Currently
electronic equipment for all types of electronic devices and systems continue
to be the main printed circuit board assembly methods. Practice has proved that,
even if the circuit schematic design right, improper printed circuit board
design, can also adversely affect the reliability of electronic equipment. For
example, if the PCB two thin parallel lines are very close, it will delay the
waveform signal is formed, is formed in the end of the transmission line
reflection noise. Therefore, in the design of printed circuit boards, it should
pay attention to the correct approach.
1 Ground
Ground design electronic equipment, the ground is an
important method of controlling interference. Such as proper grounding and
shielding can be used in combination, can solve most of the interference.
Electronic equipment ground structure substantially systematic and chassis
ground (shield ground), digitally (logically) and analog land. In ground design
should note the following:
1 ) Select the correct single-point and multi-point grounding grounding in the low-frequency circuit , the signal is less than the operating frequency of 1MHz, low inductance and affect its cabling and devices between the ground and the larger circulation circuit formed on the interference and should therefore adopt point grounding . When the signal is greater than the operating frequency of 10MHz, ground impedance become large, and you should try to reduce ground impedance, should be used to the nearest multi-point grounding. When the operating frequency 1 ~ 10MHz, if one-point grounding, grounding its length should not exceed 1/20 of the wavelength, or should be multi-point grounding method.
2) Digital
circuits and analog circuits separate both the high-speed logic circuit on the
circuit board, there are linear circuits, so that they should be separated as
much as possible , but do not be confused between the ground connected to the
power supply end of the ground , respectively . To try to increase the contact
area of linear circuits.
3 ) Be bold if the grounding wire grounding line is very thin , the ground potential changes with the current changes , resulting in an electronic device timing signal level instability, anti-noise performance deterioration . Therefore, the ground should be bold line , it is located by three printed circuit boards allow current . If possible, should be greater than the width of the ground wire 3mm.
4) The ground
wire constituting the printed circuit board ground closed loop system designed
exclusively by the digital circuit composed of the ground wire closed loop can
be made significantly improved noise immunity. The reason is that: the printed
circuit board has a lot of integrated circuit components, especially in case of
power more element, due to the grounding line thickness limits , will have a
greater potential differences across the junction , causing noise immunity
drops , if the ground structure into a loop , it will narrow the potential
difference and improve noise immunity of electronic equipment .
2 EMC design
Electromagnetic
Compatibility EMC design is an electronic device is still able to coordinate a
variety of electromagnetic environment, the ability to work effectively. EMC
design purpose is to make the electronic device can inhibit a variety of
outside interference , the electronic device can work in a specific
electromagnetic environment , while reducing the electronic device itself
electromagnetic interference with other electronic devices.
1) Choose the
right wire width due to the impact of interference in the printed line
transients generated primarily by printed wire inductance component caused, it
should minimize the printed wire inductance. Its length is proportional to the
inductance of printed conductors, and inversely proportional to its width, so
short and fine wire is advantageous for suppressing interference. Clock lead ,
drive or bus line drive signal lines often contain large transient currents ,
printed wire as short as possible . For discrete element circuits , printed
wire width at about 1.5mm , can fully meet the requirements ; for integrated
circuits, printed wire width can be selected between 0.2 ~ 1.0mm .
2) Using the
correct wiring traces equality strategy adopted to reduce lead inductance,
mutual inductance and distributed capacitance increases but the wire between
the layout allows, preferably well-shaped mesh wiring structure, which would be
the lateral side of the printed circuit board wiring, the other side of the
longitudinal wiring and connected by metallized holes intersecting hole.
In order to
suppress crosstalk between conductors on printed boards in the wiring design
should try to avoid long-distance traces equality , as far as possible distance
between the lines , the signal line and ground as much as possible and do not
cross the power cord . Some are susceptible to interference between the signal
line of a ground traces, crosstalk can be effectively suppressed.
To avoid
high-frequency electromagnetic radiation through the printed wire signal
generated when the printed circuit board layout, should note the following:
- Printed conductors to minimize the discontinuity, e.g., conductor width not mutated, the corners of the wire should be greater than 90 degrees prohibited cyclic traces and the like.
- Clock signal leads most likely to produce electromagnetic radiation interference , should go close to the line and ground loop phase , the drive should be next to the connector .
- Bus driver should immediately want to drive their buses. For those who lead away from the printed circuit board, the drive should be tightly next to the connector.
- Data bus wiring should be caught between a signal ground every two signal lines. Preferably tightly next to the most important place to address the lead circuit, because the latter often having a high frequency current.
