Types Of Solder Pastes Research Articles

Effect of PVD-coated wall aperture roughness on the life span of fine-pitch stencil printing

PurposeThis paper aims to investigate the effect of physical vapor deposition (PVD)-coated stencil wall aperture on the life span of fine-pitch stencil printing.Design/methodology/approachThe fine-pitch stencil used in this work is fabricated by electroform process and subsequently nano-coated using the PVD process. Stencil printing process was then performed to print the solder paste onto the printed circuit board (PCB) pad. The solder paste release was observed by solder paste inspection (SPI) and analyzed qualitatively and quantitatively. The printing cycle of up to 80,000 cycles was used to investigate the life span of stencil printing.FindingsThe finding shows that the performance of stencil printing in terms of solder printing quality is highly dependent on the surface roughness of the stencil aperture. PVD-coated stencil aperture can prolong the life span of stencil printing with an acceptable performance rate of about 60%.Originality/valueStencil printing is one of the important processes in surface mount technology to apply solder paste on the PCB. The stencil’s life span greatly depends on the type of solder paste, stencil printing cycles involved and stencil conditions such as the shape of the aperture, size and thickness of the stencil. This study will provide valuable insight into the relationship between the coated stencil wall aperture via PVD process on the life span of fine-pitch stencil printing.

Investigations on High-Power LEDs and Solder Interconnects in Automotive Application: Part I—Initial Characterization

Thermo-mechanical reliability is one major issue in solid-state lighting. Mismatches in the coefficients of thermal expansion (CTE) between high-power LED packages and substrates paired with temperature changes induce mechanical stress. This leads to a thermal degradation of LED modules by crack formation in the solder interconnect and/or delamination in the substrate, which in turn increases junction temperature and thus decreases light output and reduces lifetime. To investigate degradation and understand influence of LED package design and solder material, a reliability study with a total of 1800 samples − segmented in nine LED types and five solder pastes − is performed. First of all, in this paper a state-of-the-art review of high-power LED packages is performed by analyzing and categorizing the packaging technologies. Second, the quality inspection after assembly is realized by transient thermal analysis (TTA), scanning acoustic microscopy (SAM) and X-ray. For TTA, a new method is introduced to separate the thermal resistance of the LED package from solder interconnect and substrate by applying the transient dual interface method (TDI) on samples with different solder interconnect void ratios. Further measurement effort is not required. The datasheet values for thermal resistance are verified and the different LED package types are benchmarked. The void ratio of the solder interconnects is determined by X-ray inspection combined with an algorithm to suppress disruptive internal LED package structures. TTA and TDI revealed that initial thermal performance is independent of solder paste type and that voiding is more critical to smaller LED packages. In addition, lower silver proportion in the paste is found to increase voiding. SAM is less sensitive for initial void detection than X-ray, but it’s applied to monitor crack propagation while aging in combination with TTA. The results of the reliability study, i.e., the crack growth under temperature shock test for the different SAC solders, will be presented in a second independent paper.

Analyzing the Overfitting of Boosted Decision Trees for the Modelling of Stencil Printing

Stencil printing is one of the key steps in reflow soldering technology, and by the spread of ultra-fine-pitch components, analysis of this process is essential. The process of stencil printing has been investigated by a machine learning technique utilizing the ensemble method of boosted decision trees. The phenomenon of overfitting, which can alter the prediction error of boosted decision trees has also been analyzed in detail. The training data set was acquired experimentally by performing stencil printing using different printing speeds (from 20 to 120 mm/s) and various types of solder pastes with different particle sizes (particle size range 25–45 µm, 20–38 µm, 15–25 µm) and different stencil aperture sizes, characterized by their area ratio (from 0.35 to 1.7). The overfitting phenomenon was addressed by training by using incomplete data sets, which means that a subset of data corresponding to a particular input parameter value was excluded from the training. Four cases were investigated with incomplete data sets, by excluding the corresponding data subsets for: area ratios of 0.75 and 1.3, and printing speeds of 70 mm/s and 85 mm/s. It was found that the prediction error at input parameter values that have been excluded from the training can be lowered by eliminating the overfitting; though, the decrease in the prediction error depends on the rate of change in the output parameter in the vicinity of the respective input parameter value.

