There are many cases where a well-designed circuit with a PCB fails at the POC stage or the production stage. There is a more common reason is: PCB ground rebound. Ground bounce, where the grounding return path of the circuit does not have the appropriate copper area and causes resistance in the circuit, can be difficult to deal with op amps in this case, because they are very sensitive in terms of measurement (if used as a sensing amplifier) or for RF circuits that tend to generate EMI interference in high-frequency operation. An important design fault that leads to ground rebound is that there is no ground stitch. So let's talk a little bit more about suture holes.
1. What are suture holes
Before explaining what a suture hole is, let's understand the difference between a suture hole and a pass hole:
1.1 PCB board holes
If someone wants to connect from the top copper plane to the bottom copper plane, they will have to add a pass hole. A through-hole is simply a hole drilled in the PCB substrate during the PCB production stage, with the inner layer of the hole filled with copper to enable it to pass the copper trace through the different layers of the PCB.
1.2 PCB suture hole
Through-hole stitching, a technique used to join together larger copper areas on different layers, can actually create strong vertical connections through the board structure, helping to maintain low impedance and short return loops. Through-hole stitching can also be used to connect copper areas that may be isolated from their network to that network.
2. What is the use of suture holes
2.1 Increase the current capacity of the suture hole
The copper track on the PCB layer has a constant thickness. The amount of current a track can safely carry depends on its resistance, which in turn depends on its cross-sectional area. If the cross-sectional area increases and its resistance decreases, the track can carry a larger current. When laying a track on a PCB, the designer can only adjust its width to allow it to carry the expected current, because the copper thickness on the PCB layer surface remains the same. However, there is a limit to the track width that can be achieved, as additional tracks may also have to be laid out in the same space.
By connecting two tracks in parallel, the track resistance can be reduced to half that of a single track, effectively doubling its current carrying capacity. For this, the tracks must be orthogonal on the adjacent layers, and they must be joined or stitched together using multiple through-holes. Designers usually do not use large holes for splicing, but use a large number of small holes to reduce the overall resistance.
2.2 Suture holes improve heat transfer
As a rule of thumb, half of most electronic system failures are caused by runaway heat. By implementing better thermal management, the number of electronic system failures can be significantly reduced. The use of SMT is one way to improve heat transfer, but the thickness and area of the copper foil on the board and its composition material and thickness have a greater impact. Typically, the heat generated in the active device core moves down to the thermal pad and then transfers to the PCB material. Multiple heat dissipation mechanisms can be combined to eliminate the heat generated by the components on the PCB. Typically, these take the form of a horizontal heat conduction path through the copper foil, followed by a vertical heat conduction path through the heat dissipation hole, and finally escape from the strategically placed heat sink.
Tightly spaced or stitched hot through holes provide a low thermal resistance path from the top copper side of the PCB to the bottom copper side. On this side, a radiator on a copper plane helps dissipate heat into the surrounding air. The thermal resistance of the holes depends directly on their number and location. Placing the through-hole closer to the heat source reduces thermal resistance and significantly improves heat dissipation. These heat dissipation holes are used to connect the top and bottom copper layers of the PCB, but they can also be connected to multiple layers.
2.3 Improve signal integrity
By closing the track carrying the high-speed signal with a tightly stitched through hole, its impedance can be precisely defined. To this end, the designer closed the wiring with brazing pads on both sides and placed a ground plane at the bottom. The two brazing pads are connected to the bottom ground plane through several stitched holes, effectively turning the track into an external microstrip line.
Place a ground plane in the layer above the track and connect it to the two brazing pads and bottom ground plane by more stitching holes, turning the track into an internal microstrip line. Because the through-hole and ground layer shield the wiring, they closely define the impedance of the wiring, and the wiring can transmit high-speed signals while improving signal integrity. The same plan applies to isolating the memory bus, masking the analog portion, and any circuit that handles the sensor. Defining the edges of the circuit with through-holes helps prevent irregular shapes from inadvertently becoming radiating antennas.
