Contents
A product designed by an excellent engineer must meet both design requirements and production processes, and defects in any aspect cannot be considered a perfect product design. Standardize the relevant process parameters of circuit design, process design, and PCB design of products, make the produced physical products meet the technical specification requirements of producibility, Testability, maintainability, etc., and build the advantages of product process, technology, quality, and cost in the process of product design.
This article will take you from a beginner’s perspective to quickly understand the commonly used basic concepts in PCB design:
01 FR4 Material
FR-4 is a glass fiber epoxy resin copper clad plate, a kind of substrate in the circuit board, which can be divided into general FR4 plate and high TG FR4 plate. Tg is the Glass transition temperature, that is, melting point. The circuit board must be flame resistant and cannot burn at a specific temperature, but can only be softened. The temperature point at this point is called the glass transition temperature (Tg point), which is related to the dimensional stability of the PCB board.
Generally, Tg plates are above 130 degrees, high Tg plates are generally greater than 170 degrees, and medium Tg plates are approximately greater than 150 degrees. Usually, PCB printed boards with Tg ≥ 170 ℃ are called high Tg printed boards. The Tg of the substrate has been improved, and the heat resistance, moisture resistance, chemical resistance, stability resistance, and other characteristics of the printed board will be improved. The higher the TG value, the better the temperature resistance of the board, especially in lead-free processes where high Tg is commonly used.
02 Impedance matching
Impedance matching is mainly used on transmission lines to achieve the goal of all high-frequency microwave signals being transmitted to the load point, with almost no signal reflecting back to the source point, thereby improving energy efficiency. The internal resistance of the signal source is equal in magnitude and phase to the characteristic impedance of the connected transmission line, or the characteristic impedance of the transmission line is equal in magnitude and phase to the connected load impedance, respectively, which is called impedance matching. The standard impedance control values in PCBs include 100-ohm differential impedance, 90-ohm differential impedance, and single-ended 50-ohm impedance.
03 Surface Treatment
The surface treatment of PCB boards is generally divided into several types. In order to better understand various issues in one’s PCB design, a brief introduction is now provided:
- HASL and LF_HAL
HASL and LF_HAL are the most common surface treatment process in the circuit board industry, which has good solderability and can be used for most electronic products. For other surface treatments, the advantages of low cost and good weldability of HASL and LF_HAL; Its disadvantage is that the surface is not flat with gold deposition, especially when opening large windows, which is more prone to tin unevenness.
2) Immersion tin
The difference between immersion tin and HASL lies in its good flatness, but the disadvantage is that it is easily oxidized and blackened.
3) Immersion Gold
As long as it is “Immersion”, its flatness is better than the “spray” process. Immersion Gold is lead-free and is generally used for gold fingers and keyboards. Due to its low resistance, contact with gold is necessary, such as the keyboard lights of mobile phones. Immersion Gold is a soft gold, and for those that often need to be inserted and unplugged, gold plating should be used. Immersion Gold is mainly nickel gold.
4) Platting Hard Gold
Gold plating has already been mentioned in Immersion Gold, but it has a fatal flaw: poor weldability, but its hardness is better than Immersion Gold. This process is generally not used in the design of MID and VR
Little assistant tips: if there are requirements for flatness, such as impedance circuit boards (such as Microstrip) with requirements for frequency, try to use the Immersion Gold process; Generally, MID cards with BGA are processed using the Immersion Gold process.
5) OSP
It mainly relies on the reaction between the liquid medicine and the welded copper sheet to generate weldability, with the only advantage of fast production and low cost; However, due to its poor solderability and easy oxidation, circuit boards are generally less commonly used in the industry.
04 Core/PP sheet (semi-cured sheet)
A plate-like material made by immersing the reinforcing material in resin, covering one or both sides with copper foil, and hot pressing, is called a copper-clad laminate. It is the basic material for making PCBs, often referred to as substrate. When it is used for multi-layer board production, it is also called core board (CORE)
A sheet-like bonding material synthesized by resin and carrier is called a PP sheet.
