Low cte pcb material. In general, a circuit material with a lower CTE will be more robust and handle the thermal stress of PCB assembly better than a material with a higher overall CTE. If the components of the substrates have different CTE, they may expand at different Isola is an additional company that offers a wide variety of PCB materials. FR1 served simpler radio and pre-war telecommunications applications well. It is measured in parts per million per degree Celsius [ppm/°C]. Tachyon ® 100G Ultra Low Loss Laminate and Prepreg Tachyon 100G laminate materials are designed for very high-speed digital applications up to and beyond data rates of 100 Gb/s. It is a glass-reinforced epoxy laminate that offers a good balance of mechanical, electrical, and thermal properties at a relatively low cost. Understanding the different types of substrate materials is crucial for selecting the most suitable option for a particular PCB design. In the past, non-woven aramid (such as Kevlar) was used in to control the CTE of PCB. Low CTE for reliability • T260: 60 minutes • T288: 30 minutes • RoHS Compliant • UV Blocking and AOI Fluorescence High throughput and accuracy during PCB fabrication and assembly • Superior Processing Closest to conventional FR-4 processing • Core Material Standard Availability Thickness: 0. Smooth paste consistency. Also, this material can hardly expand when exposed to varying temperature degrees. 0037 at 10 GHz; Low Z-axis coefficient of thermal expansion at 32 ppm/°C; Benefits. g. radio and television), mobile communication industry, computers, avionics, military hardware, and others. The CTE of PIs are well controlled and match that of copper. High Tg, Low CTE, Multifunctional Filled Epoxy Resin and Phenolic-Cured Laminate & Prepreg IT-180A is an advanced high Tg (175℃ by DSC) multifunctional filled epoxy with low CTE, high thermal reliability and CAF resistance. These materials exhibit varying electrical, mechanical, and thermal properties, influencing the overall performance and reliability of the PCB. Introduction The printed circuit board (PCB) substrate, also known as the base material, serves as the foundation upon which the entire board is built. Rogers materials have a coefficient of thermal expansion of nearly 17ppm/°C. Jun 13, 2011 · Additional materials examined and incorporated in the same PCB manufacturing process included a graphite composite thermal control material [4] in commercial PCBs, a long-walled carbon nanotube (LWCNT) sheet material [5], and others. Materials with similar Dk: Make sure the substrate is made of materials with a similar dielectric constant (Dk). , perpendicular to the surface layer of a circuit board. The unit of CTE is PPM/°C i. FR4 maintains its dimensions and shape even under varying environmental conditions, contributing to the precision and reliability of PCBs. But performance limits quickly demanded improved substrates. 125″ (3. Two similar materials they offer are FR406 and 370HR. These both offer higher performing thermal characteristics than a general purpose FR4. PCB substrate materials are available in various forms, each offering unique properties and characteristics to suit specific application requirements. com evolves the demands for low CTE packages increases, thus the need for a PCB that can tailor the CTE to match ceramic packages or even DDA/flip chip components. Key parameters to evaluate include the dielectric constant (Dk) and the loss tangent (Df). Development PCB materials with these thermal control cores were manufactured successfully, as well as a control Apr 27, 2024 · Low Z-Axis CTE: The RO3000 series features a low Z-axis linear coefficient of thermal expansion (CTE), meaning that the material expands less when heated in the direction perpendicular to the material’s surface. Automotive Mobile products Td is the temperature at which a PCB material chemically decomposes and the change is completely irreversible; once the material crosses Td, it cannot return to its original state after cooling down and therefore becomes totally non-functional. Learn about the different types, manufacturing processes, properties, and applications of CCL, as well as tips for selection and handling. The peel strength of Isola P95/P25 PCB material after thermal stress Mar 21, 2017 · What are NASA’s Unique PCB needs? The PCB materials for space applications are chosen to optimize performance of the final printed circuit board assembly, in many cases, over relatively large swings in temperature, to minimize thermal vacuum outgassing and, to reduce the accumulation of stress over the many thermal cycles and exposures to temperature soaks associated both with ground testing The CTE of a substrate is usually much higher than copper, which can cause interconnection issues as a PCB is