Humidity-dependent loss IN PCB substrates: the in-use environment needs to be part of the model during system design and validation.Moisture in electronics has been and continues to be a significant concern for system reliability due to its impact on mechanical stresses during thermal cycles, ability to alter adhesion properties, and its corrosive corrosive /cor·ro·sive/ (kor-o´siv) producing gradual destruction, as of a metal by electrochemical reaction or of the tissues by the action of a strong acid or alkali; an agent that so acts. nature when mixed with ionic compounds In chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. The positively charged ion is usually a metal ion and the negatively charged ion is non-metallic element or molecule. such as salts. Moisture also affects the electrical properties of a dielectric dielectric (dī'ĭlĕk`trĭk), material that does not conduct electricity readily, i.e., an insulator (see insulation). A good dielectric should also have other properties: It must resist breakdown under high voltages; it should not by altering its dielectric constant dielectric constant n. See permittivity. and loss tangent tangent, in mathematics. 1 In geometry, the tangent to a circle or sphere is a straight line that intersects the circle or sphere in one and only one point. (1). As IO bus speeds exceed 1 GHz and move toward 5 GHz and beyond, it is becoming critical to system designs that the change in loss with respect to moisture be quantified. At these signaling speeds, small changes in the dielectric material properties can result in failure of electronics systems to communicate across an IO bus. A complicating com·pli·cate tr. & intr.v. com·pli·cat·ed, com·pli·cat·ing, com·pli·cates 1. To make or become complex or perplexing. 2. To twist or become twisted together. adj. 1. factor for designers is that the electrical impact due to the moisture concentration within the dielectric is time-dependent and atmospheric condition dependent. The electrical properties of a material are partially a function of the amount of moisture in the material. The extent to which a material absorbs moisture is characterized by its moisture diffusivity Dif`fu`siv´i`ty n. 1. Tendency to become diffused; tendency, as of heat, to become equalized by spreading through a conducting medium. and saturated moisture concentration. Moisture diffusivity describes the rate of change of a material's moisture concentration. The saturated moisture concentration provides an expression for the limit to the amount of moisture that a material can contain. It is important to note that both are a function of temperature and relative humidity relative humidity n. The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage. . Another measure of a material's susceptibility susceptibility the state of being susceptible. Refers usually to infectious disease but may be to physical factors such as wetting or to psychological factors such as harassment. to moisture is its maximum moisture uptake. This is typically reported on material data sheets as %weight and is related to the saturated moisture concentration. System designers need to be able to determine the typical, minimum and maximum impact of moisture on transmission line loss. This will depend on the system design. Surface microstrips and embedded Inserted into. See embedded system. microstrips are more susceptible to humidity conditions in the environment than striplines. Measurable changes in microstrip losses occur in hours as moisture concentrations change quickly. Transmission lines placed between reference planes, such as striplines, are significantly slower to respond to humidity conditions in the environment. The response time of striplines is measured in weeks or months and is dependent on the continuity and perforation per·fo·ra·tion n. 1. The act of perforating or the state of being perforated. 2. An abnormal opening in a hollow organ or viscus, as one made by rupture or injury. Perforation A hole. of the reference planes as these provide a barrier to the diffusion of moisture. Moisture Impact on Transmission Line Loss A set of test boards was designed to determine the maximum impact of moisture on transmission line loss. In actual computing designs, the power and ground planes are not continuous barriers to moisture absorption perpendicular to the plane of the board. There are anti-pads around each via pad that carry signals between layers and those that provide attach points for through-hole connectors. These provide a path for moisture to diffuse diffuse /dif·fuse/ 1. (di-fus´) not definitely limited or localized. 