Yuav ua li cas tswj PCB impedance?

Without impedance control, considerable signal reflection and distortion will be caused, resulting in design failure. Common signals, such as PCI bus, PCI-E bus, USB, Ethernet, DDR memory, LVDS signal, etc., all need impedance control. Kev tiv thaiv kev tiv thaiv thaum kawg yuav tsum tau ua tiav los ntawm PCB tsim, uas tseem tso rau pem hauv ntej qhov xav tau ntau dua rau PCB tshuab thev naus laus zis. Tom qab kev sib txuas lus nrog PCB lub Hoobkas thiab ua ke nrog kev siv EDA software, qhov cuam tshuam ntawm kev thaiv tau raug tswj raws li qhov xav tau ntawm kev teeb tsa kev ncaj ncees.

Txoj kev thaiv sib txawv tuaj yeem suav tau kom tau txais tus nqi cuam tshuam.

Microstrip lines

• It consists of a strip of wire with the ground plane and dielectric in the middle. Yog tias qhov hluav taws xob tsis tu ncua, qhov dav ntawm kab, thiab nws qhov deb ntawm lub dav hlau hauv av tuaj yeem tswj tau, tom qab ntawd nws tus yam ntxwv tsis zoo yog tswj tau, thiab qhov raug yuav nyob hauv ± 5%.

What is impedance control how to perform impedance control on PCB

Kev Qhuab Qhia

Ib txoj kab hlua yog ib txoj hlua tooj liab nyob hauv nruab nrab ntawm dielectric nruab nrab ntawm ob lub dav hlau uas ua. If the thickness and width of the line, the dielectric constant of the medium, and the distance between the ground planes of the two layers are controllable, the characteristic impedance of the line is controllable, and the accuracy is within 10%.

What is impedance control how to perform impedance control on PCB

The structure of multi-layer board:

Txhawm rau tswj PCB qhov tsis zoo, nws yog qhov tsim nyog kom nkag siab cov qauv ntawm PCB:

Feem ntau qhov peb hu ua pawg thawj coj saib ntau lub tsev yog ua los ntawm cov phaj tub ntxhais thiab cov ntawv ib nrab khov ua ke ua ke. Lub rooj tsavxwm yog tawv, tshwj xeeb tuab, ob lub phaj tooj liab tooj liab, uas yog cov khoom siv yooj yim ntawm cov ntawv luam tawm. Thiab daim ib nrab kho tau yog qhov hu ua infiltration txheej, ua lub luag haujlwm ntawm kev sib koom ua ke ntawm cov phaj phaj, txawm hais tias muaj qee qhov pib tuab, tab sis nyob rau hauv cov txheej txheem ntawm nias nws cov tuab yuav tshwm sim qee qhov kev hloov pauv.

Feem ntau txheej ob txheej txheej txheej sab nrauv ntawm txheej ntau txheej yog txheej txheej ntub dej, thiab cais cov txheej tooj liab tooj liab sab nraud sab nraud ntawm ob txheej no raws li txheej txheej tooj liab sab nraud. Thawj qhov tshwj xeeb tuab ntawm cov ntawv ci tooj liab sab hauv thiab cov ntawv tooj liab sab hauv feem ntau yog 0.5oz, 1OZ, 2OZ (1OZ yog li 35um lossis 1.4mil), tab sis tom qab ua ntu ntu ntawm kev kho saum npoo, qhov kawg ntawm txheej txheej txheej tooj liab sab nraud feem ntau yuav nce los ntawm 1 ZOS. Cov ntawv ci tooj liab sab hauv yog tooj liab npog rau ob sab ntawm lub hauv paus phaj. Qhov tuab tuab sib txawv me ntsis los ntawm qhov tuab tuab, tab sis nws feem ntau txo los ntawm ob peb um vim los ntawm kev kos.