heated. e. The ceramic, PTFE woven-glass composite material has higher thermal conductivity than conventional PCB dielectrics and is ideal for thin multi-layer boards, providing the option of the highest degree of Many Panasonic materials have excellent low coefficient of thermal expansion (CTE). Mostly, materials with a high Tg value also have a low CTE-z. FR-4 is the most widely used PCB laminate material. Normally, we only care about expansion along the z-axis, i. Low Coefficient of Thermal Expansion (CTE): PCB materials with a low CTE exhibit minimal dimensional changes with temperature fluctuations. FR-4 is composed of interlaced fiberglass fabric and a binder made of epoxy resin, offering elevated mechanical robustness and stiffness while retaining a reduced mass. Low CTE is good for the through hole reliability. For PCBs, it is important to select a material with a low CTE to reduce the risk of stress and damage from thermal cycling during operation. Nov 17, 2022 · Input a specific material property or properties, and the tool will give you a list of high and low Dk PCB materials that meet your requirements. IT180A High-Tg Low CTE and Phenolic-Cured Laminate & Prepreg. The ceramic PCB is extremely versatile and can replace a complete traditional PCB with a less complex design and increased performance. 05 ; Dissipation factor of 0. Thus it is necessary to have low CTE printed circuit boards in order to keep solder joint intact with such low CTE packages. A low Z-axis coefficient of thermal expansion (CTE) is necessary for the superior reliability of plated through-holes. Sep 28, 2023 · Its high glass transition temperature (Tg) of around 280°C and low coefficient of thermal expansion (CTE) make it suitable for high-temperature applications and provide excellent dimensional stability during thermal cycling. Dielectric Constant (Dk): Affects signal speed and impedance. Low coefficient of thermal expansion. Very close match with copper CTE. It is necessary to consider total balance of material properties and appropriate process conditions Dec 1, 2020 · Glass Transition Temperature and Coefficient of Thermal Expansion (CTE) These two PCB material properties are also related. Easily etchable Apr 6, 2022 · FR-4 is a commonly employed laminate material in the PCB sector owing to its exceptional performance across various attributes. 002″ (0. When the ambient temperature goes up, the PCB will want to expand. The coefficient of thermal expansion (CTE) is the rate at which a printed circuit board material expands while it heats. IT-180A is an advanced High-Tg (175 by DSC) multifunctional filled epoxy with low CTE, high thermal reliability and CAF resistance. All materials have some coefficient of thermal expansion (CTE), which happens to be an anisotropic quantity in a PCB substrate (i. PTFE: Low CTE of 17 ppm/°C. 3: CTE with Low Tg and High Tg Materials Nov 2, 2023 · Stable board dimensions during fabrication and operation are ensured by a low Z-axis coefficient of thermal expansion CTE and robust construction. CTE：Coefficient of thermal expansion热胀系数，通常衡量 PCB板材性能的是线性膨胀系数，定义为：单位温度改变下长度的增加量与的原长度的比值，如Z-CTE。CTE值越低，尺寸稳定性越好，反之越差。 材料的热膨胀系… To put some numbers to these concepts, Table 5 shows the CTE behavior of two common materials used in the PCB industry. FR4: Typical values of 14-20 ppm/°C. 2 days ago · The use of RF PCBs plays a dominant role in high-speed electronic products with high-volume data transmission. In addition, it has a thermal conductivity of 0. Processes like FR-4 at lower fabrication cost; Competitively priced; Excellent dimensional stability Sep 1, 2023 · Coefficient of Thermal Expansion (CTE) Measure of dimensional change per degree temperature change. See full list on mclpcb. Low thermal expansion is achieved by providing the temperature is characterized by a PCB material’s coefficient of thermal expansion (C TE). , expansion rate is different along different directions). Unlimited working life. Prepregs provide electrical isolation between the conductive copper layers in a PCB while […] Nov 9, 2020 · Coefficient of Thermal Expansion (CTE): This tells you how the board expands as its temperature increases. When temperatures get high, the materials used in PCB manufacturing can expand. Stay up-to-date on the latest trends and future developments in CCL technology. 