile and helps reduce the try-and-error time in practical application. Due to the complex nonlinear relationship between reflow thermal profile and process parameters in this problem, BPNN is adopted to establish the model for description of this intricate relationship between inputs and outputs. According to the requirements of the reflow soldering profile, GA is used to calculate the best input parameters with its strong global research ability. Thus, a combined model with BPNN and GA is proposed for parameters optimization and contributes to reduction of decision time of input parameter for reflow soldering profile. In addition, a case study is given to prove the accuracy of the proposed method.

Microstructure, Defects, and Reliability of Mixed Pb-Free/Sn-Pb Assemblies

The results of an intensive reliability study on Pb-free ball grid array (BGA)/Sn-Pb solder assemblies as well as some lessons learnt dealing with mixed assembly production at Celestica are described in this paper. In the reliability study, four types of Pb-free ball grid array components were assembled on test vehicles using the Sn-Pb eutectic solder and typical Sn-Pb reflow profiles with 205°C to 220°C peak temperatures. Accelerated thermal cycling (ATC) was conducted at 0°C to 100°C. The influence of the microstructure on Weibull plot parameters and the failure mode will be shown. Interconnect defects such as nonuniform phase distribution, low-melting structure accumulation, and void formation are discussed. Recommendations on mixed assembly and rework parameters are given.

Advances in Vapor Pressure Modeling for Electronic Packaging

Two advanced techniques have been developed for modeling vapor pressure within the plastic IC packages during solder reflow. The first involves the extension of the wetness technique to delamination along multimaterial interface and during dynamic solder reflow. Despite its simplicity, this technique is capable of offering reliable and accurate prediction for packages with high flexural rigidity. For packages with low flexural rigidity, the new decoupling technique that integrates thermodynamics, moisture diffusion, and structural analysis into a unified procedure has been shown to be more useful. The rigorous technique has been validated on both leadframe-based as well as laminate-based packages. With high accuracy and computational efficiency, these dynamic modeling tools will be valuable for optimization of package construction, materials, and solder reflow profile against popcorn cracking for both SnPb and Pb-free solders

Lead-Free Chip Scale Packages: Assembly and Drop Test Reliability

Three underfill options compatible with lead-free assembly have been evaluated: capillary underfill, fluxing underfill, and corner bond underfill. Chip scale packages (CSPs) with eutectic Sn/Pb solder were used for control samples. Without underfill, lead-free and Sn/Pb eutectic drop test results were comparable. Capillary flow underfills, dispensed and cured after reflow, are commonly used in CSP assembly with eutectic Sn/Pb solder. With capillary flow underfill, the drop test results were significantly better with lead-free solder assembly than with Sn/Pb eutectic. Fluxing underfill is dispensed at the CSP site prior to CSP placement. No solder paste is printed at the site. The CSP is placed and reflowed in a standard reflow cycle. A new fluxing underfill developed for compatibility with the higher lead-free solder reflow profiles was investigated. The fluxing underfill with lead-free solder yielded the best drop test results. Corner bond underfill is dispensed as four dots corresponding to the four corners of the CSP after solder paste print, but before CSP placement. The corner bond material cures during the reflow cycle. It is a simpler process compared to capillary or fluxing underfill. The drop test results with corner bond were intermediate between no underfill and capillary underfill and similar for both lead-free and Sn/Pb eutectic solder assembly. The effect of aging on the drop test results with lead-free solder and either no underfill or corner bond underfill was studied. Tin/lead solder with no underfill was used for control. This test was to simulate drop performance after the product has been placed in service for some period of time. There was degradation in the drop test results in all cases after 100 and 250 h of storage at 125/spl deg/C prior to the drop test. The worst degradation occurred with the lead-free solder with no underfill.

