Water Soluble Flux Research Articles

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.

Wave Soldering Using Sn/3.0Ag/0.5Cu Solder and Water Soluble VOC-Free Flux

Environmental and health concerns, due to the leaching of lead from landfills into ground water, have necessitated legislation that restricts the use of lead in electronics. The transition from the eutectic tin-lead composition used in electronic solders to lead-free solder is imminent. Understanding the impact of this transition on lead-free wave soldering is crucial because a large segment of printed circuit boards (PCBs) used in electronic home appliances are wave soldered. The usage of volatile organic compound (VOC)-free flux chemistry is expected to gather momentum in conjunction with lead-free wave soldering because of process requirements and environmental considerations. A thorough review of published literature indicated that there is limited information available on the application of water-soluble VOC-free flux chemistries for lead-free wave soldering. Consequently, the objectives of this research were to select a preferred VOC-free water-soluble flux chemistry, understand the process window of wave soldering using a 96.5Sn/3.0Ag/0.5Cu lead-free alloy, and study the impact of the lead-free wave soldering process on different surface finishes. Many of the earlier process recommendations were based on a solder pot temperature of 260 degC and higher. However, the packaging of through hole components may not withstand the higher pot temperature and longer contact time. Hence, assembly at a lower solder pot temperature and shorter contact time are highly desired. Consequently, experiments were conducted to verify the feasibility of reducing the solder pot temperature (250 degC) and the contact time (2 s). Three VOC-free fluxes, from three different vendors, were evaluated and the best flux chosen was used to establish the process window. It was demonstrated through this study that it is possible to wave solder, using a lead-free solder with low silver content, different surface finishes with a wide process window. The effect of lead-free process temperatures on the cleaning of the VOC-free water-soluble flux residues was evaluated. Residues were washed and cleaned using the existing equipment sets without any change in the process parameters. The process developed based on the initial set of experiments has been validated by the successful assembly of a number of lead-free prototype assemblies

Wetting and reaction of Sn-2.8Ag-0.5Cu-1.0Bi solder with Cu and Ni substrates

The wettability of newly developed Sn-2.8Ag-0.5Cu-1.0Bi lead-free solder on Cu and Ni substrates was assessed through the wetting balance tests. The wettability assessment parameters such as contact angle (ϑc) and maximum wetting force (Fw) were documented for three solder bath temperatures with three commercial fluxes, namely, no-clean (NC), nonactivated (R), and water-soluble organic acid flux (WS). It was found that the lead-free Sn-2.8Ag-0.5Cu-1.0Bi solder exhibited less wetting force, i.e., poorer wettability, than the conventional Sn-37Pb solder for all flux types and solder bath temperatures. The wettability of Sn-2.8Ag-0.5Cu-1.0Bi lead-free solder on Cu substrate was much higher than that on Ni substrate. Nonwetting for Sn-2.8Ag-0.5Cu-1.0Bi and Sn-Pb solders on Ni substrate occurred when R-type flux was used. A model was built and simulations were performed for the wetting balance test. The simulation results were found very close to the experimental results. It was also observed that larger values of immersion depth resulted in a decrease of the wetting force and corresponding meniscus height, whereas the increase in substrate perimeter enhanced the wettability. The wetting reactions between the solder and Cu/Ni substrates were also investigated, and it was found that Cu atoms diffused into the solder through the intermetallic compounds (IMCs) much faster than did the Ni atoms. Rapid formation of IMCs inhibited the wettability of Sn-2.8Ag-0.5Cu-1.0Bi solder compared to the Sn-Pb solder.

Synthesis of REE and Y phosphates by Pb-free flux methods and their utilization as standards for electron microprobe analysis and in design of monazite chemical U-Th-Pb dating protocol

