{"id":3285,"date":"2025-07-11T20:50:20","date_gmt":"2025-07-11T12:50:20","guid":{"rendered":"https:\/\/www.flywing-tech.com\/blog\/?p=3285"},"modified":"2025-09-04T13:49:53","modified_gmt":"2025-09-04T05:49:53","slug":"why-decoupling-capacitors-placement-matter-in-pcb-design","status":"publish","type":"post","link":"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/","title":{"rendered":"Decoupling Capacitor Placement: Stop Random MCU Resets"},"content":{"rendered":"<div class=\"fsc_text\"><div id=\"ez-toc-container\" class=\"ez-toc-v2_0_76 counter-hierarchy ez-toc-counter ez-toc-custom ez-toc-container-direction\">\r\n<div class=\"ez-toc-title-container\">\r\n<h2 class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/h2>\r\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #023a85;color:#023a85\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #023a85;color:#023a85\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\r\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/#why_decoupling_capacitor_placement_kills_more_designs_than_value_selection\" >Why Decoupling Capacitor Placement Kills More Designs Than Value Selection<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/#why_doesnt_just_picking_a_01%e2%80%af%c2%b5f_capacitor_solve_power_noise_problems\" >Why doesn\u2019t just picking a 0.1\u202f\u00b5F capacitor solve power noise problems?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/#why_does_my_microcontroller_need_caps_right_next_to_it_cant_the_power_plane_handle_it\" >Why does my microcontroller need caps right next to it? Can\u2019t the power plane handle it?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/#how_do_decoupling_capacitor_placement_stop_high-frequency_noise\" >How do decoupling capacitor placement stop high-frequency noise?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/#decoupling_capacitor_placement_why_do_engineers_use_more_than_one_capacitor_per_chip\" >decoupling capacitor placement:Why do engineers use more than one capacitor per chip?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/#what_happens_if_i_skip_small-value_capacitors_like_001%e2%80%af%ce%bcf\" >What happens if I skip small-value capacitors like 0.01\u202f\u03bcF?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/#why_is_using_y5v_capacitors_risky_in_digital_designs\" >Why is using Y5V capacitors risky in digital designs?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/www.flywing-tech.com\/blog\/why-decoupling-capacitors-placement-matter-in-pcb-design\/#is_there_a_problem_with_using_only_large_1206-size_capacitors\" >Is there a problem with using only large 1206-size capacitors?<\/a><\/li><\/ul><\/nav><\/div>\r\n<h2><span class=\"ez-toc-section\" id=\"why_decoupling_capacitor_placement_kills_more_designs_than_value_selection\"><\/span>Why Decoupling Capacitor Placement Kills More Designs Than Value Selection<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Your microcontroller just reset again. The culprit? Poor decoupling capacitor placement. That 0.1\u00b5F ceramic cap sitting 2cm from your MCU might as well not exist. At 100MHz switching speeds, decoupling capacitor placement is everything\u2014the difference between a working prototype and an expensive paperweight<\/p>\n<p><span style=\"font-weight: 400\">Decoupling capacitors are one type of electrical component that appears in almost all digital electronic circuits\/systems found today. They are active devices in <a href=\"https:\/\/www.flywing-tech.com\/blog\/power-management-ic-selection-guide-optimizing-efficiency-in-complex-systems\/\">electronic systems<\/a> (transistors, <a href=\"https:\/\/www.flywing-tech.com\/blog\/gan-and-sic-semiconductors-for-power-engineers\/\">integrated circuits,<\/a> vacuum tubes, etc.) connect to their power supplies through conductors that exhibit finite resistance and inductance. <\/span><\/p>\n<p><span style=\"font-weight: 400\">When the current drawn by an active device changes, the voltage drop from the power supply to the device also changes due to these impedances. If several active devices share a common path to the power supply, changes in the current drawn by one element may produce voltage changes large enough to affect the operation of others \u2013 voltage spikes or ground bounce, for example \u2013 so the change of state of one device is coupled to others through the <a href=\"https:\/\/www.sciencedirect.com\/topics\/engineering\/common-impedance\">common impedance<\/a> to the power supply. A decoupling capacitor placement provides a bypass path for transient currents, instead of flowing through the common impedance.