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.

Ungayilawula kanjani impedance ye-PCB

In practice, it is necessary to control trace impedance when digital marginal speed exceeds 1ns or analog frequency exceeds 300Mhz. One of the key parameters of a PCB trace is its characteristic impedance (the ratio of voltage to current as the wave travels along the signal transmission line). I-impedance yesici yocingo ebhodini lesifunda eliphrintiwe iyinkomba ebalulekile yokwakhiwa kwebhodi lesifunda, ikakhulukazi ekwakhiweni kwe-PCB yesekethe ephezulu, kufanele kubhekwe ukuthi ngabe impedance yesici yocingo iyahambisana yini ne-impedance yesici edingwa yidivayisi noma isinali. This involves two concepts: impedance control and impedance matching. This paper focuses on impedance control and lamination design.

Ukulawulwa kwe-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. I-PCB impedance isukela ku-25 kuye ku-120 ohm.

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. I-PCBS imvamisa izothanjiswa kaningi, futhi impedance yokulawula ingakhiwa ngezindlela ezahlukahlukene. 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

Ukucushwa kokulandela kanye neplate

Insulation constants of core and prefilled materials

Imigqa yokudlulisa ye-PCB iza ngezindlela ezimbili eziyinhloko: iMicrostrip neStripline.

Microstrip:

Ulayini we-microstrip ungumqhubi womugqa onendiza eyisirejista ohlangothini olulodwa kuphela, ngenhla nangaphezulu kuvulwe emoyeni (noma kugqokiwe), ngaphezu kobuso bebhodi lesifunda eliyi-Er, elinikezwa ugesi noma isisekelo njengesethenjwa. Njengoba kukhonjisiwe ngezansi:

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. Njalo njalo.

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:

I-CoaTIng isho i-coaTIng, futhi uma ingekho i-coaTIng, gcwalisa u-0 ngobukhulu no-1 ku-dielectric (i-dielectric constant) (umoya).

I-substrate imele ungqimba lwe-substrate, okungukuthi, ungqimba we-dielectric, imvamisa usebenzisa i-fr-4, ubukhulu obubalwa yisoftware yokubala ye-impedance, i-dielectric engaguqukiyo engu-4.2 (imvamisa engaphansi kwe-1GHz).

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:

I-PP (i-Prepreg) iwuhlobo lwezinto ezisebenza nge-dielectric, ezakhiwe ngengilazi yefayibha ne-epoxy resin. ICore empeleni iyi-TYPE ye-PP medium, kepha izinhlangothi zayo zombili zimbozwe ucwecwe lwethusi, kuyilapho i-PP ingekho. Lapho kwenziwa amabhodi ama-multilayer, umongo ne-PP kuvame ukusetshenziswa ndawonye, ​​futhi i-PP isetshenziselwa ukubopha phakathi komgogodla nomgogodla.

10. Izindaba ezidinga ukunakwa ku-PCB lamination design

(1) Warpage problem

Idizayini yesendlalelo se-PCB kufanele ibe ngeyokulingana, okungukuthi, ukushuba kongqimba olumaphakathi nongqimba lwethusi lwesendlalelo ngasinye kufanele kube ngokulingana. Thatha izendlalelo eziyisithupha ngokwesibonelo, ubukhulu be-top-GND ne-low-power medium kufanele buhambisane nobukhulu bethusi, KANYE nalokho kwe-GND-L2 ne-L3-POWER medium kufanele kuhambisane nobukhulu bethusi. Lokhu ngeke kuqubeke lapho kunamathiswa.

(2) Isendlalelo sesiginali kufanele sihlanganiswe ngokuqinile nendiza eyinkomba eseduze (okungukuthi, ubukhulu obuphakathi nendawo phakathi kwesendlalelo sesiginali kanye nongqimba olumayelana nokumboza ithusi kufanele kube kuncane kakhulu); Ukugqoka ithusi elinamandla nokugqoka ithusi lomhlabathi kufanele kuhlanganiswe ngokuqinile.

(3) Esimweni sejubane elikhulu kakhulu, kungangezwa izingqimba ezingeziwe ukwehlukanisa ungqimba lwesiginali, kepha kunconywa ukuthi kungabi khona izingqimba zamandla amaningi, ezingadala ukuphazamiseka komsindo ngokungadingekile.

(4) Ukusatshalaliswa kwezendlalelo zomklamo wokulaminethi kubonisiwe kuthebula elilandelayo:

(5) General principles of layer arrangement:

Ngezansi kwengxenye yento (isendlalelo sesibili) yindiza yasemhlabeni, enikezela ngqimba lokuvikela idivayisi nendiza eyinkomba yocingo lwengqimba ephezulu;

All signal layers are adjacent to the ground plane as far as possible.

Gwema ukusondelana okuqondile phakathi kwezendlalelo ezimbili zamasignali ngangokunokwenzeka;

Amandla kagesi amakhulu kufanele abe seduze ngangokunokwenzeka;

I-symmetry yesakhiwo se-laminate iyacatshangelwa.

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

(Ngemibandela engaphansi kwama-50MHZ, sicela ubheke kuyo bese uyiphumuza ngokufanele), kuphakanyiswa umgomo wokuhlelwa:

Ingxenye yobuso nengxenye yokushisela yizindiza zomhlaba ezigcwele (isihlangu);

Akunasendlalelo esincikene nentambo;

All signal layers are adjacent to the ground plane as far as possible.

Isiginali yokhiye iseduze nokwakheka futhi ayiweleli indawo yokuhlukaniswa.