STRAGTEGIES FOR ADDRESSING WHITE RESIDUE AND LOCALIZED CONTAMINATION

Transcription

1 STRAGTEGIES FOR ADDRESSING WHITE RESIDUE AND LOCALIZED CONTAMINATION Mike Bixenman, D.B.A. Phil Zhang Chris Shi Kyzen Corporation Nashville, TN, USA ABSTRACT Electrochemical migration is an occurrence of a conductive metal bridge forming between conductors when they are subjected to a DC voltage bias. Metal conductors grow from a positively charged conductor (cathode) to a negatively charged adjacent conductor (anode) creating a short circuit between the conductors. Electrochemical migration consists of metals plating in reverse. An ionic contaminant combines with water, generally forming a localized source of acid, which dissolves metal ions. Under the influence of the electrical potential, the metal ions move across the intervening space, plating out on the laminate as it goes, forming a metal filament. A more contaminated assembly has greater risk than a clean assembly. The purpose of this research is to document the root causes that promote electrochemical migration white residue, localized contamination, end use environment, and electric potential and strategies for addressing the problem. customers increasing demands for faster, smaller, and higher performance. WHITE RESIDUE FAILURE MECHANISMS Early circuit board failure represents a dominant concern for assemblers. Leading edge design failures are attributed to feature size reduction, which increases the risk of defects randomly induced by process flaws. 2 White residue sandwiched under highly dense low standoff components creates the potential for material defects caused by the presence of ionic residue (Figure 1). Traditional approaches to verifying reliability based on screening the output of a process are no longer effective. Rather, electronic assemblers are moving towards building in reliability by controlling and monitoring cleaning factors that assure complete removal of flux residue. Figure 1: White Residue Electro Migration Growth INTRODUCTION Electronic device complexity brings to light an increasingly important phenomenon: designing for reliability. Product longevity or the lack thereof - becomes more difficult when designing and manufacturing leading edge electronic assemblies. 1 As electronic assembly manufacturing processes become more advanced, consumer demand for greater performance and system functions increase. Consistent reliability over the expected life of the product is a driving factor for designers and assemblers. Cleaning flux residues post soldering has been a high reliability criterion practiced by assemblers of military, aerospace, automotive, medical devices and other value offerings. High dense designs reduce component spacing and standoff heights. The complexity of removing flux residue increases, while elevating the risk of white residue under low standoff components. Reliability is a major concern that places increased importance on manufacturing processes that completely remove all flux residues on the surface of the assembly as well as under the Z-axis. Assemblers are experiencing difficult cleaning challenges that must be overcome before they can produce reliable products that meet The rapid proliferation of portable electronic products increases the risk factor for failures. 4 Feature size reduction opens the market to faster and smaller devices. Cleaning these devices has become highly complex due to low standoff heights. Many industry standard cleaning processes do not remove the residue under the Z-axis (Figure 2). Flux residue bridges conductors, and in some cases heat during soldering and in use operation causes spherical solders 1

2 patterns to join and form a solder bridge across the conductors. For many of these devices, wear out related failure is less critical in determining product reliability over the devices useful lifetime. The critical factors move upstream to designing a process that assures reliability. number of I/Os, decreased area array pitches, and tighter component standoff heights (Figure 4). Figure 4: Component Cleaning Challenges Figure 2: White Residue under Chip Caps ELECTROCHEMICAL MIGRATION RISK Ion migration in an electric field is propagated by the charge balance at the interface where the total current density entering and leaving the electrolyte causes metal ions to split and form dendrites. The spatial coupling depends on the distance of the conductors with closer areas coupled more strongly. 3 The input / output current decreases for smaller electrodes while the current density increases. This phenomenon can create high current densities in high I/O devices. Highly dense assemblies and the spatial coupling are connected and depend strongly on the geometry of the electrolyte and electrode. Electrochemical migration is mediated by removing all flux residues, which eliminates ion migration under the influence of the electric field. 3 Partially cleaned flux residue leaves an ionic oxide film (white residue) that creates a path for metal dissolution reactions (Figure 3). Moisture from humidity and electrical current allows ionic residue to disassociate and propagate between conductors. Figure 3: White Residue Migration CLEANING LEADING EDGE ASSEMBLIES Technology-based market pressures increase reliability demands as electronic assemblers move upstream from conventional designs and toward threshold and leading edge technologies. 5 Over the past two decades, conventional surface mount technologies successfully adopted low residue no-clean soldering practices. Today s challenge for printed circuit board manufacturers hinges on density and miniaturization. High performance electronic assembly designs will be driven by multi-chip density, increasing Higher density, smaller components, and lower standoffs are changing the definition of circuit board cleanliness. The current or traditional normal view of quality assurance equated circuit board reliability to visual residue and the resistivity of solvent extract measurements. With the reduction in component size and low standoff clearances, the ability to extract and see measurable residues that correlate to product quality is much more suspect. Cleaning processes take on a whole new cleaning definition of removing residue that can be seen visually and residue entrapped under components that is commonly out of sight. The design and qualification of the cleaning process takes on new definitions. Assemblies with tighter spacing, small amounts of flux contamination can create increased circuit leakage, cross talk, and electrochemical migration. 6 These issues are increased from chemical changes in today s fluxes. Many new flux compositions reduce the level of high solids rosin needed to seal and encapsulate fabrication residues. Some of today s flux compositions use less resistant activators. Low solid flux compositions, commonly formulated with weak organic acids, leaves a chemical residue that does not directly correlate to the historical response mechanisms. Additionally, mixed technology circuit assemblies may be subjected to multiple soldering processes. It is not uncommon for an assembly to see SMT reflow for top and bottom components, wave soldering, selective soldering, rework, and localized brush cleaning that may allow pockets of residue in precise and critical areas due to secondary processing. The impact of these residues will 2