\u00a0<\/span><\/p>\n<div id=\"attachment_3312\" style=\"width: 2072px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3312\" class=\"size-full wp-image-3312\" src=\"https:\/\/www.flywing-tech.com\/blog\/wp-content\/uploads\/2025\/07\/Screenshot-2025-07-11-at-09.00.13.png\" alt=\"decoupling capacitor placement\" width=\"2062\" height=\"1436\" \/><p id=\"caption-attachment-3312\" class=\"wp-caption-text\">decoupling capacitor placement<\/p><\/div>\n<p><span style=\"font-weight: 400\">The <a href=\"https:\/\/learn.sparkfun.com\/tutorials\/capacitors\/application-examples\">decoupling capacitor<\/a> functions as the device&#8217;s local energy storage. The decoupling capacitor placement\u00a0 should be between the power line and the ground to the circuit where the current is to be provided. According to the capacitor current\u2013voltage relation i(t)=Cdv(t)dt, a voltage drop between a power line and the ground results in a current drawn out from the capacitor to the circuit. When capacitance C is large enough, sufficient current is supplied to maintain an acceptable range of voltage drop.<\/span><\/p>\n<p>&nbsp;<\/p>\n<div id=\"attachment_3289\" style=\"width: 946px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3289\" class=\"size-full wp-image-3289\" src=\"https:\/\/www.flywing-tech.com\/blog\/wp-content\/uploads\/2025\/07\/Picture1.png\" alt=\"A zoomed-in illustration of a capacitor between VCC and GND pins on a PCB\" width=\"936\" height=\"890\" \/><p id=\"caption-attachment-3289\" class=\"wp-caption-text\">A zoomed-in illustration of a capacitor between VCC and GND pins on a PCB<\/p><\/div>\n<h3>Decoupling Capacitor Placement Layout Guidelines<\/h3>\n<p><span style=\"font-weight: 400\"> The capacitor stores a small amount of energy that can compensate for the voltage drop in the power supply conductors to the capacitor. To reduce undesired parasitic equivalent series inductance, small and large <a href=\"https:\/\/electronics.stackexchange.com\/questions\/21686\/whats-the-purpose-of-two-capacitors-in-parallel\">capacitors are often placed in parallel<\/a>, adjacent to individual integrated circuits.<\/span><\/p>\n<div id=\"attachment_3314\" style=\"width: 2040px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3314\" class=\"size-full wp-image-3314\" src=\"https:\/\/www.flywing-tech.com\/blog\/wp-content\/uploads\/2025\/07\/Screenshot-2025-07-11-at-09.01.58.png\" alt=\"functions of a decoupling capacitor placement\" width=\"2030\" height=\"1412\" \/><p id=\"caption-attachment-3314\" class=\"wp-caption-text\">functions of a decoupling capacitor placement<\/p><\/div>\n<h3><b>When a microcontroller resets randomly, the root cause is often traced to poor\u00a0 decoupling capacitor placement<\/b><\/h3>\n<p><span style=\"font-weight: 400\">I remember when I was working on a small sensor device that I used a button cell battery to power it. The setup used a PIC microcontroller and would wake up every now and then to send data. On my desk, everything worked fine. But in the field, sometimes it wouldn\u2019t turn back on right after changing the battery. It looked dead. Then, suddenly, it would start working again after 20\u201330 seconds.<\/span><\/p>\n<p><span style=\"font-weight: 400\">At first, I thought it was a power or code issue. But after checking everything, I found the real cause: the<\/span><a href=\"https:\/\/www.flywing-tech.com\/category\/capacitors\"><span style=\"font-weight: 400\"> decoupling capacitors <\/span><\/a><span style=\"font-weight: 400\">were holding charge even after the battery was removed. Since the microcontroller used very little power in sleep mode, the voltage didn\u2019t drop low enough to fully reset it. So when power came back, the chip did not restart, but it remained in a partially working state.<\/span><\/p>\n<p><span style=\"font-weight: 400\">To fix it, I added a resistor to slowly drain the leftover charge and used a reset chip to make sure the microcontroller always restarted cleanly.<\/span><\/p>\n<p><span style=\"font-weight: 400\">That\u2019s when I learned that just removing the battery doesn\u2019t always mean your system is completely powered off; those small capacitors can keep it &#8220;half-awake&#8221; and cause strange problems.