Printed Circuit Board Design Tips

At present, electronic equipment is still used in printed circuit boards as the main assembly method for various electronic devices and systems. Practice has proved that even if the circuit schematic design is correct, improper design of the printed circuit board will also have an adverse effect on the reliability of electronic equipment. For example, if the two fine parallel lines of the printed board are in close proximity, a delay in the signal waveform is formed, and reflection noise is formed at the end of the transmission line. Therefore, when designing a printed circuit board, it is recommended that grounding be an important method of controlling interference.

If you can use the correct combination of grounding and shielding, you can solve most of the interference problems.

The ground structure of the electronic device is roughly systematic, chassis ground (shielded ground), digital ground (logical ground), and analog ground. The following points should be noted in the grounding design:

  1. The correct choice of single-point grounding and multi-point grounding In low-frequency circuits.

the operating frequency of the signal is less than 1MHz, its influence on the inductance between the wiring and the device is small, and the circulation current formed by the grounding circuit has a greater impact on the interference, and should be used Grounded at one point. When the operating frequency of the signal is greater than 10 MHz, the impedance of the ground wire becomes very large. At this time, the resistance of the ground wire should be reduced as much as possible, and the multi-point grounding should be used nearby. When the operating frequency is from 1 to 10 MHz, if a point is grounded, the length of the ground wire should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be adopted.

  2. Separate the digital circuit from the analog circuit. There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground lines of the two should not be mixed. They are connected to the power supply ground. Try to increase the grounding area of ​​the linear circuit.

  3. Try to make the grounding wire as thick as possible.

If the grounding wire is very thin, the grounding potential changes with the change of the current, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass three allowable currents on the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

  4. When the ground wire is used as a closed-loop design for a printed circuit board consisting of a digital circuit only, making the ground wire a closed loop can significantly improve the noise immunity. The reason for this is that there are many integrated circuit components on the printed circuit board, especially when there are many components that consume too much power, due to the limitation of the thickness of the grounding wire, a large potential difference will be generated in the ground junction, and the anti-noise ability will decrease. If the grounding structure is looped, the potential difference will be reduced and the anti-noise capability of the electronic equipment will be improved.

Design points related to PCB Layout combined production

Being able to apply and produce, and then become a formal and effective product is the ultimate goal of PCB layout, the work of layout is a paragraph. Then, when laying out, what are the general points that should be paid attention to in order to make the documents you draw valid in line with the general PCB processing factory rules, so as not to cause unnecessary extra expenses for the enterprise?

This article is for everyone to summarize the current PCB layout generally to follow the seven rules:

First, the outer circuit design rules:

(1) Welding ring (Ring ring): The welding ring of the PTH (copper hole) hole must be 8 mils larger than the single side of the hole, that is, the diameter must be 16 mils larger than the hole.

The Via hole must be 8 mils larger than the drilled side and 16 mils larger than the hole. In short, whether it is through-hole PAD or Via, the inner diameter must be greater than 12 mil and the outer diameter must be greater than 28 mil. This is very important!

(2) The line width and line spacing must be greater than or equal to 4 mils, and the distance between the holes and the holes should not be less than 8 mils.

(3) The etched word line width of the outer layer is greater than or equal to 10 mils. Note that the word is etched instead of silk screen.

(4) The circuit layer is designed with a grid of plates (plated with copper in a grid shape), and the rectangle of the grid space is greater than or equal to 10*10 mil, that is, the line spacing should not be less than 10 mil when the copper is set, and the grid line width is larger than Equal to 8mil. When laying a large area of ​​copper, it is recommended to set it into a mesh.

It can prevent the substrate and copper foil of the PCB from being volatile gas and heat when it is dipped or heated. It causes the copper foil to expand and fall off; secondly, the grid-like paving has its heating performance, and the high-frequency conductivity performance is much better than the solid paving of the whole block.

However, I believe that in terms of heat dissipation, the advantages of grid-plated copper cannot be used.

It should be considered that in the case of local heating, which will cause deformation of the PCB, the grid should be paved with copper for the purpose of preserving the integrity of the heat dissipation. The advantage of this copper plating relative to the copper is that the surface temperature is constant. Improvements, but within the scope of commercial or industrial standards, damage to components is limited; but if the direct result of bending the PCB is the occurrence of a virtual solder joint, it may directly lead to line failure. The result of comparison is to use small damage as the best. The real cooling effect should still be the best with solid copper. In practical applications, the copper in the middle layer is rarely grid-like, that is, the uneven force caused by temperature is not as obvious as the surface layer, and the solid copper with better heat dissipation effect is basically adopted.

(5) The distance between the NPTH hole and the copper is greater than or equal to 20 mils.

(6) The board formed by the seesaw (milling cutter), the distance of the copper from the forming line is 16 mil or more; therefore, in the layout, the distance of the trace from the frame should not be less than 16 mil. For the same reason, when slotting, the distance from copper should be 16 or more.

