Basic Knowledge of PCB Prototyping

writer: G June 21, 2019

The substrate of the board itself is made of a material that is insulated and heat-insulating and is not easily bent. The thin circuit material that can be seen on the surface is copper foil. The original copper foil is covered on the whole board, and part of the manufacturing process is etched away, and the remaining part becomes a mesh-like small line. These lines are called conductor patterns or wires and are used to provide electrical connections to the parts on the PCB.

If there are some parts on the PCB that need to be removed or replaced after the production is completed, the part will be installed with a socket. Since the socket is directly soldered to the board, the parts can be detached at will. ZIF (Zero Insertion Force) socket, which allows parts (here referred to as CPU) to be easily inserted into the socket or removed. A fixing rod next to the socket that secures the part after you insert it.

If you want to connect two PCBs to each other, we usually use the edge connector called "Golden Finger". The gold finger contains a lot of bare copper pads, which are in fact part of the PCB layout. Usually when connecting, we insert the gold finger on one of the PCBs into a suitable slot on another PCB (generally called the expansion Slot). In a computer, such as a display card, a sound card or other similar interface card, it is connected to the motherboard by a gold finger.

The green or brown color on the PCB is the color of the solder mask. This layer is an insulating protective layer that protects the copper wire and prevents the part from being soldered to an incorrect location. A layer of silk screen is additionally printed on the solder mask. Text and symbols (mostly white) are usually printed on this to indicate the position of each part on the board. The screen printing surface is also referred to as the legend.

Layer concept

The "layer" of Protel is not virtual, but the actual copper foil layer of the printed board material itself. Some of the newer electronic products used in printed boards not only have upper and lower sides for routing, but also a special layer of intercalated copper foil in the middle of the board. For example, the current printing board used in computer motherboards is mostly in 4 layers above. These layers are mostly used to set up a simpler power wiring layer (such as GroundDever and PowerDever in software) because of the relatively difficult processing, and are often routed by large-area filling (such as ExternaIP1a11e and Fill in software). The surface layer in the upper and lower positions and the intermediate layers need to communicate with each other by the so-called "via" mentioned in the software. With the above explanation, it is not difficult to understand the concepts of "multilayer pads" and "wiring layer settings". For a simple example, a lot of people have finished wiring. When they print out, they find that many connected terminals have no pads. In fact, this is the concept of ignoring the "layer" when adding the device library. The pad characteristics are defined as "multiple (Multi-Layer). It is important to note that once the number of layers used is selected, be sure to close those unused layers.

1. Through hole

Through-hole, in a circuit board, one line jumps from one side of the board to the other side, and the hole connecting the two wires is also called a via hole (different from the pad, there is no solder layer on the side).

Via holes are also called metallized holes. In the double-panel and multi-layer boards, in order to connect the printed conductors between the layers, a common hole, that is, a via hole, is drilled at the intersection of the wires that need to be connected at each layer. On the cylindrical surface of the via hole, a layer of metal is plated by chemical deposition to connect the copper foil to be connected in the middle layer, and the upper and lower sides of the via hole are formed into a circular pad shape, and the via hole is formed. The parameters are mainly the outer diameter of the hole and the size of the hole.

The via hole can be not only a via hole but also a buried type. The so-called through-hole via refers to the through-hole through all the copper-clad layers; the buried vias only pass through the middle copper-clad layers, as if buried by other copper-clad layers.

In order to connect the lines between the layers, a common hole is drilled at the intersection of the wires that need to be connected at each layer, which is the via. In the process, a layer of metal is deposited by chemical deposition on the cylindrical surface of the via hole of the via hole to connect the copper foil to be connected in the middle layer, and the upper and lower sides of the via hole are formed into a common pad shape, which can be directly It can also be connected to the lines on the upper and lower sides.

In general, the following principles apply to the processing of vias when designing a circuit: (1) Minimize the use of vias. Once a via is selected, it must be handled with the gaps between the surrounding entities, especially those that are easily overlooked. The gap between the line and the via that is not connected to the via is automatically resolved by selecting the “on” item in the “Via Minimization” submenu. (2) The larger the required current carrying capacity, the larger the required via size, such as the vias used to connect the power and ground layers to other layers.

2. Copper layer

Copper is one or several layers of copper on the PCB. Just like the paint on the door, after special processing, the copper becomes a strip on the board.

