For a number of years electronic manufacturers of printed circuit assemblies have used rosin‐based soldering fluxes. Post‐solder cleaning was accomplished with chlorinated or…
Abstract
For a number of years electronic manufacturers of printed circuit assemblies have used rosin‐based soldering fluxes. Post‐solder cleaning was accomplished with chlorinated or chlorofluorocarbon (CFC) solvents. With the elimination of these solvent options due to their destructive effect on the stratospheric ozone layer, manufacturers are considering alternative cleaners for rosin flux or new flux choices which can be cleaned with water or left uncleaned. Many of the flux formulations are relatively new and their long‐term effect on the performance of products manufactured with them is unknown. Although ionic contamination testers can alert one to the ionic levels remaining on an assembly, there is no direct relationship between the total ionic level and the corrosivity of the soldering flux. Surface insulation resistance testing is used in the industry, but the results are misunderstood by many. This is due to the fact that SIR data represent a complex dependency on a number of factors including (1) the test conditions (temperature, humidity, bias), (2) the area of interactions (often referred to as the number of squares), (3) the separation between lines on the interdigitated comb pattern, (4) the presence or absence of bias voltage during the test and (5) the nature of the substrate. All of these factors have been the driving force to develop a quantitative screening test for soldering flux residues. This test, originally reported by Dr David Bono, is being modified and developed at Georgia Tech to provide a quantitative evaluation of flux residue corrosivity. This work, in collaboration with the work being performed by the French UTE, will result in a new international standard. This paper reports the latest data on this important test development.
The 1980s and 1990s have seen the development of new andinteresting soldering flux formulations. Many of these fluxes exhibit improved solderingperformance or are favoured because…
Abstract
The 1980s and 1990s have seen the development of new and interesting soldering flux formulations. Many of these fluxes exhibit improved soldering performance or are favoured because of their reduced environmental impact. In order to further the understanding of these new fluxes and their interaction with the metallisation on the printed wiring board, as well as the substrate itself, one needs to examine test methods carefully and begin to correlate the data among the existing test methods. At Georgia Tech a variety of data have been collected on a number of fluxes including water soluble, low solids and activated rosin fluxes. Test methods for flux characterisation include surface insulation resistance testing, corrosion test measurements and recently impedance spectroscopy at low frequencies. This paper will review the variety of fluxes available, report on results of testing these fluxes using the techniques mentioned above and will define the important information related to soldering flux interactions which each test method uncovers.
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Kamila Piotrowska, Morten Stendahl Jellesen and Rajan Ambat
The aim of this work is to investigate the decomposition behaviour of the activator species commonly used in the wave solder no-clean flux systems and to estimate the residue…
Abstract
Purpose
The aim of this work is to investigate the decomposition behaviour of the activator species commonly used in the wave solder no-clean flux systems and to estimate the residue amount left after subjecting the samples to simulated wave soldering conditions.
Design/methodology/approach
Changes in the chemical structure of the activators were studied using Fourier transform infrared spectroscopy technique and were correlated to the exposure temperatures within the range of wave soldering process. The amount of residue left on the surface was estimated using standardized acid-base titration method as a function of temperature, time of exposure and the substrate material used.
Findings
The study shows that there is a possibility of anhydride-like species formation during the thermal treatment of fluxes containing weak organic acids (WOAs) as activators (succinic and DL-malic). The decomposition patterns of solder flux activators depend on their chemical nature, time of heat exposure and substrate materials. Evaporation of the residue from the surface of different materials (laminate with solder mask, copper surface or glass surface) was found to be more pronounced for succinic-based solutions at highest test temperatures than for adipic acid. Less left residue was found on the laminate surface with solder mask (∼5-20 per cent of initial amount at 350°C) and poorest acid evaporation was noted for glass substrates (∼15-90 per cent).
Practical implications
The findings are attributed to the chemistry of WOAs typically used as solder flux activators. The results show the importance WOA type in relation to its melting/boiling points and the impact on the residual amount of contamination left after soldering process.
Originality/value
The results show that the evaporation of the flux residues takes place only at significantly high temperatures and longer exposure times are needed compared to the temperature range used for the wave soldering process. The extended time of thermal treatment and careful choice of fluxing technology would ensure obtaining more climatically reliable product.