<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"why_doesnt_just_picking_a_01%e2%80%af%c2%b5f_capacitor_solve_power_noise_problems\"><\/span><span style=\"font-weight: 400\">Why doesn\u2019t just picking a 0.1\u202f\u00b5F capacitor solve power noise problems?<\/span><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400\">Most Engineers choose a decoupling capacitor by looking at its value, for example,\u00a0 0.1\u202f\u00b5F, 1\u202f\u00b5F, or 10\u202f\u00b5F. That\u2019s a good start, but it does not tell you the whole story.<\/span><\/p>\n<p><span style=\"font-weight: 400\">The Capacitance value tells you how much <\/span><b>charge <\/b><span style=\"font-weight: 400\">the capacitor can store. It purpose is to help smooth out <\/span><b>slow voltage dips<\/b><span style=\"font-weight: 400\">, because as switching speeds increase, the noise gets faster and higher in frequency. And by noise i mean voltage Spikes or overshoot<\/span><\/p>\n<p><span style=\"font-weight: 400\">To put it candidly, every time a digital IC switches states ( from 0 to 1), it draws a sudden high amount of current from the power source. \u00a0 So,\u00a0 If your<a href=\"https:\/\/transientspecialists.com\/blogs\/blog\/coupling-decoupling-networks-conducted-rf-transient-immunity?srsltid=AfmBOoqx9i34yUiS0BWU42Ht6bkdijcQ3tYdkD0DsmqRhudWaDajl72W\"> decoupling network<\/a> and layout is not good, this causes the noise we mentioned above<\/span><\/p>\n<p><span style=\"font-weight: 400\">Always be mindful that If your microcontroller or FPGA switches at 100\u202fMHz, the power supply doesn\u2019t just need a bulky capacitance. It requires a capacitor that can <\/span><b>respond fast enough<\/b><span style=\"font-weight: 400\"> to those high-frequency events. And that&#8217;s where <\/span><b>SRF and ESR<\/b><span style=\"font-weight: 400\"> come in.<\/span><\/p>\n<h3><b><i>What Frequency Range Are We Talking About?<\/i><\/b><\/h3>\n<ul>\n<li style=\"font-weight: 400\"><i><span style=\"font-weight: 400\">Low-speed switching (basic MCUs): <\/span><\/i><b><i>100\u202fkHz to a few MHz<\/i><\/b><\/li>\n<\/ul>\n<ul>\n<li style=\"font-weight: 400\"><i><span style=\"font-weight: 400\">Medium-speed digital (Wi-Fi chips, Cortex-M4): <\/span><\/i><b><i>10\u202fMHz\u2013100\u202fMHz<\/i><\/b><\/li>\n<\/ul>\n<ul>\n<li style=\"font-weight: 400\"><i><span style=\"font-weight: 400\">High-speed logic (FPGAs, DDR memory): <\/span><\/i><b><i>100\u202fMHz\u2013500+\u202fMHz<\/i><\/b><\/li>\n<\/ul>\n<h3>Decoupling Capacitor Placement vs. Power Plane Performance<\/h3>\n<p><i><span style=\"font-weight: 400\">At those frequencies, capacitances like 10\u202f\u00b5F are <\/span><\/i><b><i>too slow to respond<\/i><\/b><i><span style=\"font-weight: 400\">. This is why you need small, high-SRF ceramic capacitors <\/span><\/i><b><i>placed close to the IC<\/i><\/b><i><span style=\"font-weight: 400\"> to remove that noise before it damages your\u00a0 logic levels or radiates as EMI.<\/span><\/i><\/p>\n<div id=\"attachment_3311\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3311\" class=\"size-full wp-image-3311\" src=\"https:\/\/www.flywing-tech.com\/blog\/wp-content\/uploads\/2025\/07\/pic4.png\" alt=\"decoupling capacitor placement\" width=\"1024\" height=\"1024\" \/><p id=\"caption-attachment-3311\" class=\"wp-caption-text\">A graph showing current spike during an IC switching event.<\/p><\/div>\n<h3><b>SRF: Self-Resonant Frequency is The \u201cSpeed Limit\u201d of a decoupling capacitor placement<\/b><\/h3>\n<p><span style=\"font-weight: 400\">Let me tell you a secret: Every capacitor behaves like more than just a capacitor. It also has these two unique quality:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">A bit of <\/span><b>series resistance<\/b><span style=\"font-weight: 400\"> (ESR)<\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">A bit of <\/span><b>inductance<\/b><span style=\"font-weight: 400\"> (ESL \u2014 Equivalent Series Inductance)<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400\">You need to know that a capacitor&#8217;s performance is governed by its total impedance (Z), which includes its Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). The formula for a\u00a0 capacitor&#8217;s impedance at a given frequency (f) is:<\/span><span