PCB News - Summary of experience: mistakes made in PCB design

1. Common errors in schematic diagrams

1) There is no signal connected to the ERC report pin:

a. I/O attributes are defined for the pins when the package is created;

b. Inconsistent grid attributes are modified when components are created or placed, and the pins and wires are not connected;

c. When creating a component, the pin direction is reversed, and it must be connected to the non-pin name end;

d. The most common reason is that there is no project file, which is the most common mistake for beginners.

2) The component goes out of the drawing boundary: no component is created in the center of the diagram paper in the component library.

3) The network table of the created project file can only be partially imported into the PCB: when the netlist is generated, global is not selected.

4) When using multi-part components created by yourself, never use annotate.

2. Common errors in PCB board

1) It is reported that NODE is not found when the network is loaded:

a. The components in the schematic diagram use packages that are not in the PCB library;

b. The components in the schematic diagram use packages with inconsistent names in the PCB library;

c. The components in the schematic diagram use packages with inconsistent pin numbers in the PCB library. For example, a triode: the pin numbers in sch are e, b, and c, while those in PCB are 1, 2, and 3.

2) It is always impossible to print on one page when printing:

a. It is not at the origin when creating the PCB library;

b. The components have been moved and rotated many times, and there are hidden characters outside the boundaries of the PCB board. Choose to show all hidden characters, shrink the PCB, and then move the characters to the boundary.

3) The DRC reporting network is divided into several parts:

Indicates that this network is not connected. Look at the report file and use CONNECTED COPPER to find it.

If you make a more complicated design, try not to use automatic wiring.

3. Common errors in the PCB manufacturing process

1) Pad overlap:

a. Causes heavy holes, and breaks the drill and damages the holes due to multiple drilling in one place during drilling.

b. In the multi-layer board, there is both a connecting plate and an isolating plate at the same position, and the board is shown as: • Isolation and connection errors.

2) Irregular use of graphics layer:

a. Violation of conventional design, such as the component surface design in the Bottom layer, and the welding surface design in the TOP layer, causing misunderstandings.

b. There are a lot of design rubbish on each layer, such as broken lines, useless borders, labels, etc.

3) Unreasonable characters:

a. The characters cover the SMD solder tabs, which brings inconvenience to PCB on-off detection and component soldering.

b. The characters are too small, which makes screen printing difficult. If the characters are too large, they will overlap each other and be difficult to distinguish. The font is generally >40mil.

4) Setting aperture of single-sided pad:

a. Single-sided pads are generally not drilled, and the hole diameter should be designed to be zero. Otherwise, when the drilling data is generated, the coordinates of the hole will appear at this position. Special instructions should be given for drilling.

b. If a single-sided pad needs to be drilled, but the aperture is not designed, the software treats this pad as an SMT pad when outputting electrical and ground data, and the inner layer will lose the isolation disk.

5) Draw pads with filling blocks:

Although it can pass the DRC inspection, the solder mask data cannot be directly generated during processing, and the pad is covered with solder mask and cannot be soldered.

6) The electrical ground layer is designed with both a heat sink and a signal line. The positive and negative images are designed together, and errors occur.

7) Large area grid spacing is too small:

The grid line spacing is less than 0.3mm. During the PCB manufacturing process, the pattern transfer process will cause film breakage after development, which will increase the processing difficulty.

8) The graphics are too close to the outer frame:

At least 0.2mm or more spacing should be ensured (V-cut 0.35mm or more), otherwise the copper foil will be warped and the solder resist will fall off during the exterior processing, which will affect the appearance quality (including the inner copper skin of the multilayer board).

9) The outline frame design is not clear:

Many layers are designed with frames and do not overlap, which makes it difficult for PCB manufacturers to determine which line to use. The standard frame should be designed on the mechanical layer or the BOARD layer, and the internal hollowed-out parts should be clear.

10) Uneven graphic design:

When the pattern is electroplated, the current distribution is uneven, which affects the uniformity of the coating, and even causes warpage.

11) Short shaped hole:

The length/width of the special-shaped hole should be> 2:1, and the width should be> 1.0 mm, otherwise the CNC drilling machine cannot process it.

12) No milling profile positioning hole is designed:

If possible, design at least two positioning holes with a diameter of> 1.5mm in the PCB board.

13) The aperture is not clearly marked:

a. The aperture marking should be marked in metric system as far as possible, and in increments of 0.05.

b. Combine the apertures that may be combined into a reservoir area as much as possible.

c. Whether the tolerances of metallized holes and special holes (such as crimp holes) are clearly marked.