2. (di-fuz´) to pass through or to spread widely through a tissue or substance. dif·fuse adj. through the PCB PCB: see polychlorinated biphenyl. PCB in full polychlorinated biphenyl Any of a class of highly stable organic compounds prepared by the reaction of chlorine with biphenyl, a two-ring compound. . To simulate actual product designs in this experimental design there were via anti-pads regularly distributed across the ground planes, except directly under the transmission lines as shown in FIGURE 1. Multiple test boards were fabricated fab·ri·cate tr.v. fab·ri·cat·ed, fab·ri·cat·ing, fab·ri·cates 1. To make; create. 2. To construct by combining or assembling diverse, typically standardized parts: and tested with varying distributions of anti-pads to simulate the different levels of plane coverage. All data presented in this section is from test boards having 90% ground plane coverage. [FIGURE 1 OMITTED] Test boards for this experiment were eight layers with symmetry across the midplane of the board. Three stackups were used: 1080 FR-4, 7628 FR-4 and Rogers 4350 High Frequency Circuit Material. All single-ended (SE) lines were designed to have 50-ohm characteristic impedance This article is about impedance in electronics. For characteristic acoustic impedance, see acoustic impedance. The characteristic impedance or surge impedance of a uniform transmission line, usually written . Striplines were connected to the surface layers using microvias to minimize via parasitics and improve the frequency response of the test structures. All boards were preconditioned pre·con·di·tion n. A condition that must exist or be established before something can occur or be considered; a prerequisite. tr.v. for one week at 65[degrees]C to remove any residual moisture. All boards were then stored in a dry box at less than 10% relative humidity (RH) and room temperature while dry measurements of S-parameters were made. When this was complete the boards were placed in an environmental chamber kept at 38[degree]C and 95% RH. Measurements of S-parameters were made at regular increasing intervals until the loss reached equilibrium. Measurements of S-parameters were made using an Agilent 8722ES S-parameter analyzer (VNA VNA abbr. Visiting Nurse Association ). It was calibrated cal·i·brate tr.v. cal·i·brat·ed, cal·i·brat·ing, cal·i·brates 1. To check, adjust, or determine by comparison with a standard (the graduations of a quantitative measuring instrument): daily using an SOLT calibration calibration /cal·i·bra·tion/ (kal?i-bra´shun) determination of the accuracy of an instrument, usually by measurement of its variation from a standard, to ascertain necessary correction factors. standard. The VNA connections to the PCB trace were made using microprobes. The $11 measurements were not significantly affected by moisture. FIGURE 2 shows the $21 measurements from the microstrip transmission lines for the three materials at their preconditioned dry state and saturated equilibrium state. The measured loss of microstrips on Rogers RO4350 demonstrated the smallest magnitude shift and smallest percentage change due to moisture. The 7628 FR-4 had a lower overall loss compared to the 1080 FR-4 at each state and demonstrated similar increased loss due to moisture. The measured line losses equilibrated within days to a couple of weeks, depending on material type and thickness of microstrip structure. [FIGURE 2 OMITTED] FIGURE 3 shows $21 measurements for striplines on layer three of the same boards as those in Figure 2. Equilibrium delay time for striplines was affected by both their depth into the board and the coverage of the power and ground planes between them and the surface. The measured loss on some combinations of material and ground plane coverage had not reached loss equilibrium after five months of testing. As with the microstrip structures, the Rogers RO4350 demonstrated the smallest magnitude shift and smallest percentage change due to moisture. The longer equilibrium delay time for the stripline structures was expected as the striplines were partially shielded by copper planes. Also, striplines are typically positioned further from the surface of the PCB, forcing moisture to diffuse through the surface substrates to reach the stripline structure. Fick's Law of Diffusion
[FIGURES 3-4 OMITTED] Impact of Moisture on Loss Tangent and Dielectric Constant To characterize the loss of only the PCB substrate, a split-post dielectric resonator A dielectric resonator is an electronic component that exhibits resonance for a narrow range of frequencies, generally in the microwave band. The resonance is similar to that of a circular hollow metallic waveguide, except that the boundary is defined by large change in (SPDR SPDR The Standard and Poor's depositary receipt. This is a tracking stock which trades like an index mutual fund which follows the S&P 500. It trades continuously. SPDR See Standard amp; Poor's Depositary Receipt (SPDR). ) was used to measure loss tangent (tan[delta]) and dielectric constant ([epsilon]r) of bare PCB substrate materials (2). No copper or soldermask was present on these PCB samples. The core PCB material was conditioned in two different environmental