Solder Flux Residue Research Articles

Assessment of Solder Flux Corrosivity and Flux Residue Level in Clip Bonding Assembly

Abstract Most fluxes and their flux residue are corrosive, and their presence on the surface of assemblies may cause reliability concerns due to electrochemical migration. This study firstly refers to the test method IPC-TM-650 2.3.32D and 2.6.15C and the requirements in J-STD-004A to evaluate flux-induced corrosion and the corrosivity of their flux residue. Then thermogravimetric analysis is used to evaluate the flux residue level after soldering under identical conditions. These comparison data can be utilized to compare flux performance of solder pastes. For the cleanliness evaluation after flux cleaning, the qualitative analysis refers to IPC-A-610G and adopts optical inspection to observe the flux cleaning effect. The quantitative analysis refers to IPC-TM-650 2.3.28B ion chromatography test method to analyze the level of ionic species after flux cleaning. In addition to IPC-TM-650 2.3.28B ion chromatography method, the effectiveness of other methods has been well demonstrated to characterize the fluxes and their flux residue in clip bonding assembly.

Transformation of reflow solder flux residue under humid conditions

The use of reflow flux system for soldering process of SMT electronics results in the concealed residues located underneath of the components, which changes its characteristics upon exposure to humidity. In this paper, the transformation of reflow process-related solder flux residues was investigated under varied humidity and temperature conditions. Evolution of potentially corrosive active components from the residue under high humidity and temperature conditions was investigated for different solder pastes using the acidity indication gel test. Changes in the concealed morphology of the residue were visualized using light optical and scanning electron microscopes, and the observations were correlated to the results from the gel indication test. Quantification of the release of active components from the residue was performed using the extract conductivity measurements. The results show that the evolution of acidic residues from the reflow-soldered pads occurs upon exposure to humidity and temperature due to opening up of the concealed residue, however, temperature alone was found to have no effect on this phenomenon. The hygroscopic nature of the evolved solder flux activators cause the absorption of moisture under humid conditions, accelerating the formation of a conductive electrolyte that provides a potential chance for an undesired corrosion process.

Investigation of the Effect of Temperature on the Properties of No-Clean Reflow Soldering Fluxes

With the decreasing size of microelectronics, corrosion-related reliability issues have become more abundant. The presence of no-clean soldering flux residues has been commonly identified as an accelerating factor for electrochemical corrosion. This paper discusses the investigation of five different commercially available no-clean fluxes, used in reflow solder pastes. At first, the initial composition and thermal behavior of the systems were characterized with gas chromatography (GC–MS) and thermogravimetric analysis. Based on the results, no differences were identified between the evaporation characteristics of the fluxes in nitrogen and in air up to 250°C. After initial investigation, the real-life thermal decomposition behavior, pH- and conductivity-changing capability of the fluxes were emulated in the temperature range of 50–250°C. To identify the potential specific effect of copper, the experiments were conducted with both direct bonded copper and glass substrates. No specific effect of copper was detected regarding the remaining amount of flux residue on the surface. Furthermore, after treating the samples at 250°C, no significant pH- or conductivity-changing capability was identified in the case of any of the investigated flux systems regardless of the substrate. Based on these results, the necessity of the follow-up cleaning steps must be reconsidered.

Effect of Organic Acids on the Electrochemical Migration of Tin in Thin Electrolyte Layer

Background and Objective: The effects of representative solder flux residue weak organic acids on electrochemical migration (ECM) of tin in thin electrolyte layer were studied using a technique based on the coupling of in situ electrochemical measurements and optical observations, as well as ex situ characterizations. Methods and Results: The results showed that the increasing amount of weak organic acid decreased the probability of ECM and dendrites formed were mainly composed of metallic tin. Tin ions reacted with organic compound ions from hydrolysis of weak organic acids to form complexes with electronegativity, which retarded the transfer of tin ions. Some complexes can be oxidized to the insoluble tin oxides on the anode surface and blocked the dissolution of anode during tin ECM. Conclusion: The growth rate of tin dendrite was found to be limited by the dissociation of complexes. Mechanisms involved were proposed to explain the role of weak organic acid in the tin ECM.

