Circuito stampato (PCB) si trovano in quasi tutti i tipi di dispositivi elettronici. If there are electronic components in a piece of equipment, they are also embedded in various sizes of PCB. In addition to fixing various small parts, the main function of the PCB is to provide electrical connections between the components. As electronic equipment becomes more and more complex, more and more parts are needed, and the wiring and parts on the PCB become more and more dense. A standard PCB looks something like this. Bare Board (without parts on it) is also often referred to as “Printed Wiring Board (PWB).

The substrate of the board itself is made of a material that is insulated and resistant to bending. The small line material that can be seen on the surface is copper foil. Originally, the copper foil is covered on the whole board, and the middle part is etched away in the manufacturing process, and the remaining part becomes a network of small lines. These lines are called conductors or conductors and are used to provide electrical connections to parts on the PCB.

To secure parts to the PCB, we solder their pins directly to the wiring. Su un PCB di base, le parti sono concentrate su un lato e i fili sono concentrati sull’altro. So we need to make holes in the board so that the pins can go through the board to the other side, so the pins of the parts are welded to the other side. Because of this, the front and back sides of a PCB are called Component Side and Solder Side respectively.

Se ci sono parti sul PCB che possono essere rimosse o rimontate dopo la produzione, Socket verrà utilizzato per installare le parti. Because the socket is directly welded to the board, the parts can be arbitrarily disassembled. A ZIF (Zero InserTIon Force) plug allows parts to be inserted and removed easily. The lever next to the socket can hold the parts in place after you insert them.

To connect two PCBS to each other, an edge connector is commonly used. Il dito d’oro contiene un numero di pad in rame nudo che in realtà fanno parte del cablaggio del PCB. Normalmente, per connettere, inseriamo il dito dorato su un PCB nell’apposito Slot (comunemente chiamato Slot di espansione) sull’altro PCB. In computers, display cards, sound cards, and similar interface cards are connected to the motherboard by means of a gold finger.

The green or brown color on the PCB is the color of the solder mask. This layer is an insulating shield that protects the copper wire and prevents parts from being welded to the wrong place. Un’altra serigrafia verrà stampata sullo strato di resistenza della saldatura. It is usually printed with words and symbols (mostly white) to indicate the position of the parts on the board. Screen printing surface is also known as icon surface

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Tavole a un lato

Come abbiamo detto, su un PCB di base, le parti sono concentrate da un lato e i fili sono concentrati dall’altro. Because the wire appears on only one side, we call this TYPE of PCB single-sided. Poiché i singoli pannelli avevano molte restrizioni rigorose sulla progettazione del circuito (poiché c’era solo un lato, il cablaggio non poteva incrociarsi e doveva prendere un percorso separato), solo i primi circuiti utilizzavano tali schede.

Tavole a doppia faccia

Il circuito ha cablaggio su entrambi i lati. But in order to use both wires, there must be proper electrical connections between the two sides. Questo “ponte” tra i circuiti è chiamato foro guida (VIA). I fori guida sono piccoli fori nel PCB riempiti o rivestiti di metallo che possono essere collegati ai fili su entrambi i lati. Because a dual panel has twice the area of a single panel, and because the wiring can be interlaced (it can be wound around to the other side), it is better for more complex circuits than a single panel.

Multi-layer Boards

Per aumentare l’area cablabile vengono utilizzate più schede di cablaggio singole o bifacciali. Il pannello multistrato utilizza diversi doppi pannelli e uno strato di isolamento viene posizionato tra ciascun pannello e incollato (pressato). The number of layers of the board represents several independent wiring layers, usually an even number of layers, including the outermost two layers. Most motherboards are built with four to eight layers, but it is technically possible to build up to 100 layers of PCBS. La maggior parte dei grandi supercomputer utilizza diversi strati di schede madri, ma sono caduti in disuso in quanto possono essere sostituiti da cluster di computer ordinari. Because the layers in a PCB are so tightly integrated, it’s not always easy to see the actual number, but if you look closely at the motherboard, you might be able to.

The guide hole (VIA) we just mentioned, if applied to a double panel, must be through the entire board

But in a multilayer, if you only want to connect some of the lines, the guide holes may waste some of the line space in the other layers. Buried vias and Blind vias avoid this problem because they only penetrate a few layers. Blind holes connect several layers of internal PCBS to surface PCBS without penetrating the entire board. Buried holes are only connected to the internal PCB, so light is not visible from the surface.

In a multilayer PCB, the entire layer is directly connected to the ground wire and the power supply. So we classify the layers as Signal, Power or Ground. If the parts on the PCB require different power supplies, they usually have more than two power and wire layers.

Tecnologia di confezionamento delle parti

Tecnologia a foro passante

The technique of placing parts on one side of the board and welding the pins to the other side is called “Through Hole Technology (THT)” encapsulation. This part takes up a lot of space and one hole is drilled for each pin. Quindi i loro giunti occupano effettivamente spazio su entrambi i lati e i giunti di saldatura sono relativamente grandi. D’altra parte, le parti THT sono meglio collegate al PCB rispetto alle parti Surface Mounted Technology (SMT), di cui parleremo più avanti. Sockets like wired sockets and similar interfaces need to be pressure-tolerant, so they are usually THT packages.

Surface Mounted Technology

For Surface Mounted Technology (SMT) parts, the pin is welded on the same side with the parts. This technique does not drill holes in the PCB for each pin.

Le parti adesive superficiali possono anche essere saldate su entrambi i lati.

SMT ha anche parti più piccole di THT. Compared to PCB with THT parts, PCB with SMT technology is much denser. SMT package parts are also less expensive than THT’s. So it’s no surprise that most of today’s PCBS are SMT.

