About PCB Layout And Routing

Regarding PCB layout and routing, today we will not talk about signal integrity analysis (SI), electromagnetic

compatibility analysis (EMC), and power integrity analysis (PI). Only talking about manufacturability analysis (DFM), unreasonable manufacturability design will also lead to product design failure.

Successful DFM in PCB layout starts with setting the design rules to account for important DFM constraints. The DFM rules shown below reflect some of the contemporary design capabilities found by most manufacturers. Make sure that the constraints set in the PCB design rules do not violate them so that you can ensure compliance with most standard design constraints.

The DFM problem of PCB layout depends on a good PCB layout, and the wiring rules can be preset, including the number of bending times of the traces, the number of via holes, and the number of steps. Generally, exploratory routing is performed first, and the short lines are quickly connected, and then maze routing is performed. First, the wiring to be routed is optimized for the global routing path, and try to re-wiring to improve the overall effect and DFM manufacturability problems .

First, DFM of PCB layout

1. SMT components

The device layout spacing meets the assembly requirements. Generally, the surface mount device is greater than 20mil, the IC type is greater than 80mil, and the BGA type is greater than 200mi. Only when the device spacing meets the assembly requirements during layout can the quality and yield of the production process be improved.

The spacing between the SMD pads of the device pins generally needs to be greater than 6mil, and the manufacturing capacity of the solder mask bridge is 4mil. If the spacing between the SMD pads is less than 6mil, the spacing of the solder mask openings is less than 4mil, and the solder mask bridge cannot be retained, resulting in the assembly process. Large lumps of solder appear (especially between the pins), causing short circuits.

2. DIP components

The pin spacing, device direction, and device spacing of devices processed by wave soldering must take into account the requirements of wave soldering processing. Insufficient spacing between the pins of the device can lead to solder joints, which can lead to short circuits.

Many designers try to reduce the use of through-hole devices (THT) as much as possible, or put the through-hole devices on the same side of the board. However, in-line devices are often unavoidable. In the case of combination, if the in-line device is placed on the top layer and the chip device is placed on the bottom layer, in some cases, it will affect the single-sided wave soldering. At this point, a more expensive soldering process, such as selective soldering, is used.

3. The distance from the component to the edge of the board

If it is soldered on the machine, the distance between the electronic components and the edge of the board is generally 7mm (different welding manufacturers have different requirements), but you can also add a process edge when making the PCB, so that the electronic components can be placed on the edge of the PCB, as long as it is convenient Wiring is fine.

However, when the components on the edge of the board are welded by the machine, they may touch the guide rail of the machine and damage the components. The device pads on the edge of the board will be cut off during the manufacturing process. If the pads are relatively small, the soldering quality will be affected.

4. Distance between high/short components

There are many types of electronic components, different shapes, and various lead wires, so there are differences in the assembly methods of printed boards. A good layout can not only make the machine performance stable, shockproof, and reduce damage, but also get the inside of the machine. Neat and beautiful effect.

Around tall components, short components need to keep a certain distance. The distance between components and the height of components will cause uneven thermal waves, which may cause poor soldering or the risk of failure to repair after soldering.

5. The spacing between electronic components and components

Generally, during SMT processing, it is necessary to consider that there is a certain error in machine placement, and to facilitate maintenance and visual appearance inspection, two adjacent components should not be too close, and a certain safety distance should be left.

The spacing between chip components, between SOTs, and between SOIC and chip components is 1.25mm. The distance between PLCC and chip components, SOIC, and QFP is 2.5mm. 4mm between PLCCs. When designing the PLCC socket, care should be taken to reserve the size of the PLCC socket (the pins of the PLCC are inside the bottom of the socket).

Second, DFM of PCB layout:

1. Line width/line spacing

For designers, we cannot only consider the precision and perfection requirements of the design in the design process, but also a big constraint is the production process. It is impossible for a board factory to rebuild a production line for the birth of an excellent product.

Under normal circumstances, the line width and line spacing are controlled to 4/4mil, and the via hole selection is 8mil (0.2mm). Basically, more than 80% of PCB manufacturers can produce it, and the production cost is the lowest. The minimum line width and line spacing is controlled to 3/3mil, and the via hole selection is 6mil (0.15mm). Basically, more than 70% of PCB manufacturers can produce it, but the price is slightly more expensive than the first case, not too expensive.

2. Acute angle/Right angle

Acute-angle routing is generally forbidden to appear during wiring, and right-angle routing is generally a situation that is required to be avoided in PCB wiring, and it has almost become one of the standards for measuring the quality of wiring. Due to affecting the integrity of the signal, the right-angle routing will generate additional parasitic capacitance and parasitic inductance.

In the process of PCB plate making, PCB wires intersect to form an acute angle, which will cause a problem called acid angle. In the PCB line etching process, the "acid angle" will cause excessive corrosion of the PCB line, resulting in the problem of virtual disconnection of the PCB line. Therefore, PCB engineers need to avoid sharp or strange angles in the traces, and the corners of the traces should be kept at 45 degrees.

3. Copper strip/island copper skin

If it's a large enough island of copper, it becomes an antenna, which can cause noise and other interference within the board (since its copper is not grounded - it will be a signal collector).

Copper strips and island copper skins are free floating copper on many plane layers, which can cause some serious problems in acid baths. Tiny copper specks are known to break off PCB panels and travel to other etched areas on the panel, causing short circuits.

4. Drilled grommets

Annulus refers to the ring of copper around the drilled hole, due to tolerances in the manufacturing process, the remaining copper ring around the drilled hole after drilling, etching, copper plating, the drill bit does not always hit the pad perfectly center point, thus possibly causing rupture of the annular ring.

The annular ring of the via hole needs to be larger than 3.5mil on one side, and the annular ring of the plug-in hole is larger than 6mil. The annular ring is too small. During the manufacturing process, there are tolerances for drilling and line alignment. The offset of the tolerance will cause the annular ring to break open.

5. Teardrops of Wiring

Adding teardrops to PCB wiring can make the connection of the circuit on the PCB board more stable and reliable, and the system produced by doing this will be more stable, so it is necessary to add teardrops to the circuit board.

Adding teardrops can prevent the contact point between the wire and the pad or the wire and the guide hole from being disconnected when the circuit board is subjected to a huge external force. When adding teardrop soldering, it can protect the pad, avoid multiple soldering to make the pad fall off, and avoid uneven etching and cracks caused by via hole deviation during production.

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