- When PCB arranged high, medium and low-speed logic circuit, arranged reflection interference suppression device 3 in order to suppress the occurrence of reflection interference printed line terminal, in addition to special needs as far as possible to shorten the length of the tracks and using a slow circuit. Termination necessary, to add that at the end of the transmission line to ground and power increase by one end of each of termination resistors of equal value. According to experience, the general faster TTL circuitry printed on the termination of measures should be used when the line is longer than 10cm or more. Matching resistor should be based on an integrated circuit output drive current and the maximum current drawn to decide.
3
decoupling capacitor configuration
In the DC power supply circuit, a change in the
load will cause the power supply noise. For example, in a digital circuit, when
the circuit switches from one state to another state, it will produce a large
peak current in the power line, the formation of noise voltage transients.
Configuring decoupling capacitor can suppress the noise generated by load
changes, is a conventional practice printed circuit board reliability design,
configuration principles are as follows:
- Power input electrolytic capacitor connected across a 10 ~ 100uF , if the position of the printed circuit board to allow using 100uF electrolytic capacitor above interference effect will be better .
- Configuring a 0.01uF ceramic capacitors for each IC chip. When the case of a small printed circuit board space and fit , may every 4 to 10 chips to configure a 1 ~ 10uF tantalum electrolytic capacitors , high frequency impedance of such devices are very small, in the range of 500kHz ~ 20MHz impedance of less than 1Ω, and low leakage current (0.5uA or less ) .
- For noise weak , when the off current variation in device and ROM, RAM and other storage -type device , the chip should be in the power cord (Vcc) ground (GND) between direct access and decoupling capacitors.
- Decoupling capacitor lead not too long, especially in the high-frequency bypass capacitor can not Leaded.
4 printed
circuit board size and arrangement of the device
Printed circuit board size should be moderate, printed lines long, resistance increases too large, not only decrease noise immunity, the cost is high ; too small, the heat is not good , but susceptible to interference near the line .
In the arrangement of the device with other logic circuits, it should be related to each other as much as possible to put too close to some devices , so you can get a better anti- noise effect . As shown in picture 2. A clock input terminal species generator, crystal oscillator and CPU are easy to produce noise, some to be close to each other. The device is easy to produce noise, low current circuit, high current circuit should be kept away from the logic circuit and, if possible, to do another circuit board, it is very important.
5 thermal
design allows for heat dissipation from the perspective
Printed
version is best upright distance between the board and the board should
generally not be less than 2cm, and the device arrangement in the printed
version should follow certain rules : for the use of free air convection
cooling device , preferably the integrated circuit ( or other device ) by
lengthwise arranged ; for the use of forced air cooling equipment, the best is
an integrated circuit ( or other devices ) are arranged horizontally long way ,
with the device on a printed circuit board as far as possible according to
their size and the heat radiating Zoning arrangement , little heat or poor heat
resistance devices (e.g., small signal transistors and small -scale integrated
circuits , electrolytic capacitors , etc.) on the cooling air upstream (
entrance ) , heat or heat-resistant components ( such as power transistors ,
LSI , etc.) on the most downstream cooling airflow .
On other devices when the temperature in the vertical direction, the power device as close as possible above the PCB layout, in order to reduce these devices work ; in the horizontal direction , high-power devices PCB edge is arranged as close as possible in order to reduce the heat transfer path of the temperature sensitive device is preferably arranged in the coldest regions ( such as the bottom of the device ) , do not put it directly above heating devices , preferably a plurality of devices are staggered layout in the horizontal plane . Cooling equipment within the PCB rely mainly on air flow, so to study the design of the air flow path, the rational allocation of the device or printed circuit board. Always tend to place a small flow resistance when the air flow, so the printed circuit board to configure the device, to avoid leaving a large airspace in an area. The whole configuration of the printed circuit board should also pay attention to multiple the same problem.
6 Summary
A lot of practical experience show that reasonable device arrangement can effectively reduce the temperature rise of the printed circuit so that the failure rate of devices and equipment decreased more than just the printed circuit board some general principles of reliability design , printing circuit board reliability and is closely related to the specific circuit does not need to be dealt with accordingly in the design according to the specific circuit , to the greatest extent possible to ensure the reliability of the printed circuit board.
A lot of practical experience show that reasonable device arrangement can effectively reduce the temperature rise of the printed circuit so that the failure rate of devices and equipment decreased more than just the printed circuit board some general principles of reliability design , printing circuit board reliability and is closely related to the specific circuit does not need to be dealt with accordingly in the design according to the specific circuit , to the greatest extent possible to ensure the reliability of the printed circuit board.
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