(7) The die-formed board, the distance between the copper and the forming line is 20 mil or more;

if the board you draw may be mass-produced in the future, in order to save costs, it may be required to open the mold, so it must be foreseen at the time of design. To.

(8) V-CUT (generally draw a line in the Bottom Mask and Top Mask layers,

preferably mark the V-CUT here), the board should be designed according to the thickness of the board.

[1] The plate thickness is 1.6 mm, and the distance of copper from the V-CUT line is 0.8 mm or more.

[2] The plate thickness is 1.2 mm, and the distance between the copper and the V-CUT line is 0.7 mm or more.

[3] The thickness of the plate is 0.8 mm to 1.0 mm, and the distance between the copper and the V-CUT line is 0.6 mm or more.

[4] The plate thickness is 0.8 mm or less, and the distance between the copper and the V-CUT line is 0.5 mm or more.

[5] Gold hand board, the distance between copper and V-CUT line is greater than or equal to 1.2mm.

Note: Don’t set such a small spacing when you are jigsaw, try to be bigger.

Second, the inner layer design rules:

(1) Welding ring (Ring ring): The welding ring of the PTH hole must be 8 mils larger than the single side of the hole, that is, the diameter must be 16 mils larger than the hole. The Via hole must be 8 mils larger than the drilled side and 16 mils larger than the hole.

(2) Line width and line spacing must be greater than or equal to 4 mils.

(3) The inner layer has an etched word line width of 10 mils or more.

(4) The distance between the NPTH hole and the copper is greater than or equal to 20 mils.

(5) The board formed by the seesaw (milling cutter), the distance of copper from the forming line is greater than or equal to 30 mil (generally 40 mil).

(6) The inner layer of the non-weld ring PTH drilled to the copper foil is kept at least 10 mil (four-layer board), and the six-layer board is at least 11 mil.

(7) When the line width is less than or equal to 6 mil lines and there are holes in the pads; teardrops must be added between the lines and the pads.

(8) The isolation area between the two large copper faces is 12 mil or more.

(9) Heat dissipation PAD (plum pad), the distance from the edge of the drill hole to the inner circle is greater than or equal to 8 mil (ie Ring ring), the distance from the inner circle to the outer circle is greater than or equal to 8 mils, and the width of the opening is greater than or equal to 8 mils. Generally there are four openings, at least two openings are required.

Third, the drilling design rules

(1) The PCB board factory in principle designs the “8” shaped hole into a slot (annular hole). Therefore, it is recommended to make a ring as much as possible in the layout. There is no such function. You can put more than N circles and stack as many mistakes as possible. In this way, the final ring groove will not appear “dog teeth”, and the board factory will not break the drill bit because of your slot!

(2) The minimum mechanical drilling aperture is 0.25mm, and the general aperture design is greater than or equal to 0.3mm. If it is smaller than this or just 0.25mm, the people at the board factory will definitely find you.

(3) The minimum slot diameter is 0.25mm, and the general aperture design is greater than or equal to 0.3mm.

(4) In general practice, only the mechanical drilling unit is mm; the remaining units are mil.

My habit of drawing is that in addition to doing the library because of the size of the mm, the rest are mil units, mil’s unit is small, it is convenient.

(5) Laser drilling aperture is generally 4mil-8mil.

This type of technology is generally used for boards with more than 6 layers and very dense, such as mobile phone motherboards. Of course, the price will definitely increase by N levels. More important is the minimum processing factor of the PCB: one to three-stage blind buried holes, laser drilling minimum 4mil, minimum line width 4mil, minimum gap 4mil. Buried holes, as the name implies, are buried in the middle of the slab, but they are only used as general guides; blind holes, one that is exposed outside and hidden inside, are usually only used as guides. The laser drilling, the penetration thickness is less than or equal to 4.5 mil, and the round hole is punched out. So don’t think about using the laser drilling process to get through the PAD. Via is hard to use. So be careful when placing the PAD, don’t forget the 0.25mm limit.

Fourth, the text design principles:

The line width is 6 mils or more, the text is 32 mils higher, and the line width of the text box is 6 mils or more.

Five, hole copper and copper design principles

(1) The board of general finished copper 1OZ, the hole copper is 0.7mil.

(2) General finished copper 2OZ board, hole copper 0.7mil-1.4mil.

Sixth, anti-weld design principles

(1) The solder resist is 3 mils larger than the pad. A lot of software is set by default, you can find it yourself.

(2) The soldering distance line (copper skin) is greater than or equal to 3 mils.

(3) Green oil bridge ≥ 4mil, which is the gap between the solder joints of the IC pins.

(4) The BGA position window and cover line are greater than or equal to 2 mils, and the green oil bridge is designed. If the distance is not enough, the skylight window is made.

(5) The gold finger part of the gold finger board must be solder-proof open, including fake fingers.

(6) The wire diameter of the anti-welding form is ≥8mi, and the word height is ≥32mil.

Seven, of course, all of the above are general principles, and specific issues still need to be specifically analyzed.