3 need to be assessed; appropriate test protocols will have to be established to assure product reliability. Figure 5: Airborne Connector Flux Entrapment CLEANING PROCESS DESIGN Process engineers looking to clean leading edge designs typically start by identifying the cleaning equipment and cleaning material. Many use a small subset of test cards to qualify the process. This technique worked when qualifying a cleaning process for conventional designs but typically fails to address numerous factors when designing leading edge cleaning processes. The design of the cleaning process must dig into critical factors such as the designs of the circuit cards being cleaned, through-put requirements, materials compatibility, cleaning material, process parameters, cleaning equipment, spray impingement, time in the wash, temperature of the wash, controlling the chemistry, ventilation, water management, environmental constraints, and cost of cleaning. Flux Residue Problem Area Design for manufacturability (DFM) includes a set of techniques to modify and improve printed circuit and cleaning process designs in order to match the cleaning process to the substrate, contaminant, and available cleaning methods. Designing printed circuit assemblies for ease of manufacture needs to begin with the CAD designer s awareness of the cleaning process and its limitations. As a general rule, through-hole assemblies offer fewer problems than surface mount assemblies do and, in particular, those on high-density interconnection structure substrates. The problem is that the driving force toward smaller components makes effective removal of flux residues more difficult. With a clear understanding of the unique part consideration and restrictions, the next step in the design for manufacturability considers the effects of contamination left on the circuit board after the assembly typically the soldering process. Upstream and downstream processing conditions have an effect on both cleaning effects and requirements. Subsequent processing steps may also influence cleaning effects and requirements. Figure 6 illustrates charred flux residue that was oxidized from heat used to reflow the board. Figure 6: Charred Flux Residue Best practice in designing the cleaning process is a thorough review of part composition, function, components, size/geometry/configuration, handling issues, part unique considerations/restrictions, and compatibility. Part composition, size, and geometry can create low clearances and, sandwiched components with little egress, resulting in residues that are very difficult to remove. Small and light weight parts increase the need to fixture assemblies as they track through the cleaning process. Figure 5 illustrates an airborne connector board where wave flux entraps into the end screws holding the connector flush to the board. Flux remaining under the connector expands and weeps to the side of connector when exposed to heat in the cleaning process drying step. To address this issue, board designers need to develop spacing that allows the cleaning material to penetrate and remove the flux residue. Materials compatibility represents an important consideration when designing the cleaning process. Factors of concern are board laminate, surface finishes, components, metal alloys, adhesive bond, part markings, plastics, the mix and configuration of materials in the assembly and the impact of entrapped contaminants. The critical considerations for designing the cleaning process are components; assembly materials, chemistries (including concentration), cleaning force (impingement), anticipated process time, temperature and equipment design. Cleaning factors that may impact the 3

