PCB Technical - Common problems in FPC circuit design

1. The overlap of PCB pads

1. The overlap of the pads (except the surface mount pads) means the overlap of the holes. During the drilling process, the drill bit will be broken due to multiple drilling in one place, resulting in damage to the holes.

2. The two holes in the multilayer board overlap. For example, one hole is an isolation plate and the other hole is a connection plate (flower pad), so that the film will appear as an isolation plate after drawing, which will cause scrap.

2. The abuse of the graphics layer

1. Some useless connections were made on some graphics layers. It was originally a four-layer board but designed with more than five layers of wiring, which caused misunderstandings.

2. Save trouble during design. Take the Protel software as an example to draw the lines on each layer with the Board layer, and use the Board layer to mark the lines. In this way, when performing light drawing data, because the Board layer is not selected, it is omitted. The connection is broken, or it may be short-circuited due to the selection of the label line of the Board layer, so the integrity and clarity of the graphics layer is maintained during design.

3. Violation of conventional design, such as component surface design on Bottom layer and soldering surface design on Top, causing inconvenience.

3. Random placement of characters

1. The SMD soldering pad of the character cover pad brings inconvenience to the continuity test of the printed board and the soldering of the components.

2. If the character design is too small, it will be difficult for screen printing. If it is too large, the characters will overlap each other and be difficult to distinguish.

Four, single-sided pad aperture setting

1. Single-sided pads are generally not drilled. If the drilling needs to be marked, the hole diameter should be designed to be zero. If the numerical value is designed, then when the drilling data is generated, the coordinates of the hole appear at this position, and there is a problem.

2. Single-sided pads such as drilling should be specially marked.

5. Draw pads with filler blocks

Drawing pads with filler blocks can pass the DRC inspection during PCB design, but it is not good for processing. Therefore, similar pads cannot directly generate solder mask data. When solder resist is applied, the filler block area will be covered by solder resist. It is difficult to solder the device.

6. The electrical ground is also a flower pad and a connection

Because the power supply is designed as a pattern pad, the ground layer is opposite to the image on the actual printed board. All the connections are isolated lines. The designer should be very clear about this. By the way, when drawing several sets of power supplies or ground isolation lines, you should be careful not to leave gaps to short-circuit the two sets of power supplies, or to block the connection area (to separate a set of power supplies).

Seven, the processing level is not clearly defined

1. The single-sided board is designed on the TOP layer. If the front and back are not specified, the manufactured board may not be easy to be soldered with components installed.

2. For example, a four-layer board is designed with four layers of TOP mid1 and mid2 bottom, but it is not placed in this order during processing, which requires explanation.

8. There are too many filling blocks in the design or the filling blocks are filled with very thin lines

1. The gerber data is lost, and the gerber data is incomplete.

2. Because the filling blocks are drawn one by one with lines when processing the light drawing data, the amount of light drawing data generated is quite large, which increases the difficulty of data processing.

Nine, the surface mount device pad is too short

This is for continuity testing. For surface mount devices that are too dense, the spacing between the two pins is quite small, and the pads are also quite thin. To install the test pins, they must be staggered up and down (left and right), such as pads. The design is too short, although it does not affect the device installation, but it will make the test pin staggered.

10. The spacing of large-area grids is too small

The edges between the same lines that make up a large-area grid are too small (less than 0.3mm). During the manufacturing process of the printed board, the image transfer process will easily produce a lot of broken films attached to the board after the image is developed, causing the line to break.

11, the distance between the large area copper foil and the outer frame is too close

The distance between the large area copper foil and the outer frame should be at least 0.2mm, because when milling the shape, if it is milled to the copper foil, it is easy to cause the copper foil to warp and the solder resist falling off caused by it.

12. The design of the outline frame is not clear

Some customers have designed contour lines in Keep layer, Board layer, Top over layer, etc. and these contour lines do not overlap, which makes it difficult for PCB manufacturers to judge which contour line shall prevail.

XIII. Uneven graphic design

In the process of pattern plating, the plating layer is not uniform, which affects the quality.

14 When the copper area is too large, grid lines should be used to avoid blistering during SMT.

Main items to be checked after the design is completed:

The following inspections include all aspects related to the design cycle, and some additional items should be added for special PCB applications.

1) Has the circuit been analyzed? Is the circuit divided into basic units to smooth the signal?

2) Is the circuit allowed to use short or isolated key leads?

3) Where must be shielded, are they effectively shielded?

4) Have you made full use of the basic grid graphics?

5) Is the size of the printed board the best size?

6) Do you use the selected wire width and spacing as much as possible?

7) Has the preferred pad size and hole size been used?

8) Are the photographic plates and sketches appropriate?

9) Do you use the least jumper wires? Do jumper wires go through components and accessories?

l0) Are the letters visible after assembly? Are their size and model correct?

11) In order to prevent blistering, is there a window on the large area of copper foil?

12) Are there tool positioning holes?