No-Clean Solder Flux Chemistry and Temperature Effects on Humidity-Related Reliability of Electronics

The use of no-clean flux technology for the wave soldering process of a printed circuit board assembly (PCBA) influences the humidity-related robustness of the electronic devices due to the ionic residues remaining on the PCBA surface after soldering. This paper investigates the effect of various no-clean solder flux chemistries on the formation of a water layer on the PCBA surface studied as a function of climatic conditions relevant for the electronics industry. The activating part of the commercial flux formulations used for the investigations was based on single weak organic acid (WOA) and WOA mixtures. The hygroscopic nature of flux residues, reflecting their ability to uptake moisture and lead to water film build-up, was investigated using the gravimetric moisture sorption test and AC electrochemical impedance. The effect of flux composition and ambient climatic conditions on the corrosion reliability was studied using DC leakage current measurements upon varying potential bias, humidity, and temperature levels. The study shows that solder flux residues containing a mixture of WOA activators in their formulation facilitate the formation of water layer on the PCBA surface at lower humidity levels, compared to the residues composed of a single activator, and absorb a higher amount of moisture. Corrosion occurrence is accelerated by the presence of highly hygroscopic residues of WOA mixtures, compared to significantly less corrosive contaminants comprised of single activator. Increasing ambient temperature enhances moisture interaction with flux residues and changes its deliquescent behaviour, leading to the formation of a thicker water layer and severe ion-induced corrosion at lower humidity levels.

Parametric Study of Solder Flux Hygroscopicity: Impact of Weak Organic Acids on Water Layer Formation and Corrosion of Electronics

The presence of solder flux residues on the printed circuit board assembly surface is an important factor contributing to humidity-related reliability issues that affect device lifetime. This investigation focuses on understanding the hygroscopic nature of typical wave solder flux activators—weak organic acids—under varied temperature conditions. In situ x-ray diffraction measurements assessed the effect of high temperature on the crystal structure of organic activators. The hygroscopicity studies were carried out under relative humidity (RH) levels varying from 30% to ∼ 99% and at temperatures 25°C, 40°C, and 60°C. Water absorption levels were determined using the gravimetric method, and the influence on reliability was assessed using electrochemical impedance and leak current measurements performed on the surface insulation resistance comb patterns. The corrosion studies were correlated with the hygroscopicity results and solubility data. Corrosion morphology was analysed using the optical microscopy and scanning electron microscopy. The results show that the hygroscopic nature of typical solder flux residue depends on its chemical structure and temperature. An increase of temperature shifts the critical RH level for water vapour absorption towards lower RH range, accelerating the formation of a conductive electrolyte and the occurrence of ion transport-induced electrochemical migration. The overall ranking of flux activators with the increasing order of aggressivity is: palmitic < suberic < adipic < succinic < glutaric < dl-malic acid.

OPTIMIZATION OF CHEMISTRY FOR A VAPOUR PHASE PROCESS TO DEFLUX NO CLEAN LEAD FREE MATERIALS ON PCBS

Abstract The cleaning action to remove baked-on solder flux residues after reflow, also known as defluxing, from printed circuit boards has become more and more challenged because of the higher densities in today's electronics, miniaturization, lower stand off components, and complex geometries where the packaging has evolved from 2D to 3D. This situation becomes even more difficult in No Clean lead free assembly where higher process temperatures are applied causing charring to flux residues. In addition, traditional chemicals like CFC's, HCFC's, brominated solvents, detergents and glycols cannot longer do a good cleaning job anymore because most flux formulations have changed. An hydrocarbon solvent used as co-solvent in vapour phase application has been re-formulated under three main criterias: environmental legislations, toxicity, and defluxing performance The results from the laboratory stage have been applied to clean the fluxes of three No Clean lead free solder pastes used in mass production of printed circuit boards assemblies with high densities interconnections and exposed to harsh environmental conditions.

Analysis of surface insulation resistance related failures in electronics by circuit simulation

