PCB Technical - What related issues should be considered in pcb design

I have encountered quite a lot of PCB design related problems, the assembly density is too high, and high components such as tantalum capacitors, chip inductors, and fine-pitch SOIC, TSOP and other devices must be distributed on the PCB soldering surface. In this case, you can only Double-sided printing solder paste is used for reflow soldering, and plug-in components should be distributed as concentrated as possible to accommodate manual soldering. Another possibility is that the perforated components on the component surface should be distributed as much as possible in several main straight lines. In order to adapt to the latest selective wave soldering process, manual soldering can be avoided to improve efficiency and ensure soldering quality. Discrete solder joint distribution is a taboo for selective wave soldering, which will increase the processing time exponentially.

When adjusting the position of the components in the printed board file, you must pay attention to the one-to-one correspondence between the components and the silk-screen symbols. If the components are moved without correspondingly moving the silk-screen symbols next to the components, it will become a major quality hazard in manufacturing., Because in actual production, silk screen symbols are industry language that can guide production.

1. The PCB must be equipped with clamping edges, positioning marks, and process positioning holes necessary for automated production.

At present, electronic assembly is one of the industries with the highest degree of automation. The automation equipment used in production requires the automatic transmission of PCB. This requires that in the transmission direction of the PCB (usually the long-side direction), there should be no less than 3- The 5mm wide clamping edge facilitates automatic transmission and prevents components close to the edge of the board from being unable to be automatically assembled due to clamping.

The function of the positioning mark is that for the currently widely used optical positioning assembly equipment, the PCB needs to provide at least two to three positioning marks for the optical identification system to accurately position the PCB and correct the PCB processing errors. Among the commonly used positioning marks, two marks must be distributed on the diagonal of the PCB. The selection of positioning marks generally uses standard graphics such as solid round pads. For easy identification, there should be an open area without other circuit features or marks around the mark. The size should preferably not be less than the diameter of the mark. The mark should be 5mm away from the edge of the board. above.

In PCB manufacturing, semi-automatic plug-in, ICT testing and other processes in the assembly, the PCB needs to provide two to three positioning holes in the corners.

2. Reasonable use of puzzles to improve production efficiency and flexibility.

When assembling PCBs with small or irregular shapes, there are many restrictions. Therefore, the method of splicing several small PCBs is generally used to splice several small PCBs into PCBs of suitable size for assembly. Generally, for PCBs with a single side size less than 150mm, you can consider using the boarding method. Through two, three, four, etc., the size of the large PCB can be assembled to the appropriate processing range, usually 150mm~250mm in width and 250mm~350mm in length. The PCB is a more suitable size in automated assembly.

Another way of splicing is to assemble the PCB with SMD on both sides into a large board. This kind of splicing is commonly known as Yin-Yang splicing. It is generally for the purpose of saving the cost of the network board, that is, through this Jigsaw puzzles originally required two screens, but now only one screen is needed. In addition, when the technicians compile the operation program of the placement machine, the PCB programming efficiency of using Yin and Yang spelling is also higher.

The connection between the sub-boards can be double-sided engraved V-slots, long slots and round holes when joining the board, but the design must consider making the separation line as possible as possible to be in a straight line to facilitate the final split. At the same time, it should be considered that the separation edge should not be too close to the PCB trace, so that the PCB is easily damaged when the board is split.

There is also a very economical jigsaw, which does not refer to the PCB jigsaw, but the mesh pattern of the stencil. With the application of fully automatic solder paste printers, the current more advanced printers (such as DEK265) have allowed the opening of multi-sided PCB mesh patterns on a steel mesh with a size of 790*790mm, which can be used for multiple stencils. The printing of individual products is a very cost-saving approach, especially suitable for manufacturers whose products are characterized by small batches and multiple varieties.

3. Consideration of testability design

The testability design of SMT is mainly aimed at the current ICT equipment situation. The test problems of later product manufacturing are taken into consideration in the design of the circuit and surface mount printed circuit board SMB. To improve the design of testability, two aspects of process design and electrical design should be considered.

4. Requirements of process design

Positioning accuracy, substrate manufacturing procedures, substrate size, and probe type are all factors that affect the reliability of detection.

(1) Precise positioning holes. Set precise positioning holes on the substrate. The error of the positioning holes should be within ±0.05mm. At least two positioning holes should be set, and the distance is better. Use non-metallized positioning holes to reduce the thickening of the solder plating layer and fail to meet the tolerance requirements. If the substrate is manufactured as a whole piece and then tested separately, the positioning holes must be provided on the main board and each individual substrate.

(2) The diameter of the test point is not less than 0.4mm, and the distance between adjacent test points is preferably above 2.54mm, not less than 1.27mm.

(3) Do not place components with a height exceeding *mm on the test surface. Excessive components will cause poor contact between the on-line test fixture probe and the test point.

(4) It is best to place the test point 1.0mm away from the component to avoid impact damage to the probe and component. There should be no components or test points within 3.2mm around the ring of the positioning hole.

(5) The test point cannot be set within 5mm of the edge of the PCB. The 5mm space is used to ensure the clamping of the fixture. Usually the same process side is required in conveyor belt production equipment and SMT equipment.

(6) It is best to plate all the detection points with tin or use a soft, easily penetrated, and non-oxidized metal conductive material to ensure reliable contact and prolong the service life of the probe.

(7) The test point cannot be covered by solder resist or text ink, otherwise the contact area of the test point will be reduced and the reliability of the test will be reduced.

5. Electrical design requirements

(1) It is required to lead the SMC/SMD test points on the component surface to the soldering surface through via holes as much as possible. The diameter of the via holes should be greater than 1mm. In this way, the online test can be tested with a single-sided needle bed, thereby reducing the cost of online testing.

(2) Each electrical node must have a test point, and each IC must have POWER and GROUND test points, and be as close to this component as possible, preferably within 2.54mm from the IC.

(3) When setting test points on the traces of the circuit, the width can be enlarged to 40 mils.

(4) The test points are evenly distributed on the printed board. If the probes are concentrated in a certain area, the higher pressure will deform the board to be tested or the needle bed, which will further cause some probes to not touch the test point.

(5) The power supply line on the PCB circuit board should be set with test break points in different areas, so that when the power supply decoupling capacitor or other components on the circuit board are short-circuited to the power supply, it is faster and more accurate to find the fault point. When designing breakpoints, you should consider restoring the test