0ppm/degK, resulting in the package performance with +/-0. uA higher Tg does not mean a higher Td (better thermal resistance). As a result of mismatched CTEs, Rogers circuit boards suffer thermal strain. Rogers CLTE Series materials offer tight dielectric constant and thickness tolerance, low loss tangent, low out-gassing and low insertion loss. Non conductive. Synthesized and sintered ZrW 2 O 8 along with ZrO 2 ceramic has been used in Fiber Bragg Reflectors (FBG) successfully with an overall isotropic CTE of -9. Also, at the Z axis, the CTE is 20/120 ppm/℃. Therefore ultra low CTE (coefficient of thermal expansion) core materials have been needed as a key solution for the reduction of the warpage for PoP. Materials for applications in data center & wired infrastructure environments. With a low CTE-z value, several reliability issues are reduced, such as cracks within the via, corner cracks, and pad lifting. Flip chip type packages for exam-ple have very low CTE compared to traditional PCB material. As a material’s temperature rises past T g, the CTE will rise as well. 6 W/m·k. EMC is ranked among top 5 laminate producers in the world, and is the leading provider of the green halogen-free laminate. When a Circuit […] Aug 30, 2016 · Application in Optoelectronic Packaging. 05 mm) to 0. This property is crucial to maintain the integrity of the circuit and prevent issues like solder joint failures or stress-induced cracks. Therefore, less thermal expansion along the Z-axis will mean a greater degree of reliability and less stress exerted by this expansion on the plating of the holes. The difficulty of the The electrical performance of a PCB material directly impacts the functionality and efficiency of the electronic device. New Materials in PCB Tg Low CTE; Automotive components (Engine room ECU), Electronic devices that require high reliability (use of lead-free solder), etc. Coefficient of thermal expansion (CTE) X/Y and Z direction Mar 18, 2024 · Types of PCB Substrate Materials. Tg and CTE relationships of common PCB laminates. Copper Clad Laminate (CCL) is the material on which all circuitry and printed circuit boards (PCBs) are based. This advantage is because these PCBs provide suitable ceramic substrates for electronic circuits with high thermal conductivity and low coefficient of thermal expansion (CTE). e parts per million per Celsius degree. Many chip packages have a low CTE compared to traditional PCB materials. MCL-HS100, which has the properties of both low thermal expansion material and low transmission loss material. Therefore, this PCB material can survive in high temperatures in an operational environment. CTE along the X and Y axes are generally low – around 10 to 20 ppm per degree Celsius. There are cur-rently several materials available Jun 16, 2020 · PCB material selection best practices. Nov 18, 2021 · PCB stress absorption prepreg <TD-002> Sort by application. The CTE increases when the temperature of the material climbs past Tg. It’ s design for high layer PCB and can pass 260 Lead free assembly and sequential lamination process. Thermal conductivity is the ability of a material to transfer heat. It is also vital to select a PCB laminate material with a CTE value that is suitable for your intended use. 3. Low-loss, high frequency advanced materials with well-controlled dielectric properties to enable microwave and RF applications, including mobile networks. Low Loss / Low CTE / Halogen Free EM-526 / EM-526B Low Df, excellent electrical performance Low CTE for X / Y / Z -axis direction For high speed sever, network and telecom application For HDI, MEMS, SiP, eMMC Application Basic Laminate Property Property Item IPC-TM-650 Test Condition Unit Typical Value Thermal Tg The coefficient of thermal expansion is a measure of how much a material will expand or contract when heated, and it is crucial to know the CTE value of the PCB laminate materials you’re using. Jul 11, 2024 · Common PCB Laminate Materials. At excessive temperature changes, the system can experience excessive deformation, which should create a mechanical reliability concern for a designer. Coefficient of thermal expansion: Use materials with similar CTEs CTE measures the expansion rate of a PCB material as it heats up. How do material choices impact the performance of an RF PCB? A. The material worked well with in-plane CTE (9 to 12ppm/C). Material choices are critical in RF PCBs as they directly affect the board's ability to transmit high-frequency signals. Other properties such as Tg, Td and interlayer adhesion are significantly important to the reliability as well. 2. Mechanical Properties: The laminate material should have sufficient mechanical strength and dimensional stability to withstand handling and operation. Wireless Infrastructure. Features. Mar 24, 2020 · A lot of designers tend to focus on potential signal integrity problems when selecting PCB substrate materials. The rate at which a PCB substrate expands when the material heats up is known as CTE or Coefficient of Thermo Expansion. This helps avoid distortions or deformations in the position and size of vias during PCB manufacturing due to temperature variations. Coefficient of Thermal Expansion (CTE): A PCB’s expansion rate is known as its CTE. cn One component flowable epoxy for bonding and sealing applications. Parts on each million or ppm expanded for each degree Celsius of heating is how CTE is stated. This is one reason why multilayer PCBs typically use more than one type of circuit material. The tool also provides an easy-to-use comparison table that lets you compare the detailed properties of 50+ rigid and 10+ flex materials. Environmental Resistance May 14, 2019 · Temperature rises results in materials expanding, however, the principal materials that PCB are made of have disparate thermal expansion coefficients. 