conditions until equilibrium before measuring er and tan[delta]. The measured values of tan[delta] and [epsilon]r measured after equilibrium at 20[degrees]C and 10% RH and measured again after equilibrium at 38[degrees]C and 95% RH are shown in TABLE 1. As seen in Table 1, moisture had a significant impact on the loss tangent. The dielectric constant increased slightly with absorption of moisture. The measured loss tangent increased for each measured frequency by 43 to 58% between the two conditions. The change in loss tangent is significant as its magnitude is high and it is directly proportional (Math.) proportional in the order of the terms; increasing or decreasing together, and with a constant ratio; - opposed to See also: Directly to the dielectric loss component of measured transmission line loss. Modeling Moisture Dependent Loss Fick's Law of Diffusion is a close analog of the method of Fourier for characterizing and predicting heat flow (3). Fick's law characterizes the time and distance dependent concentration of a solute solute /so·lute/ (sol´ut) the substance dissolved in solvent to form a solution. sol·ute n. in a solvent. In the case of modeling moisture dependent loss in PCB substrates, atmospheric moisture is the solute and PCB materials are the solvent. Fick's law assumes a homogeneous isotropic Refers to properties that do not differ no matter which direction is measured. For example, an isotropic antenna radiates almost the same power in all directions. In practice, antennas cannot be 100% isotropic. material as the solvent, but it has been shown to adequately model PCB composites. Fick's law for single-dimensional diffusion is shown in EQUATION 1: [partial derivative partial derivative In differential calculus, the derivative of a function of several variables with respect to change in just one of its variables. Partial derivatives are useful in analyzing surfaces for maximum and minimum points and give rise to partial differential ]C/[partial derivative]t = D [[partial derivative].sup.2]C/[partial derivate der·i·vate adj. Derivative. ][chi square chi square (kī), n a nonparametric statistic used with discrete data in the form of frequency count (nominal data) or percentages or proportions that can be reduced to frequencies. ] Equation 1 states mathematically that the time rate of change of concentration "C" of a solute in a solvent is proportional by constant "D" to the derivative of the concentration gradient concentration gradient n. The graduated difference in concentration of a solute per unit distance through a solution. Noun 1. of the solute. The constant "D" is known as diffusivity and has units of area/time. Solving Equation 1 by method of separation of variables In mathematics, separation of variables is any of several methods for solving ordinary and partial differential equations, in which algebra allows one to re-write an equation so that each of two variables occurs on a different side of the equation. leads to the general form shown in EQUATION 2: C = [[infinity].summation summation n. the final argument of an attorney at the close of a trial in which he/she attempts to convince the judge and/or jury of the virtues of the client's case. (See: closing argument) over (m=1)] ([A.sub.m] sin [[lambda].sub.m]x + [B.sub.m] cos [[lambda].sub.m]x)exp exp abbr. 1. exponent 2. exponential (-[[lambda].sup.2.sub.m]Dt) where exp(n) denotes the base of natural logarithms Natural logarithm Logarithm to the base e (approximately 2.7183). , "e", raised to the power of "n" The solution of Equation 2 is dependent on the boundary and initial conditions and the geometric dimensions of the sample being modeled. In the case for which the material starts out dry and is placed in an environment of temperature and humidity, the diffusion of moisture occurs from the outer boundary of the material inward. Since the material is much wider than thick, all diffusion is treated as if it were in the x direction only. The material is modeled as an infinite plane of thickness twice "l" as shown in Figure 4. Solution of Equation 2 using partial fractions and the boundary and initial conditions for two-sided diffusion into an infinite plane is shown in EQUATION 3: [MATHEMATICAL EXPRESSION A group of characters or symbols representing a quantity or an operation. See arithmetic expression. NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] Where: C = concentration of solute at time "t" and distance from material center "x" D = diffusivity ([mm.sup.2]/sec) x = distance from the center of the material (mm) as in Figure 4 l = 1/2 thickness of the material (mm) as in Figure 4 t = time (seconds) Co = equilibrium concentration of the solute at the surface (2) As seen in Equation 1, diffusion is gradient-driven. The center of the plane (x = 0) has a zero gradient gradient In mathematics, a differential operator applied to a three-dimensional vector-valued