Sådan bestemmer du antallet af lag i PCB'er

Før man beslutter sig for antallet af lag til et printkort, er det vigtigt at identificere det formål, som printkortet skal bruges til. Dette vil påvirke antallet af lag, der kræves, ligesom kompleksiteten af det elektroniske kredsløb og den mængde strøm, det vil forbruge. Generelt set kræver højteknologiske applikationer et højt antal lag.

Brug af signallags-estimatoren

Estimering af antallet af PCB-lag er et afgørende trin i fremstillingen af printkort. Jo flere lag et printkort har, jo dyrere bliver det. Flere lag kræver også flere produktionstrin, materialer og tid. Ved at bruge estimatoren til signallag kan du bestemme det rigtige antal lag til dit printkort. Derefter kan du justere printkortet i overensstemmelse hermed for at få et effektivt design.

Signallaget er det første lag i en PCB-stackup med to lag. Det kobbermateriale, der bruges til lag et, er 0,0014 tommer tykt. Det vejer cirka en ounce. Dette lags effekt vil variere afhængigt af printpladernes størrelse.
Brug af estimatoren for jordoverfladen

Antallet af lag, der kræves til et givet design, afhænger af kredsløbets effektniveau og kompleksitet. Flere lag øger produktionsomkostningerne, men de giver også mulighed for flere spor og komponenter. Derfor er estimering af lagantal et vigtigt trin i designprocessen. Sierra Circuits har skabt et værktøj kaldet Signal Layer Estimator, som kan hjælpe dig med at bestemme antallet af lag, der kræves til dine PCB'er.

PCB-design er afgørende for din enheds ydeevne. Designprocessen skal specificere antallet af lag til strøm, jord, routing og særlige hensyn. PCB'er kan have helt op til fire lag, og signallagene skal være tæt på hinanden. Dette arrangement reducerer uønskede signaler og holder modstanden mellem strømme og kredsløb inden for acceptable grænser. Det ideelle område for denne modstand er 50 til 60 ohm. Hvis impedansen er for lav, kan du opleve spidser i den strøm, der trækkes. På den anden side vil en for høj impedans generere mere elektromagnetisk interferens og udsætte kortet for fremmed interferens.

Håndtering af en god stackup

Managing a good stackup in PCBA design requires an understanding of the various demands on stackup. The three main demands are controlled impedance, crosstalk control, and interplane capacitance. Fabricators cannot account for the first two demands, because only the design engineer knows what they need.

The layers of a PCB must be stacked in such a way that they are compatible and can transmit signals. In addition, the layers must be coupled to each other. The signal layer must be adjacent to the power plane, mass plane, and ground plane. To achieve these objectives, the best mode is an 8-layer stackup, but you can customize this to suit the requirements of your design.

Good stackup can reduce crosstalk, which is energy that moves from one PCB trace to the next. There are two types of crosstalk: inductive and capacitive. Inductive crosstalk is dominated by return currents, which generate magnetic fields in the other traces.

Considering component keep-out or head-room restrictions

When determining the number of layers on your PCB, keep in mind any head-room or component keep-out restrictions that may apply. Head-room restrictions refer to areas on a board where the physical shape of the components are too close to the board or where the board is not large enough to accommodate a particular component. These are usually noted on the schematic. The type of components on the board and the overall layout will determine the number of layers.

Calculating microstrip and stripline impedance for high-speed signals

Using the same mathematical formula, we can calculate the impedance of both striplines and microstrips for high-speed signals. Unlike a stripline, a microstrip’s characteristic impedance is dependent on the width of its trace, not its height. As a result, the higher the frequency, the higher the microstrip’s characteristic impedance.

In circuit design, controlled-impedance lines are most often set up in a microstrip configuration. The edged-coupled microstrip configuration uses a differential pair on an external layer of the circuit board with a reference plane adjacent. The Embedded microstrip, on the other hand, utilizes additional dielectric materials such as Soldermask. In addition to this, stripline routing is commonly symmetrical.

The values of impedance are not always accurate because the circuits are influenced by a variety of factors and parameters. Incorrectly calculated values can lead to PCB design errors and can interfere with the operation of the circuit. In order to avoid such a situation, use an impedance calculator. It is a powerful tool to tackle impedance problems and to get accurate results.