Qanday nazorat qilish kerak PCB impedans?

Without impedance control, considerable signal reflection and distortion will be caused, resulting in design failure. PCI bus, PCI-E bus, USB, Ethernet, DDR xotira, LVDS signali va boshqalar kabi umumiy signallarning barchasi impedans nazoratini talab qiladi. 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.

Tegishli impedans qiymatini olish uchun simlarni ulashning turli usullarini hisoblash mumkin.

Microstrip lines

• Bu er tekisligi va o’rtada dielektrik bo’lgan simli chiziqdan iborat. Agar dielektrik doimiyligi, chiziqning kengligi va uning er tekisligidan masofasi nazorat qilinsa, uning xarakterli empedansi nazorat qilinadi va aniqlik ± 5%atrofida bo’ladi.

What is impedance control how to perform impedance control on PCB

Stripline

Lenta chizig’i – bu ikkita o’tkazgich tekisligi orasidagi dielektrik o’rtasida joylashgan mis tasmasi. 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 impedansini yaxshi nazorat qilish uchun PCB tuzilishini tushunish kerak:

Odatda biz ko’p qatlamli taxta deb ataydigan narsa yadroli plastinkadan va bir-biri bilan laminatlangan yarim qotib qolgan varaqdan iborat. Yadro taxtasi – qattiq, o’ziga xos qalinligi, bosilgan taxtaning asosiy materiali bo’lgan ikkita nonli mis plastinka. 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.

Odatda ko’p qatlamli tashqi ikki dielektrik qatlam namlangan qatlam bo’lib, tashqi mis folga sifatida bu ikki qatlamning tashqi tomonida alohida mis folga qatlamlari ishlatiladi. Tashqi mis folga va ichki mis plyonkaning asl qalinligi spetsifikatsiyasi odatda 0.5oz, 1OZ, 2OZ (1OZ taxminan 35um yoki 1.4 mil) ni tashkil qiladi, lekin bir qator sirt ishlov berishdan so’ng tashqi mis folga qatlamining oxirgi qalinligi odatda taxminan ortadi. 1OZ. Ichki mis folga – bu yadro plastinkasining har ikki tomonidagi mis qoplamasi. Oxirgi qalinligi dastlabki qalinligidan unchalik farq qilmaydi, lekin u odatda gravitatsiya tufayli bir necha umrga kamayadi.

Ko’p qatlamli taxtaning eng tashqi qatlami payvandlash qarshiligining qatlami bo’lib, biz tez -tez “yashil yog ‘” deymiz, albatta, u sariq yoki boshqa ranglar ham bo’lishi mumkin. 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. Quyidagilar odatda 6 qavatli laminatlangan tuzilishdir:

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:

Yuzaki mis folga:

There are three thicknesses of copper foil that can be used: 12um, 18um and 35um. Tugatgandan so’ng oxirgi qalinligi taxminan 44um, 50um va 67um.

Yadro plitasi: S1141A, standart FR-4, ikkita nonli mis plastinka ishlatiladi. Ixtiyoriy spetsifikatsiyalarni ishlab chiqaruvchi bilan bog’lanish orqali aniqlash mumkin.

Semi-cured tablet:

Texnik xususiyatlari (asl qalinligi) 7628 (0.185 mm), 2116 (0.105 mm), 1080 (0.075 mm), 3313 (0.095 mm). Bosishdan keyin haqiqiy qalinligi odatda dastlabki qiymatdan taxminan 10-15um kam bo’ladi. Bir xil infiltratsiya qatlami uchun maksimal 3 ta yarim quritilgan planshetdan foydalanish mumkin, va 3 ta yarim quritilgan tabletkaning qalinligi bir xil bo’la olmaydi, kamida bitta yarim davolangan planshetdan foydalanish mumkin, lekin ba’zi ishlab chiqaruvchilar kamida ikkitasini ishlatishi kerak. . Agar yarim quritilgan bo’lakning qalinligi etarli bo’lmasa, yadro plastinkasining har ikki tomonidagi mis plyonkani qirib tashlash mumkin, so’ngra yarim quritilgan bo’lakni ikki tomondan bog’lab qo’yish mumkin, shunda qalinroq infiltratsiya qatlami bo’lishi mumkin. erishildi.

Resursli payvandlash qatlami:

Mis folga ustidagi lehim qarshilik qatlamining qalinligi C2≈8-10um. Mis folga bo’lmagan sirtdagi lehim qarshilik qatlamining qalinligi C1 bo’lib, u yuzadagi mis qalinligiga qarab o’zgaradi. Sirtdagi mis qalinligi 45um, C1≈13-15um va sirtdagi mis qalinligi 70um bo’lganda, C1≈17-18um.