The outermost layer of the multilayer board is the welding resistance layer, which is what we often say “green oil”, of course, it can also be yellow or other colors. The thickness of the solder resistance layer is generally not easy to determine accurately. The area without copper foil on the surface is slightly thicker than the area with copper foil, but because of the lack of copper foil thickness, so the copper foil is still more prominent, when we touch the printed board surface with our fingers can feel.

When a particular thickness of the printed board is made, on the one hand, reasonable choice of material parameters is required, on the other hand, the final thickness of the semi-cured sheet will be smaller than the initial thickness. The following is a typical 6-layer laminated structure:

What is impedance control how to perform impedance control on PCB

PCB parameters:

Different PCB plants have slight differences in PCB parameters. Through communication with circuit board plant technical support, we obtained some parameter data of the plant:

Cov ntawv tooj liab tooj liab:

There are three thicknesses of copper foil that can be used: 12um, 18um and 35um. Qhov tuab tuab tom qab ua tiav yog kwv yees li 44um, 50um thiab 67um.

Core plate: S1141A, standard FR-4, two breaded copper plates are commonly used. The optional specifications can be determined by contacting the manufacturer.

Semi-cured tablet:

Kev qhia tshwj xeeb (thawj qhov tuab) yog 7628 (0.185mm), 2116 (0.105mm), 1080 (0.075mm), 3313 (0.095mm). Qhov tuab tuab tom qab nias feem ntau yog li 10-15um tsawg dua tus nqi qub. Qhov siab tshaj ntawm 3 ntsiav tshuaj ib nrab tuaj yeem siv rau tib txheej kev nkag mus, thiab tuab ntawm 3 ntsiav tshuaj ib nrab kho tsis tuaj yeem zoo ib yam, tsawg kawg ib nrab ntsiav tshuaj tuaj yeem siv tau, tab sis qee cov tuam ntxhab yuav tsum siv tsawg kawg ob . If the thickness of the semi-cured piece is not enough, the copper foil on both sides of the core plate can be etched off, and then the semi-cured piece can be bonded on both sides, so that a thicker infiltration layer can be achieved.

Kuj vuam txheej:

The thickness of the solder resist layer on the copper foil is C2≈8-10um. The thickness of the solder resist layer on the surface without copper foil is C1, which varies with the thickness of copper on the surface. When the thickness of copper on the surface is 45um, C1≈13-15um, and when the thickness of copper on the surface is 70um, C1≈17-18um.

Hla ntu:

We would think that the cross section of a wire is a rectangle, but it’s actually a trapezoid. Taking the TOP layer as an example, when the thickness of copper foil is 1OZ, the upper bottom edge of trapezoid is 1MIL shorter than the lower bottom edge. Piv txwv, yog tias kab dav yog 5MIL, tom qab ntawd sab saum toj thiab hauv qab yog txog 4MIL thiab sab hauv qab thiab hauv qab yog txog 5MIL. The difference between top and bottom edges is related to copper thickness. The following table shows the relationship between top and bottom of trapezoid under different conditions.

What is impedance control how to perform impedance control on PCB

Permittivity: The permittivity of semi-cured sheets is related to thickness. The following table shows the thickness and permittivity parameters of different types of semi-cured sheets:

What is impedance control how to perform impedance control on PCB

The dielectric constant of the plate is related to the resin material used. The dielectric constant of FR4 plate is 4.2 — 4.7, and decreases with the increase of frequency.

Dielectric poob qhov tseem ceeb: cov khoom siv dielectric raws li kev hloov pauv hluav taws xob, vim cua sov thiab kev siv hluav taws xob hu ua dielectric poob, feem ntau yog qhia los ntawm dielectric poob qhov zoo Tan. Tus nqi raug rau S1141A yog 0.015.

Yam tsawg kawg kab dav thiab kab sib nrug kom ntseeg tau kev ua haujlwm: 4mil/4mil.

