Computers and Technology

How to Adjust PCB Screen Printing with One Click?

For later component assembly, especially manual assembly of components, PCB assembly drawings are generally obtained for component placement and positioning. At this time, the silk screen position number shows its necessity.

Friends who are beginners to draw PCB must have done such an operation. Manually adjusting the silk screen position number of the component one by one is very tedious and inefficient…

Is there any good solution?

The screen-printed position number on the PCB can be displayed or hidden during production, but it does not affect the output of the assembly drawing. Press the shortcut key “L”, press the all layers close button to close all layers, and then individually check to open only the silk screen layer and the corresponding solder mask layer to adjust the silk screen.

The following are the principles and conventional recommended sizes for silk screen tag adjustment:

(1) The silk screen position number is not on the solder mask, and it is missing after placing the silk screen for production.

(2) The screen printing position number is clear, and the recommended font size is 4/25mil, 5/30mil, and 6/45mil.

(3) Keep the direction uniformity. Generally, one PCB should not be placed in more than two directions. It is recommended that the letters be on the left or the bottom, as shown in Figure 11-21.

Figure 11-21 Screen printing position number display direction

(4) For some silk-screened logos at the mercy, you can place 2D auxiliary lines or place squares to mark them for easy reading, as shown in Figure 11-22.

Figure 11-22 Auxiliary lines and squares

How to adjust the silk screen number

AltiumDesigner provides a method to quickly adjust the silk screen, that is, the “component text position” function, which can quickly place the silk screen of the component around the component or in the center of the component.

(1) Select the component to be operated.

(2) Press the shortcut key “AP” to enter the “Component Text Position” dialog box, as shown in Figure 11-23. The dialog box provides two placement methods: “Identifier” and “Comment”. “Identifier” as an example.

(3) “Identifier” provides up, down, right, left, upper left, lower left, upper right, and lower right directions, which can correspond to the number keys on the small keyboard. By setting the shortcut key for the “component text position” command, when you want to quickly place the silkscreen number of the selected component on top of the component, press the number keys “5” and “2” on the small keyboard. Complete this operation, as shown in Figure 11-24. The other directions are similar. For example, press the number keys “5” and “6” to place it to the right of the component, and press the number keys “5” and “8” to place it under the component.

Figure 11-23 “Component Text Position” dialog box

Figure 11-24 The silk screen position number is quickly placed on top of the component

Some tips for PCB design

1. How to choose PCB board?

The choice of PCB board must strike a balance between meeting design requirements and mass production and cost. The design requirements include both electrical and mechanical parts. Usually, this material problem is more important when designing very high-speed PCB boards (frequency greater than GHz). For example, the commonly used FR-4 material, the dielectric loss (dielectric loss) at a frequency of several GHz will have a great influence on the signal attenuation, and may not be suitable. As far as electricity is concerned, pay attention to whether the dielectric constant and dielectric loss are suitable for the designed frequency.

2. How to avoid high frequency interference?

The basic idea of ​​avoiding high-frequency interference is to minimize the electromagnetic field interference of high-frequency signals, which is called crosstalk. Can increase the distance between the high-speed signal and the analog signal, or add groundguard/shunttraces next to the analog signal. Also pay attention to the noise interference from the digital ground to the analog ground.

3. How to solve the problem of signal integrity in high-speed PCB design?

Signal integrity is basically a problem of impedance matching. The factors that affect impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace. The solution is to rely on the topology of termination and adjustment of the wiring.

4. Can a ground wire be added to the middle of the differential signal line?

It is generally not possible to add a ground wire in the middle of the differential signal. Because the most important point of the application principle of differential signals is to use the benefits of coupling between differential signals, such as flux cancellation and noise immunity. If you add a ground wire in the middle, it will destroy the coupling effect.

5. Is it necessary to add ground wire shields on both sides when fabricating the clock?

Whether to add a shielded ground wire or not depends on the crosstalk/EMI situation on the board, and if the shielded ground wire is not handled well, it may make the situation worse.

6. How to deal with a section of a line segment (with a small box) during allegro wiring?

The reason for this is that after we reuse the module, an automatically named group automatically generated, so the key to solving this problem is to break up the group again, select the group in the placementedit state and then break it up.

After completing this command, move the traces of all the small boxes and tap the ix00 coordinates.

7. How to meet EMC requirements as much as possible without causing too much cost pressure?

The cost increase on the PCB due to EMC is usually due to the increase in the number of ground layers to enhance the shielding effect and the addition of ferritebead, choke and other high-frequency harmonic suppression devices. In addition, it is usually necessary to match the shielding structure on other institutions to make the entire system pass the EMC requirements. The following only provides several design techniques for PCB board to reduce the electromagnetic radiation effect produced by the circuit:

1) Try to choose devices with slower signal slewrate to reduce the high frequency components generated by the signal.

2) Pay attention to the placement of high-frequency components, not too close to the external connector.

3) Pay attention to the impedance matching of high-speed signals, the wiring layer and its return current path to reduce high-frequency reflection and radiation.

4) Place sufficient and appropriate decoupling capacitors on the power supply pins of each device to alleviate the noise on the power plane and ground plane. Pay special attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements.

5) The ground near the external connector can be properly separated from the ground, and we can connect the ground of the connector to the chassisground nearby.

6) Appropriate use of groundguard/shunttraces beside some special high-speed signals. But pay attention to the influence of guard/shunttraces on the characteristic impedance of the trace.

7) The power layer is 20H inward than the ground layer, and H is the distance between the power layer and the ground layer.

8. What rules we should follow in the design of microstrip for high frequency PCB design above 2G?

For the design of RF microstrip lines, we need three-dimensional field analysis tools to extract the transmission line parameters. All rules should be specified in this field extraction tool.

9. Which end of the AC coupling on the high-speed signal on the PCB board is better?

We often see different processing methods, some near the receiving end and some near the transmitting end.

Let’s take a look at the role of AC coupling capacitors. There are three points: ①The source and sink terminals have different DCs, so DC is blocked; ②The DC component may be crosstalked during signal transmission, so DC blocking makes the signal eye diagram better; ③AC coupling Capacitors can also provide protection against DC bias and overcurrent. After all, the role of the AC coupling capacitor is to provide a DC bias.

Then why add this AC coupling capacitor? Of course there are benefits. Increasing the AC coupling capacitor will definitely enable better communication between the two stages and improve the noise tolerance. We should understand that AC coupling capacitors will cause the signal edges to become slow.

1) Some agreements or manuals will provide design requirements, and we will place them in accordance with the designguideline requirements.

2) There is no requirement of the first article. If it is from IC to IC, please place it close to the receiving end.

3) If it is an IC to a connector, please place it close to the connector.

10. How to check whether the PCB meets the design process requirements before leaving the factory?

Many PCB manufacturers have to go through a power-on network continuity test. This test to ensure that all connections are correct before they completed the PCB processing. At the same time, manufacturers also adopt X-ray testing to check some failures. For boards we finished, we use ICT testing, which requires to add ICT test points during PCB design. If there is a problem, we can use a special X-ray inspection equipment to rule out whether the processing causes the fault.

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