Because the solder joints and pins of parts are very small, it is very difficult to weld them manually. However, given that current assembly is fully automated, this problem will only occur when repairing parts.

The design process

Nella progettazione di PCB, ci sono in realtà passaggi molto lunghi da eseguire prima del cablaggio formale. Quello che segue è il processo di progettazione principale:

The system specifications

First of all, the system specifications of the electronic equipment should be planned. It covers system functionality, cost constraints, size, operation and so on.

System function block diagram

Il prossimo passo è creare un diagramma a blocchi funzionale del sistema. Anche la relazione tra i quadrati deve essere contrassegnata.

Divide the system into several PCBS

Dividing the system into several PCBS not only reduces the size, but also gives the system the ability to upgrade and swap parts. The system function block diagram provides the basis for our segmentation. Computers, for example, can be divided into motherboards, display cards, sound cards, floppy disk drives, power supplies, and so on.

Determinare il metodo di confezionamento da utilizzare e la dimensione di ciascun PCB

Once the technology and the number of circuits used for each PCB has been determined, the next step is to determine the size of the board. If the design is too large, then packaging technology will have to change, or re-split the action. Anche la qualità e la velocità dello schema elettrico dovrebbero essere prese in considerazione quando si seleziona la tecnologia.

Draw schematic circuit diagrams of all PCB’s

The details of the interconnections between the parts should be shown in the sketch. I PCB in tutti i sistemi devono essere descritti e la maggior parte di essi utilizza attualmente CAD (Computer Aided Design). Here is an example of a CircuitMakerTM design.

Schema schematico del circuito PCB

Preliminary design of simulation operation

To ensure that the designed circuit diagram works, it must first be simulated using computer software. Such software can read blueprints and show how the circuit works in many ways. This is much more efficient than actually making a sample PCB and then measuring it manually.

Place the parts on the PCB

The way parts are placed depends on how they are connected to each other. Devono essere collegati al percorso nel modo più efficiente. Efficient wiring means the shortest possible wiring and fewer layers (which also reduces the number of guide holes), but we’ll come back to this in actual wiring. Here is what the bus looks like on a PCB. Placement is important in order for each part to have perfect wiring.

Test wiring possibilities with correct operation at high speed

Some of today’s computer software can check whether the placement of each component can be correctly connected, or check whether it can operate correctly at high speed. Questo passaggio è chiamato arrangiamento delle parti, ma non andremo troppo oltre. If there is a problem with the circuit design, parts can also be rearranged before the circuit is exported in the field.

Circuito di esportazione su PCB

The connections in the sketch will now look like wiring in the field. This step is usually fully automated, although manual changes are usually required. Below is the wire template for 2 laminates. The red and blue lines represent the PCB parts layer and the welding layer respectively. The white text and squares represent the markings on the screen printing surface. I punti e i cerchi rossi rappresentano i fori di perforazione e guida. On the far right we can see the gold finger on the welding surface of the PCB. The final composition of this PCB is often referred to as the working Artwork.

Each design must conform to a set of rules, such as minimum reserved gaps between lines, minimum line widths, and other similar practical limitations. Queste specifiche variano in base alla velocità del circuito, alla potenza del segnale da trasmettere, alla sensibilità del circuito al consumo di energia e al rumore e alla qualità del materiale e delle apparecchiature di produzione. If the strength of the current increases, the thickness of the wire must also increase. In order to reduce PCB costs, while reducing the number of layers, it is also necessary to pay attention to whether these regulations are still met. Se sono necessari più di 2 strati, lo strato di potenza e lo strato di massa vengono solitamente utilizzati per evitare che il segnale di trasmissione sullo strato di segnale sia interessato e possono essere utilizzati come scudo dello strato di segnale.

Wire after circuit test

In order to be sure that the line is working properly behind the wire, it must pass the final test. Questo test verifica anche la presenza di collegamenti errati e tutti i collegamenti seguono il diagramma schematico.

Stabilire e archiviare

Because there are currently many CAD tools for designing PCBS, manufacturers must have a profile that meets the standards before they can manufacture boards. There are several standard specifications, but the most common is the Gerber Files specification. A set of Gerber files includes a plan of each signal, power and ground layer, a plan of the solder resistance layer and the screen printing surface, and specified files of drilling and displacing.

Electromagnetic compatibility problem

È probabile che i dispositivi elettronici non progettati secondo le specifiche EMC emettano energia elettromagnetica e interferiscano con gli apparecchi vicini. L’EMC impone limiti massimi alle interferenze elettromagnetiche (EMI), ai campi elettromagnetici (EMF) e alle interferenze in radiofrequenza (RFI). This regulation can ensure the normal operation of the appliance and other nearby appliances. EMC imposes strict limits on the amount of energy that can be scattered or transmitted from one device to another, and is designed to reduce susceptibility to external EMF, EMI, RFI, and so on. In other words, the purpose of this regulation is to prevent electromagnetic energy from entering or emanating from the device. This is a very difficult problem to solve, and is usually solved by using power and grounding layers, or putting PCBS into metal boxes. The power and ground layers protect the signal layer from interference, and the metal box works equally well. We won’t go too far into these issues.

The maximum speed of the circuit depends on EMC compliance. L’EMI interno, come la perdita di corrente tra i conduttori, aumenta all’aumentare della frequenza. If the current difference between the two is too large, make sure to lengthen the distance between them. This also tells us how to avoid high voltage and minimize the current consumption of the circuit. Anche il tasso di ritardo nel cablaggio è importante, quindi minore è la lunghezza, meglio è. Quindi un piccolo PCB con un buon cablaggio funzionerà meglio ad alte velocità rispetto a un grande PCB.