(REE,Y) phosphates were synthesized in a 1 atm furnace by flux-growth methods involving Pb-free fluxes. Microcrystalline (REE,Y) phosphate was precipitated from a solution of (REE,Y) chlorides or nitrates plus ammonium dihydrogen phosphate, and mixed with a M2CO3 (M = Li or Na)-MoO3 flux (75:25:2 molar ratio M2CO3:MoO3:REEPO4). Crystal growth was achieved over the temperature range 1350–870 °C, with extended (15 h) high- T soaking, and cooling rates of 3 °C/h. Crystals grown by this method are clear, generally inclusion-free, up to several mm in length, and are easily extracted from the water-soluble flux. Crystal size and habit are influenced by the alkali component of the flux. Successful LREE phosphate synthesis is favored by both Li-and Na-bearing fluxes, whereas better results for Y and MREE-HREE phosphates were achieved with Na-bearing fluxes. Li-bearing fluxes produced LREE phosphates with an overall platy habit, in contrast to more prismatic-to-equant LREE-MREE phosphates produced with Na-bearing fluxes. Structural refinements of single-crystal X-ray diffraction data show that LREEPO4 (La-Gd) crystallize with the monoclinic monazite structure, and HREEPO4 (Tb-Lu) plus YPO4 crystallize with the tetragonal xenotime structure. Element distribution maps of synthetic REEPO4 reveal homogeneous distribution of REE and P at grain-size scale and below, and both wavelength-dispersive (WD) spectral scans and quantitative electron microprobe analyses show no other elements (e.g., flux inclusions) present at significant levels. The synthetic phosphates grown with this method are suitable for use as electron microprobe standards and as compositionally simplified monazite analogs for use in the design of U-Th-Pb chemical dating protocols. WD scans of synthetic unary and ternary (La-Ce-Nd) phosphates reveal that Ar X-ray detectors produce non-filterable LREE escape peaks with pulse-height analyzers optimized for Pb analysis. Such LREE escape peaks complicate collection of both U and Pb peak and background counts. Due to the larger energy difference between LREE peaks and Xe (relative to Ar), LREE escape peaks produced in Xe detectors are filterable with pulse height analyzers, resulting in LREE-free spectra over the wavelength range sampled for Pb peak and background collection.

Influence of titanium dioxide nanopowder addition on microstructural development and hardness of tin–lead solder

This paper presents the microstructure and hardness of composite solders obtained by the addition of nanopowders of titanium dioxide to a conventional solder. Titanium dioxide powders-reinforced lead (Pb)–tin (Sn) composite solders were prepared by thoroughly blending nano-sized titanium dioxide powders with powders of a eutectic solder and using a water-soluble flux. The blended solder paste was melted and allowed to re-solidify in a crucible placed on a hot plate and maintained at a constant temperature. Optical microscopy observations revealed that for additions of titanium dioxide up to 1 wt.%, the grain size and width of the grain boundary decreased. For the addition of 2 wt.% of titanium dioxide, nanopowders microporosity was observed both at and along the grain boundary regions coupled with the presence of second-phase particles. Microhardness measurements revealed that the addition of titanium dioxide nanopowders is helpful in enhancing the overall strength of the eutectic solder.

The effect of flux chemistry, applied voltage, conductor spacing, and temperature on conductive anodic filament formation

Conductive anodic filament (CAF) formation, a failure mode in printed wiring boards (PWBs) exposed to high humidity and high voltage gradient, has caused catastrophic field failures. This study quantified the effect of flux chemistry, applied voltage (V), spacing (L), and temperature on the failure rate. Test vehicles, which had hole-to-hole spacing of 0.5 mm or 0.75 mm, were processed with one of three water-soluble fluxes (WSFs), and a heated control was also evaluated. The samples were placed in a temperature humidity chamber at 85%RH, at one of three temperatures: 75°C, 85°C, or 95°C. A voltage of 150 V or 200 V was applied to the test vehicle and periodically removed so that a measurement could be taken. A specially designed linear circuit was used to determine when the insulation resistance dropped significantly, indicating a failure. Activation energies were determined. The mean time to failure was a function of L 4 /V 2 .

The influence of copper nanopowders on microstructure and hardness of lead–tin solder

This paper presents the microstructure and hardness of composite solders obtained by the addition of nanopowders of copper to a conventional solder. Copper powders-reinforced lead (Pb)–tin (Sn) composite solders were prepared by thoroughly blending nano-sized copper powders (average powder particle size 100 nm) with a powder of a eutectic solder and using a water-soluble flux. The blended solder paste was melted and allowed to re-solidify in a crucible placed on a hot plate and maintained at a constant temperature. Optical microscopy observations revealed the as-solidified microstructure of the composite solder to be altered by the addition of nanopowders to the eutectic Sn–Pb solder. The copper powders precipitated as intermetallic compounds that were non-uniformly distributed through the microstructure. Microhardness measurements revealed a 30–40% increase in hardness of the composite solder over the conventional unreinforced eutectic counterpart.

