Kuidas kontrollida PCB takistus?

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. Impedance control ultimately needs to be realized through PCB design, which also puts forward higher requirements for PCB board technology. After communication with PCB factory and combined with the use of EDA software, the impedance of wiring is controlled according to the requirements of signal integrity.

Vastava impedantsi väärtuse saamiseks saab arvutada erinevaid juhtmestiku meetodeid.

Microstrip lines

• See koosneb traadiribast, mille maandustasand ja dielektrik on keskel. Kui dielektriline konstant, joone laius ja kaugus maapinnast on reguleeritavad, siis on selle iseloomulik takistus reguleeritav ja täpsus jääb ± 5%piiresse.

What is impedance control how to perform impedance control on PCB

Stripline

Lintjoon on vaskriba dielektriku keskel kahe juhtiva tasapinna vahel. 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:

PCB impedantsi hästi juhtimiseks on vaja mõista PCB struktuuri:

Tavaliselt koosneb see, mida me nimetame mitmekihiliseks plaadiks, üksteisest lamineeritud südamikplaadist ja pooltahke lehest. Põhiplaat on kõva, kindla paksusega kahe leivaga vaskplaat, mis on trükiplaadi põhimaterjal. And the semi-cured piece constitutes the so-called infiltration layer, plays the role of bonding the core plate, although there is a certain initial thickness, but in the process of pressing its thickness will occur some changes.

Usually the outermost two dielectric layers of a multilayer are wetted layers, and separate copper foil layers are used on the outside of these two layers as the outer copper foil. Välise vaskfooliumi ja sisemise vaskfooliumi esialgne paksuse spetsifikatsioon on tavaliselt 0.5, 1 oz, 2 OZ, 1 OZ (35 OZ on umbes 1.4 um või 1, XNUMX ml), kuid pärast mitmeid pinnatöötlusi suureneb välise vaskfooliumi lõplik paksus üldiselt umbes XNUMXOZ. Sisemine vaskfoolium on südamikuplaadi mõlemal küljel olev vaskkate. Lõplik paksus erineb esialgsest paksusest vähe, kuid söövitamise tõttu väheneb see tavaliselt mitme um võrra.

Mitmekihilise plaadi välimine kiht on keevitustakistuskiht, mida me sageli ütleme “roheline õli”, muidugi võib see olla ka kollane või muud värvi. 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:

Pinna vaskfoolium:

There are three thicknesses of copper foil that can be used: 12um, 18um and 35um. Lõplik paksus pärast viimistlemist on umbes 44um, 50um ja 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:

Spetsifikatsioonid (algne paksus) on 7628 (0.185 mm), 2116 (0.105 mm), 1080 (0.075 mm), 3313 (0.095 mm). Tegelik paksus pärast pressimist on tavaliselt umbes 10-15um väiksem kui algne väärtus. Sama infiltratsioonikihi jaoks võib kasutada maksimaalselt 3 poolkõvastatud tabletti ja 3 poolkõvastatud tableti paksus ei pruugi olla sama, võib kasutada vähemalt ühte poolkõvastunud tabletti, kuid mõned tootjad peavad kasutama vähemalt kahte . 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.

Resistance welding layer:

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.

Läbilõige:

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. Näiteks kui joone laius on 5 MIL, siis ülemine ja alumine külg on umbes 4 MIL ning alumine ja alumine külg on umbes 5 MIL. 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 loss factor: dielectric materials under the action of alternating electric field, due to heat and energy consumption is called dielectric loss, usually expressed by dielectric loss factor Tan δ. S1141A tüüpiline väärtus on 0.015.

Minimaalne rea laius ja reavahe töötlemise tagamiseks: 4mil/4mil.

Impedantsi arvutamise tööriista sissejuhatus:

When we understand the structure of the multilayer board and master the required parameters, we can calculate the impedance through EDA software. Selleks saate kasutada Allegro’i, kuid soovitan Polar SI9000, mis on hea tööriist iseloomuliku takistuse arvutamiseks ja mida nüüd kasutavad paljud PCB tehased.

Nii diferentsiaaljoone kui ka ühe klemmliini sisemise signaali iseloomuliku takistuse arvutamisel leiate Polar SI9000 ja Allegro vahel vaid mõningaid üksikasju, näiteks juhtme ristlõike kuju. Kui aga arvutada Surface signaali iseloomulik takistus, soovitan teil valida Surface mudeli asemel Coated mudel, sest sellised mudelid võtavad arvesse jootetakistuse olemasolu, nii et tulemused on täpsemad. 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

Kuna jootekindla kihi paksust ei ole lihtne kontrollida, võib kasutada ka ligikaudset lähenemisviisi, nagu on soovitanud plaadi tootja: lahutage pindmudeli arvutusest konkreetne väärtus. Soovitav on, et diferentsiaaltakistus oleks miinus 8 oomi ja ühe otsa takistus miinus 2 oomi.

PCB diferentsiaalnõuded juhtmestikule

(1) Determine the wiring mode, parameters and impedance calculation. There are two kinds of difference modes for line routing: outer layer microstrip line difference mode and inner layer strip line difference mode. Impedance can be calculated by related impedance calculation software (such as POLAR-SI9000) or impedance calculation formula through reasonable parameter setting.

(2) Paralleelsed isomeetrilised jooned. Määrake joone laius ja vahe ning järgige marsruutimisel rangelt arvutatud joone laiust ja vahekaugust. Kahe rea vahekaugus peab alati jääma muutumatuks, st paralleelseks. 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. Üldiselt proovige vältida kihtide vahelise erinevussignaali kasutamist, nimelt seetõttu, et PCB tegelikul töötlemisel on kaskaadse lamineeritud joondamise tõttu täpsus söövitamise täpsuse ja lamineeritud dielektrilise kadu vahel palju väiksem, ei saa garanteerida erinevust, et reavahe on võrdne kihtidevahelise dielektriku paksusega, põhjustab impedantsimuutuse erinevuse kihtide erinevuse. Soovitatav on kasutada erinevust sama kihi piires nii palju kui võimalik.