style=\"font-weight: 400\">\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400\">Where:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">X_L is the inductive reactance:<\/span><span style=\"font-weight: 400\"><br \/>\n<\/span>$$X_L = 2\\pi f \\cdot \\text{ESL}$$<\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">X_C is the capacitive reactance:<\/span><span style=\"font-weight: 400\"><br \/>\n<\/span>$$X_C = \\frac{1}{2\\pi f \\cdot C}$$<\/li>\n<\/ul>\n<p><span style=\"font-weight: 400\">The Self-Resonant Frequency (f_SRF) is the point where the inductive reactance equals the capacitive reactance (X_L = X_C), making the capacitor&#8217;s impedance reach its absolute minimum (equal to its ESR). This is calculated as:<\/span><\/p>\n<p>$$f_{\\text{SRF}} = \\frac{1}{2\\pi \\sqrt{\\text{ESL} \\cdot C}}$$<\/p>\n<p><span style=\"font-weight: 400\">At this frequency, the capacitor is most effective at shunting high-frequency noise.<\/span><\/p>\n<p><span style=\"font-weight: 400\">This means that while operating it at some frequency, the capacitor\u2019s inductive behavior <\/span><b>cancels out<\/b><span style=\"font-weight: 400\"> its capacitive behavior. That frequency is called the <\/span><b>Self-Resonant Frequency (SRF)<\/b><span style=\"font-weight: 400\">.<\/span><\/p>\n<p><b>Below SRF<\/b><span style=\"font-weight: 400\">: the cap behaves like a capacitor<\/span><span style=\"font-weight: 400\"><br \/>\n<\/span> <b>At SRF<\/b><span style=\"font-weight: 400\">: impedance is lowest \u2014 best noise filtering<\/span><span style=\"font-weight: 400\"><br \/>\n<\/span> <b>Above SRF<\/b><span style=\"font-weight: 400\">: the cap acts like an <\/span><b>inductor<\/b><span style=\"font-weight: 400\">, and becomes useless<\/span><\/p>\n<p><span style=\"font-weight: 400\">So if you pick a 10\u202f\u00b5F cap thinking you have made the best choice, not realising that its SRF is 300\u202fkHz, it won\u2019t work to block a 50\u202fMHz voltage spike. You need a smaller-value cap (like 0.1\u202f\u00b5F or 0.01\u202f\u00b5F) with a <\/span><b>higher SRF<\/b><span style=\"font-weight: 400\"> to suppress that kind of noise.<\/span><\/p>\n<h4><b>Use this to make your decision<\/b><\/h4>\n<table>\n<tbody>\n<tr>\n<td><b>Cap Value<\/b><\/td>\n<td><b>Typical SRF<\/b><\/td>\n<td><b>Best For Filtering<\/b><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400\">10\u202f\u00b5F<\/span><\/td>\n<td><span style=\"font-weight: 400\">~0.3 MHz<\/span><\/td>\n<td><span style=\"font-weight: 400\">Low-speed drop \/ startup transients<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400\">1\u202f\u00b5F<\/span><\/td>\n<td><span style=\"font-weight: 400\">~3 MHz<\/span><\/td>\n<td><span style=\"font-weight: 400\">Low-frequency switching noise<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400\">0.1\u202f\u00b5F<\/span><\/td>\n<td><span style=\"font-weight: 400\">~30 MHz<\/span><\/td>\n<td><span style=\"font-weight: 400\">Medium-speed digital noise<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400\">0.01\u202f\u00b5F<\/span><\/td>\n<td><span style=\"font-weight: 400\">~200 MHz<\/span><\/td>\n<td><span style=\"font-weight: 400\">High-speed edge spikes (EMI)<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><span style=\"font-weight: 400\">At its core, the capacitor\u2019s impedance is frequency dependent:<\/span><\/p>\n<p>$$Z(f) = \\frac{1}{2\\pi f C}$$<\/p>\n<p><span style=\"font-weight: 400\">But real capacitors also include ESL (L) and ESR (R), turning it into an RLC network gives:<\/span><\/p>\n<p><span style=\"font-weight: 400\">\u00a0 $$Z(f) = \\sqrt{R^2 + \\left(2\\pi f L &#8211; \\frac{1}{2\\pi f C}\\right)^2}$$<\/span><\/p>\n<p><span style=\"font-weight: 400\">The frequency at which inductive and capacitive reactance cancel out is called the Self-Resonant Frequency (SRF):<\/span><\/p>\n<p>$$f_{\\text{SRF}} = \\frac{1}{2\\pi \\sqrt{L C}}$$<span style=\"font-weight: 400\">\u2003\u2003 <\/span><\/p>\n<p><b>Equivalent Series Resistance is use to Controls Damping<\/b><\/p>\n<p><b>ESR<\/b><span style=\"font-weight: 400\"> determines how \u201cresistive\u201d the capacitor is at its working frequencies.