4 effectiveness of the cleaning process include the cleaning agent, concentration, equipment, pressure, speed, and temperature. These same factors may influence materials compatibility. Figure 7 illustrates a blue (etched) solder joint attacked from by the cleaning agent. Figure 7: Blue (etched) Solder Joint Oxygenated organic materials dissolve rosin and resin (solute) structures naturally present in many flux types. The interaction of resin/rosin with solvent(s) increases the dissolution rate (Figure 9). Solvents are selected on the basis of like dissolves like commonly referred to as the solvated state, whereby organic resin flux residues are dissolved by oxygenated solvent molecules. Solvent dissolution is a kinetic process and is quantified by its rate. The rate of dissolution depends on the solvent and solute, temperature, impingement pressure, and interfacial surface tension. Figure 9: Like Dissolves Like ELECTRONIC ASSEMBLY CLEANING MATERIALS Electronic assembly cleaning material options cover three classes that include solvent, co-solvent, bi-solvent, semiaqueous, and aqueous compositions. During the pre-ozone depletion era, CFC-113 and rosin-based flux compositions were the standard. In today s environment, the cleaning agent of choice is selected based on the soil to be cleaned, throughput, readily-available cleaning equipment, compatibility with materials of construction, cost, and environmental regulations. All cleaning material types comprise strengths and weaknesses. In most cases, the application drives the cleaning material type. Best in Class Cleaning Agents Electronic assembly cleaning materials are designed to remove a broad array of flux technologies including organic acid, rosin, resin, and polymeric structures from mixed technology circuit boards. The building blocks (Figure 8) used to formulate best in class cleaning agents are solvency to dissolve resin structures; reactive agents to buffer and saponify soils; wetting agents to lower surface tension and improve penetration under low standoff components; and minor ingredients to improve materials compatibility and control foam propagation under high pressure. For electronic assembly defluxing, the solvent materials interact with the polar solvent, water. Dissolving the flux residue involves different types of intermolecular interaction: hydrogen bonding, polarity and dispersive attractions. The hydrogen bonding, ion-dipole, and dipole-dipole interactions occur from the water and water s interaction with oxygenated solvents (Figure 10). The oxygenated solvents interact with water to improve dissolution of organic acid ions necessary to leave an ionically clean assembly. Figure 10: Solubility of the Flux Residue 7 Figure 8: Cleaning Material Building Blocks For aqueous cleaning fluid designs, mild alkalinity provides two important functions: 1. Improving the cleaning rate and 2. Maintaining a consistent ph by forming a strong buffer. 4

5 Rosin, commonly used in flux compositions, is a solid form of resin obtained from pine trees and some other plants. To improve the cleaning rate, alkaline materials are used to react (saponify) with the rosin/resin to increase the rate of dissolution (Figure 11). Additionally, the alkaline source is used to react with a weak acid to form a buffer that keeps the ph at a nearly constant value. An optimal ph range of 9-11 prevents redeposition of flux soils and ionic constituents onto the circuit assembly after the cleaning process. by trapping many gas bubbles at the liquid interface. Rapid turn over of the wash tank and high pressure jets create a condition to trap and grow gaseous tight foam. To break or retard foam, antifoaming agents are added to the engineered composition to inhibit foam formation (Figure 13). Figure 13: Controlling Foaming Figure 11: Flux Dissolution Driven by Reactivity Wetting agents lower the surface tension of the cleaning fluid, by reducing the droplet size, improving spreading, and lowering the interfacial surface tension. Surface active agents form micelles that contain a lipophilic end to dissolve oily soils and hydrophilic ends to hydrogen bond with water. Wetting agents reduce surface tension of water by adsorbing at the liquid-gas interface (Figure 12). These materials reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface. When micelles form in the cleaning solution, their tails encapsulate an oil droplet, and their ionic polar heads form an outer shell that maintains favorable contact with water. Wetting agents improve penetration under the Z-axis to remove flux residues under components. The second class of minor ingredients includes materials that decrease the corrosion rate of tin, lead, aluminum, and yellow metals. Alkaline saponified cleaning materials chemically react with soft metals. Corrosion inhibitors form a passivation layer - a thin film on the surface of the alloy(s) that stops access of the corrosive substance to the metal. Properly designed cleaning agent inhibition packages reduce oxidation and reduction reactions. Solder joints, aluminum heat sinks, anodized aluminum and copper are protected from exposure to the cleaning media (Figure 14). Figure 14: Cleaning Agent Inhibition Figure 12: Wetting <90 >90 Minor ingredients formulated into the cleaning agent address two important functions: Control wash bath foam when processing in high pressure equipment and decrease the rate of metal alloy corrosion. Foam is a substance that is formed Cleaning Equipment Cleaning equipment designs are categorized into two classifications: batch and inline. 8 Increased complexity of board and component geometry coupled with more difficult solder paste and flux formulations, drives the need for improved mechanical and chemical energy. The objective mechanical cleaning systems is to reduce time by using by maximizing the physical energy delivered at the surface to be cleaned. 5

