PCB Tech - Precautions for PCB drawing and crystal soldering

Common specifications for PCB drawing

PCB contains four files: schematic diagram, schematic library, package library file, PCB file

First create a new PCB project: File->New->Project->PCBProject

1. Schematic file name.SchDoc: File->new->Schmatic

2. Schematic library file name.SchLib: File->New->Library->Schematic Library

3. Package library file name.PCBLib: File->New->Library->PCB Library

4. PCB file name.PCBDoc: File->New->PCB

PCB common units

1mil = 0.0254mm

100mil = 2.54mm

1inch = 1000mil = 25.4mm

The typical via sizes used in PCB design and production are as follows:

The size of the via hole used for grounding or other special needs on the PCB is: the hole diameter is 16mil, the pad diameter is 32mil, and the anti-pad diameter is 48mil;

The size of the via hole used when the board density is not high is: the hole diameter is 12mil, the pad diameter is 25mil, and the anti-pad diameter is 37mil;

The size of the via hole used when the board density is higher is: the hole diameter is 10mil, the pad diameter is 22mil or 20mil, and the anti-pad diameter is 34mil or 32mil;

The via size used under 0.8mm BGA is: hole diameter 8mil, pad diameter 18mil, anti-pad diameter 30mil.

The spacing of circuit lines is generally not less than 6mil

The distance between copper and copper is generally set to 20mil

The distance between copper skin and trace, copper skin and via (via) is generally 10mil

Power cord generally choose 30mil

All line widths are generally not less than 6mil

The conventional routing of the board factory is 8mil, and the processing capacity is: the minimum line width/line spacing is 4mil/4mil. From the perspective of cost, the width of the signal line is usually 8mil

The minimum size of the via is 10/18mil, and the other options are 10/20mi or 12/24mil. It is best to use commonly used vias.

All characters should be consistent in the X or Y direction. The size of characters and silk screen should be unified, generally with=6mil, size=60mil

Parasitic capacitance of via

The via itself has a parasitic capacitance to the ground. If it is known that the diameter of the isolation hole on the ground layer of the via is D2, the diameter of the via pad is D1, the thickness of the PCB board is T, and the dielectric constant of the board substrate is ε, the parasitic capacitance of the via is approximately: C=1.41εTD1/(D2-D1)

The main effect of the parasitic capacitance of the via on the circuit is to extend the rise time of the signal and reduce the speed of the circuit.

In the design of high-speed digital circuits, the damage caused by the parasitic inductance of the via is often greater than the impact of the parasitic capacitance. Its parasitic series inductance will weaken the contribution of the bypass capacitor and weaken the filtering effect of the entire power system. We can simply calculate the approximate parasitic inductance of a via with the following formula: L=5.08h[ln(4h/d)+1] where L refers to the inductance of the via, h is the length of the via, and d is the center The diameter of the hole. It can be seen from the formula that the diameter of the via has a small influence on the inductance, and the length of the via has the greatest influence on the inductance.

Using the above example, the inductance of the via can be calculated as: L=5.08x0.050[ln(4x0.050/0.010)+1]=1.015nH. If the rise time of the signal is 1ns, then its equivalent impedance is: XL=πL/T10-90=3.19Ω. Such impedance can no longer be ignored when high-frequency currents pass. Special attention should be paid to the fact that the bypass capacitor needs to pass through two vias when connecting the power plane and the ground plane, so that the parasitic inductance of the vias will increase exponentially.

Precautions for soldering circuit board crystal oscillator

First of all, the temperature of the soldering tin for circuit board welding should not be too high, and the soldering time for circuit board welding should not be too long, so as to prevent the crystal from internal deformation and instability. When the crystal case needs to be grounded, it should be ensured that the case and the pins are not accidentally connected to cause a short circuit. As a result, the crystal does not vibrate. Ensure that the solder points of the two pins are not connected, otherwise it will cause the crystal to stop vibrating. For the crystal oscillator that needs to be cut, attention should be paid to the influence of mechanical stress. After the circuit board is soldered and soldered, it must be cleaned to prevent the insulation resistance from not meeting the requirements.

How do we solder the circuit board about the crystal oscillator

First of all, we know that the soldering method of the quartz crystal oscillator PCB is related to its packaging. The plug-in and the patch are two different soldering methods. The chip crystal oscillator is divided into manual circuit board welding and automatic circuit board welding. The soldering of the plug-in crystal oscillator circuit board is not very complicated. First put the crystal oscillator on the circuit board with tweezers and melt the solder with a hot air gun.

SMD crystal oscillator manual welding is relatively complicated

1. First, add a proper amount of solder to the chisel-shaped (flat spade-shaped) or knife-edge soldering iron tip, use a fine brush to dip the flux or use a soldering pen to apply a small amount of flux on the pads at both ends, and plate them on the pads Soldering; Use tweezers to hold the chip crystal oscillator with one hand, place it on the corresponding pad in the center, and do not move it after alignment; with the other hand, pick up the soldering iron and heat one of the pads for about 2 seconds, and then remove the soldering iron; then Use the same method to heat the pad on the other end for about 2 seconds.

Special reminder: During the soldering process, pay attention to keeping the SMD crystal oscillator close to the pad and place it upright to avoid one end of the crystal oscillator from lifting or soldering crookedly. If the solder on the pad is insufficient, you can use the soldering iron in one hand and the solder wire to repair the soldering. The time is about 1 second.

2. Plating a proper amount of solder on the pad first, use a small nozzle nozzle for hot air q1an9, adjust the temperature to 200 degree Celsius～300 degree Celsius, and adjust the wind speed to 1～2 block. When the temperature and wind speed are stable, use tweezers to hold it with one hand. Place the components on the welding position, and pay attention to it. Hold the hot air q1an9 with the other hand, keep the nozzle vertical to the components to be removed, a distance of 1cm~3cm, and heat it evenly. After the solder around the crystal oscillator is melted, remove the hot air q1an9, and remove the tweezers after the solder cools.

In order to save money, many factories will use automatic placement machines for automatic placement. We should pay attention to several issues when soldering circuit boards. If you are soldering surface crystals, it is recommended to use automatic placement machines as much as possible, because the wafers for inserting crystal quartz crystal oscillators are relatively thin., The volume is relatively small, it is easier to weld the ceramic crystal oscillator by hand, and the quartz crystal oscillator is generally controlled in

1. Generally, the temperature of the soldering iron tip is controlled at about 300 degree Celsius, and the hot air gun is controlled at 200 degree Celsius～400 degree Celsius;

2. It is not allowed to directly heat the part above the heel of the crystal oscillator pin during soldering, so as not to damage the internal capacitance of the crystal oscillator;

3. It is necessary to use solder wire of ∮0.3mm～∮0.5mm; the tip of the soldering iron is always smooth, free of hooks and thorns; the tip of the soldering iron must not touch the pad again and again, and do not repeatedly heat a pad for a long time. The operating temperature of the conventional crystal oscillator Generally, it is between -40 and +85°C. Heating the PCB pads for a long time may exceed the operating temperature range of the crystal oscillator, resulting in a reduction in the life of the quartz crystal or even damage. In order to avoid damage to the resonator, pay more attention to time control during the welding process to avoid unstable product performance.