<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400\"><b>Low ESR<\/b><span style=\"font-weight: 400\"> = better noise suppression<\/span><\/li>\n<li style=\"font-weight: 400\"><b>Zero ESR<\/b><span style=\"font-weight: 400\"> = can cause <\/span><b>ringing or anti-resonance<\/b><span style=\"font-weight: 400\"> when you use multiple capacitors<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400\">If you mix several capacitors (say, 0.1\u202f\u00b5F and 10\u202f\u00b5F), their SRFs can interfere and <\/span><b>create spikes<\/b><span style=\"font-weight: 400\"> on your power rail unless their ESRs are balanced. That\u2019s why you sometimes want <\/span><b>a bit<\/b><span style=\"font-weight: 400\"> of ESR to absorb that energy without resonance.<\/span><\/p>\n<p><b>Why even picking\u00a0 the Best Capacitor Can Still Fail<\/b><\/p>\n<p><span style=\"font-weight: 400\">Even if you pick the perfect capacitor value, SRF, and ESR, it can still <\/span><b>fail altogether<\/b><span style=\"font-weight: 400\"> if you place it wrong on the PCB. this is why decoupling capacitor placement matters<\/span><\/p>\n<p><span style=\"font-weight: 400\">Here\u2019s what kills decoupling performance:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Long traces from capacitor to IC pin = high inductance<\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Shared vias or power routing = noisy return path<\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Far away placement = slow energy delivery = transient spikes<\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Poor ground = loop area increases \u2192 more radiated EMI<\/span><\/li>\n<\/ul>\n<p><i><span style=\"font-weight: 400\">Think about this: A 0.1\u202f\u00b5F capacitor placed 2\u202fmm from the VDD pin is effective.<\/span><\/i><i><span style=\"font-weight: 400\"><br \/>\n<\/span><\/i><i><span style=\"font-weight: 400\"> The same cap placed 2\u202fcm away? Might as well not be there.<\/span><\/i><\/p>\n<p><span style=\"font-weight: 400\">The capacitor should be placed <\/span><b>as close as possible to the IC\u2019s power pin<\/b><span style=\"font-weight: 400\">, with a <\/span><b>short, direct connection to<\/b> <b>ground.<\/b><span style=\"font-weight: 400\"> Ideally, this connection should utilize multiple vias and a solid ground plane underneath.<\/span><\/p>\n<p><span style=\"font-weight: 400\">The effectiveness of the bypass loop depends on the loop inductance (L), which increases with trace length and area.\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400\">Inductance roughly scales as:<\/span><\/p>\n<p><span style=\"font-weight: 400\">\u2003\u2003$$L \\approx \\frac{\\mu \\cdot l}{w}$$<br \/>\n<\/span><\/p>\n<p><span style=\"font-weight: 400\">Where:<\/span><\/p>\n<p>$$\\begin{align*}<br \/>\nl &amp; = \\text{loop length} \\\\<br \/>\nw &amp; = \\text{trace width} \\\\<br \/>\n\\mu &amp; = \\text{permeability of the material}<br \/>\n\\end{align*}$$<\/p>\n<p><span style=\"font-weight: 400\">Smaller loop = less inductance = faster response.<\/span><\/p>\n<p><span style=\"font-weight: 400\">Let me give a simple analogy to explain this concept.\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400\">Your power source is like a water pipe supplying water(current). While Fast-switching ICs are like someone opening and closing the pipe 100 million times a second. which will create pressure ripples (voltage noise) that bounce back unless you have &#8220;shock absorbers&#8221; (decoupling capacitors) tuned to the right frequency.<\/span><\/p>\n<div id=\"attachment_3293\" style=\"width: 946px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3293\" class=\"size-full wp-image-3293\" src=\"https:\/\/www.flywing-tech.com\/blog\/wp-content\/uploads\/2025\/07\/Picture6.png\" alt=\"decoupling capacitor placement\" width=\"936\" height=\"626\" \/><p id=\"caption-attachment-3293\" class=\"wp-caption-text\">Multilayer Ceramic Capacitors (MLCCs)<\/p><\/div>\n<h3><b>\u00a0What does a decoupling capacitor do on a PCB?<\/b><\/h3>\n<p><span style=\"font-weight: 400\">We have mentioned previously that when an IC quickly switches its logic states, it needs a sharp burst of curren right?. But the problem is that the current can\u2019t travel from your power supply fast enough, it\u2019s too far, and the traces (thin copper lines on a printed circuit board (PCB)) act like inductors. At this point that\u2019s where the decoupling capacitor steps in. It\u2019s works like a battery sitting right next to the IC. It supplies energy instantly, smoothing out the voltage and keeping everything stable.