PurposeThe purpose of this study is to show that the humidity levels for surface insulation resistance (SIR)-related failures are dependent on the type of activators used in no-clean flux systems and to demonstrate the possibility of simulating the effects of humidity and contamination on printed circuit board components and sensitive parts if typical SIR data connected to a particular climatic condition are available. This is shown on representative components and typical circuits.Design/methodology/approachA range of SIR values obtained on SIR patterns with 1,476 squares was used as input data for the circuit analysis. The SIR data were compared to the surface resistance values observable on a real device printed circuit board assembly. SIR issues at the component and circuit levels were analysed on the basis of parasitic circuit effects owing to the formation of a water layer as an electrical conduction medium.FindingsThis paper provides a summary of the effects of contamination with various weak organic acids representing the active components in no-clean solder flux residue, and demonstrates the effect of humidity and contamination on the possible malfunctions and errors in electronic circuits. The effect of contamination and humidity is expressed as drift from the nominal resistance values of the resistors, self-discharge of the capacitors and the errors in the circuits due to parasitic leakage currents (reduction of SIR).Practical/implicationsThe methodology of the analysis of the circuits using a range of empirical leakage resistance values combined with the knowledge of the humidity and contamination profile of the electronics can be used for the robust design of a device, which is also important for electronic products relying on low current consumption for long battery lifetime.Originality/valueExamples provide a basic link between the combined effect of humidity and contamination and the performance of electronic circuits. The methodology shown provides the possibility of addressing the climatic reliability of an electronic device at the early stage of device design by using typical SIR data representing the possible climate exposure.

Relative effect of solder flux chemistry on the humidity related failures in electronics

Purpose – This paper aims to investigate the effect of no-clean flux chemistry with various weak organic acids (WOAs) as activators on the corrosion reliability of electronics with emphasis on the hygroscopic nature of the residue. Design/methodology/approach – The hygroscopicity of flux residue was studied by quartz crystal microbalance, while corrosive effects were studied by leakage current and impedance measurements on standard test boards. The measurements were performed as a function of relative humidity (RH) in the range from 60 to ∼99 per cent at 25°C. The corrosiveness of solder flux systems was visualized by the ex situ analysis using a gel with tin ion indicator. Findings – The results showed that the solder flux residues are characterized by different threshold RH, above which a sudden increase in direct current leakage by 2–4 orders of magnitude and a significant reduction in surface resistance in the impedance measurements were observed. Practical implications – The findings are attributed to the deliquescence RH of the WOA(s) in the flux and chemistry of water-layer formation. The results show the importance of WOA type in relation to its solubility and deliquescence RH on the corrosion reliability of printed circuit boards under humid conditions. Originality/value – The classification of solder flux systems according to IPC J-STD-004 standard does not specify the WOAs in the flux; however, ranking of the flux systems based on the hygroscopic property of activators would be useful information when selecting no-clean flux systems for electronics with applications in humid conditions.

Solder Flux Residues and Humidity-Related Failures in Electronics: Relative Effects of Weak Organic Acids Used in No-Clean Flux Systems

This paper presents the results of humidity testing of weak organic acids (WOAs), namely adipic, succinic, glutaric, dl-malic, and palmitic acids, which are commonly used as activators in no-clean solder fluxes. The study was performed under humidity conditions varying from 60% relative humidity (RH) to ∼99%RH at 25°C. The following parameters were used for characterization of WOAs: mass gain due to water adsorption and deliquescence of the WOA (by quartz crystal microbalance), resistivity of the water layer formed on the printed circuit board (by impedance spectroscopy), and leakage current measured using the surface insulation resistance pattern in the potential range from 0 V to 10 V. The combined results indicate the importance of the WOA chemical structure for the water adsorption and therefore conductive water layer formation on the printed circuit board assembly (PCBA). A substantial increase of leakage currents and probability of electrochemical migration was observed at humidity levels above the RH corresponding to the deliquescence point of WOAs present as contaminants on the printed circuit boards. The results suggest that use of solder fluxes with WOAs having higher deliquescence point could improve the reliability of electronics operating under circumstances in which exposure to high humidity is likely to occur.

Contamination profile on typical printed circuit board assemblies vs soldering process

Purpose – The purpose of this paper was to analyse typical printed circuit board assemblies (PCBAs) processed by reflow, wave or selective wave soldering for typical levels of process-related residues, resulting from a specific or combination of soldering processes. Typical solder flux residue distribution pattern, composition and concentration are profiled and reported. The effect of such contaminants on conformal coating was tested. Design/methodology/approach – Presence of localized flux residues was visualized using a commercial residue reliability assessment testing gel test and chemical structure was identified by Fourier transform infrared spectroscopy, while the concentration was measured using ion chromatography, and the electrical properties of the extracts were determined by measuring the leak current using a twin platinum electrode set-up. Localized extraction of residue was carried out using a commercial critical contamination control extraction system. Findings – Results clearly show that the amount and distribution of flux residues are a function of the soldering process, and the level can be reduced by an appropriate cleaning. Selective soldering process generates significantly higher levels of residues compared to the wave and reflow process. For conformal coated PCBAs, the contamination levels generated from the tested wave and selective soldering process are found to be enough to generate blisters under exposure to high humidity levels. Originality/value – Although it is generally known that different soldering processes can introduce contamination on the PCBA surface, compromising its cleanliness, no systematic work is reported investigating the relative levels of residue introduced by various soldering processes and its effect on corrosion reliability.