function to yield a vector whose three components are the partial derivatives of the function with respect to its three variables. The symbol for gradient is ∇. . Hence, the concentration at any depth except the center at x = 0 is from diffusion from one side of the plane. One could place an impermeable impermeable /im·per·me·a·ble/ (-per´me-ah-b'l) not permitting passage, as of fluid. im·per·me·a·ble adj. Impossible to permeate; not permitting passage. plane in the middle of the sample and have no effect on diffusion and concentration at any location for [absolute value of x] > 0. In other words Adv. 1. in other words - otherwise stated; "in other words, we are broke" put differently , all diffusion into the top half of the material is from the top surface downwards. All diffusion into the bottom half of the material is from the bottom surface upwards. Thus, an impermeable reference plane below a measured trace can be considered a "virtual center" for the purposes of this model and will be at the location x = 0 for the purposes of modeling change in transmission line loss due to moisture absorption with Fick's law. Measuring Diffusivity Test boards were designed to obtain the diffusivity constant [D.sub.l] describing moisture dependent transmission line loss for 2116 FR-4 at different constructions. By adjusting the layer placement of the transmission line and the reference plane, various depths within the board were obtained for testing. In this design, the distance from the top of a trace to the surface of the board was varied to obtain three unique structures with the distance between a trace and its corresponding reference plane held constant (FIGURE 5). By keeping the distance between the trace and reference plane constant, the electric field between the trace and plane was held fairly consistent between the three structures. The magnitude of the measured loss of each structure was expected to increase as the representative traces moved further from the surface. There were multiple traces of different lengths designed into each structure. The traces located within Structure A are closest to the board surface and expected to respond quicker to changes in environmental conditions. The traces of Structure C are farthest from the surface and thus expected to respond slowest to changes in environmental conditions. The ground planes were continuous with the exception of the anti-pads required for the connecting vias of the transmission line termination points, thus moisture diffusion was limited to one direction. [FIGURE 5 OMITTED] S-parameter measurements were made using a HP 8510C VNA. Calibration of the VNA was accomplished using a standard coaxial co·ax·i·al adj. Having or mounted on a common axis. coaxial Adjective 1. Electronics (of a cable) transmitting by means of two concentric conductors separated by an insulator SOLT kit prior to each experimental run. Measurement of fixed attenuators during preliminary testing indicated that calibration drift was negligible for the planned test period. The test boards were connected to the VNA through a Kiethley RF Mux that allowed in-situ measurements at the desired environmental conditions. This allowed finer time resolution between measurements. The system also allowed the measurement of multiple structures within the same test board and measurements across multiple test boards without changing termination conditions, and without removing test samples from an environmental chamber for measurement. As a result, it was possible to measure changes in transmission line loss during short environmental transitions, such as changes in temperature. The test boards were placed in an environmental chamber and connected to the measurement system. Prior to the boards being exposed to high humidity conditions, the boards were baked at 105[degrees]C and RH <10% for approximately nine days. This was to ensure the boards contained no initial moisture. After the preconditioning preconditioning preparation of 6 to 8 months old range-reared, recently weaned beef calves for entry into a feedlot and an intensive fattening program. Includes castration, dehorning and branding 3 weeks before and all vaccinations 2 weeks before weaning, and weaning 3 to 4 weeks bake, the environmental chamber conditions were ramped down over eight hours to 15[degrees]C to measure the temperature impact on transmission line loss in the dry PCB. The environmental chamber was then set to 85[degrees]C/85RH until saturation saturation, of an organic compound saturation, of an organic compound, condition occurring when its molecules contain no double or triple bonds and thus cannot undergo addition reactions. across all structures was obtained. The temperature in the environmental chamber was then ramped down again over eight hours to 15[degrees]C, which provided measurement of temperature impact on transmission line loss in a moisture saturated