4[W/m*K], a Tg of 180C, and a CTE for Z-axis of 45 [ppm/C]. So lets assume a PCB is made out of FR4 which has a CTE of 17 (usually it is 14 to 17 ppm/°C). Match Coefficient of Thermal Expansion (CTE): CTE is the most critical thermal characteristic for substrates. Factors to consider when choosing an RF PCB material. Feb 25, 2021 · Coefficient of Thermal Expansion (CTE): The coefficient of thermal expansion, or CTE, is the rate of expansion of a PCB material as it heats up. If the CTE for the board material is significantly different from the copper on the board, the expansion can place solder joints on stress. A higher Tg contributes to a lower thermal expansion in the z-axis thus better through hole reliability. Thermal Properties: The material should have excellent thermal conductivity and a low coefficient of thermal expansion (CTE) to dissipate heat effectively and prevent overheating. 2 mm) Jul 11, 2012 · Table 3: Typical CTE values for materials commonly used in high frequency PCBs. This is usually thanks to the woven glass that constrains the material in the X and Y directions, and the CTE doesn’t change much even as Apr 26, 2022 · The coefficient of thermal expansion is a measurement of how robust a circuit board is under high temperatures. The CTE of a substrate is Jul 30, 2018 · Substrate material selection impacts circuit performance and potentially circuit material choice. CTE is anisotropic, meaning the board will expand at different rates in different directions. This is definitely understandable; high speed/high frequency devices require low losses and flat dispersion within the relevant bandwidth to prevent signal distortion, and this is often a starting point for selecting PCB substrate materials. 48 +/- 0. These excel in high-precision applications where dimensional stability is essential. 1. What is Ajinomoto's Insulation film (Ajinomoto Build-up Film® ABF) ? Ajinomoto Build-Up Film® (ABF) is used as an insulating material for high-performance semiconductors (CPUs), which are the heart of personal computers. Achieved low Dk and low Df, as well as keeping high heat resistance, low CTE, low shrinkage, and high peel strength with copper, equivalent to conventional BT laminates. If CTE is low, deformation will begin earlier and cannot be used at high temperatures as it will deform, causing malfunctions or affecting the result. One of the key advantages of FR-4 is its flame resistance, which improves the reliability and safety The coefficient of thermal expansion (CTE) is the rate at which a material expands or contracts based on changes in temperature. fineprint. By considering thermal, electrical, mechanical, and chemical Innovation, Teamwork, Excellence, Quality Low CTE High Tg Material IT-180A ITEQ Corporation Your Partner of Advanced Materials & Mass Lam Service PDF 文件使用 "pdfFactory Pro" 试用版本创建 ÿwww. For example, knowing the CTE of substrate materials becomes a high priority because problems can occur if two substrate materials (or even the substrate and components) across the same printed circuit board have a CTE mismatch. Jul 30, 2022 · Coefficient of Thermal Expansion (CTE) Although CTE is not important for high-frequency PCB materials, it can be critical. RF PCBs should be made of materials with a low CTE. At the X. Rogers RO4000 hydrocarbon ceramic laminates and prepregs are the industry leader. Apr 24, 2020 · The Coefficient of Thermal Expansion (CTE) specifies how much a printed circuit board will expand/contract when it is heated or cooled. Jun 10, 2013 · Next build-up material in demand Demand for build-up material Fine line & space (high adhesion strength with low roughness) Fine via pitch Low warpage during cure and reflow Low CTE, High insulation reliability (Layer to Layer, and circuit to circuit) Current PKG structure • Downsizing • Use thinner core • High function Apr 27, 2022 · Compared to thermal conductivity of the copper foil, that of the dielectric material is quite low. There are several common types of PCB laminate materials, each with its own unique set of properties: FR-4. . Parts per million, also known as (ppm), are the units of deformation. evolves the demands for low CTE packages increases, thus the need for a PCB that can tailor