<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"why_does_my_microcontroller_need_caps_right_next_to_it_cant_the_power_plane_handle_it\"><\/span><b>Why does my microcontroller need caps right next to it? Can\u2019t the power plane handle it?<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400\">Let&#8217;s be very clear, your <\/span><a href=\"https:\/\/resources.pcb.cadence.com\/blog\/2024-power-plane-and-ground-pane-pcb-design-best-practices\"><span style=\"font-weight: 400\">central power plane<\/span><\/a><span style=\"font-weight: 400\"> (<\/span><b>solid layer of copper<\/b><span style=\"font-weight: 400\"> for carrying either <\/span><b>power (VDD)<\/b><span style=\"font-weight: 400\"> or <\/span><b>ground (GND)<\/b><span style=\"font-weight: 400\">) cannot handle the current demands of your <a href=\"https:\/\/www.techtarget.com\/iotagenda\/definition\/microcontroller\">Microcontroller<\/a>, because at high speeds, it\u2019s too slow. The inductance in the traces and power planes adds delay. A decoupling capacitor placed just 1\u20132\u202fmm\u00a0 away provides a much faster, low-impedance path to get the charge it needs. That\u2019s why proximity isn\u2019t optional, it\u2019s essential.<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"how_do_decoupling_capacitor_placement_stop_high-frequency_noise\"><\/span><span style=\"font-weight: 400\">How do decoupling capacitor placement stop high-frequency noise?<\/span><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400\">We have all agreed that When an IC switches it states, it creates high-frequency voltage spikes on the power rail. They appear tiny and fast, and they ride on top of your voltage line. A decoupling capacitor will filter them out by acting like a shortcut to the ground. At high frequencies, it has very low impedance, so it &#8220;sinks&#8221; the spike before it can cause any trouble. The smaller the cap, the better it works at higher frequencies.<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"decoupling_capacitor_placement_why_do_engineers_use_more_than_one_capacitor_per_chip\"><\/span><span style=\"font-weight: 400\">decoupling capacitor placement:<\/span><b>Why do engineers use more than one capacitor per chip?<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400\">No single capacitor can clean all the noise your chip will create. A 10\u202f\u00b5F capacitor is great for slow voltage dips but becomes useless above a few MHz. A 0.01\u202f\u00b5F capacitor will not help at startup but will kill high-frequency spikes. That\u2019s why engineers stack them by using multiple values together to cover the whole frequency range. In essence, bulk capacitors handle slow changes, and smaller capacitors deal with fast ones.<\/span><\/p>\n<p><span style=\"font-weight: 400\">Each capacitor handles a portion of the frequency spectrum.<\/span><\/p>\n<p><span style=\"font-weight: 400\">\u2003\u2003$$f_{\\text{cutoff}} \\approx \\frac{1}{2\\pi R C}$$<\/span><\/p>\n<p><span style=\"font-weight: 400\">Using this, a 0.1\u202f\u00b5F capacitor with a 1\u202f\u03a9 path impedance filters noise around:<\/span><\/p>\n<p><span style=\"font-weight: 400\">\u2003\u2003$$f \\approx 1.6\\,\\text{MHz}$$<\/span><\/p>\n<p><span style=\"font-weight: 400\">This shows why you need a mix of smaller capacitors\u00a0 that cutoff frequency higher.<\/span><\/p>\n<h3><b>Common Failure Cases and How to Spot Them<\/b><\/h3>\n<h3><b>Why is placing all decoupling caps near the regulator a bad idea?<\/b><\/h3>\n<p><span style=\"font-weight: 400\">Let me share an experience, I once saw a board where every decoupling capacitor was placed neatly around the LDO, but none near the actual microcontroller. The board passed functional tests, but occasionally glitched during Wi-Fi transmission. It turns out that the switching noise was never being filtered at the source. The capacitors were just too far from the IC. Decoupling capacitor placement matters, they will help you if they\u2019re right next to the switching node. Otherwise, you will just be feeding noise through a long, delayed power rail.<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"what_happens_if_i_skip_small-value_capacitors_like_001%e2%80%af%ce%bcf\"><\/span><span style=\"font-weight: 400\">What happens if I skip small-value capacitors like 0.01\u202f\u03bcF?