Impedance xam cov cuab yeej qhia:

Thaum peb nkag siab cov qauv ntawm pawg thawj coj saib ntau tus neeg thiab paub txog qhov tsis xav tau, peb tuaj yeem suav qhov cuam tshuam los ntawm EDA software. Koj tuaj yeem siv Allegro los ua qhov no, tab sis kuv pom zoo rau Polar SI9000, uas yog lub cuab yeej zoo rau suav cov yam ntxwv tsis zoo thiab tam sim no siv los ntawm ntau lub chaw tsim khoom PCB.

Thaum xam tus yam ntxwv tsis zoo ntawm lub teeb liab sab hauv ntawm ob kab sib txawv thiab kab ib kab dav dav, koj yuav pom tsuas yog qhov sib txawv me ntsis ntawm Ncov qaumteb qabteb SI9000 thiab Allegro vim muaj qee cov ntsiab lus, xws li cov duab ntawm ntu ntu ntawm cov hlau. Txawm li cas los xij, yog tias nws yog los xam tus yam ntxwv tsis zoo ntawm lub teeb liab Surface, Kuv hais qhia koj xaiv Tus Qauv Coated tsis yog siv tus qauv Surface, vim tias cov qauv no coj mus rau hauv tus lej ntawm cov kab ua haujlwm tsis kam, yog li cov txiaj ntsig yuav raug dua. The following is a partial screenshot of the surface differential line impedance calculated with Polar SI9000 considering the solder resistance layer:

What is impedance control how to perform impedance control on PCB

Txij li cov tuab ntawm cov txheej txheem tiv thaiv txheej txheem tsis yooj yim los tswj, ib qho kev kwv yees kwv yees kuj tseem tuaj yeem siv tau, raws li tau pom zoo los ntawm pawg thawj coj saib xyuas kev lag luam: rho tawm tus nqi tshwj xeeb los ntawm kev suav qauv qauv. Nws tau pom zoo tias qhov sib txawv ntawm qhov tsis txaus yuav tsum tau rho tawm 8 ohms thiab qhov ib leeg-kawg qhov cuam tshuam yuav raug rho tawm 2 ohms.

Kev sib txawv PCB xav tau rau kev thaiv

(1) Determine the wiring mode, parameters and impedance calculation. Muaj ob hom kev sib txawv rau txoj kab kab: txheej txheej microstrip kab sib txawv hom thiab txheej txheej txheej kab sib txawv. Impedance tuaj yeem suav los ntawm kev cuam tshuam cuam tshuam txog kev suav software (xws li POLAR-SI9000) lossis tus lej ntsuas qhov tsis txaus los ntawm qhov ntsuas tsim nyog.

(2) Kab sib npaug isometric kab. Txiav txim siab txoj kab dav thiab qhov sib nrug, thiab nruj me ntsis ua raws cov kab dav dav thiab qhov sib nrug thaum taug kev. Qhov sib nrug nruab nrab ntawm ob kab yuav tsum ib txwm tsis hloov pauv, uas yog, kom sib npaug. There are two ways of parallelism: one is that the two lines walk in the same side-by-side layer, and the other is that the two lines walk in the over-under layer. Feem ntau sim kom tsis txhob siv qhov sib txawv teeb liab ntawm cov txheej, uas yog vim nyob hauv kev ua tiav ntawm PCB hauv cov txheej txheem, vim qhov cascading laminated tuab si lug raug yog qis dua li muab nruab nrab ntawm qhov ua kom raug, thiab hauv txheej txheem laminated dielectric poob, tsis tuaj yeem lav qhov sib txawv kab sib nrug yog sib npaug rau cov tuab ntawm interlayer dielectric, yuav ua rau qhov sib txawv ntawm cov khaubncaws sab nraud povtseg ntawm qhov sib txawv ntawm kev hloov pauv. Nws raug pom zoo kom siv qhov sib txawv hauv tib txheej kom ntau li ntau tau.