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.

Effect of stencil wall aperture on solder paste release via stencil printing

Solder paste printing is a process by which the correct amount of solder paste is applied to the printed circuit board via a stencil. The solder release from the stencil printing process very much depends on the type of solder paste and stencil conditions such as the shape of the aperture, size, and thickness of the stencil. This paper investigates the stencil condition in particular the stencil wall aperture and its relationship to the solder release ability. In this work, two types of stencil wall openings A and B were used, which differ in a different ways of cutting to achieve the wall aperture. The cutting process produced different surface roughness of the wall aperture of the stencil. Stencil printing was performed to print the solder paste onto the PCB pad. The release of solder paste was observed by solder paste inspection (SPI) and analyzed qualitatively and quantitatively. The results show that stencil B gives a better solder compared to stencil A. This is due to the smoother wall aperture compared to stencil A which has a roughened wall aperture. This shows that the performance of stencil printing in terms of solder printing quality is highly dependent on the surface roughness of the stencil aperture. Stencil quality is important as it affects the performance of solder paste printing, and this process is mainly carried out in the electronics industry. Therefore, understanding stencil conditions is important for electronic technology that uses solder printing.

The influence of soldering process parameters on the optical and thermal properties of power LEDs

PurposeThis paper aims to present the results of investigations that show the influence of soldering process parameters on the optical and thermal parameters of power LEDs.Design/methodology/approachThe power LEDs were soldered onto metal core printed circuit board (MCPCB) substrates in different soldering ovens: batch and tunnel types, characterized by different thermal profiles. Three types of solder pastes based on Sn99Ag0.3Cu0.7 with the addition of TiO2 were used. The thermal and optical parameters of the diodes were measured using classical indirect electrical methods. The results of measurements obtained were compared and discussed.FindingsIt was shown that the type of oven and soldering thermal profile considerably influence the effectiveness of the removal of heat generated in the LEDs tested. This influence is characterized by thermal resistance changes. The differences between the values of this parameter can exceed 20%. This value also depends on the composition of the soldering paste. The differences between the diodes tested can exceed 15%. It was also shown that the luminous flux emitted by the diode depends on the soldering process used.Practical implicationsThe results obtained could be useful for process design engineers for assembling power LEDs for MCPCBs and for designers of solid-state light sources.Originality/valueThis paper presents the results of investigations into the influence of the soldering profiles and soldering pastes used on the effectiveness of the removal of heat generated in power LEDs. It shows and discusses how the factors mentioned above influence the thermal resistance of the LEDs and optical parameters that characterize the light emitted.

Soldering Electronics to Smart Textiles by Pulsed Nd:YAG Laser.

Experts attest the smart textiles market will have high growth potential during the next ten years. Laser soldering is considered to be a good contacting method because it is a contactless process. For this reason, it is intended to investigate the contacting process of printed circuit boards (PCB) to isolated conductive textile strips by means of a ytterbium-doped fiber laser (1064 nm). During the investigation, the copper strands in the textile tape were stripped by the laser and soldered to the PCB without any transport of the textile. Therefore, we investigated different sets of parameters by means of a design of experiment (DoE) for different types of solder pastes. Finally, the joinings were electrically analyzed using a contact resistance test, optically with a REM examination, and mechanically using a peeling test.