the CTE to match ceramic packages or even DDA/flip chip components. It’s design for high layer PCB and can pass 260℃ Lead free assembly and sequential lamination process. Peel strength. The MEGTRON 6 Family is the original PCB industry standards, ideal for mobile, networking, and wireless applications requiring high-speed, ultra-low loss circuit materials. One important thermomechanical property of PCB materials is the CTE value, or the rate of change in a material’s volume with temperature. It is the basic material used in the PCB industry with wide applications in consumer electronics (e. RF PCBs are used in a wide range of industries, including wireless satellite stations, signal-receiving base stations, wireless products, millimetre wave and automotive radar applications, avionics, aerospace, and digital data communications such as servers, routers, and semiconductors. It is a fibrous reinforcement that has been pre-impregnated with a resin system prior to laminating PCB layers together. Cryogenically serviceable. Isola P95/P25 PCB material features a low CTE. Coefficient of thermal expansion (CTE) mismatch is one key issue for the reliability of the dielectric materials in the advanced package. With a wide variety of substrate materials available, from FR-4 to high-frequency laminates […] Aug 23, 2022 · FR-4 is a dielectric glass-reinforced epoxy laminate material used as a base material for insulation in printed circuit boards. Materials with low dielectric constants and loss tangents are preferred to reduce signal loss and support higher frequency ranges. Recently, we have developed new ultra low CTE core material named E-770G for next generation thin CSP, applying new resin systems, featuring low shrinkage and low residual stress. Jul 1, 2016 · The electronics industry is also facing issues with solder joint stress and coefficient of thermal expansion (CTE) mismatch between PCB laminates and electronic components. Dk of 3. Automotive Mobile products: Halogen-free multi-layer circuit board materials: series: Lineup of environment-friendly materials corresponding to the various application of mobile products. Jun 17, 2024 · Choosing the right PCB substrate requires a deep understanding of the material properties and their impact on the PCB's performance. Moderate CTE mismatch with copper. Fig. The z-axis coefficient of thermal expansion increases sharply (as much as four to fourteen times that of the x-y axis) when the temperature rises close to the Tg. PCB makers can apply the same manufacturing process as conventinal BT laminates. Their primary advantage is dimensional stability over a wide temperature range. This guide provides a detailed overview of RO4350B material properties, key datasheet parameters, design considerations, and dielectric constant specifications that engineers need to know when designing with this high speed PCB substrate Get an in-depth understanding of Copper Clad Laminate (CCL) materials used in printed circuit board (PCB) fabrication. Aug 17, 2023 · SLP progression of substrate technology. Table 5. The final point being, that specialist PCB fabricators can introduce SLP technology using a unique family of raw materials with low CTE (=Coefficient of Thermal Expansion@ PPM/C0), enabling a Chip-on-Board (CoB) to be assembled on the substrate alongside regular SMD, press-fit, or wire bonding (AL, Cu or Au wires). There One important thermomechanical property of PCB materials is the CTE value, or the rate of change in a material’s volume with temperature. The PCB material can expand or contract Introduction Prepreg, also referred to as pre-impregnated composite fibers, is a key material used in the manufacture of multilayer printed circuit boards (PCBs). You will find RO3000 series in power amplifiers and surface mount technology RF components. In this comparison, the alpha 1 test was done in the temperature range of 50ºC to 185ºC and the alpha 2 was 185ºC to 260ºC. The Td of FR4 material FR370HR is 340 °C. Supreme 10ANHT-LO. Therefore it is necessary to have low CTE printed circuit boards in order to keep solder joints intact. The properties of the substrate have a major influence on the performance, manufacturability and reliability of the PCB. To achieve a narrow range of positional tolerances, it is imperative to match the X and Y coefficient expansion of the PCB substrate to copper. 