PCB. Since loss diffusivity, [D.sub.l], was derived from trace loss measurements, the variability of the method across different frequencies, trace-to-trace within the same structure, and between structures was examined. FIGURE 6 shows the results of measuring six individual microstrip traces on 7628 FR-4 at three frequencies. This data reflects a change from 20[degrees]C and dry to 38[degrees]C and 95% RH. The [D.sub.l] derived from loss measurements was consistent between different traces and across selected frequencies. FIGURE 7 shows the variance in derived [D.sub.l] for 2116 FR-4 using different line lengths and structures. No significant length or structure dependencies were observed. As a result, loss diffusivity was calculated for each tested material that adequately described the loss behavior over time and between different structures. For the 2116 FR-4 material, the typical diffusivity was calculated to be 1.31 X10-6 [mm.sup.2]/sec at 85[degrees]C/85RH. Using a weight-based method, Fremont et al. obtained moisture diffusivity values for an unspecified FR-4 at 85[degrees]C/85RH of 1.51 X [10.sup.-6] [mm.sup.2]/[sec.sup.4]. This initial comparison shows close agreement between loss diffusivity derived from fitting hloss hloss (the humidity-induced change in loss from the dry state) to Fick's law and moisture diffusivity. [FIGURES 6-7 OMITTED] The loss diffusivity measured for 7628 FR-4 and 2116 FR4, shown in Figures 6 and 7, respectively, cannot be directly compared as each was derived from different environmental conditions. Diffusivity is a function of temperature and relative humidity. At lower temperatures and lower relative humidity the moisture diffusion through the PCB substrate is slower. This was observed in the diffusivities calculated for 2116 FR-4 at 85[degrees]C/85RH of 1.3x[10.sup.-6] [mm.sup.2]/sec and the diffusivity calculated for 7628 FR-4 at 38[degrees]C/95RH at 2.3x[10.sup.-7] [mm.sup.2]/sec. It is noted, that even at the same environmental conditions, 2116 FR-4 and 7628 FR-4 would have slightly different diffusivities due to differences in glass fabric construction. The transmission line loss is a function of temperature and humidity, and must be considered during system design and validation. As noted, the temperature was ramped prior to any change in relative humidity. This provided a measurement of the transmission line loss as a function of temperature at the dry and saturated states as shown in FIGURE 8. For comparison, the measured loss for the 2116 FR-4 as received was 0.95dB/inch at 25[degrees]C. At 25[degrees]C, drying improved the transmission line loss by ~0.3dB/inch and moisture increased the loss by 0.65dB/inch when hilly hill·y adj. hill·i·er, hill·i·est 1. Having many hills. 2. Similar to a hill; steep. hill saturated. Temperature had a significant affect on loss across the range 15[degrees]C to 85[degrees]C. And when saturated with moisture, the impact of changing temperature was more pronounced. When dry, the loss changed 0.5dB/ inch between 15[degrees]C and 85[degrees]C. When saturated, the loss changed by 0.85dB/inch between 15[degrees]C and 85[degrees]C. In relatively small temperature ranges, the critical loss factor was moisture. [FIGURE 8 OMITTED] Conclusions It has been shown that moisture has a significant impact on the transmission line loss characteristics within a PCB substrate. It has also been shown that temperature affects transmission line loss and influences the significance of humidity dependent loss. As a result, expected environmental conditions need to be considered during system design and validation. PCB substrates respond to changes in temperature faster than changes in relative humidity. Microstrip transmission lines are more sensitive than striplines, as they respond quickly to changes in the relative humidity of the environment and reach saturation within a few days. Striplines change more slowly, with moisture saturation taking weeks and sometimes months. As a result, striplines may not be susceptible to seasonal changes, but variations would exist between two locations due to the average environmental condition. It has been demonstrated that Fick's Law of Diffusion can be used to model the humidity dependent losses of a PCB trace. The method provided can be used to determine the loss diffusivity of a desired PCB substrate. The loss diffusivity allows the designer to model the rate of change in transmission line loss due to humidity, and thus predict the electrical performance risk across different environmental conditions. The loss diffusivity also appears to be close to moisture diffusivity, but additional experimental work is required to confirm. It is also shown that the humidity dependent loss does vary with frequency. Due to the magnitude of humidity dependent loss, end user environmental conditions must be taken into account as they affect board performance. The impact