Effects of solder paste on thermal and optical performance of high-power ThinGaN white LED

PurposeThermal management of high-power (HP) light-emitting diodes (LEDs) is an essential issue. Junction temperature (TJ) and thermal resistance (Rth) are critical parameters in evaluating LEDs thermal management and reliability. The purpose of this paper is to study thermal and optical characteristics of ThinGaN (UX:3) white LED mounted on SinkPAD by three types of solder paste (SP): No-Clean SAC305 (SP1), Water-Washable SAC305 (SP2) and No-Clean Sn42/Bi57.6/Ag0.4 (SP3).Design/methodology/approachThermal transient tester (T3Ster) machine is used to determine TJ and total thermal resistance (Rth–JA). In addition, the LED’s optical properties are measured via thermal and radiometric characterization of power LEDs (TeraLED) system. The LED is mounted on SinkPAD using SP1, SP2 and SP3 by stencil printing to control a thickness of SP and reflow soldering oven to minimize the number of voids. The LED with SP1, SP2 and SP3 is tested at various input currents and ambient temperatures.FindingsThe results indicate that at high input current, which equals to 1,200 mA, Rth–JA and TJ, respectively, are reduced by 30 and 17 per cent between SP1 and SP2. At same current value, Rth–JA and TJ are minimized by 42 and 25 per cent between SP1 and SP3, respectively. In addition, at an ambient temperature of 85°C, Rth–JA and TJ are decreased by 34 and 7 per cent between SP1 and SP2, respectively. Similarly, the reduction in Rth–JA and TJ between SP1 and SP3 is 44 and 10 per cent, respectively. Luminous flux, luminous efficacy and color shift of the LED with the three types of SPs are compared and discussed. It is found that the SP1 improves the chromatic properties of the LED by increasing the overall light efficiency and decreasing the color shift.Originality/valueThermal and optical performance of ThinGaN LEDs mounted on SinkPAD via three types of SPs is compared. This investigation can assist the research on thermal management of HP ThinGaN-based LEDs.

X-ray inspection and Six-Sigma in analysis of LED thermal pad coverage

PurposeThe purpose of this paper is to evaluate selected methods of reduction voidings in lead-free solder joints underneath thermal pads of light-emitting diodes (LEDs), using X-ray inspection and Six Sigma methodology.Design/methodology/approachOn the basis of cause and effect diagram for solder voiding, the potential causes of voids and influence of process variables on void formation were found. Three process variables were chosen: the type of reflow soldering, vacuum incorporation and the type of solder paste. Samples of LEDs were mounted with convection and vapour phase reflow soldering. Vacuum was incorporated into vapour phase soldering. Two types of solder pastes OM338PT and LFS-216LT were used. Algorithm incorporated into X-ray inspection system enabled to calculate the statistical distribution of LED thermal pad coverage and to find the process capability index (Cpk) of applied soldering techniques.FindingsThe evaluation of selected soldering processes of LEDs in respect of their thermal pad coverage and statistical Cpk indices is presented. Vapour-phase soldering with vacuum is capable (Cpk > 1) for OM338PT and LFS-216LT paste. Convection reflow without vacuum with LFS-216LT paste is also capable (Cpk = 1.1). Other technological soldering processes require improvements. Vacuum improves radically the capability of a reflow soldering for an LED assembly. When vacuum is not accessible, some improvement of capability to a lower extent is possible by an application of void-free solder pastes.Originality/valueSix Sigma statistical methodology combined with X-ray diagnosis was used to check whether applied methods of void reduction underneath LED thermal pads are capable processes.

Effects of nano-copper particles on the properties of Sn58Bi composite solder pastes

PurposeThis paper aimed to investigate the effects of nano-copper particles on the melting behaviors, wettability and defect formation mechanism of the Sn58Bi composite solder pastes.Design/methodology/approachIn this paper, the mechanical stirring method was used to get the nano-composite solder pastes.FindingsExperimental results indicated that the addition of 3 wt.% (weight percentage) 50 nm copper particles showed limited effects on the melting behaviors of the Sn58Bi composite solder paste. The spreading rate of the Sn58Bi composite solder paste showed a decreasing trend with the increase of the weight percentage of 50 nm copper particles from 0 to 3 wt.%. With the addition of copper particles of diameters 50 nm, 500 nm or 6.5 μm into the Sn58Bi solder paste, the porosities of the three types of solder pastes showed a similar trend. The porosity increased with the increase of the weight percentage of copper particles. Based on the experimental results, a model of the void formation mechanism was proposed. During reflow, the copper particles reacted with Sn in the matrix and formed intermetallic compounds, which gathered around the voids produced by the volatilization of flux. The exclusion of the voids was suppressed and eventually led to the formation of defects.Originality/valueThis study provides an optimized material for the second and third level packaging. A model of the void formation mechanism was proposed.

