PCB News - What are the common mistakes in PCB design work?

People make mistakes, and PCB design engineers are no exception. Contrary to popular belief, as long as we can learn from these mistakes, making mistakes is not a bad thing. The following will briefly summarize some common mistakes in PCB design.

1. Common errors in PCB

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

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

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

The components in the schematic 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 onto one page when printing

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

The components have been moved and rotated many times, and there are hidden characters outside the boundaries of the PCB board. Select 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.

Two, the common mistakes of the schematic diagram

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

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

Inconsistent grid attributes were modified when creating components or placing components, and the pins and wires were not connected;

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

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

(2) The component went out of the drawing boundary: no component was created in the center of the diagram paper of 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.

Three, common errors in the PCB manufacturing process

(1) Pad overlap

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

In a multi-layer board, there are both connection plates and isolation plates at the same position, and the board is shown as: • Isolation and connection errors.

(2) Irregular use of graphics layer

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.

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

(3) Unreasonable characters

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

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

(4) Single-sided pad setting aperture

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

If the 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 filler 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 frame

The distance should be at least 0.2mm (more than 0.35mm at the V-cut), 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) Milling profile positioning hole is not 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

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

The possible apertures are merged into a reservoir area as much as possible.

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

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

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

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

(15) Buried blind hole plate design problem

The significance of designing buried and blind vias:

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

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

Improve PCB design freedom

Reducing raw materials and costs is conducive to environmental protection.

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