Core boards and semi-cured sheets are common materials used for making laminated multilayer boards.
05 Differential line
Differential signal refers to the driver sending two equal and opposite phase signals, and the receiver determines the logical state “0” or “1” by comparing the difference between these two voltages. The pair of wires carrying differential signals is called differential wiring. Differential signals, also known as differential signals, use two identical signals with opposite polarity to transmit one channel of data, relying on the level difference between the two signals for judgment. To ensure that the two signals are completely consistent, parallel wiring should be maintained, and the line width and spacing should remain unchanged.
06 Signal Integrity
Signal integrity refers to the quality of the signal on the transmission line. A signal with good Signal integrity means that it has the required voltage level value when needed. Poor Signal integrity is not caused by a single factor but by multiple factors in board-level design. The main Signal integrity problems include reflection, oscillation, ground bounce, crosstalk, etc.
07 Signal Reflection
Reflection is the echo on the transmission line. A portion of the signal power (voltage and current) is transmitted online and reaches the load, but a portion is reflected. If the source and load have the same impedance, the reflection will not occur. The impedance mismatch between the source and load terminals can cause online reflection, and the load will reflect a portion of the voltage back to the source. If the load impedance is less than the source impedance, the reflection voltage is negative; otherwise, if the load impedance is greater than the source impedance, the reflection voltage is positive. The geometric shape of the wiring, incorrect wire termination, transmission through connectors, and discontinuity in the power plane can all cause such reflections.
Reflection can cause signal overshoot, undershoot, ringing, and edge lag, which are also stepped voltage waves.
08 Crosstalk
Cross talk is the coupling between two signal lines, and mutual inductance and capacitance between signal lines cause noise on the line. Capacitive coupling triggers coupling current, while inductive coupling triggers coupling voltage. The parameters of the PCB board layer, signal line spacing, electrical characteristics of the driver and receiver ends, and wire termination methods all have a certain impact on crosstalk.
09 Inner Layer
The inner layer is a negative layer of a PCB, mainly used as a layer for power or ground, and is divided into power sources when necessary
10 Blind and Burial Hole
1) Blind hole
A through hole extending from the middle layer to a surface layer of the PCB. Commonly referred to as first-order or second-order, for example, first-order blind holes refer to vias from the second layer to the TOP layer or from the penultimate layer to the Bottom.
2) Buried hole
The through hole from one middle to another does not extend to the surface layer of the PCB.
11 Test point
Generally, it refers to the independent PTH hole, SMT PAD, Goldfinger, Bonding finger, IC finger, BGA welding point, and the Test point tested by the customer after the plug-in.
12 Mark point
Mark points are the position recognition points, also known as optical point points, used in PCB design on automatic placement machines. The selection of Mark points directly affects the placement efficiency of automatic placement machines. The selection of general Mark points is related to the model of the automatic placement machine. Mark points are generally designed as Ф A circular shape of 1 mm (40 mil). Considering the contrast between the material color and the environment, a solder-free zone 0.5 mm (19.7 mil) larger than the optical positioning reference symbol is left, and no characters are allowed, as shown in the figure. The optical positioning reference symbols on the same board should have the same background as their adjacent inner layers, that is, the presence or absence of copper foil under the three reference symbols should be consistent. The isolated optical positioning symbol without wiring around 10mm should be designed with a protective ring with an inner diameter of 2mm and a ring width of 1mm, and surrounded by an 8-sided isolation copper ring with a diameter of 2.8mm on the upper and lower sides.
13 PTH (Plating Through Hole)/NPTH (Non-Plating Through Hole)
A hole with a metal layer deposited on its wall is called a metalized hole, which is mainly used for the electrical connection of interlayer conductive patterns. On the contrary, it is a non-metallic hole, usually used as a positioning hole or installation hole.