Real time joint resistance monitoring during solder reflow

During solder reflow of smaller and smaller microelectronic interconnections, a robust soldering process can become challenging, resulting in opens or bridging. In this work, a customized real time resistance monitoring (RTRM) setup is developed to observe the resistance of a solder connection in real-time during reflow to better understand the challenges of soldering. Two types of solder paste are studied with RTRM. Solder A was fresh while solder B was one year old. We found solder B in general has a lower room temperature resistance before reflow than solder A. During reflow, the resistance signals behave differently between solder A and solder B. For example, in solder B, an episode of high resistance (24.40 ± 16.04 kΩ) is observed for 4–58 s not long before melting, while solder A mainly shows resistance drops in the early stage of reflow. Microscopic images and cross-sectioning studies with respect to the special events in the resistance signal show solder compacting, solder sintering, flux segregation, and final open or connection. RTRM can be a helpful tool when developing smaller solder processes for thin film substrate with miniaturized passive and active components.

Wettability study of lead free solder paste and its effect towards multiple reflow

Nowadays, wafer bumping using solder paste has come into focus as it provides a low cost method. However, since the industries are moving towards lead-free electronic packaging, a new type of no-clean flux was produced specifically for lead-free solder paste. Therefore, this study is used to evaluate the wettability of two different types of no-clean flux onto copper substrate. Besides, its effect towards multiple reflow was also studied. Reflow soldering was conducted for both types of solder paste that contained different type of no-clean flux for up to double reflow. Two different reflow profile was used. The results showed that the Flux A exhibit better soldering performance after first and second reflow soldering. In addition, type of intermetallic compound (IMC) found after first reflow remain the same even after second reflow which was Cu-Sn based. This is shows that Flux A manage to control the diffusion process which will finally leads to a better solder joint performance. Nevertheless, mechanical testing should be carried out in order to evaluate the solder joint strength.

Vapour phase soldering used for quality improvement of semiconductor thermogenerators (TEGs) assembly

The processing of harvested waste heat obtained from industrial or domestic installations directly converted into electrical energy using thermoelectric generators (TEG) is an interesting and future-proof solution. The TEG generator involves the serial connection of even tens of p–n thermocouples, through which a current of even a dozen amperes can flow. Its reliability strongly depends also on the quality of the solder joints, especially on the types of solder alloys, their quality, the soldering techniques applied and other factors. An additional difficulty is the functioning of thermogenerators and their solder joints at high temperature of up to several hundred degrees Celsius and with large temperature gradients between cold and hot plates. This paper presents the technical results of research investigation of semiconductor TEG model with particular reference to the technological aspects of the solder joint performances. Two types of solder pastes with different compositions and melting temperatures were examined. The main soldering method used was vapour phase soldering (VPS). VPS uses the latent heat of liquid vaporization to provide heat for soldering. A vacuum option during the soldering process was also applied. The results showed that the number of voids is strongly dependant on alloy composition and the presence of the vacuum.