6 Fluid management is critical in maintaining an electronic assembly cleaning process. 9 Individual module containment and specifically with the wash chemistry is essential. Fluid delivery is critical for penetrating and rapidly breaking the flux dam under low standoff components. Air management is critical to reducing chemical odors in the workspace while minimizing the amount of wash fumes exhausted from the machine. Fluid storage is critical for long wash bath life. Fluid control is critical in maintaining the proper wash bath concentration within the cleaning process tolerance. Cleaning equipment design issues in any of these areas can and will upset the cleaning process over time. 9 Issues such as high wash consumption, steam out of the machine, foaming in the wash and or rinse, exhaust losses, and poor cleaning all result from an imbalance caused by one or more of these factors. Process issues may not show up when the machine is initially charged with cleaning chemistry and started up, but slowly creep in over time. Lack of process optimization results in higher defect rates, which typically render white residue formation and unacceptable levels of ionic residues on the surface and under component gaps. To improve cleaning under low standoff components, research data indicates that fluid flow, pressure at the board surface, directional forces, and time in the wash improve the process cleaning rate. 9 The wash section of the cleaning machine is highly important. Research data findings indicate that flux not adequately removed in the wash will not be removed in the rinse sections. Cleaning data studies show that high levels of fluid across the board surface decrease needed cleaning time. Directional forces that provide a 360 impingement pattern during the wash exposure decreases time in the wash. Maintaining pressure with flow also decreases the amount of time required in the wash section. Wash impingement effects can be generated using various nozzle and pump technologies. 9 To improve cleaning efficacy, boards are initially sprayed in the pre-wash section using fan jets. The pre-wash zone brings the circuit card up to process temperature, which starts the flux softening process. In the wash section, nozzle jets provide uniform wash coverage. Board geometry, density, and component types are impinged upon using a combination of nozzle technologies that provide various levels of fluid flow, pressure at the board surface, and directional forces. Printed circuit boards with increased density and component shadowing require a longer wash time to allow wash fluid to penetrate blind gaps. absence of impingement energy) of the wash chemistry is driven by the cleaning material compatibility with flux soil, rate of dissolving the flux soil, concentration, part fixturing, and wash temperature effects. The cleaning material static cleaning rate may vary on different flux residues. To address these complexities, best in class cleaning material designs are formulated to work on most flux residue types, but the rate varies for both hard and soft flux residues, with the key variable representing the length of the wash section, the nozzle design, and wash time. Figure 15: Impingement Needed to Break Flux Dam Controlling the Wash Process As cleaning complexities increase, the cleaning process window narrows. Wash bath consistency over time is critically important. High energy cleaning machines drive with pressure, heat, and fluid flow. Fluid losses in the wash must be monitored and controlled to assure consistency over time. Lack of wash tank control leads to cleaning agent depletion and eventually to incomplete cleaning (Figure 16). Figure 16: Poor Wash Tank Control To remove all flux residues under gaps less than 2 mils, time in the wash and wash temperature are critical parameters. 9 The wetting effects of flux during the reflow soldering process cause the flux to penetrate under small component gaps and create a flux dam (Figure 15). To break the flux dam, the cleaning fluid and impingement energy must first dissolve the residue to create an opening for the wash fluid to flow under the component. Hard flux residues take longer time to dissolve than do soft flux residues, which increases wash complexity. The static cleaning rate (dissolution in the 6

