The problem of near holes that cannot be ignored in multilayer PCB design

Posted on 2020-02-18

When designing the PCB board, the most consideration for wiring is how to connect the various layers to the network signal lines. The denser the high-speed PCB board lines, the higher the density of vias (VIA), and the vias can play to each layer. The role of electrical connections. Multi-layer circuit board PCB proofing often receives feedback from the board factory that “holes are too close to the line, which exceeds the process capability.” So, if the vias are too close, will it be difficult to produce and will it affect product reliability?

1. Two holes too close will affect the aging of PCB drilling process. When the first hole is drilled after drilling the second hole, the material in one side is too thin, the drill bit is not uniformly stressed and the drill bit is not heat-dissipated, which will cause the drill bit to break, which will cause the PCB hole to collapse or be missed The hole is not conducting.

3. The hole position tolerance of the drilled hole is ≤0.05mm. When the tolerance is at the upper limit, the following conditions will occur in the multilayer board.

(1) When the lines are dense, small gaps appear irregularly through 360 ° to other elements. To ensure a safe distance of 3mil, the pads may have multi-directional PAD.

(2) According to the data of the source file, the hole edge to line edge is 6mil, the hole ring is 4mil, and the ring to line is only 2mil. To ensure that there is a 3mil safety distance between the ring and the line, the 1mil solder ring needs to be cut, and the pad after cutting is only 3mil. . When the hole position tolerance offset is the upper limit of 0.05mm (2mil), the hole ring has only 1mil left

4. PCB production will produce a small amount of deviation in the same direction, the direction of the pad being cut is irregular, and in the worst case, individual holes will be broken.

5. The influence of lamination deviation in multilayer boards. Taking a six-layer board as an example, two core boards + copper foil are laminated to form a six-layer board (Figure 5-1). During the pressing process, the core plate 1 and core plate 2 may have a deviation of ≤0.05mm when they are pressed, and the inner layer holes will also have irregular deviations of 360 ° after the pressing

To sum up, the drilling process affects the PCB yield and PCB production efficiency. There is no complete copper protection around the small hole of the ring, and the PCB open and short test can pass. There is no problem in the early use of the product, but the long-term reliability is not enough.

Multilayer PCB board, high speed PCB board hole-to-hole, hole-to-line spacing:

(1) Multilayer board inner hole to wire to copper:

4 layers: you can ignore

6 layers: ≥6mil

8 layers: ≥7mil

10 layers or more: ≥8mil

(2) Edge spacing of via diameter:

Same network via: ≥8mil (0.2mm)

Different network vias: ≥12mil (0.3mm