Continue discuss about the several practical tips for PCB layoutlet’s when you doing pcb designing

Hey guys today let’s Continue discuss about the several practical tips for PCB layoutlet’s when you doing pcb designing before you use QiHe smt equipments like smt pick and place machine , reflow oven, stencil printer

In the previous article we discussed five points ow let’s move on to the rest points

6. The wires used at the input and output terminals should avoid being adjacent to and parallel to each other as much as possible.
   It is best to add a ground wire between the lines to avoid feedback coupling.

7. The corner of the printed wire is generally arc-shaped,
   However, right angles or included angles will affect electrical functions in high-frequency circuits.
   If it is necessary to take a right angle, two 135-degree angles are generally used instead of a right angle.

8. Power cord design
   According to the size of the printed circuit board current,
   Try to increase the width of the power line and reduce the loop resistance.
   At the same time, make the direction of the power line and ground line and the direction of data transmission the same,
   This helps to enhance the ability to resist noise.

9. Ground wire design

The principles of ground wire design are:

(1) The digital ground is separated from the analog ground.
If there are both logic circuits and linear circuits on the circuit board,
They should be separated as much as possible.
The ground of the low-frequency circuit should be selected as a single-point parallel connection ground as much as possible.
When the actual wiring is difficult, it can be partially connected in series and then connected to the ground in parallel.
High-frequency circuits should use multi-point series grounding, and the ground wire should be short and rented.
Use grid-shaped large-area foil as much as possible around high-frequency components.

(2) The ground wire should be as thick as possible.
If the grounding wire is made of a thin wire, the grounding potential changes with the change of the current,
degrades noise immunity.
Therefore, the ground wire should be thickened,
Make it pass three times the allowable current on the printed board.
If possible, the ground wire should be above 2~3mm.

(3) The ground wire forms a closed loop.
Printed boards consisting only of digital circuits,
Most of its grounding circuits are grouped into loops to improve the ability to resist noise.

10. Decoupling capacitor configuration

One of the conventional practices in PCB design is to configure appropriate decoupling capacitors in various key parts of the printed board.

The general configuration principle of the decoupling capacitor is:

(1) Connect the electrolytic capacitor of 10 ~ 100uf across the input terminal of the power supply. If possible, it is better to connect more than 100uF.

(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor,
If there is not enough space in the printed board,
A 1-10pF capacitor can be placed every 4-8 chips.

(3) Weak anti-noise ability,
Equipment with large power changes when turned off
, such as RAM, ROM storage devices,
The decoupling capacitor should be connected directly between the power line and the ground line of the chip.

(4) The capacitor lead should not be too long,
In particular, high frequency bypass capacitors cannot have leads.

11. In addition, the following two points should be noted:

(1) When there are touch devices, relays, buttons and other components in the printed board.
When operating them, a large spark discharge will be generated,
The RC circuit shown in the attached figure must be used to absorb the discharge current.
Generally, R takes 1 ~ 2K, and C takes 2.2 ~ 47UF.

(2) The input impedance of CMOS is very high and is susceptible to induction.
Therefore, when in use, the unnecessary terminals must be grounded or connected to the positive power supply.

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