Residues in printed wiring boards and assemblies

This paper is an examination of residues on printed wiring boards and printed wiring assemblies. Sources of residues are illustrated and the effects of various residues are discussed. Case studies are presented for bare board cleanliness issues, water soluble flux and aqueous cleaning processes, and low solids flux (no‐clean) processes, with and without cleaning. The case studies reflect lessons learned in various process troubleshooting efforts. Residues in this paper were characterized using advanced ion chromatography procedures. In addition, some data on surface insulation resistance (SIR) are presented.

A comparison of hourly versus daily testing methods for evaluating the reliability of water soluble fluxes

This paper presents background on surface insulation resistance (SIR) testing and reports on its application to six water-soluble fluxes, and two hot-air solder leveling (HASL) fluids. This work evaluates two electrical reliability-testing methods. One method recorded SIR readings on a daily basis, the other on an hourly basis. During the twenty-eight day test, increased frequency of testing was more successful in detecting discrete electrical events, such as surface dendrites but not subsurface conductive anodic filaments (CAF). These failure mechanisms are sporadic in nature as they initiate, grow, short out and disintegrate, reform, then short out again. Unless a measurement is taken during the time when shorting is about to occur, they will not be detected electrically. Surface dendrites due to water condensation were easily detected with the hourly SIR method but not with the daily measurements. Unfortunately, neither the daily nor hourly SIR methods detected the presence of conductive anodic filaments (CAF). These results demonstrate the importance of visual examination of circuit assemblies after environmental testing, as well as highlighting the need for a more sensitive electrical reliability test method.

Characterizing the weak organic acids used in low solids fluxes

The elimination of chlorofluorocarbons (CFCs) and other chlorinated cleaning solvents due to their long-term environmental impact has lead electronic assemblers to examine soldering fluxes that reduce or eliminate the need for post-solder cleaning. Today, low solids fluxes are replacing more traditional rosin-based and water-soluble fluxes because many of them can be used in a no-clean process. Most low solids fluxes use weak organic acids as active ingredient. It has been reported that some of these weak organic acids leave behind residues that are corrosive to copper. Surface Insulation Resistance (SIR) measurements of flux-processed comb patterns have been the main test method used to determine the corrosivity of flux residues. This test has been performed with test samples exposed to accelerated temperature and humidity conditions of 85°C and 85%RH and a 50 V bias. Recent data on some weak organic acids suggests that they slowly disappear at this temperature and a lower test temperature of 65°C has been introduced into the new Bellco Standard. In Europe, this test is normally performed at 40°C and 93% RH. This paper reports on the application of SIR tests to study the corrosive behavior of three carboxylic acids (succinic, glutaric, and adipic acids) that are commonly used as the active ingredients in soldering fluxes. Coupons treated with equi-molar solutions of the acids were either exposed to reflow-soldering conditions or wave soldered face-up to create partially heated residues. Both tests were run under two different accelerating conditions, 85°C/85%RH for 7 days or 40°C/93%RH for 20 days. This latter condition is being considered for inclusion in an IPC standard. At the end of the test period, both corrosion and SIR test samples were examined under a microscope and any residues or dendritic growth were documented. SEM and EDX characterization was also performed to determine the residue and dendrite composition.

LASER REMOVAL OF FLUX RESIDUE BEFORE CHIP ATTACHMENT TO BOARD IN A FLIP CHIP ASSEMBLY

The occurrence of flux residue is a manufacturing and reliability concern for flip chip assemblies. In this paper, removal of flux residue by laser cleaning before chip attachment to board is carried out. In the flip chip process, 63%Sn37%Pb solder paste with a water soluble flux was printed on wafer by stencil and subjected to infrared reflow. Instead of washing with water, flux residue on the chips after wafer sawing was removed by ablation of pulsed ultraviolet (UV) lasers with wavelengths of 248 nm and 355 nm respectively. Some parameters in the cleaning process such as the energy density of laser and number of pulses irradiated on the chips had been optimized. It was found that the laser beams also etched polyimide passivation on the chips. After laser etching, the passivation surface became cleaner. The rate of etching was measured by a surface profilometer. To conclude, we discuss the advantages of laser cleaning over wet chemical cleaning in the removal of flux residue.

