Electronic Design - How to design and imposition PCB to save money?

Normally, when a hardware designer designs a PCB, he considers the arrangement of electrical signals and components on the circuit board, and focuses on the functional issues of the product. Less consideration is given to PCB manufacturing and assembly. To realize the smooth manufacturing of PCB, especially in SMT assembly, special attention needs to be paid to the PCB design.

1. The purpose and significance of the puzzle

To improve SMT assembly efficiency, the more single boards contained in a panel, the shorter the average time for a single PCB to travel on the track, and the higher the utilization rate of the placement equipment, the higher the production efficiency of SMT can be greatly improved.

Improve the quality of SMT production and prevent bad problems in the production process. The distribution density of the components on the PCB is high. Some components such as the edge connector may extend beyond the edge of the PCB. These components are movable before the reflow soldering or wave soldering is completed, so additional processes are required to increase the entire PCB. The area of the jigsaw board prevents these board edge components from being affected by external factors and lowering the assembly quality.

Easy to operate and prevent damage. The design of the jigsaw is also to facilitate the operation of the production line. The assembly of a PCB requires many steps. The finished or semi-finished PCB is put into the turnover box, and the turnover rack is transferred, stored, stored, and transported. These operations require specific process edges to prevent damage to the components on the PCB.

Two, PCB puzzle design rules

No matter what kind of PCB, a jigsaw structure is usually required. It’s just that there are many ways to design jigsaws, and sometimes it is difficult to determine which jigsaw method and the number of jigsaws to use, and many factors need to be considered comprehensively.

Jigsaw type

1. ABAB, the design feature of this board is that the distribution of components on both sides of the board is symmetrical and exactly the same. The advantage of this design is that the equipment configuration is simple and easy, and the preparation and operation before production are simple and easy. All SMT assembly only needs to be prepared once: a steel mesh, a set of patch procedures, SPI inspection procedures, and reflow soldering temperature curves. Wait. However, for products with large differences in the distribution density of components on both sides of the PCB, this may cause additional problems. In some product designs, most of the components may be concentrated on one side of the PCB, and there are only a few, simple packaged components on the other side. This ABAB splicing method is not very good. This design sometimes leads to printing and mounting problems for some fine-pitch components. If there are large-mass components on the board, sometimes it will also bring the risk of components falling during the secondary reflow. Finally, for high-volume production, this kind of splicing causes the utilization rate of the equipment to be not too high.

2. AAAA/BBBB, this kind of non-female board design, its advantage is based on the distribution characteristics of its board components: size, density, reasonable arrangement of equipment configuration, production process, and improve equipment utilization. Also because the components of the same structure are on the same side of the panel, the production process can be reasonably arranged, such as avoiding the printing short circuit of close-pitch components or the problem of high-profile, large-quality components dropping during secondary reflow. The shortcomings of this form of jigsaw are mainly that the production and assembly of the two sides of the jigsaw are completely different. Contrary to ABAB, all preparations such as steel mesh, SPI inspection procedures, placement procedures, AOI inspections, The reflow temperature curve, etc., are different on the front and back, and even the production line needs to prepare two. This is not suitable for small batches of multi-model products. Of course, for large quantities, if the production line configuration is tight, it is not recommended as the preferred design. Especially in the NPI stage, it brings troubles to the production plan.

Number of jigsaw unit boards

How many unit boards should be included in a jigsaw board? This quantity problem needs to be determined according to the actual situation. In principle, the more the number, the better, the more the number, the less time a single PCB will lose on the track, and the use of equipment The higher the rate; at the same time, the more the number of panels, the higher the utilization rate of materials used in PCB manufacturing, and the cost will be reduced accordingly. Of course, this also has a limit. Sometimes with the increase in the number of puzzles, the material cost of the unit board will increase instead, which is mainly due to the limitation of the equipment capacity of the PCB manufacturer. How many unit boards need to be included in a specific puzzle may need to be considered from the following aspects:

1. The capacity of the equipment, the maximum PCB size that the equipment can produce (print or mount). Within this maximum allowable size range, it is possible to include as many unit boards as possible.

2. PCB thickness, if the thickness of the PCB board is small, it is necessary to consider the deformation of the entire board during reflow or wave crest. Therefore, the size of the jigsaw board needs to be considered to reduce the impact of PCB deformation on the assembly quality.

3. Whether fixture assisted production. For PCBs with small thickness, if there is fixture assisted production, the size of the panel can also be relaxed, but this may involve cost growth.

4. Whether the production operation is easy or not, too large a size may cause operational safety issues, which are also issues that need to be considered.

Other design rules:

1. Keep a 1.6mm or 2.4mm space between each unit board in the jigsaw to facilitate router or Punch splitting. If a width less than 1.6mm is used, it is necessary to consider whether the equipment is suitable for such small gaps.

2. On the four corners of the jigsaw, design tool holes that do not require plating. The entire jigsaw must ensure at least two holes, and the size of the holes is 3.05±0.05mm. The center of the hole should be positioned at the corner of the PCB, 5mm each in the X and Y directions.

3. The four corners of the jigsaw should be rounded or chamfered with a radius of 5mm to prevent jamming on the track conveyor belt.

4. The jigsaw design must ensure that the edge of the reference point (including the green oil opening) is at least 5mm away from the edge of the jigsaw. Do not place the reference points on the front and back sides symmetrically, so that the identification function of the device itself can prevent the PCB from entering the machine in the reverse direction.

5. A direction mark can be designed on the edge of the jigsaw board, such as the semi-circular hole at the edge of the picture below, to prevent the PCB direction from being wrong during production.

6. The maximum shape tolerance of the PCB panel is controlled within the range of .05mm.

3. Materials needed to complete the puzzle design

If you want to design a good jigsaw puzzle, you must have enough information to support it, otherwise, it may cause subsequent SMT assembly or sub-board quality problems due to the unreasonable design of the jigsaw puzzle. To complete a qualified jigsaw design, the following data information is usually required.

Gerber file, including the distribution of PCB components, consider the spacing of the board and the placement of the connection points;

PCB outline drawing, understand the size and specifications of PCB;

For large-scale components, especially the specifications of board edge components, it is necessary to design the spacing and avoiding positions of the board.

4. The impact of poor panel design on production

In actual work, there are often some puzzle design problems due to various reasons. These problems may cause the production to not be carried out normally, and it is difficult to divide the boards.

Without the position avoidance design, the components on the side of the board cannot be installed normally;

The distance between the unit plates is too small, causing difficulty in mold manufacturing or easy damage to the tool;

The process edge is too narrow, and the solder paste or components near the board edge are erased during track transmission;

The size of the jigsaw is too large, and the high temperature deformation causes poor welding;

Insufficient process edges or less connection points result in insufficient rigidity of the panel and affect production.

5. Summary

The above is mainly to discuss the design requirements of rigid PCB panels. For flexible FPCs, the requirements for panels may be different due to different production methods.