Influence of sodium chloride and weak organic acids (flux residues) on electrochemical migration of tin on surface mount chip components

The electrolytic properties of sodium chloride and no-clean solder flux residue, and their effects on electrochemical migration and dendrite growth on surface mount chip capacitors were investigated. The leakage current dependency on concentration of contaminants was measured by a solution conductivity method and compared with current measurements using DC voltage. The effect of electrolyte concentration and potential bias on the probability of electrochemical migration was investigated using a water droplet method on chip capacitors. The results from leakage current and conductivity measurement showed a difference which is caused by polarization effects, and demonstrated existing issues when indexing contamination levels on printed circuit board assemblies using a standardised solvent extract method. The experimental results showed that dendrite growth was dependent on the type and amount of contamination. The probability of migration becomes less dependent on the amount of contamination for sodium chloride at high concentrations. However, for organic acids from flux residues the migration probability shows an abrupt decrease with increasing concentration, which is attributed to a pH change in the condensed electrolyte phase.

Effect of solder flux residue on the performance of silicone conformal coatings on printed circuit board assemblies

Conformal coatings are applied on printed circuit board assemblies (PCBAs) in order to protect the assembly from environmental influence and silicone-based coating is commonly used. A systematic study on the performance of silicone conformal coating in connection with process-related contaminants is the focus of this paper. Different test substrates such as plain laminate surface and a test PCBA with and without conformal coatings were exposed to high humid environment at ambient temperature and the performance of the coating was evaluated using various parameters such as increase in leakage current across the components, morphology of the coating, and analysis of dendrite formation due to electrochemical migration under the coating. The morphology of the coating before and after exposure was investigated using scanning electron microscopy, and energy dispersive X-ray spectroscopy. Results show that the presence of flux residues greatly influences the electrical functionality of the components and also degrades the performance of the coatings compared to the clean substrates.

Corrosion failure due to flux residues in an electronic add-on device

Corrosion of components and sub-assemblies on an electronic Printed Circuit Board Assembly (PCBA) is a major reliability concern. Both process and user related contamination will influence the corrosion reliability of a PCBA and the electronic device as a whole. An important process related contamination is solder flux residues which can act as a corrosion promoter in humid atmosphere due to the presence of ionic substances and a resin component. The presence of ionic substances will increase the conductivity of a condensed water layer and influence corrosion processes, depending on the species present. The resin component can easily attract dust during operation, which will eventually make surfaces hydrophilic and are thus become a potential source for ions. This paper describes the failure analysis of tactile switches, used in PCBAs mounted in wind turbines. More detailed investigation of the electrochemical behavior metallic materials (alloys) used in the switch and risk of electrochemical migration (ECM) between the switch components in presence of flux residues was also carried out. Investigations included potentiodynamic polarization measurements on the switch electrodes using a micro-electrochemical technique, in situ ECM studies, and scanning electron microscopy (SEM). Failure of the switches was found to be either due to the flux residue acting as an insulating layer or as a corrosion accelerator causing ECM.

Structural reliability assessment of multi-stack package (MSP) under high temperature storage (HTS) testing condition

Wire ball open failure at the interface of the gold wire and bonding pad of a multi-stack package (MSP) under high temperature storage (HTS) condition of 150 °C is studied. Failure analysis using FIB-SEM was conducted by in-plane moiré interferometry and FEA to clarify the failure mechanism. The ball open failure due to Kirkendall void that results from metal diffusion at high temperature was accelerated by the tensile stress imposed at the gold wire. The tensile stress developed at the gold wire when packages showing different warpage behaviours were stacked. Mechanical interaction between top and bottom packages caused unstable warpage, readily twisted and saddled. The wire came in contact with the photo-sensitive solder resist (PSR) dam because of the unstable warpage and this contact resulted in tensile stress at the gold wires. Solder flux residues reacted with the encapsulant, and as a result, the encapsulant of the top package adhered to the chip of the bottom package, and this adherence created additional tensile stress at the gold wires. To reduce the tensile stress at the wires, the PSR dam was removed, loop shape was altered from 45° to 90°, water soluble flux was applied, and cleaning process was added. HTS reliability was significantly improved and guaranteed after reducing the tensile stress at the wires.

