[Experience sharing] Some common problems in PCB design

PCB design is not a simple task and requires both technical knowledge and design experience. As the saying goes, “You can see further by standing on the shoulders of your predecessors.” Similarly, as PCB design beginners, we can also avoid taking some detours by summarizing the experience of our predecessors.
Some common problems in PCB design

Pad overlap

1. The overlap of solder pads (except for surface-mounted pads) means the overlap of holes. During the drilling process, drilling multiple holes in one place will cause the drill bit to break, resulting in damage to the hole.

2. Two holes in a multilayer board overlap. For example, one hole is an isolation plate and the other hole is a connection pad (flower pad). This will appear as an isolation plate after drawing the negative, causing scrapping.

Misplacement of characters

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

2. If the characters are designed too small, it will be difficult to screen print. If they are too large, the characters will overlap each other and be difficult to distinguish.

There are too many fill blocks in the design or the fill blocks are filled with extremely thin lines

1. The light painting data is lost and the light painting data is incomplete.

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

Abuse of Graphics Layer

1. Some useless connections were made on some graphics layers. It was originally a four-layer board but more than five layers of circuits were designed, causing misunderstanding.

2. Violate the conventional design, such as the component surface is designed on the Bottom layer and the welding surface is designed on the Top, causing inconvenience.

The spacing of the large area grid is too small

The edges between the same lines that make up a large-area grid are too small (less than 0.3mm). In the printing board manufacturing process, after the image transfer process is completed, a lot of film fragments are likely to adhere to the board, causing breakage.

The large area of copper foil is too close to the outer frame

A large area of copper foil should be at least 0.2mm away from the outer frame, because milling onto the copper foil during contour milling can easily cause the copper foil to warp and cause the solder resist to fall off.

The outline and border design is unclear

Some customers have designed outline lines for Keep layer, Board layer, Top over layer, etc., and these outline lines do not overlap, making it difficult for PCB manufacturers to determine which outline line should be used.

The impact of solder pads on high-speed signals

In PCB, from a design perspective, a via mainly consists of two parts: the drill hole in the middle and the pad around the drill hole. The pad has an impact on high-speed signals, which is similar to the impact of the device’s packaging on the device. A detailed analysis is that after the signal comes out of the IC, it passes through the binding wire, pins, packaging shell, pad, and solder to the transmission line. All joints in this process will affect the quality of the signal.
However, in actual analysis, it is difficult to give the specific parameters of the pad, solder and pins. Therefore, they are generally summarized using the package parameters in the IBIS model. Of course, such analysis can be received at lower frequencies, but for higher-frequency signals, higher-precision simulations are not accurate enough. A current trend is to use IBIS’s V-I and VT curves to describe Buffer characteristics, and use SPICE models to describe packaging parameters.


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