PCB News - PCB layout design process requirements for PCB design

Last time we talked about "PCB Design Principles of Circuit Board Layout", which described the layout principles and precautions of printed circuit boards in the PCB design process. Today we will talk about the process requirements of the circuit board layout design in this design process.

When we start to design a PCB circuit board project, we should consider the following according to the PCB design process:

1. Establish a basic PCB design circuit diagram (as shown in the figure)

1. Establish a basic PCB design circuit diagram

It should contain:

1) PCB board size, frame and wiring area

A. The size of the board should strictly comply with the requirements of the structure.

Note: At present, the maximum size of multi-layer PCB circuit boards that Benqiang Circuit has been able to produce is immersion gold board: 520*800mm, vertical immersion tin PCB board is 500*600mm, horizontal immersion tin board: single side less than 500mm; horizontal immersion silver Board: single side less than 500mm; leaded/lead-free tinned PCB: 520*650mm; OSP: single side less than 500mm; electroplated hard gold circuit board: 450*500mm; in addition, single side is not allowed to exceed 520mm

B. The board outline of the PCB is usually drawn with a 10mil line.

C. The distance between the wiring area and the edge of the board should be greater than 5mm.

2) The stacked arrangement edge of the PCB board

A. Based on PCB manufacturing process considerations: the following figure is an example of a four-layer PCB circuit board, the first method is recommended.

Four-layer PCB circuit board stack structure diagram

For a six-layer PCB board, the layer arrangement is as shown in the figure below; for a multi-layer PCB, the same applies.

B. Laminated arrangement based on consideration of electrical characteristics.

In the design of multi-layer circuit boards, the signal layer should be separated by the ground layer and the power layer as much as possible, and the traces of the adjacent signal layers that cannot be separated should adopt the orthogonal direction. The following picture shows the arrangement of a four-layer circuit board:

Recommended stack structure for 10-layer PCB

The following figure shows a recommended 10-layer PCB stack structure, and other layers of the PCB are analogized in turn.

10-layer PCB stack structure diagram

3) PCB mechanical positioning holes and optical positioning points for SMC.

A. For PCB mechanical positioning holes, the following rules should be followed: Requirements

■ The size requirements of the mechanical positioning hole

The size of the PCB board mechanical positioning hole must be standard (see the following table and figure), and the following unit is mm.

Standard size table of PCB board mechanical positioning hole

Reminder: When you place an order in our company Benqiang PCB Procurement Mall, if there are special sizes, please be sure to indicate separately in the uploaded Gerber file.

B. Positioning of mechanical positioning holes

The positioning of the mechanical positioning hole is on the diagonal of the PCB as shown in the figure:

Mechanical positioning holes on the diagonal of the PCB

■ For ordinary PCB circuit boards, our company especially recommends: the diameter of the mechanical positioning hole is 3mm, and the distance between the center of the mechanical positioning hole and the edge of the circuit board is 5.08mm.

■ For circuit boards with components (objects, connectors, etc.) on the edges, the mechanical positioning holes will move in the X direction. The diameter of the mechanical positioning holes is recommended to be 3mm.

■ The mechanical positioning holes are non-hole holes.

C. For the optical positioning point of the SMC of the PCB board, the following rules should be followed:

■ Optical positioning point of PCB board

In order to meet the needs of SMC's automated production and processing, optical positioning points must be added on the surface and bottom layers of the PCB circuit board, as shown in the figure below:

Optical positioning point

Note:

1) The distance between the edge of the board and the mechanical positioning hole is ≥7.5mm.

2) The positioning holes of these mechanisms must have the same X or Y coordinates.

3) The optical positioning point must be added with solder mask.

4) There are at least 2 optical positioning points, and they are placed diagonally.

5) The size of the optical positioning point is shown in the figure below.

Optical anchor point size list

6) They are surface pads placed on the top and bottom layers.