14) Unreasonable wiring in the inner layer of the multilayer board:

a. The heat dissipation pad is placed on the isolation tape, and it is easy to fail to connect after drilling.

b. There are gaps in the design of the isolation belt, which is easy to misunderstand.

c. The isolation band design is too narrow to accurately judge the network

15) Design problems of buried and blind vias:

The significance of designing buried and blind vias:

a. Increase the density of the multilayer board by more than 30%, reduce the number of layers and reduce the size of the multilayer board

b. Improve PCB performance, especially the control of characteristic impedance (shortened wires and reduced aperture)

c. Improve PCB design freedom

d. Reduce raw materials and costs, which is conducive to environmental protection.

Some people generalize these problems to work habits. People who have problems often have these bad habits.

4. Lack of planning

As the saying goes, "If a person has no plan beforehand, he will find trouble and will come to his door." Of course, this also applies to PCB design. One of the many steps to make PCB design successful is to choose the right tool. Today's PCB design engineers can find many powerful and easy-to-use EDA kits on the market. Each model has its own unique capabilities, advantages and limitations. In addition, it should also be noted that no piece of software is foolproof, so problems such as mismatched component packaging are bound to occur. It is possible that there is no single tool that can meet all your needs. However, you still have to work hard in advance to find the best product that best suits your needs. Some information on the Internet can help you get started quickly.

5. Poor communication

Although the practice of outsourcing PCB design to other manufacturers is becoming more and more common and often very cost-effective, this approach may not be suitable for high-complexity PCB design, because in this design, performance and reliability Sex is extremely critical. With the increase of design complexity, in order to ensure accurate component placement and routing in real time, face-to-face communication between engineers and PCB designers has become very important. This face-to-face communication will help save expensive future Do (rework) work.

It is also important to invite PCB board manufacturers to join in the early stages of the design process. They can provide initial feedback on your design, and they can maximize efficiency according to their processes and procedures. In the long run, this will help you save considerable time and money. By letting them know your design goals and inviting them to participate in the early stages of the PCB layout, you can avoid any potential problems before the product is put into production and shorten the time to market.

6. Failure to thoroughly test early prototypes

The prototype board allows you to prove that your design is operating according to the original specifications. Prototype testing allows you to verify the function and quality of the PCB and its performance before mass production. Successful prototype testing requires a lot of time and experience, but a strong test plan and a clear set of goals can shorten the evaluation time and also reduce the possibility of production-related errors. If any problems are found during the prototype test, a second test needs to be performed on the reconfigured circuit board. By incorporating high-risk factors in the early stages of the design process, you will benefit from multiple iterations of the test, identify any potential problems early, reduce risks, and ensure that the plan can be completed on schedule.

7. Use inefficient layout techniques or incorrect components

Smaller and faster devices allow PCB design engineers to lay out complex designs. This design will use smaller components to reduce the footprint, and they will also be placed closer together. The use of some technologies, such as embedded discrete devices on the internal PCB layer, or ball grid array (BGA) packaging with a smaller pin pitch, will help reduce the size of the circuit board, improve performance, and reserve space for You can redo after the problem. When used with components with high pin counts and smaller pitches, it is important to select the correct circuit board layout technology at design time, so as to avoid problems in the future and reduce manufacturing costs as much as possible.

In addition, you must carefully study the value range and performance characteristics of the replacement components you intend to use, even those that are marked as drop-in replacements. A small change in the characteristics of the replacement component may be enough to mess up the performance of the entire design.

8. Forget to back up your work

Back up important data. Do I still need to remind you? At the very least, you should back up your most important work results and other hard-to-replace files. Although most companies back up all the company's data every day, some smaller companies may not do this, or if you work from home, neither will you. Nowadays, backing up data to the cloud is so convenient and cheap. There is really no excuse not to back up the data and keep the data in a safe place to prevent it from being stolen, encountering fires, and other local disasters.

9. Become a single island

Although you may think that your design is flawless, and making mistakes is not your style at all, but many times, your peers will see some mistakes in your design that you did not notice. Sometimes, even if you know the intricate details of the design, people with less exposure to it may be able to maintain a more objective attitude and provide valuable insights. Checking your design frequently with your peers can help find unforeseen problems and keep your plan on the right track, keeping costs within budget.

Of course, making mistakes is inevitable, but as long as you can learn the lesson, you can design an excellent product next time.