Copper must be on the top and bottom. Also set the grid and line width. It is a kind of filling, and the copper is applied to the blank. This is good for signal shielding and board sturdiness.

Generally, the copper is fully applied, and the network is set to GND, so it is also called paving.

3. Silk screen layer (Overlay)

In order to facilitate the installation and maintenance of the circuit, the required logo pattern and character code, etc., such as the component number and the nominal value, the component outline shape and the manufacturer logo, the date of manufacture, and the like are printed on the upper and lower surfaces of the printed board. When many beginners design the relevant content of the silk screen layer, they only pay attention to the neat and beautiful text symbols, ignoring the actual PCB effect. On the plates they design, the characters are not blocked by the components or invaded the soldering area, and the components are marked on the adjacent components. Such designs will bring great assembly and maintenance. The correct silk screen layer character layout principle is: "No ambiguity, see the stitches, beautiful and generous."

4. The particularity of SMD

The Protel package has a large number of SMD packages, surface soldered devices. The biggest feature of this type of device in addition to its small size is the single-sided distribution element pin hole. Therefore, the choice of such devices should define the surface of the device to avoid "missing pins". In addition, the relevant text labels for such components can only be placed with the surface of the component.

5. Grid Filling Area (External Plane) and Filling Area (Fill)

As the name suggests, External Plane treats large areas of copper foil into a mesh, and Fill is only a complete copper foil. In the design process of beginners, the difference between the two is often not seen on the computer. In essence, as long as you enlarge the drawing, you can see at a glance. It is precisely because it is not easy to see the difference between the two, so the use of it does not pay attention to the distinction between the two, it should be emphasized that the former has a strong role in suppressing high-frequency interference in the circuit characteristics, suitable for the need to do Large areas of filling, especially when using certain areas as shields, partitions, or high-current power lines. The latter is often used in places where a small area is required, such as a general line end or a turning area.

6. Pad

Pad is the most common and important concept in PCB design, but beginners tend to ignore its choices and corrections, using circular Pads in the design. The type of Pad of the selected component should take into account factors such as the shape, size, arrangement, vibration and heat, and direction of the force. Protel presents a series of Pads of different sizes and shapes in the package library, such as circles, squares, octagons, circles and positioning pads, but sometimes this is not enough, you need to edit it yourself. For example, a pad that generates heat and has a large force and a large current can be designed as a "teardrop shape". In the design of the output transformer pin Pad of the familiar color TV PCB, many manufacturers use it. In general, in addition to the above, should consider the following principles when editing your own Pad:

(1) When the length of the shape is inconsistent, the difference between the width of the line and the length of the specific side of the Pad should not be considered too large;

(2) It is necessary to use a long and short asymmetric Pad when routing between the component lead angles;

(3) The size of the Pad holes of each component should be edited and determined according to the thickness of the component pins. The principle is that the size of the holes is 0.2-0.4 mm larger than the diameter of the pins.

7. Various types of Mask

These films are not only essential for the PcB manufacturing process, but also necessary for component soldering. According to the position and function of the "film", the "film" can be divided into two types: the surface or the welding surface (Top or Bottom Paste Mask). As the name implies, the solder mask is a film that is applied to the pad to improve solderability, that is, a light colored spot on the green board that is slightly larger than the pad. In the case of a solder mask, the opposite is true. In order to adapt the fabricated board to a soldering form such as wave soldering, it is required that the copper foil on the non-pad on the board cannot be tinned, so a coating is applied to each part other than the pad. Used to block tin on these parts. It can be seen that the two membranes are a complementary relationship. From this discussion, it is not difficult to determine the settings of items like "solder Mask Enlargement" in the menu.

8. Flying lines, have two meanings

When the automatic wiring is used to observe the rubber band-like network connection, after the components are transferred through the network table and the preliminary layout is made, the crossover status of the network connection under the layout can be seen by the Show command, and the components are continuously adjusted. The location minimizes this crossover to achieve maximum auto-routing throughput. In addition, the automatic routing is over, and which networks are not yet connected, can also be found by this function. After finding out the unrouted network, you can use manual compensation. If you can't compensate, you need to use the second meaning of "flying line", which is to connect these networks with wires on future printed boards. If the board is a high-volume automatic line production, the flying line can be designed as a resistance element with a uniform resistance of 0 ohms.

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