Basic concepts commonly used in PCB design
This article, China PCB manufacturer will take you from a beginner's point of view, quickly understanding of the basic concepts commonly used in PCB design.
1. FR4 Substrate
FR-4 is the glass fiber epoxy resin copper clad board, a substrate in the circuit board, can be divided into general TG FR4 and high TG FR4, Tg is the glass transition temperature, that is, the melting point. The circuit board must be flame-resistant and cannot be burned at a certain temperature, but only softened. The temperature point at this point is called the glass transition temperature, and this value is related to the dimensional stability of the PCB board.
General Tg of the FR4 board is 130 degrees or more, high Tg is generally greater than 170 degrees, medium Tg is about greater than 150 degrees. Usually Tg≥ 170 degrees Celsius PCB printed board, called high Tg printed board. The Tg of the substrate is improved, the heat resistance of the printed board, humidity resistance, chemical resistance, stability resistance and other characteristics will be increased and improved. the higher the TG value, the better the temperature resistance of the board, especially in the lead-free process, high Tg application is more.
2. Impedance matching
Impedance matching is mainly used in transmission lines to achieve all high frequency microwave signals can be transmitted to the point of load, and there is almost no signal reflection back to the source, thereby improving energy efficiency. The signal source internal resistance and the connected transmission line characteristic impedance of equal size and phase, or the transmission line characteristic impedance and the connected load impedance of equal size and phase, respectively, known as the input or output of the transmission line in the impedance matching state, referred to as impedance matching. PCB common impedance control values are: 100 Ohm differential impedance, 90 Ohm differential impedance and single-ended 50 Ohm impedance.
3. Surface treatment
PCB board surface treatment is generally divided into several, in order to better understand their own PCB design issues, is briefly introduced:
1) HASL, HASL is the most common surface treatment process within the circuit board industry, it has good solderability and can be used for most electronic products. HASL board for other surface treatment, it is low cost, good weldability advantages; its shortcomings are no Immersion gold surface flat, especially when a large area of open windows, more likely to appear tin uneven phenomenon.
2) Immersion tin, Immersion tin and HASL difference is that it is good flatness, but the shortcoming is very easy to oxidation black.
3) Immersion gold, as long as it is "Immersion" its flatness is better than the "spray" process. Immersion gold is lead-free, Immersion gold is generally used for gold fingers, keypad, because the gold resistance is small, so the contact must be used gold, such as cell phone keypad light. Immersion gold is soft gold, for often to plug to use gold plating, Immersion gold is mainly Immersion nickel gold.
Gold plating has been mentioned in the Immersion gold, gold plating has a fatal deficiency when its poor solderability, but its hardness is better than Immersion gold.
Little helper tip: If the flatness requirements, such as the frequency requirements of the impedance circuit board (such as microstrip lines) try to use the Immersion gold process; general with BGA MID board are used to Immersion gold process.
It mainly relies on the reaction between the reaction between the flux and the solder copper skin to produce solderability, the only advantage is the fast production, low cost; but because of its poor solderability, easy to oxidation, the circuit board industry is generally used less.
4. Core board (Core) / PP sheet (semi-cured sheet)
The reinforcement material is dipped in resin, one or both sides are covered with copper foil, and a board material is made by hot pressing, called copper foil laminate. It is the basic material for making PCBs and is often called the substrate. When it is used in the production of multilayer boards, it is also called core board (CORE)
The resin is synthesized with a carrier in a sheet bonding material called PP sheet.
Core board and semi-cured sheets are common materials used to make laminated multilayer boards.
5. Differential line
Differential signal is the driver side to send two equal and opposite signals, the receiver side by comparing the difference between the two voltages to determine the logic state "0" or "1". The pair of lines carrying the differential signal is called differential lines. Differential signals, some also known as differential signals, with two identical, opposite polarity signal transmission of data along the way, relying on the two signal level difference for the judgment. In order to ensure that the two signals are identical, the wiring should be kept in parallel, line width and line spacing remain the same.
6. Signal integrity
Signal integrity refers to the quality of the signal on the transmission line. A signal with good signal integrity means that when needed, it has the necessary voltage level value. Poor signal integrity is not caused by a single factor, but by a combination of factors in the board-level design. Major signal integrity issues include reflections, oscillations, ground bounce, crosstalk, etc.
7. Signal Reflections
A reflection is an echo on a transmission line. A portion of the signal power (voltage and current) is transmitted to the line and reaches the load, but a portion is reflected. If the source and the load have the same impedance, reflections do not occur. A mismatch between the impedance of the source and the load will cause reflection on the line, and the load will reflect part of the voltage back to the source. If the load impedance is less than the source impedance, the reflected voltage is negative, and vice versa, if the load impedance is greater than the source impedance, the reflected voltage is positive. Variations in the geometry of the wiring, incorrect line termination, transmission through connectors and discontinuities in the power plane can cause such reflections.
Reflections can cause signal overshoot, undershoot, ringing, and edge retardation, also known as stepped voltage waves.
Crosstalk is the coupling between two signal lines, the signal lines between the mutual inductance and mutual capacitance caused by the noise on the line. Capacitive coupling triggers coupling current, while inductive coupling triggers coupling voltage. the parameters of the PCB layer, signal line spacing, the electrical characteristics of the driver and receiver side and line termination have a certain impact on crosstalk.
9. Inner layer
Inner layer is a negative layer of the PCB, the main role is to act as a layer of power or ground, when necessary for power split.
10. Blind buried holes
1) blind holes: from the middle layer extends to the PCB a surface layer of the over hole. Commonly known as first-order, second-order, such as first-order blind hole is the second layer to the TOP layer of the hole or the penultimate layer to the Bottom hole.
(2) buried holes: from one middle to another middle between the perforations, will not extend to the PCB surface layer.
11. Test points.
Generally refers to independent PTH holes, SMT PAD, gold fingers, Bonding fingers, IC fingers, BGA soldering points, and test points tested by customers after plug-in.
12. Fiducial point
Fiducial point is a circuit board design PCB applied to the automatic placement machine location identification points, also known as optical point location point. fiducial point selection directly affects the automatic placement machine placement efficiency. fiducial point is generally designed as Ф1 mm (40 mil) circular figure. Considering the contrast between the material color and the environment, leaving 0.5 mm (19.7 mil) larger than the optical positioning reference symbols without solder resistance area, also does not allow any characters, see the figure. The optical positioning reference symbol on the same board and its adjacent inner layer background should be the same, that is, the presence or absence of copper foil under the three reference symbols should be consistent. Around 10mm no wiring isolated optical positioning symbols should be designed with an inner diameter of 2mm ring width of 1mm protective circle, and surrounded by the upper and lower sides of the diameter of 2.8mm 8-sided isolated copper ring.
13. PTH (metallized hole) / NPTH (non-metallized hole)
Hole wall deposited with a metal layer of the hole is called metallized hole, which is mainly used for the electrical connection between the layers of conductive graphics. Conversely, they are non-metallic holes, which are generally used as positioning holes or mounting holes.