Investigation of the effect of solder flux residues on RF signal integrity using real circuits

PurposeProvide information on the effects of flux residues from surface mount assembly on radio frequency (RF) performance.Design/methodology/approachA series of test vehicles designed to evaluate the RF performance on various test patterns and some simple circuits. Empirical testing is used in determining the data.FindingsProvides a methodology for checking the performance of flux residues as well as information on the performance of a few different flux residue types.Research limitations/implicationsThis is not an all encompassing project and the results may not extrapolate out to higher frequency ranges.Practical implicationsA good source of reference that can be used to understand the impacts of the assembly process on RF performance.Originality/valueThis paper shows the effect of assembly materials (flux residues) on a real circuit and not just test patterns. It can give a basic understanding to process engineers of the potential impact of the assembly process on a RF circuit.

Processing and reliability of CSPs with underfill

The use of chip scale packages (CSPs) is rapidly expanding, particularly in portable electronic products. Many CSP designs will meet the thermal cycle or thermal shock requirements for these applications. However, mechanical shock and bending requirements often necessitate the use of underfills to increase the mechanical strength of the CSP-to-board connection. This paper examines the assembly process with capillary and fluxing underfills. Issues of solder paste versus flux only, solder flux residue cleaning and reworkability are investigated with the capillary flow underfills. Fluxing underfills eliminate the issues of flux-underfill compatibility, but require placement into a predispensed underfill. Voiding during placement is discussed. To evaluate the relative performance of the underfills, a drop test was performed and the results are presented. All of the underfills significantly (5-6x) improved the reliability in the drop test compared to nonunderfilled parts. Test vehicles were also subjected to liquid-to-liquid thermal shock testing. The use of underfill improved the thermal shock performance by /spl ges/5x.

Ethylene glycol ether free solder paste development

There are growing concerns in the electronics industry for not only finding alternatives to lead but also other potentially hazardous materials as well. This paper summarizes the development of ethylene glycol ether (EGE)-free solder flux for the formulation of lead-free solder pastes. Replacing the toxic components in the flux was only the first challenge, the criteria of commercially proven pastes also had to be met. Both commercial and in-house solder paste formulations were evaluated for printability, reflow, wetting, flux residue removal, and solder void characteristics. Two critical issues, solder bump boids and flux residue removal, were identified and associated with the high temperature reflow of Pb-free pastes. These issues were not effectively improved by the existing commercial EGE-free solder pastes. New solder paste formulations were developed utilizing alternative chemistry than those found in traditional solder paste fluxes. These pastes, some, of which are also water soluble, reduced void frequency and size by more than 4x as compared to vendors' pastes. Solder bump height uniformity of 135 4 m within each die was consistently achieved. Thermal-mechanical reliability tests were performed on various lead-free solder alloys using the new flux formulations. The reliability of flip chip assembled DCA on organic boards with both OSP/Cu and Cu/Ni/au pad metallizations were comparable to eutectic Sn63Pb37 bumped assemblies using commerical pastes.

An evaluation of the effect of ageing on the cleanability of solder flux residues

The efficiency of cleaning of flux residues after various periods of ageing was assessed by measuring the ionic contamination removed in an Ionograph 500 SMD. The flux residues were removed from bare boards, and boards with through hole and surface mount components. The effect of different ageing temperature was also investigated. The work has shown that there is a maximum time interval following assembly during which cleaning should be carried out. The ionic contamination of aged assemblies with through hole and surface mount components were cleaned with varying efficiencies. The surface mount components were more difficult to clean. The use of brushing and scrubbing proved particularly beneficial for the through hole components. A proprietary cleaner proved more effective than the generic alternatives considered.

Use of surface insulation resistance and contact angle measurements to characterize the interactions of three water soluble fluxes with FR-4 substrates

Soldering flux chemistry and its interaction with the printed wiring board have been important reliability concerns for a number of years. Post Vietnam investigation of military hardware revealed corrosion in some areas. The test method most frequently used to assess the corrosion potential of flux residues is surface insulation resistance (SIR) testing. This paper gives some background on surface insulation resistance testing and reports on its application to three different water soluble fluxes. The appearance of surface dendrites is linked to test procedures that allowed water condensation on the board surface. Subsurface conductive anodic filament formation is associated with the use of fluxes which contained polyglycols. The use of contact angle measurements to assess the effect of the soldering flux residues on the board is demonstrated.