Lead-free 0201 manufacturing, assembly and reliability test results

The trend towards smaller, faster and cheaper electronic devices has led to an increase in the use of 0201 ( L ∼ 0.02 in.; W ∼ 0.01 in.) and even smaller sized passive components. The size advantages of the 0201 component make it a popular choice among design engineers but not among manufacturing engineers. From a manufacturing perspective, the size of the 0201 package poses significant challenges to the printed circuit board (PCB) assembly process. The many challenges with 0201 assembly can be attributed to the solder paste volume, pad design, aperture design, board finish, type of solder paste, pick-and-place and reflow profile. If these factors are not optimized, they will introduce undesirable manufacturing defects. The small size of 0201 packages and undetected manufacturing defects will also raise concerns about their second level interconnect reliability, especially for lead-free solder alloys and surface finishes, with new processes and higher reflow requirements. To determine the optimum conditions, a design-of-experiment (DOE) study was carried out to investigate the effects of these parameters on assembly defects and solder joint reliability. This paper presents the test results and comparative literature data on the influence of a few key manufacturing parameters and defects associated with the 0201 component using lead-free and tin–lead solder alloys. Data pertaining to component shear strength before and after isothermal aging at 150 °C and intermetallic growth up to 500 h of aging are presented. A number of test vehicles were also subjected to thermal cycling (1500 cycles) in the range of −55/100 °C to determine the solder fatigue behavior. Shear test results for test vehicles subjected to thermal cycling is also presented. In addition, optical microscopy analysis of solder joint behavior during thermal cycling showing the progress of the solder damage and cross-sectional photos taken at 1500 cycles is included.

Study of Interfacial Delamination by Considering the Effect of Temperature and Moisture during Solder Reflow for QFN Package

Experiment has been conducted to measure the solubility Csat of mold compound and to obtain the moisture weight loss curves at various temperature. Moisture desorption modeling has been conducted to calculate the moisture diffusivity for desorption D(T) by matching with the experimental results. Finite Element Analysis (FEA) has also been conducted to calculate the transient development of strain energy release rate (ERR) at the interfacial crack tip due to thermal stress only (Gt), hygrostress only (Gh), and the combined effect (Gtotal) during solder reflow. ERR is computed based on the Virtual Crack Closure Technique (VCCT). It is found that Gh is significantly smaller than Gt, however the effect of hygrostress significantly increases the total strain energy release rate when combined with the thermal effect. The maximum Gtotal occurs at the peak of the solder reflow profile. The effects of crack size and geometrical parameters have been studied. The result imply that the interfacial crack is unstable and has a high tendency of growing to a significant extent. ERR is strongly influenced by the thickness of the package while length between the edge of die and pad has a moderate affect on the ERR.

Effects of process conditions on reliability, microstructure evolution and failure modes of SnAgCu solder joints

In this study, microstructure evolution at intermetallic interfaces in SnAgCu solder joints of an area array component was investigated at various stages of a thermal cycling test. Failure modes of solder joints were analyzed to determine the effects of process conditions on crack propagation. Lead-free printed-circuit-board (PCB) assemblies were carried out using different foot print designs on PCBs, solder paste deposition volume and reflow profiles. Lead-free SnAgCu plastic-ball-grid-array (PBGA) components were assembled onto PCBs using SnAgCu solder paste. The assembled boards were subjected to the thermal cycling test (−40 °C/+125 °C), and crack initiation and crack propagation during the test were studied. Microstructure analysis and measurements of interface intermetallic growth were conducted using samples after 0, 1000, 2000 and 3000 thermal cycles. Failures were not found before 5700 thermal cycles and the characteristic lives of all solder joints produced using different process and design parameters were more than 7200 thermal cycles, indicating robust solder joints produced with a wide process window. In addition, the intermetallic interfaces were found to have Sn–Ni–Cu. The solder joints consisted of two Ag–Sn compounds exhibiting unique structures of Sn-rich and Ag-rich compounds. A crystalline star-shaped structure of Sn–Ni–Cu–P was also observed in a solder joint. The intermetallic thicknesses were less than 3 μm. The intermetallics growth was about 10% after 3000 thermal cycles. However, these compounds did not affect the reliability of the solder joints. Furthermore, findings in this study were compared with those in previous studies, and the comparison proved the validity of this study.

Reflow profile study of the Sn‐Ag‐Cu solder

A study of the Sn‐Ag‐Cu lead‐free solder reflow profile has been conducted. The purpose of the work was to determine the Sn‐Ag‐Cu reflow profile that produced solder bumps with a thin intermetallic compound (IMC) layer and fine microstructure. Two types of reflow profiles were studied. The results of the experiment indicated that the most significant factor in achieving a joint with a thin IMC layer and fine microstructure was the peak temperature. The results suggest that the peak temperature for the Sn‐Ag‐Cu lead‐free solder should be 230°C. The recommended time above liquidus is 40 s for the RSS reflow profile and 50‐70 s for the RTS reflow profile.