Process characterization and reliability for the assembly of 01005 chip components

PurposeThe purpose of this paper is to optimize assembly processes in order to minimize defects in the assembly of 01005 chip components.Design/methodology/approachDuring the study, solder paste printing process‐related variables, such as solder paste type, stencil type, and stencil opening ratio, and pick and place process‐related methods, such as vision camera type and vacuum pickup nozzle type were evaluated with the goal of achieving a high‐yield assembly solution for 01005 chip components. A test board was used in a series of designed experiments to optimize the solder paste printing, pick and placement, and reflow processes. Assembly defects were analyzed as a function of the stencil design and the assembly processes.FindingsThe results of the study indicated that both electroformed and electropolished laser‐cut stencils had a comparable print quality with respect to the solder volume delivered to the pads. In terms of assembly yield performance, type 4 (size range: 20‐38 μm) solder paste with a smaller sphere size gave a better overall yield and better paste deposition on the pad, if used on a 0.08‐mm thick electroformed stencil with a 90 per cent aperture. Temperature cycling between −65 and 150°C, with up to 1,500 cycles, showed that no cracks were observed at the solder joints due to temperature cycling. The process and design change required for achieving a robust manufacturing process have been indicated and reported.Originality/valueThe results of this work provide process recommendations for the implementation of 01005‐sized chip components assembly in mass production processes.

Thermal Cycling Reliability of Lead-Free Solders (SAC305 and Sn3.5Ag) for High-Temperature Applications

Applications with temperatures higher than the melting point of eutectic tin-lead solder (183°C) require high-melting-point solders. However, they are expensive and not widely available. With the adoption of lead-free legislation, first in Europe and then in many other countries, the electronics industry has transitioned from eutectic tin-lead to lead-free solders that have higher melting points. This higher melting point presents an opportunity for the manufacturers of high-temperature electronics to shift to mainstream lead-free solders. In this paper, ball grid arrays (BGAs), quad flat packages, and surface mount resistors assembled with SAC305 (96.5%Sn+3.0%Ag+0.5Cu) and Sn3.5Ag (96.5%Sn+3.5%Ag) solder pastes were subjected to thermal cycling from -40°C to 185°C. Commercially available electroless nickel immersion gold board finish was compared to custom Sn-based board finish designed for high temperatures. The data analysis showed that the type of solder paste and board finish used did not have an impact on the reliability of BGA solder joints. The failure analysis revealed the failure site to be on the package side of the solder joint. The evolution of intermetallic compounds after thermal cycling was analyzed.

レーザ変位法を用いたソルダペーストぬれ性評価装置の開発

Equipment for evaluating the wettability of solder paste has been developed using a laser displacement method. Using the equipment, the descent behavior of an electronic part in reflow soldering can be measured with a laser displacement meter. From the measured height displacement curve, the wetting time and spread rate of the solder paste onto the electrodes of an electronic part and a substrate can be investigated. The developed equipment can detect the difference in the wettability of solder paste which depends on the solder paste type, the electrode type, and the degradation of the solder paste over time.

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.

Quality Optimisation of Fine Pitch Surface Mount Solder Joints using Taguchi Experimental Design Techniques

The use of Taguchi experimental design techniques to examine the effects of package type, solder paste type and solder reflow technique on the quality of fine pitch surface mount IC package solder joints is described. In particular, the effect of the use of ceramic or plastic packages, copper or Alloy 42 leadframes, silver loaded or non‐silver loaded solder paste and infra‐red, laser or hot‐bar reflow on solder joint metallurgical structure, electrical resistance and mechanical strength is evaluated. In addition to these solder joint parameters, an associated visual inspection was used to find the best process parameters to minimise solder balling, bridging etc. and a correlation between paste contacts at placement and solder bridges after reflow was also conducted. The experiment used an L9 array to find the optimum parameters from three factors, each at three levels. An extension to the basic Taguchi array was included in the form of an outer (noise) factor to include the effect of climatic stress on the solder joints under investigation. Response tables separate out the contribution of each factor level to the mechanical strength and electrical resistance of the assemblies. By comparing the response tables before and after climatic testing it is possible to estimate the effect of each factor level on the long‐term quality of the solder joints. It is shown how Taguchi experimental design techniques can be used to minimise the number of experiments required to predict optimum solder assembly process parameters. The accuracy of the prediction is shown by the results of a confirmation run which yielded mechanical strengths very close to those predicted, both before and after highly accelerated stress testing of the solder assemblies.