PCB News - Design requirements for surface mount printed boards

The PCB printed board design specifications of surface mount technology (SMT) and through-hole insertion technology (THT) are quite different. When determining the shape, land pattern and wiring method of the surface mount printed board, full consideration should be given to the type of PCB circuit board assembly, mounting method, placement accuracy, and soldering process. Only in this way can the welding quality be guaranteed and the reliability of the functional modules can be improved.

1. Surface mount printed board shape and positioning design

The shape of the printed board must be processed by CNC milling. If calculated according to the placement machine accuracy ±0.02mm, the vertical parallel accuracy around the printed board, that is, the form and position tolerance should reach ±0.02mm.
For printed boards with an external dimension less than 50mm*50mm, it is advisable to adopt the form of paneling. The specific size of the panel should be determined according to the specifications and specific requirements of the placement machine and screen printing machine.
The printed board needs to be positioned during the process of missing printing, and a positioning hole must be provided. Take the UK-made DEK screen printing machine as an example. The machine is equipped with a pair of D3mm positioning pins. Correspondingly, at least two D3mm positioning holes should be set on the opposite sides or diagonal lines of the PCB, relying on the machine's vision system (Vision) and The positioning hole ensures the positioning accuracy of the printed board.
The circumference of the printed board should be designed with a process clamping edge whose width is generally (5±0.1) mm, and there should not be any land patterns and devices in the process clamping edge. If it is true that the board size is limited and the above requirements cannot be met, or the panel assembly method is adopted, the production method of adding a frame around the periphery can be adopted, leaving the process clamping edge, and manually breaking and removing the frame after the welding is completed.

2. The wiring method of the printed board

Try to go as short as possible, especially for small-signal circuits. The shorter the line, the smaller the resistance and the smaller the interference. At the same time, the length of the coupling line should be as short as possible.
When changing the direction of the signal lines on the same layer, you should avoid turning at right angles, and go diagonally as much as possible, and the radius of curvature should be larger.
Trace width and center distance
The width of the printed board lines is required to be as consistent as possible, which is conducive to impedance matching. In terms of the printed board manufacturing process, the width can be 0.3mm, 0.2mm or even 0.1mm, and the center distance can also be 0.3mm, 0.2mm, 0.1mm. However, as the lines become thinner, the spacing becomes smaller, In the production process, the quality will be more difficult to control, the scrap rate will rise, and the manufacturing cost will increase. Unless the user has special requirements, the wiring principle of 0.3mm line width and 0.3mm line spacing is more appropriate, which can effectively control the quality.

Design of power cord and ground wire
For power lines and ground lines, the larger the wiring area, the better, in order to reduce interference. For high-frequency signal lines, it is best to shield them with ground wires.
Multilayer board routing direction
The wiring of the multilayer board should be separated according to the power layer, ground layer and signal layer to reduce interference between power, ground and signals. Multi-layer board wiring requires that the lines of two adjacent layers of printed boards should be as perpendicular to each other as possible, or slanted or curved, and not parallel, so as to help reduce the coupling and interference between the substrate layers. The large-area power layer and the large-area ground layer should be adjacent to each other, and its function is to form a capacitor between the power supply and the ground to play a filtering role.

3 Pad design control
Because there is no unified standard for surface mount components, different countries and different manufacturers have different component shapes and packages. Therefore, when choosing the pad size, it should be consistent with the package shape of the components you choose. Compare the size of the pins and other related soldering to determine the length and width of the pad.

Land length
The length of the pad plays a more important role in the reliability of the solder joint than the width of the solder joint. The reliability of the solder joint mainly depends on the length rather than the width. As shown in Figure 1.

Figure 1 Solder joint
The selection of L1 and L2 size should be conducive to the formation of a good meniscus profile when the solder is melted, and also to avoid the bridging phenomenon of the solder, and take into account the deviation of the components (the deviation is within the allowable range) to facilitate the increase Adhesion of solder joints improves soldering reliability. Generally, L1 takes 0.5mm and L2 takes 0.5-1.5mm.
Land width
For RC components above 0805, or SD, SOJ, and other IC chips with a pin pitch above 1.27mm, the pad width is generally based on the component pin width plus a value, and the value range is 0.1- Between 0.25mm. For an IC chip of 0.65mm including a pin pitch of 0.65mm or less, the width of the pad should be equal to the width of the pin. For fine-pitch QFPs, sometimes the pad width should be appropriately reduced relative to the pins, such as when there is a lead passing between the two pads.
Requirements for lines between pads
It is necessary to avoid crossing wires between pads of fine-pitch components as much as possible. If it is necessary to cross wires between pads, solder masks should be used to reliably shield them.
Requirement of pad symmetry
For the same component, all pads used symmetrically, such as QFP, SOIC, etc., should be designed to strictly ensure their overall symmetry, that is, the shape and size of the pad pattern are exactly the same to ensure that the solder will act on the component when the solder melts. The surface tension protection of all solder joints on the device is balanced to facilitate the formation of ideal high-quality solder joints and ensure no displacement.

4 Benchmark standard (Mark) design requirements

A fiducial mark must be set on the printed circuit board to serve as a reference point for the placement machine during placement operations. Different types of placement machines have different requirements for the shape and size of the reference point. Generally, 2-3 D1.5mm bare copper solids are set on the diagonal of the printed board as the reference mark.
For multi-pin components, especially fine-pitch mounted ICs with a pin pitch below 0.65mm, a fiducial mark should be added near the pad pattern, and two symmetrical fiducial points should be set on the diagonal of the pad pattern. The mark is used for the optical positioning and calibration of the placement machine.

5 Other requirements

Transition hole treatment
Transition holes are not allowed in the pad, and the filter hole should be avoided to connect with the pad to avoid poor soldering caused by the loss of solder. If the transition hole does need to be interconnected with the pad, and the distance between the transition hole and the edge of the pad is greater than 1mm.
Characters and graphics requirements
Symbols such as characters, graphics, etc. must not be printed on the pad to avoid poor soldering.

6 concluding remarks
As a surface mount printed circuit board design technician, in addition to being familiar with the relevant theoretical knowledge of circuit design, you must also understand the surface mount production process, familiar with the component outline packaging of various companies that are often used, and many soldering quality problems are related to Poor design is directly related. According to the concept of whole production process control, surface mount printed circuit board design is a key and important link to ensure the quality of the surface mount.

The above is an introduction to the design requirements of surface mount printed boards. Ipcb also provides PCB manufacturers and PCB manufacturing technology.