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Kamila Piotrowska, Feng Li and Rajan Ambat
The purpose of this paper is to investigate the decomposition behavior of binary mixtures of organic activators commonly used in the no-clean wave flux systems upon their exposure…
Abstract
Purpose
The purpose of this paper is to investigate the decomposition behavior of binary mixtures of organic activators commonly used in the no-clean wave flux systems upon their exposure to thermal treatments simulating wave soldering temperatures. The binary blends of activators were studied at varying ratios between the components.
Design/methodology/approach
Differential scanning calorimetry and thermogravimetric analysis were used to study the characteristics of weak organic acid (WOA) mixtures degradation as a function of temperature. The amount of residue left on the surface after the heat treatments was estimated by gravimetric measurements as a function of binary mixture type, temperature and exposure time. Ion chromatography analysis was used for understanding the relative difference between decomposition of activators in binary blends. The aggressivity of the left residue was assessed using the acidity indication gel test, and effect on reliability was investigated by DC leakage current measurement performed under varying humidity and potential bias conditions.
Findings
The results show that the typical range of temperatures experienced by electronics during the wave soldering process is not sufficient for the removal of significant activator amounts. If the residues contain binary mixture of WOAs, the final ratio between the components, the residue level and the corrosive effects depend on the relative decomposition behavior of individual components. Among the WOA investigated under the conventional wave soldering temperature, the evaporation and removal of succinic acid is more dominant compared to adipic and glutaric acids.
Practical implications
The findings are attributed to the chemistry of WOAs typically used as flux activators for wave soldering purposes. The results show the importance of controlling the WOA content and ratio between activating components in a flux formulation in relation to its tendencies for evaporation during soldering and the impact of its residues on electronics reliability.
Originality/value
The results show that the significant levels of flux residues can only be removed at significantly higher temperatures and longer exposure times compared to the conventional temperature range used for the wave soldering process. The potential corrosion issues related to insufficient flux residues removal will be determined by the residue amount, its composition and ratio between organic components. The proper time of thermal treatment and careful choice of fluxing formulation could ensure more climatically reliable product.
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Surface insulation, electrochemical migration and various other insulation resistances are terms which are often glibly used, sometimes even incorrectly. This paper categorises…
Abstract
Surface insulation, electrochemical migration and various other insulation resistances are terms which are often glibly used, sometimes even incorrectly. This paper categorises different types of insulation resistance and catalogues about twenty practical applications of insulation resistance measurement, each with its ideal general conditions of measurement (test voltage, bias voltage, bias polarity, test voltage period, test frequency, test duration, temperature, humidity, test pattern type, test pattern dimensions, voltage gradients, tolerances, etc.) This description is independent of any of the nearly forty known, often contradictory, standards, most of which no longer correspond to the practical printed circuit or assembly of today. Also discussed are the different technologies of insulation resistance measurement, starting with the original non‐electronic ‘Megger®’ types through to modern laboratory electrometers and, finally, instrumentation specific to the practical measurement of printed circuit insulation resistances, including static and dynamic types. The importance of automatic statistical analyses is emphasised, especially with production testing as well as qualification procedures. This paper is aimed not only at those wishing to learn what modern insulation resistance testing is all about, but also at experienced persons wanting to marshall their thoughts about the fundamental meanings of insulation testing for different applications and specifications.
The need to use cleaning methods other than traditional CFC‐113 solvent for hi‐rel electronics imposes more rigid cleanliness testing. In the past, this was mainly limited to…
Abstract
The need to use cleaning methods other than traditional CFC‐113 solvent for hi‐rel electronics imposes more rigid cleanliness testing. In the past, this was mainly limited to ionic contamination control, but this is probably insufficient by itself when using other methods. This paper discusses the various methods for which instrumentation is available, from the practical standpoint. This should satisfy all the requirements of both procurement agencies and manufacturers. Particular emphasis is placed on the fact that most existing standards are out‐of‐date and should be urgently revised. It is suggested that the standards be based on statistically valid test results rather than the simpler, but risky, go/no‐go methods. These probability limit levels should be modulated according to the use to which the circuitry will be put and the technology used in its manufacture. Above all, emphasis is placed on testing methods that are more scientifically based with less empirical guesswork.