Analysis of Catastrophic Field Failures Due to Conductive Anodic Filament (CAF) Formation

ABSTRACTUnder certain environmental conditions, printed wiring boards (PWB) respond to applied voltages by developing subsurface deposits of copper salts extending from anode to cathode along separated fiber / epoxy interfaces. The formation of these deposits, termed conductive anodic filaments (CAF) require high humidity (80%RH) and high voltage gradient (5V/mil). The humidity exposure during the storage environment may cause the failure in the use environment. CAF formation is enhanced by the use of certain hot air solder leveling (HASL) fluids and / or water soluble flux constituents.In this work, two catastrophic field failures were analyzed. Both failures were related to boards produced in a manufacturing process, which included HASL. One CAF failure occurred between a component through-hole and power plane held at a potential difference of 40V with a 0.005″ nominal spacing. The other occurred on an inner layer of a multi-layer board (MLB) between a via and ground plane held at a potential difference of 320V with 0.015” nominal spacing. The nature of the CAF was analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). Ion chromatography (IC) was used to identify residue extracted from a failed board. The failure phenomena known as CAF poses serious longterm reliability concerns in electronic products exposed to adverse and hostile environments, especially those with closely spaced conductors.

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.

Effect of Substrate Preheating on Solderability Performance as a Guideline for Assembly Process Development Part 1: Baseline Analysis *

A study was performed which investigated the wettability of 63Sn‐37Pb and 96.5Sn‐3.5Ag solders on copper and gold ‐nickel plated Kovar ™ using a rosin ‐based, mildly activated (RMA) flux, a water soluble organic acid flux (WS ),and a low residue (LR) flux. The quantitative metric was the contact angle, θc, measured by the meniscometer /wetting balance technique. The first part of the study (Part 1) examined wetting performance following continuous exposure to 25°C prior to testing. Then, a preheating step was introduced into the experimental procedure after flux application, but preceding the actual wettability test in order to simulate a factory reflow process; these results are presented in Part II of this study. Contact angles for the 63Sn‐37Pb solder (215°C) on copper were 22±2° with the RMA flux, 12±5° for the WS flux, and 31±6° for the LR flux. Increasing the 63Sn‐37Pb solder temperature to 245°C improved wettability with the RMA and LR fluxes, but no change was observed with the WS fulx. Theii 96.5Sn‐3.5Ag lead ‐free solder exhibited poorer wettability on copper compared with the 63Sn‐37Pb alloy, with contact angles of 41±2° (RMA), 63±15°(WS) and 39±4°(LR). For the gold ‐nickel plated Kovar™ substrates, the 63Sn‐37Pb solder at 215° had contact angles of 15±3°, 35±6° and 29±6° for the RMA, WS and LR fluxes, respectively. The values were reduced at the higher test temperature (245°). The 96.5Sn‐3.5Ag solder also exhibited good wetting performance on the gold ‐nickel plated Kovar™ specimens compared with copper. Analysis of the interfacial tension parameters, γSF‐γSLand γLF ,exemplified the importance of γLF as well as the condition of the surfaces (γSF ) on wettability performance. A so ‐called ‘combined analysis’ of the 63Sn‐37Pb and 96.5Sn‐3.5Ag wettability data on either copper or gold ‐ nickel plated Kovar™ substrates was used to predict the solder temperature dependence of wettability for the three fluxes and two base materials.

Contamination of Various Flux/Cleaning Combinations on SMT Assemblies: A Discussion of Different Analysis Methods

The contamination on SMT assemblies caused by a number of flux/cleaning variation has been investigated, with several analysis methods used to quantify the contamination. The results from analysis of test printed circuit boards (PCBs) using anion separation, conductivity, ionic contamination measurement, visual and SEM inspection, microsections and surface insulation resistance (SIR) methods were compared. The best cleaning result was observed with the combination of water soluble (WS) solder cream and water cleaning. The best measurement method for cleanliness of PCBs was considered to be SIR measurement. For WS fluxes, the iconic contamination test offers effective measurement of the amount of residues left on the PCBs.

Application Model for Organic Solderability Preservatives

Organic solderability preservative (OSP) coatings are not new. They have been used successfully with aggressive water soluble flux for assembly of through‐hole only PWBs. However, the multiple heating cycles required for mixed technology assembly and use of no‐clean low solids flux (LSF) for wave solder assembly have placed a greater demand on the solderability protection provided by OSPs. Wetting balance and float testing were used to evaluate numerous OSPs as well as the potential for these surface finishes to be used for ‘No‐Clean’ assembly. Although these laboratory evaluations revealed that OSPs are not as robust as SnPb, they did indicate the assembly processes and materials which could work with OSPs. Additional simulated assembly trials with test vehicles confirmed that thick OSP pre‐flux coatings interfere with soldering and that the solderability of surfaces with thin OSPs degrades when heated in an air environment. Since none of the OSPs evaluated outperformed the imidazole currently in use at AT&T, a no‐clean LSF assembly production trial with a mixed technology telecommunication circuit pack was conducted to compare imidazole with hot air solder levelled surfaces. The production trial and laboratory evaluations resulted in the development of an application model. The elements of the application model are not complicated: (1) use thin OSPs, (2) avoid baking, (3) use as aggressive a flux as possible, (4) apply as much flux as possible, (5) apply the flux where you want solder to wet, and (6) use nitrogen inerted processes whenever possible. Combination of these elements has led to the successful implementation of OSPs for no‐clean assembly. Funding for this effort was obtained through the National Center for Manufacturing Sciences (NCMS) Printed Wiring Board Interconnect Program.