7 The chemical wash tank has a critical soil loading level at which cleaning will drop off (Figure 17). As flux soils load into the wash tank, the level of soil will increase over time. Additionally, wash tank fluid will be lost over time to exhaust and drag-out. To maintain control between the upper and lower design limits, additional cleaning material will need to be added when the wash tank is replenished. Failure to add cleaning material when replenishing the wash tank with water will reduce bath life. Adding wash chemistry at optimal levels will extend bath life and assure cleaning consistency over time (Figure 18). Figure 19: Process Control System Loading Figure 17: Critical Soil Loading Level Bath Life Simulation Bath Life Figure 18: Bath Life Simulation CSL Process Integration Cleaning process optimization requires a balanced of chemical and mechanical effects. The job of the cleaning material is to remove flux residue and ionic contaminants. As previously discussed, aqueous cleaning material designs contain reactive materials at various concentration levels. On certain flux types, higher reactivity increases the static cleaning rate, but can cause other issues. When cleaning flux residue under low standoff components, longer wash time is needed. Highly reactive cleaning materials create several compatibility concerns in the form of solder joint attack, anodized aluminum attack, dry film solder mask removal, part marking removal, component attack, polymer/adhesive attack, and a range of other issues. Highly reactive cleaning materials saponify rosin, which can create a foam condition as the wash bath loads. Loading Bath Life Simulation Bath Life CSL BLN BLO BLBC Best in class cleaning materials exhibit other important properties. The vapor pressure of each material used in the compositional make-up influences evaporative loss rates. The dissolution rate on higher molecular weight resin structures used in low residue and lead-free flux compositions influences the static cleaning rate. The rate of cleaning material solvency in water can influence the cleaning rate and defoaming properties. The cleaning material wetting forces is critical to penetrating low standoff gaps. Properly designed, the cleaning material rapidly dissolves rosin /resin structures, wetting low standoff gaps, inhibits solder joint attack, overcomes compatibility concerns, works at low concentrations, and provides long bath. To maintain wash bath consistency over time, programmable logic process control units are design to monitor the wash bath (Figure 19). As water is added, cleaning chemistry is also added to a preset concentration level. The process control unit monitors the concentration of cleaning chemistry and water within the upper and lower pre-set requirements. Properly controlled wash baths reduce variation and consistency for cleaning leading edge designs. The cleaning machine design is equally important. 9 Fluid management is critical in maintaining an economic cleaning process. Individual module containment and specifically with the wash chemistry is essential. Fluid delivery is critical for penetrating and rapidly breaking the flux dam under low standoff components. Air management is critical to reducing chemical odors in the workspace while minimizing the amount of wash fumes exhausted from the machine. Fluid storage is critical for long wash bath life. Fluid control is critical in maintaining the proper wash bath concentration within the cleaning process tolerance. CONCLUSION Leading edge circuit board design failures are attributed to feature size reduction, which increases the risk of defects 7

8 randomly induced by process flaws. 2 White residue sandwiched under highly dense low standoff components creates the potential for material defects caused by the presence of ionic residue (Figure 1). Traditional approaches to verifying reliability based on screening the output of a process are no longer effective. Rather, electronic assemblers are moving towards building in reliability by controlling and monitoring cleaning factors that assure complete removal of flux residue. Higher density, smaller components, and lower standoffs are changing the definition of circuit board cleanliness. The current or traditional normal view of quality assurance equated circuit board reliability to visual residue and the resistivity of solvent extract measurements. With the reduction in component size and low standoff clearances, the ability to extract and see measurable residues that correlate to product quality is much more suspect. Cleaning processes take on a whole new cleaning definition of removing residue that can be seen visually and residue entrapped under components that is commonly out of sight. Cleaning and Reliability. IPC/SMTA High Performance Cleaning Symposium. 6. Gillespie, B., & DeBenedetto, M. (2007, Feb). Conformal coating and board cleanliness. Circuit Assembly, Retrieved from 7. Dishart, K.T. (2008, October). Using Hansen Solubility Parameters to Match the Cleaning Agent to Lead Free Flux Residue. IPC/SMTA High Performance Cleaning Symposium. 8. Stach, S., & Bixenman, M. (2005, Sep). Optimizing Cleaning Energy in Electronic Assembly Spray in Air Systems. SMTAI, Rosemont, IL. 9. Bixenman, M., Ellis, D., & Neiderman, J. (2008, April). Collaborative Cleaning Process Innovations from Managing Experience and Learning Curves. IPC, APEX Designers Summit. Las Vegas, NV. Cleaning process optimization requires a balance of chemical and mechanical effects. Best in class cleaning materials remove a wide range of flux materials, decrease droplet size, exhibit wide material compatibility, stable under pressure, and support process control. Best in class cleaning machines offer fluid management, fluid delivery, air management, fluid storage, and fluid control. The sum of the parts must be integrated and controlled. AUTHORS Dr. Mike Bixenman is the CTO of Kyzen Corporation. For questions and insight on this topic, please Mike at: mikeb@kyzen.com Phil Zhang is Kyzen s territory manager for Northern China. For questions and insight on this topic, please Phil at: phil_zhang@kyzen.com Chris Shi is Kyzen s territory manager for Southern China. For questions and insight on this topic, please Chris at: chris_shi@kyzen.com REFERENCES 1. Santarini, M. (2008, March 6). Consumer ICs: Designing for Reliability Moosa, M.S., Poole, K.F., & Grams, M.L. (1996). EFSIM: An Integrated Circuit Early Failure Simulator. Quality and Reliability Engineering International. 12 (1996) Christoph, J., & Elswirth, M. (2002, March). Theory of electrochemical pattern formation. American Institute of Physics. 12(1), No Author Listed. (2000, August 3). One Size Doesn t Fit All: Same is True with Embedded PCs Fisher, J. (2008, Oct). IPC Technology Roadmap Future of Interconnection Technology and Its Impact on 8