L.J. Turbini, J. Schodorf, J. Jachim, L. Lach, R. Mellitz and F. Sledd
Today's emphasis on alternative flux technology as an approach to eliminate the use of chlorofluorocarbons (CFCs) requires an understanding of the corrosion potential of the new…
Abstract
Today's emphasis on alternative flux technology as an approach to eliminate the use of chlorofluorocarbons (CFCs) requires an understanding of the corrosion potential of the new fluxes. In 1989, Dr David Bono proposed that monitoring the effect of different soldering fluxes on the rate of corrosion of a copper wire printed on a circuit board would provide quantitative information on the corrosion potential of a flux. Further analysis of this testby Turbini et al. revealed that the degradation mechanism associated with Bono's test is the growth of conductive anodic filaments along the glass fibres of the epoxy‐glass boards. The original test method has been revised, and the test coupon redesigned with the goal of developing a standard, quantitative test method to characterise soldering fluxes. This paper will describe the equipment, test coupon and electrical circuitry associated with this proposed test method. Procedures chosen to reduce error sources associated with electrical noise will be reported and explained.
Conductive anodic filament (CAF) is a failure mode in printed wiring boards (PWBS), which occurs under high humility and high voltage gradient conditions. This paper aims to…
Abstract
Purpose
Conductive anodic filament (CAF) is a failure mode in printed wiring boards (PWBS), which occurs under high humility and high voltage gradient conditions. This paper aims to review the history of CAF from its identification in the 1970s to the statistical analysis of its failure mode and the factors that enhance its formation.
Design/methodology/approach
Charts the chronology and details the developments of CAF over the last 30 years.
Findings
CAF is a conductive copper‐containing salt created electrochemically that grows from the anode toward the cathode sub‐surface along the epoxy/glass interface. It can also grow from the anode on one layer to a cathode on another. CAF was first discovered in 1976 and was identified as a catastrophic failure mode. It is enhanced by high humidity during storage or use, by high voltage gradient between anode and cathode, by certain soldering flux ingredients, by hole drilling, multiple thermal cycles during processing, and by higher processing temperatures associated with lead‐free solders. CAF is a copper hydroxy chloride salt and is a semiconducting material.
Originality/value
Our analytical tools today are far superior to those of these early researchers. Early data were obtained from chart recorders and manual plotting. Today we have computers for automated data collection and analysis and the sensitivity of the scanning electron microscope has improved significantly. The researchers of the 1970s and early 1980s characterized the basic factors associated with CAF and in many ways we are just repeating what they have done.
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Aulis Tuominen, Eero Ristolainen and Ville Lehtinen
Owing to the incessant demand for reductions in the size of portable electronics, new dense packaging technologies are required. Reflow soldering is still mainly used for…
Abstract
Owing to the incessant demand for reductions in the size of portable electronics, new dense packaging technologies are required. Reflow soldering is still mainly used for component joining on the substrate. In tiny joints such as those in flip chip (FC) assemblies the flux effect is vitally important and needs to pass a narrower performance window than in ordinary surface mount technology (SMT). The determination of the suitability of a flux, as reported in this paper, is twofold; first, the flux must perform well in its intended purpose and second, the flux must not leave harmful residues causing leakage or electromigration. The first test used was the wetting balance test for all fluxes. Fluxes accepted on the basis of the wetting tests were then subjected to the surface insulation resistance test (SIR).
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W.J. Ready, S.R. Stock, G.B. Freeman, L.L. and L.J. Turbini
Under certain environmental conditions, printed wiring boards (PWBs) respond to applied voltages by developing sub‐surface deposits of copper salts extending from anode to cathode…
Abstract
Under certain environmental conditions, printed wiring boards (PWBs) respond to applied voltages by developing sub‐surface deposits of copper salts extending from anode to cathode along separated fibre/epoxy interfaces. These deposits are termed conductive anodic filaments (CAFs) and, in this work, the dimensions and growth patterns of a CAF have been determined by serial sectioning. The CAF growth pathway is characterised and the spatial distribution of the copper salts is quantified with scanning electron microscopy (SEM) using backscattered electrons. The chemical composition of the CAF is determined using energy dispersive X‐ray analysis (EDS). Prior research using high‐resolution non‐destructive X‐ray microtomography is correlated with the serial sectioning data. The failure phenomenon known as CAF may pose serious long‐term reliability concerns in electronics applications exposed to adverse and hostile environments.