Conduction mechanisms in contaminant layers on printed circuit boards

AC impedance methods have been utilized to explore surface conduction mechanisms on printed circuit boards (PCBs) containing various types of solder flux contaminants. Residues from water-soluble, rosin-based, and no-clean fluxes were analyzed and evaluated for their potential impact on reliability. Impedance data for intentionally contaminated PCBs having several circuit line geometries were obtained at different relative humidities. An equivalent circuit model is presented that fits the data obtained. It is used to evaluate and distinguish among ohmic, kinetic, and diffusion effects and to predict the environmental conditions that may be detrimental for various line geometries.

Aqueous and Semi-Aqueous Cleaning Processes

AT&T's efforts to eliminate environmentally unsound manufacturing processes have included development of alternative fluxing and cleaning operations for use during printed-circuit board assembly. Innovations have been necessary because of health and stratospheric ozone depletion concerns. For wave-soldered through-hole assembly, elimination of solvent cleaning processes has been facilitated by detergent underbrush cleaning. Waste disposal problems associated with the detergent have been alleviated by “semi-aqueous” underbrush cleaning that uses an emulsion of water and a naturally-derived terpene hydrocarbon solvent. Improved surface insulation resistance of aggressive water soluble fluxes has allowed aqueous cleaning for products compatible with total immersion. Low solids, no-clean fluxes that leave minimal residue have eliminated the cleaning step for many products. For surface mount applications, where underbrush cleaning is not applicable, spray cleaning with terpene followed by water rinse has proven effective. Safety concerns about spray flammability and low flash temperature have been resolved, and excellent cleaning results have been obtained with a production machine running at AT&T's Merrimack Valley facility.

Water Soluble Fluxes, their Reliability and their Usefulness as a Means of Eliminating CFC‐113 Usage

This paper outlines the composition of water soluble fluxes for the electronics industry and their methods of use when wave soldering and reflowing tinned coatings and solder pastes. Process optimisation is facilitated by the Taguchi method. Three types of cleaning machinery are evoked, with varying results. It is shown that the energy/time relationship is important to ensure adequate cleaning quality. A number of fallacious arguments are debunked. Methods of water purification and the problems of effluent treatment for all sizes of installation are addressed. Doubt is expressed as to the viability of closed‐circuit water recycling except for the largest installations or where exceptional conditions prevail. It is shown that water soluble fluxes and their subsequent aqueous removal are unlikely to make any significant contribution to the Greenhouse Effect. The overall cost of their use is substantially similar to that of rosin fluxes with CFC‐113 azeotropes at 1986 prices. Cleanliness control under production and laboratory conditions is discussed with reference to both ionic contamination testing, including its use for SMDs, and SIR analysis, especially at low voltages, including non‐destructive production SIR testing. Reliability of the assembled circuits is shown to be at least as good as that with more traditional soldering and cleaning methods, frequently better, and this is the case even for military and aerospace applications. The paper concludes that, now that quality water soluble solder pastes are available, this method is most likely to become the workhorse for the majority of electronics applications.

Non‐ionic Water Soluble Flux Residue Detection by XPS

For many years the analysis of contaminant residues on PWB surfaces has been of major importance to the industry. While the identification of residues left on metallic surfaces has proven to be relatively straightforward, the analysis of organic contamination of similar composition to that of the underlying board surface has not been as successful. Through the use of modern XPS instrumentation, the non‐ionic component of water soluble flux has been identified and differentiated from the chemically similar FR‐4 and soldermask substrates. This paper presents the XPS results for a series of experiments aimed at determining the location and relative concentration of water soluble flux residues on standard surface insulation resistance (SIR) comb patterns. The data show that the water soluble flux residue is not present as a uniform coating on the board surface but appears in localised sites in high concentrations while being absent in other locations. Through more aggressive cleaning procedures the sites of high residue concentration can be significantly reduced.