PCB design specification and process

PCB Design Specifications for Manufacturability

DFM has the advantages of shortening the development cycle, reducing costs, and improving product quality. It is the way for enterprise products to succeed.

PCB (Printed Circuit Board, printed circuit board) is an important electronic component in electronic products. It is the carrier for electronic components to realize electrical connection, and PCBA (Printed Circuit Board Assembly, PCB assembly, that is, to assemble electronic components on the PCB board The formed semi-finished product) is the hardware body that realizes the function of the electronic product circuit system. This article aims at the manufacture of PCB components in electronic products, and briefly discusses some specifications of PCB design from various aspects to achieve the purpose of realizing the manufacturability of PCB components.

1. Basic principles of PCB DFM design

(1) Reduce the manufacturing process and cost of PCB assembly, and try to place the parts on the main welding surface of the PCB. (2) The same or similar components should be placed in the same column or row and the polarity should point in the same direction. (3) Distribute the components evenly according to the size and quantity on the PCB to avoid deformation of the PCBA during the reflow process and wave soldering process. (4) Connectors and sockets should be placed on the main soldering surface of the PCBA (5) Do not design through-hole devices on both sides of the PCB. (6) Automatic assembly should be considered as far as possible in the design, and manual operation should be minimized. (7) Avoid using jumpers and any additional manual operations. (8) Consider the requirements of equipment debugging in the design. (9) Errors of various variables are considered in the design.

2. PCB shape and size design

The shape and size of the PCB are determined by the PCB transmission method and the mounting range of the placement machine. According to the equipment capability index, the PCB assembly size range that the usual PCB component manufacturers can produce is: minimum 50 (length) * 50 (style) mm, maximum 520 (length) * 400 (width) mm, PCB thickness 0.2mm ~~4.5mm.

In order to improve the utilization rate and balance rate of SMT machines, it is usually required that the size of the PCB panel be as large as possible (the more single panels in the same panel, the better, as long as it does not exceed the allowable size range of the equipment), of course, the panel must also be considered The ease of handling and the hidden danger of compression deformation of the panel.

3. PCB process side

Most assembled PCBs have upper and lower track sides, also known as PCB process sides. The width of the transmission side of the PCBA assembly equipment is usually designed to be 5-10mm, which is used to drive the PCB assembly while contacting the PCB process.

Usually, components cannot be placed within 3mm from the actual PCB board to the edge of the PCB, so as to prevent the machine track from clamping the component pads during circuit board assembly and making it impossible to install. If there are components within 3mm of the edge of the single board, the board must be added with a PCB process edge.

If the body of the assembly component exceeds the edge of the board or is flush with the edge of the board, the bare board is not allowed to be placed on the conveying equipment for production, and the edge of the board is not allowed to be set in the V-Cut paneling method, but the edge of the board needs to be hollowed out according to the specific situation Partially or by means of stamp holes to connect the edge of the board. If the board must be produced using SMT production equipment, it is necessary to use a special jig carrier board operation.

If the body of the plug-in component exceeds the edge of the board or is flush with the edge of the board, the wave soldering process must be performed after board separation. The gold finger edge on the PCB cannot be used as a transmission edge.

4. Connection design of V-CUT and stamp hole of PCB panel

When the PCB board and the board are connected in a straight line, the edge is flat and does not affect the device installation, the PCB panel can be connected to each small board by V.cUT. V-CUT is a straight-through type and cannot turn in the middle.

The recommended board thickness of the PCB designed using V-CUT panel is 0.6O2.0mm. The depth of the groove is (for the commonly used FR4PCB material) the thickness of the upper and lower 1/3 boards. For the V-CUT connected PCB that needs to be automatically divided by the machine, in order to ensure that there will be no exposed copper defects during the PCB division process, it is required that all traces and copper foils be more than 0.50mm away from the edge of the board during PCB layout design, while the actual components The distance between the physical edge and v_CuT should be greater than lmm.

If the connection between the boards in the PCB panel is not a straight line, you can use the stamp hole connection to connect each small board in the panel. Generally, the connection part of the stamp hole adopts four through holes, the stamp hole cannot be directly below the part, and the minimum distance from the stamp hole to the wiring edge is 0.5ram.

5.PCB optical positioning point design

In order to facilitate accurate optical positioning of SMT production equipment, FiducialMark (optical positioning point) is a necessary design for PCB. The PCB optical positioning point is very important for SMT machine parts, and its quality will directly affect the accuracy of machine placement.

Global optical positioning point (Globalfiducial mark): Generally, the PCB is required to have at least 2 global optical positioning points (usually also called large board Mark points), and its size and shape should be consistent, while 3 to 4 global optical positioning points are necessary for SMT production. better.

At least 2 global optical positioning points should be distributed on the diagonal of the panel. If the PCB needs to be produced on both sides, there must be global optical positioning points on both sides. track edge) plus anchor points.