<\/span><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400\">A common mistake some engineers make is to think 10\u202f\u03bcF or 1\u202f\u03bcF caps are \u201cenough.\u201d But without a 0.01\u202f\u03bcF or 0.1\u202f\u03bcF cap, you leave the high-frequency end wide open. I\u2019ve seen this lead to hard-to-catch logic errors, like random GPIO glitches or memory bus noise. Once a 0.01\u202f\u03bcF cap was added near the MCU, the spikes disappeared from the scope. It\u2019s a frequency game: smaller caps cover faster noise.<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"why_is_using_y5v_capacitors_risky_in_digital_designs\"><\/span><span style=\"font-weight: 400\">Why is using Y5V capacitors risky in digital designs?<\/span><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400\">Y5V capacitors are cheap, but misleading. On paper, you will see 10\u202f\u03bcF. But under 3.3V bias, you might\u00a0 only get 2\u20133\u202f\u03bcF. That\u2019s not enough for proper decoupling. In one project, I had unexplained ripples even with &#8220;enough&#8221; capacitors. After replacing the Y5V ceramics with X7R, the rail cleaned up immediately. Always check the DC bias curve in the datasheet, especially for critical power paths.<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"is_there_a_problem_with_using_only_large_1206-size_capacitors\"><\/span><span style=\"font-weight: 400\">Is there a problem with using only large 1206-size capacitors?<\/span><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400\">Bigger isn\u2019t better when it comes to high-speed noise. 1206 capacitors might hold more charge, but they can\u2019t react fast enough. Their self-resonant frequency is lower, which means they\u2019re useless above 10\u201320\u202fMHz. In one design, switching from 1206 to 0402 for 0.1\u202f\u03bcF capacitors cut my VDD noise by half. For anything above 50\u202fMHz, is better to stick with smaller packages.<\/span><\/p>\n<p>&nbsp;<\/p>\n<p><b>Component Spotlight: Know Your Capacitors<\/b><\/p>\n<p><span style=\"font-weight: 400\">Not all capacitors are the samel. The technology inside each of the component tells us\u00a0 its performance, especially at high frequencies.<\/span><\/p>\n<div id=\"attachment_3294\" style=\"width: 946px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3294\" class=\"wp-image-3294 size-full\" title=\"Multilayer Ceramic Capacitors(MLCCs)\" src=\"https:\/\/www.flywing-tech.com\/blog\/wp-content\/uploads\/2025\/07\/Picture1-1.png\" alt=\"Multilayer Ceramic Capacitors(MLCCs)\" width=\"936\" height=\"626\" \/><p id=\"caption-attachment-3294\" class=\"wp-caption-text\">Multilayer Ceramic Capacitors(MLCCs)<\/p><\/div>\n<h4><b>1. The High-Frequency\u00a0 Multilayer Ceramic Capacitors (MLCCs)<\/b><\/h4>\n<p><span style=\"font-weight: 400\">These are the most common decoupling capacitors, especially for values from 0.01 \u00b5F to 1 \u00b5F.<\/span><\/p>\n<p><span style=\"font-weight: 400\">They have Extremely low <a href=\"https:\/\/circuitdigest.com\/tutorial\/understanding-esr-and-esl-in-capacitors\">ESL and ESR<\/a>. Their physical structure is a stack of interleaved ceramic and metal layers, making them highly efficient at supplying fast bursts of current.<\/span><\/p>\n<p><span style=\"font-weight: 400\">The ceramic material used\u00a0 as dielectric is critical. The most commonly use are:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400\"><b>Class 1 (C0G\/NP0):<\/b><span style=\"font-weight: 400\"> is\u00a0 extremely stable over temperature and voltage, but offer lower capacitance values. Best for filters or timing circuits.<\/span><\/li>\n<li style=\"font-weight: 400\"><b>Class 2 (X7R, X5R):<\/b><span style=\"font-weight: 400\"> this is the go-to choice for your\u00a0 decoupling operation. They offer high capacitance in small packages but lose some capacitance under DC voltage bias. Always check the datasheet for the &#8220;DC Bias Characteristic&#8221; curve.<\/span><\/li>\n<li style=\"font-weight: 400\"><b>Class 2 (Y5V, Z5U):<\/b><span style=\"font-weight: 400\"> these are red flags, <\/span><b>Avoid these for decoupling.<\/b><span style=\"font-weight: 400\"> They are cheap but can lose up to 80% of their capacitance when a voltage is applied. A 10 \u00b5F Y5V capacitor might only act like a 2 \u00b5F capacitor in a 3.3V circuit.