With the increasing speed of PCB signal switching, today’s PCB designers need to understand and control the impedance of PCB traces. Corresponding to the shorter signal transmission times and higher clock rates of modern digital circuits, PCB traces are no longer simple connections, but transmission lines.

Kako nadzirati impedanco PCB

In practice, it is necessary to control trace impedance when digital marginal speed exceeds 1ns or analog frequency exceeds 300Mhz. Eden od ključnih parametrov sledenja tiskanega vezja je njegova značilna impedanca (razmerje med napetostjo in tokom, ko val potuje po prenosni liniji signala). Značilna impedanca žice na tiskanem vezju je pomemben pokazatelj zasnove vezja, zlasti pri načrtovanju tiskanih vezij visokofrekvenčnega vezja je treba upoštevati, ali je značilna impedanca žice skladna z značilno impedanco, ki jo zahteva naprava ali signal. This involves two concepts: impedance control and impedance matching. This paper focuses on impedance control and lamination design.

Nadzor impedance

EImpedance Controling, the conductor in the circuit board will have all kinds of signal transmission, in order to improve the transmission rate and must increase its frequency, if the line itself due to etching, stacking thickness, wire width and other different factors, will cause impedance value change, the signal distortion. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, known as “impedance control”.

The impedance of a PCB trace will be determined by its inductive and capacitive inductance, resistance, and conductivity coefficient. The main factors affecting the impedance of PCB wiring are: the width of copper wire, the thickness of copper wire, the dielectric constant of medium, the thickness of medium, the thickness of pad, the path of ground wire, the wiring around the wiring, etc. Impedanca tiskanega vezja se giblje od 25 do 120 ohmov.

In practice, a PCB transmission line usually consists of a trace, one or more reference layers, and insulation materials. Traces and layers form the control impedance. PCBS bo pogosto večplastni, krmilno impedanco pa lahko sestavimo na različne načine. However, whatever method is used, the impedance value will be determined by its physical structure and the electrical properties of the insulating material:

Width and thickness of signal trace

The height of the core or prefill material on either side of the trace

Konfiguracija sledi in plošče

Insulation constants of core and prefilled materials

Prenosni vodi PCB so v dveh glavnih oblikah: Microstrip in Stripline.

Microstrip:

Mikrotrakasta linija je tračni vodnik z referenčno ravnino samo na eni strani, pri čemer so vrh in stranice izpostavljene zraku (ali prevlečene), nad površino vezja Er konstantne izolacije, z referenčnim napajanjem ali ozemljitvijo. Kot je prikazano spodaj:

Note: In actual PCB manufacturing, the board manufacturer usually coats the surface of the PCB with a layer of green oil, so in actual impedance calculation, the model shown below is usually used for surface microstrip line calculation:

Stripline:

A ribbon line is a ribbon of wire placed between two reference planes, as shown in the figure below. The dielectric constants of the dielectric represented by H1 and H2 can be different.

The above two examples are only a typical demonstration of microstrip lines and ribbon lines. There are many kinds of specific microstrip lines and ribbon lines, such as coated microstrip lines, which are related to the specific laminated structure of PCB.

The equations used to calculate the characteristic impedances require complex mathematical calculations, usually using field solving methods, including boundary element analysis, so using the specialized impedance calculation software SI9000, all we need to do is control the parameters of the characteristic impedances:

Dielectric constant Er, wiring width W1, W2 (trapezoid), wiring thickness T and insulation layer thickness H.

W1, W2:

The calculated value must be within the red box. In tako naprej.

SI9000 is used to calculate whether the impedance control requirements are met:

First calculate the single-end impedance control of DDR data line:

TOP layer: 0.5oz copper thickness, 5MIL wire width, 3.8mil distance from the reference plane, dielectric constant 4.2. Select the model, substitute in the parameters, and select Lossless Calculation, as shown in the figure:

OBLAST pomeni premaz, in če premaza ni, napolnite 0 v debelini in 1 v dielektriku (dielektrična konstanta) (zrak).

Substrat pomeni substratni sloj, to je dielektrični sloj, na splošno z uporabo fr-4, debelina, izračunana s programsko opremo za izračun impedance, dielektrična konstanta 4.2 (frekvenca manjša od 1 GHz).

Click on Weight (oz) to set the thickness of the copper layer, which determines the thickness of the cable.

9. Prepreg/Core concept of insulation layer:

PP (Prepreg) je neke vrste dielektrični material, sestavljen iz steklenih vlaken in epoksidne smole. Jedro je pravzaprav vrsta medija PP, vendar sta njegovi dve strani pokriti z bakreno folijo, PP pa ne. Pri izdelavi večplastnih plošč se jedro in PP običajno uporabljata skupaj, PP pa se uporablja za povezavo med jedrom in jedrom.

10. Pri oblikovanju laminiranja PCB je treba posvetiti pozornost

(1) Težava z Warpage

Zasnova plasti PCB mora biti simetrična, to pomeni, da mora biti debelina srednje in bakrene plasti vsake plasti simetrična. Vzemite na primer šest plasti, debelina medija z zgornjo GND in spodnjo močjo mora biti skladna z debelino bakra, debelina medijev GND-L2 in L3-POWER pa z debelino bakra. Pri laminiranju se to ne bo upognilo.

(2) Signalna plast mora biti tesno povezana s sosednjo referenčno ravnino (to pomeni, da mora biti debelina medija med signalno plastjo in sosednjo plastjo bakrene prevleke zelo majhna); Močna bakrena obloga in mleta bakrena obloga morata biti tesno povezana.

(3) V primeru zelo velike hitrosti je mogoče dodati dodatne plasti za izolacijo signalne plasti, vendar je priporočljivo, da ne izolirate več plasti moči, kar lahko povzroči nepotrebne motnje hrupa.

(4) Porazdelitev tipičnih laminiranih slojev je prikazana v naslednji tabeli:

(5) Splošna načela razporeditve plasti:

Pod površino komponente (drugi sloj) je ozemljitvena ravnina, ki zagotavlja zaščitni sloj naprave in referenčno ravnino za ožičenje zgornje plasti;

Vse signalne plasti čim bolj mejijo na ozemljitveno ravnino.

Izogibajte se neposrednemu stiku med dvema signalnima plastema, kolikor je to mogoče;

Glavni vir napajanja mora biti čim bližje;

Upoštevana je simetrija laminatne strukture.

For the layer layout of the motherboard, it is difficult for the existing motherboard to control the parallel long-distance wiring, and the working frequency of the board level is above 50MHZ

(Za pogoje pod 50 MHz se obrnite nanj in ga ustrezno sprostite), je predlagano načelo postavitve:

Površina sestavnega dela in varilna površina sta celotna ozemljitvena plošča (ščit);

Brez sosednjega vzporednega ožičenja;

Vse signalne plasti čim bolj mejijo na ozemljitveno ravnino.

Ključni signal je v bližini formacije in ne prečka območja segmentacije.