of moisture on the transmission line loss has been confirmed with both direct VNA measurements and split post dielectric resonators (SPDR) on unclad dielectrics. The SPDR measurements confirm that moisture absorption increases the dielectric dissipation factor In physics, the dissipation factor (DF) is a measure of loss-rate of power of a mechanical mode, such as an oscillation, in a dissipative system. For example, electric power is lost in all dielectric materials, usually in the form of heat. (tan[delta]) of a PCB substrate. This change in tan[delta] as a function of moisture can be included during electrical simulations to determine the design risk across environmental use conditions. In addition, the dielectric constant of the material has also been shown to change with moisture absorption and should be included when modeling moisture impacts during system design and validation. The magnitude of humidity dependent loss in PCB substrates is affected by material selection. It has been shown that a low moisture absorbing material such as Rogers 4350 is not as susceptible to moisture dependent losses as compared to FR-4. The work presented in this article allows the likelihood of humidity dependent loss in transmission lines under various environmental use conditions to be evaluated. During system design and validation the loss diffusivity and Fick's law can be used to predict the rate at which hloss increases. The impact of humidity dependent loss on electrical performance can be modeled by adjusting the values of dissipation factor (tan[delta]) and dielectric constant of the PCB substrate used with transmission line simulation tools. Ed.--This paper previously appeared in the IPC (1) (InterProcess Communication) The exchange of data between one program and another either within the same computer or over a network. It implies a protocol that guarantees a response to a request. Printed Circuit Expo 2007 Technical Conference Proceedings. It has been edited and is published by PCD&M with the permission of the authors. ACKNOWLEDGEMENTS Discussions with and ideas from our Intel associates have materially contributed to this work. These teammates include Dennis Miller Dennis Miller (born November 3, 1953) is an American Emmy Award-winning comedian, political commentator, television personality, and talk radio host. He rose to fame as a cast member of Saturday Night Live , Stephen H. Hall and Bryce Horine. Mark Beaudoin has spent many patient and careful hours executing the first experiment. The authors are obliged o·blige v. o·bliged, o·blig·ing, o·blig·es v.tr. 1. To constrain by physical, legal, social, or moral means. 2. for their help. REFERENCES (1.) Khan, Subhotosh; "Comparison of the dielectric constant and dissipation factors of non-woven aramid/FR-4 and glass/FR-4 laminates," Volume: 26 Issue: 2, Circuit World, June 2000 (2.) J. Krupka, A.P.; Gregory, O.C.; Rochard, R.N.; Clarke, B.; Riddle riddle, puzzling question, specifically one that consists of a fanciful description or definition of something to be guessed. A famous riddle was asked by the Sphinx: "What goes on four legs in the morning, on two at noon, on three at night?" Oedipus guessed the , J. Baker-Jarvis, "Uncertainty of Complex Permittivity Permittivity A property of a dielectric medium that determines the forces that electric charges placed in the medium exert on each other. If two charges of q1 and q2 coulombs in free space are separated by a distance r Measurements by Split-Post Dielectric Resonator Technique" Journal of the European Ceramic Society, Version 7, 2001 (3.) Crank, J.; "The Mathematics of Diffusion" Oxford Science Publications, UK, 1979 (4.) Fremont, Helene, et al., "Measurements and FE-Simulations of Moisture Distribution in FR4-based Printed Circuit Boards," 7th Int. Conf. On Thermal, Mechanical and Multiphysics Simulation and Experiments in Micro-Electronics and Micro-Systems, EuroSimE 2006 GARY BRIST is Staff PCBTechnologist with Intel; gary.a.brist@ intel.com. PAUL HAMILTON is SeniorTest Engineer with Tyco; paul.hamilton@tycoelectronics.com. GUY BARNES JR. (guy. barnes.jr@intel.com) is an RF/Signaling Engineer and JASON Jason, in Greek mythology Jason, in Greek mythology, son of Aeson. When Pelias usurped the throne of Iolcus and killed (or imprisoned) Aeson and most of his descendants, Jason was smuggled off to the centaur Chiron, who reared him secretly on Mt. Pelion. SCHRADER (jason.s.schrader@intel.com) is a PCB Technologist, both with Intel.
TABLE 1. Values of tan[delta] and [epsilon]r measured using
split-post dielectric resonator.
7628 (FR-4) 20[degrees]C 10% RH
1.2 GHz 3.2 GHz 7.2 GHz 10.2 GHz
tan[delta] 0.01255 0.0126 0.0129 0.0125
[epsilon]r 4.5 4.55 4.47 4.24
7628 (FR-4) 360[degrees]C 95% RH
1.2 GHz 3.2 GHz 7.1 GHz 10.2 GHz
tan[delta] 0.0180 0.0193 0.0197 0.0197
[epsilon]r 4.65 4.725 4.58 4.475
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