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400\">\u00a0For high-speed noise, smaller packages like <\/span><b>0402<\/b><span style=\"font-weight: 400\"> or <\/span><b>0201<\/b><span style=\"font-weight: 400\"> are superior to larger ones like <\/span><b>0805<\/b><span style=\"font-weight: 400\"> or <\/span><b>1206<\/b><span style=\"font-weight: 400\"> because their smaller physical size results in lower ESL.<\/span><\/p>\n<h4><b>2.\u00a0 Electrolytic &amp; Tantalum Capacitors<\/b><\/h4>\n<p><span style=\"font-weight: 400\">These are used for larger capacitance values (typically 10 \u00b5F and above).<\/span><\/p>\n<div id=\"attachment_3296\" style=\"width: 946px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3296\" class=\"wp-image-3296 size-full\" src=\"https:\/\/www.flywing-tech.com\/blog\/wp-content\/uploads\/2025\/07\/Picture14.png\" alt=\"decoupling capacitor placement\" width=\"936\" height=\"936\" \/><p id=\"caption-attachment-3296\" class=\"wp-caption-text\">Electrolytic &amp; Tantalum Capacitors<\/p><\/div>\n<p>&nbsp;<\/p>\n<ul>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">The Electrolytic &amp; <a href=\"https:\/\/www.flywing-tech.com\/category\/capacitors\/tantalum-polymer-capacitors-392ada81\">Tantalum Capacitors<\/a> act as large local energy reservoirs, perfect for handling slower voltage droops that occur when a major part of a chip (like a processor core) wakes up from sleep.\u00a0 They have much higher ESR and ESL than MLCCs, making them too slow to handle high-frequency switching noise.\u00a0 <\/span><i><span style=\"font-weight: 400\">NOTE:<\/span><\/i> <b>These capacitors are polarized.<\/b><span style=\"font-weight: 400\"> They have a positive and negative terminal. Installing one backward will permanently damage it, often causing it to vent or burst.<\/span><\/li>\n<\/ul>\n<h3><b>So What\u2019s the Bottom Line?<\/b><\/h3>\n<p><span style=\"font-weight: 400\">Decoupling\u00a0capacitor placement isn\u2019t about adding a few capacitors near the power rail and hoping for the best to work out. It\u2019s about knowing what the capacitor does at the frequency your circuit operates at, and most importantly, placing it exactly where it needs to be. If you get the <a href=\"https:\/\/resources.pcb.cadence.com\/blog\/2019-capacitor-self-resonant-frequency-and-signal-integrity\">SRF<\/a> wrong, and ignore layout, or pick the wrong dielectric, then the noise will find its way into your circuit.<\/span><\/p>\n<p><span style=\"font-weight: 400\">I have seen designs that pass lab tests but fail in the field because the power rail couldn\u2019t keep up when it mattered. And every single time, the fix was not just another regulator or a firmware patch; it was just using\u00a0 the right capacitor, in the right place, with the right specs.<\/span><\/p>\n<p><span style=\"font-weight: 400\">If you want clean power, don\u2019t just \u201cadd capacitors.\u201d Understand them. Stack values that cover your full frequency range. Keep the decoupling capacitor placement close to the IC. And most importantly, treat layout like it\u2019s part of the circuit, not an afterthought.<\/span><\/p>\n<p><span style=\"font-weight: 400\">That\u2019s the difference between a stable product and one that resets when the user turns on a fan.<\/span><\/p>\n<p>&nbsp;<\/p>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Why Decoupling Capacitor Placement Kills More Designs Than Value Selection Your microcontroller just reset again. The culprit? Poor decoupling capacitor placement. That 0.1\u00b5F ceramic cap sitting 2cm from your MCU might as well not exist. At 100MHz switching speeds, decoupling capacitor placement is everything\u2014the difference between a working prototype and an expensive paperweight Decoupling capacitors [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":3322,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[429,377],"tags":[430,433,431,432],"class_list":["post-3285","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-decoupling-capacitor-design","category-experience-sharing","tag-decoupling-capacitors","tag-noise-suppression","tag-pcb-layout","tag-power-integrity"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.3 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\r\n<title>Decoupling Capacitor Placement: Stop Random MCU Resets - Fly-Wing<\/title>\r\n<meta name=\"description\" content=\"Microcontroller randomly resetting? 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