The optical positioning point is usually designed as a solid circle with a diameter of 1.0mm. In the circular area where the center diameter of the optical point is the diameter of the solid circle plus 2.0mm, solder mask opening is required. The optical point manufacturing process requires the surface to be clean and flat, the edges smooth and neat, and the color to be clearly different from the surrounding background color (the key is to have a clear contrast with the color of the reference point). There are no pads, vias, test points or silk screen marks in the background of the optical dots. The optical point cannot be cut by V-Cut and cannot be recognized by the machine.

6. Design of PCB pads and connected leads

(1) When the size of the pad is asymmetrical, or the ends of the two components are designed on the same pad, suspension bridges and displacements will occur due to asymmetrical surface tension. Need to avoid.

(2) When the pad is connected to a large area of ground, the cross paving method and 45 should be preferred. Paving method. The length of the wire drawn from the large-area ground or power line should be greater than 0.5mm, and the width should be less than 0.4mm.

(3) Try to make the wiring connected to the pads symmetrically distributed. Reduces unbalanced solder flow due to asymmetric distribution, causing component rotation or misalignment.

7.PCBA test point design

For PCBs of common jfl consumer electronics products, there is usually at least one test point on each line network, at least 4 test points on each power line network, and at least 6 test points on the ground network. All test points are distributed on the bottom of the PCB as much as possible, which facilitates the manufacture of test fixtures, reduces the cost of manufacturing test fixtures and increases the stability of the fixtures.

The center distance between two adjacent test points should be at least greater than 1.25mm, the center of the test point should be greater than 2.54mm from the edge of the board (excluding the broken edge), and greater than 1.25mm from the edge of the screw hole. The test point and the periphery of the part (zero even The distance between the test point and the periphery of the part (with a height of 3mm or less) should be less than 1.27mm, the distance between the test point and the periphery of the part (with a height of 3-6mm) should be less than 3.3mm, and the distance between the test point and the periphery of the part (with a height of 6mm or more) should be more than 3.81mm.

8.PCB stack design

There are generally two designs for PCB stacking: one is the structure of copper foil plus core board (Core), referred to as the Foil stacking method; the other is the core board. (Core) stacking method, referred to as Core stacking method. The PCB stacking method is recommended to use the Foil stacking method, and the Core stacking method can be used when multi-layer boards of special materials and boards are mixed.

The outer layer of the PCB generally uses 0.5oz to Ioz (1oz=0.35 micron) copper foil electroplating, and the inner layer generally uses 1OZ copper foil: try to avoid using a core board with inconsistent copper foil thickness on both sides of the inner layer.

The PCB stackup adopts a symmetrical design. Symmetrical design refers to the insulation layer thickness, prepreg type, copper foil thickness, and graphic distribution type (large copper foil layer, circuit layer) as symmetrical as possible relative to the vertical center line of the PCB.

9.PCB surface treatment

9.1 Hot air leveling process

This process is to cover the tin-silver-copper alloy on the final exposed metal surface of the PCB. The thickness of the tin-silver-copper alloy coating required for hot air leveling is 1 micron to 25 microns. The hot air leveling process is difficult to control the thickness of the coating and the pattern of the pad. It is not recommended to be used on PCBs with fine-pitch components, because the fine-pitch components have high requirements on the flatness of the pads.

The thermal shock of the hot air leveling process may cause PCB warpage, and this surface treatment method is not recommended for ultra-thin PCBs with a thickness of less than 0.7mm.

9.2 Electroless nickel gold process

Electroless nickel gold is the abbreviation of nickel immersion gold. The electroless nickel layer coating used on the copper metal surface of PCB is 2.5 microns to 5.0 microns, and the thickness of immersion gold (99.9% pure gold) layer is 0.08 micron 0.I27 micron . It can provide a relatively flat surface, and this process is suitable for PCBs with fine-pitch components. 10.3 Organic Solderability Protective Film Process This process is abbreviated as OSP, which refers to the surface coverage of the exposed PCB copper surface with specific organic substances. At present, the only recommended protective layer for the Ke machine is Enthone'SEntekPlusCu-l06A, and its thickness requirement is 0.2 microns to 0.5 microns, because it can provide a very flat PCB surface, especially suitable for PCBs with fine-pitch components.

Summarize

In the era of electronic information, the updating speed of electronic products is very fast, which means that the market competition of electronic products is very fierce. In order to adapt to market competition and complete the assembly and manufacture of electronic products more effectively, the content of manufacturable PCB design work has become more and more abundant. It can also bring direct cost savings to manufacturers while improving production capacity and controlling quality. . Only by grasping the unity of design and production can they promote each other and achieve the goal of enhancing market competitiveness.

The above are the details of the PCB design specifications for manufacturability introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad: Songhan, Yingguang, Jieli, Ankai, Quanzhi, realtek, with MCU, voice IC, Bluetooth IC and module, wifi module. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the research and development of electronic education products.