The design department of our company recommends: usually the diameter of the optical positioning pad (PD) is 1.6m (63mil), and the diameter of the solder mask (D(SR)) is 3.2mm (126mil); when the density and precision of PB are very high, the optical The positioning point pad can be 1.0mm (must be specially marked), and the pad must be soldered.

■ Reference point for surface mount components on PCB

1) When the lead pitch of the component (SMC) is less than 0.6mm, a reference point must be added and placed at the corner of the component, as shown in the figure below. Only two reference points can be placed. The reference point should be placed on the diagonal position. After the component is placed, the reference point must be visible.

Reference point for surface mount components on PCB

2) On a high-density PCB circuit board, and there is no space to place the reference point of the component, then in the Changhe

In the area with width ≤100mm, only two common reference points can be placed, as shown in the figure below

Benchmark arrangement of high-density PCB boards

The design department of our company recommends: Lead Pitch ≥ 0.6mm, then there is no need to add component positioning points, otherwise, reference points must be added.

4) The reference point of the component and the type of the optical positioning point of the PCB board are the same, and the size of a non-porous pad is shown in (Optical positioning point of the PCB board).

2. PCB component layout requirements.

The layout rules of PCB components should strictly refer to the content of (1), and the specific requirements are as follows:

1) The orientation of component placement (orientation)

A. After considering the requirements of wiring, assembly, welding and maintenance, the placement direction of components should be unified as far as possible.

The components on the PBA are required to have a unified direction as much as possible, and the components with positive and negative poles must also have a unified direction.

B. For the wave soldering process, the component placement direction requirements are as shown in the figure:

PCB with wave soldering process, the placement direction of the components on it

Due to the shadow effect of wave soldering, the component direction is 90° to the soldering direction, and the height of the component on the wave soldering surface is limited to 4mm.

C. For the hot air reflow soldering process, the placement direction of the components has little effect on soldering.

D. For PCBs with components on both sides, larger and denser ICs, such as QFP, BGA and other packaged components are placed on the top of the board, plug-in components can only be placed on the top layer, and the other side (bottom layer) of the plug-in components can only be placed on the top layer. Place smaller components and chip components with a small number of pins and loosely arranged, and cylindrical surface mount devices should be placed on the bottom layer.

E. For the structure of the vacuum fixture, the maximum height of the components on the back of the board cannot exceed 5.5mm; if the standard acupressure test fixture is used, the maximum height of the components on the back of the board cannot exceed 10mm.

F. Considering the actual working environment and self-heating, etc., heat dissipation factors should be taken into consideration when placing components.

Note:

1) The arrangement of components should be conducive to heat dissipation. Fans and radiators should be used if necessary, and radiators must be installed for small-sized and high-heat components.

2) High-power MOSFETs and other components can be coated with copper to dissipate heat, and try not to place heat-sensitive components around these components, so as not to affect the electrical performance of these heat-sensitive components. If the power is extremely high and the heat is extremely high, a heat sink can be installed to dissipate heat.

2) Consideration of the influence of PCB layout on electrical signals.

A designer should consider the following picture when considering the distribution of PCB components.

PCB layout considerations for electrical signal factors

A. High-speed components (connected to the outside world) should be as close as possible to the connector.

B. Digital circuits and analog circuits should be separated as much as possible, preferably separated by ground.

3) The distance between the component and the positioning hole

A. The distance between the positioning hole and the nearby through-foot pad is not less than 7.62 mm (300mil).

Dimensional drawing of the distance between the components on the PCB and the positioning hole

B. The distance between the positioning hole and the edge of the surface mount device is not less than 5.08mm (200mil).

The distance between the positioning hole and the edge of the surface mount device is not less than 5.08mm

For SMD components, the minimum radius distance from the center of the positioning hole SMD component frame is 5.08mm (200mil)

4) Requirements for DIP automatic plug-in machine.

For the PB with both SMD and DIP components, in order to avoid DIP components from damaging the SMD components during automatic insertion, the layout requirements of SMD and DIP components must be considered during layout.