Chiqib ketish bo’limi:

Biz simning kesimini to’rtburchaklar deb o’ylardik, lekin bu aslida trapezoid. Misol uchun, TOP qatlamini olganda, mis folga qalinligi 1OZ bo’lganda, trapezoidning yuqori pastki qirrasi pastki pastki chetidan 1MIL qisqaroq bo’ladi. Misol uchun, agar chiziq kengligi 5MIL bo’lsa, unda yuqori va pastki tomonlari taxminan 4MIL, pastki va pastki tomonlari esa taxminan 5MIL. Yuqori va pastki qirralarning orasidagi farq mis qalinligi bilan bog’liq. Quyidagi jadvalda har xil sharoitda trapezoidning yuqori va pastki qismlari o’rtasidagi bog’liqlik ko’rsatilgan.

What is impedance control how to perform impedance control on PCB

Ruxsat berish qobiliyati: yarim quritilgan varaqlarning o’tkazuvchanligi qalinligi bilan bog’liq. Quyidagi jadvalda har xil turdagi yarim quritilgan varaqlarning qalinligi va o’tkazuvchanlik parametrlari ko’rsatilgan:

What is impedance control how to perform impedance control on PCB

Plastinaning dielektrik konstantasi ishlatilgan qatron materialiga bog’liq. FR4 plastinkasining dielektrik konstantasi 4.2 – 4.7 ni tashkil qiladi va chastota oshishi bilan kamayadi.

Dielektrik yo’qolish koeffitsienti: issiqlik va energiya sarfi tufayli o’zgaruvchan elektr maydonining ta’sirida bo’lgan dielektrik materiallar, dielektrik yo’qotish deyiladi, odatda dielektrik yo’qotish faktori Tan δ bilan ifodalanadi. S1141A uchun odatiy qiymat – 0.015.

Qayta ishlashni ta’minlash uchun minimal chiziq kengligi va chiziqlar oralig’i: 4mil/4mil.

Empedansni hisoblash vositasi:

Ko’p qavatli taxtaning tuzilishini tushunganimizda va kerakli parametrlarni o’zlashtirganimizda, EDA dasturi orqali impedansni hisoblashimiz mumkin. Buning uchun siz Allegro -dan foydalanishingiz mumkin, lekin men Polar SI9000 -ni tavsiya qilaman, bu xarakterli impedansni hisoblash uchun yaxshi vosita va hozirda ko’p PCB fabrikalarida ishlatiladi.

Differentsial chiziqning ham, bitta terminalli liniyaning ham ichki signalining xarakterli impedansini hisoblaganda, Polar SI9000 va Allegro o’rtasida ba’zi tafsilotlar, masalan, simning kesishishi shaklidagi kichik farqni topasiz. Ammo, agar bu Surface signalining xarakterli empedansini hisoblash bo’lsa, men sizga Surface modeli o’rniga Coated modelini tanlashingizni tavsiya qilaman, chunki bunday modellarda lehim qarshilik qatlamining mavjudligi hisobga olinadi, shuning uchun natijalar aniqroq bo’ladi. Quyida Lehim qarshilik qatlamini hisobga olgan holda Polar SI9000 bilan hisoblangan sirt differentsial chiziqli impedansning qisman skrinshotlari keltirilgan.

What is impedance control how to perform impedance control on PCB

Lehim qarshilik qatlamining qalinligi oson nazorat qilinmaganligi sababli, taxta ishlab chiqaruvchisi tavsiya qilganidek, taxminiy yondashuv ham qo’llanilishi mumkin: Surface modeli hisobidan ma’lum bir qiymatni olib tashlang. Differentsial empedans minus 8 ohm va bitta uchli empedans minus 2 ohm bo’lishi tavsiya etiladi.

Kabellarni ulash uchun PCB uchun differentsial talablar

(1) Ulanish rejimini, parametrlarini va impedans hisobini aniqlang. Chiziqni yo’naltirishda ikki xil farq rejimi mavjud: tashqi qatlamli mikrostripli chiziqli farq rejimi va ichki qatlamli chiziqli chiziqli farq rejimi. Empedansni tegishli impedance hisoblash dasturi (masalan, POLAR-SI9000) yoki oqilona parametrlarni sozlash orqali impedansni hisoblash formulasi yordamida hisoblash mumkin.

(2) Parallel izometrik chiziqlar. Chiziq kengligi va oralig’ini aniqlang va yo’naltirishda hisoblangan chiziq kengligi va oralig’iga qat’iy rioya qiling. Ikki chiziq orasidagi masofa har doim o’zgarishsiz qolishi kerak, ya’ni parallel bo’lishi kerak. 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. Odatda qatlamlar orasidagi farq signalini ishlatmaslikka harakat qiling, chunki jarayonda PCBni haqiqiy qayta ishlash jarayonida, kaskadli laminatlangan hizalanish aniqligi, ishlov berish aniqligi orasidagi farqdan ancha past bo’ladi va laminatsiyalangan dielektrik yo’qotish jarayonida, chiziqlar oralig’i qatlamlar oralig’idagi dielektrikning qalinligiga teng bo’lishini kafolatlay olmaydi, bu qatlamlar orasidagi farqni impedans o’zgarishi farqiga olib keladi. Farqi iloji boricha bir qatlam ichida ishlatish tavsiya etiladi.