025nm over ambient temperatures (-40 to 80degC), described in this presentation. The above graphic shows the expansion of a dielectric material in the Z-axis. Thus it is necessary to have a low CTE printed circuit board in order to keep solder joints intact with such low CTE packages. Jul 11, 2017 · This value is of importance for the stability of vias. CTE measures the expansion rate of a PCB material as it heats up. Once the temperature of the uConventional way of classifying materials is mainly based on Tg. Coefficient of Thermal Expansion — Rate of expansion or contraction of a PCB material as it heats up or cools down. Despite relatively high signal loss and low thermal rating, FR1 PCB offered a low-cost material choice. Loss Tangent (Df): Indicates the material’s ability to dissipate energy. Mar 1, 2003 · How advanced low coefficient of thermal expansion (CTE) laminates and prepregs can improve the reliability of printed circuit boards (PCBs) - Author: Michael Weinhold, George Yen This paper details the impact of “Coefficient of Thermal Expansion” (CTE) imbalances caused by the different materials used in a Printed Circuit Board (PCB). Herein, a novel polyimide (PI) with low CTE is successfully prepared with the introduce benzoxazole and silicon-oxygen groups. Types of PCB Materials. Semiconductor PKG; -Low CTE with high modulus-Standard material in FC-BGA market: 250–270: 5–7: packages have more functionality and faster speed but also have very low CTE compare to traditional PCB material. LaminateR-5785(N)/Prepreg R-5680(N) The ultra-low dielectric constant (Dk) and dissipation factor (Df) make MEGTRON 7 ideal for high speed and large data volumes associated with servers and routers required for 5G. Introduction to Various Nov 21, 2022 · What are the Materials Used in a Ceramic PCB? A variety of ceramic materials are used to make ceramic PCBs. IT180A is a Multifunctional Filled Epoxy Resin. uKey material features: Td, CTE, and Tg Class Curing Agent Tg(℃, DSC)Td (℃) CTE(%, 50~260℃) Product Low transmission loss materials for high frequency, high speed signal use. This PCB substrate is a low-cost, most commonly used material with good electrical and environmental properties. Q. Electrical Performance: Oct 15, 2020 · Combining copper with invar, a material with a low coefficient of thermal expansion, results in CIC PCBs. The CTE of these PCB materials is much lower than FR-4. Therefore, the copper foil removes more heat than the dielectric material of the PCB can. Besides, excellent thermal and mechanical properties are also proved These materials are UL 94 V-0 rated for active devices and high power RF designs. When choosing ceramic materials, the two essential characteristics to pay attention to are the PCB thermal conductivity and coefficient of thermal expansion (CTE). NASA low outgassing approved one component thermally conductive epoxy adhesive. Design concept of MCL-HS100 Generally, the reduction in the coefficient of thermal expansion of the substrate material can reduce the warpage of the thin-type package substrate. com. Because a PCB is typically formed of a dielectric laminated with copper (t o form transmission lines and a ground plane), t he material’s linear CTE in the x and y directions is usually engineered to match the CTE of copper (about 17 ppm/°C). Jul 11, 2024 · 4. Apr 23, 2024 · PCB materials encompass a diverse array of substrates, laminates, and dielectric materials that form the foundation of electronic circuits. 2: CTE Before and Beyond Tg. Rogers RO4350B is a popular high frequency circuit board laminate designed for performance-demanding wireless and radar applications from 500 MHz to 77 GHz. In the reinforcement direction, it restrains by a glass cloth having a relatively low CTE. Y axis, it has a coefficient of thermal expansion (CTE) of 10/9 ppm/℃. 370HR has a thermal conductivity of 0. When a substrate is exposed to temperatures that exceed its Tg, the material will also undergo a rise in CTE, which is measured in parts per million (ppm). 4. Used in microwave and millimeter wave frequencies, this low loss material offers easier use in circuit fabrication and streamlined properties over traditional PTFE materials. mal Expansion (CTE) mismatch between PCB and IC substrate. One important thermomechanical property of PCB materials is the CTE value, or the rate of change in a material’s volume with temperature. CTE is expressed in parts per million (ppm) expanded for every degree Celsius that it is heated. Mar 13, 2024 · Q. Sep 23, 2022 · This extension is much larger in the transversal direction. This causes mechanical stress that can create micro-cracks that may be undetected during electrical tests (Open / short tests) carried out at the end of production. Should match copper traces to avoid structural stress. Ultra-low Loss, Highly Heat Resistant Circuit Board Materials. EMC offer a wide range of products with diversified applications, including materials for HDI, multi-layer PCB, high-layer-count PCB, IC substrate, and rigid-flexible PCB. In a typical PCB laminate this means the z axis is expanding from 50 to 200 ppm/ °C at the Tg, compared to 15 ppm/ °C in the x-y axis.
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