2.4 Stitched holes improve EMI/EMC
Fully stitched through holes around the PCB helps improve its EMI/EMC characteristics. A copper plane was placed that was roughly the same shape as the board, but a few hundredths of an inch smaller than the outline of the board. Use a rule of thumb called the 3H rule to keep any signal layer three times the dielectric thickness away from the nearest ground layer. This method helps to control stray emission. This design rule results in the outer edge of the PCB containing only the ground shape, while all other copper wires remain inside, away from the edge
By splicing the top and bottom two ground planes with multiple holes, the entire PCB forms a locked Faraday cage, drastically cutting all harmonics that reduce FCC compliance requirements.
The spacing of the suture holes depends on the frequency at which they must be suppressed and the capabilities of the contract manufacturer. If the application requires very tight through-hole spacing that may be difficult for contract manufacturers to achieve, you can place a second row of grounded through-holes within the first row.
3. Difference between suture hole and shield hole
PCB design there are different types of through the hole stitching process, here are mainly three, through the following instructions you can distinguish the difference between the suture hole and the shield hole.
3.1 The grounding is fixed by stitching
This is the most commonly used Via splicing technique in most PCBS. The ground layer is completed by stitching to ensure that the ground return path in the PCB is shorter from the load device to the power supply. As a result, it maintains a healthy ground return path, resulting in low resistance in the ground layer. If the design supports more than two layers of copper plane, because the copper coating is larger and connected to the top and bottom or other layers, it produces lower heat dissipation, resulting in low drop resistance. It balances the copper resistance at all locations in the PCB.
Therefore, if one measures the voltage drop between ground strata at different locations, the different voltage drop will cause the grounding rebound problem due to the different resistance. The stitching hole is very efficient and requires very little effort compared to debugging ground rebound faults in PCBS.
In this example, we used the through-hole splicing technology on the ground plane to make multiple PCBS that were successful in the working phase. Just to give you an example, one of them successfully used Via concatenation. Here are some images shown by splicing, highlighting them in red for better understanding.
Not only in the ground plane, it can also be used in other places where perfect copper coating is required. Suture holes are used for different layers outside the ground plane.
3.2 Hot through hole suture
If designed properly, PCB-based heat sinks can be more useful in most situations. A key component of PCB-based radiators is the hot-through-hole stitching. Several projects have been carried out in which the use of hot-through-hole stitching provides excellent thermal conductivity on multiple copper planes (top and bottom).
In the PCB above or other PCBS, it is very helpful to distribute heat over the copper plane. Because PCBS are more conducive to the layer where high power components are located. It gets too hot and the heat is only distributed from the side, while the PCB core and another opposite layer stay cooler than the active wiring. Stitching at this point gives it better thermal conductivity, which further dissipates heat to the core and further to the relative plane of the connection, thereby reducing the overall junction temperature of the target high-power component.
3.3 Shielding Holes
For EMI related reasons, through-hole shielding is performed on high-frequency RF or mixed-signal circuits, primarily in WiFi, Bluetooth, and other wideband high-frequency components that may be affected by EMI interference. It is also known as PCB fence.
Typically, it is created using single or multiple rows of through-holes stitched around the perimeter of a large copper plane that is too close to the high-frequency rail. The design given below uses shielding holes on a 4-layer circuit board.
As we can see, it is done at the perimeter of the ground plane too close to the WiFi module antenna. In general, it is recommended to use shielded through-hole splicing on RF boards, where the spacing of the through-holes is at least 1/10 of the target wavelength of the highest frequency for which the shielded interface is required. In some practices, it is also done using 1/8 of the space. But the main attention is to keep the through-hole spacing smaller than the wavelength in the substrate dielectric.
4. How to operate the suture hole separately
4.1 Application of suture holes
Most EDA tools will provide some mechanism to automatically stitch two copper castings together. The first decision for users is whether to include islands in the copper flood. An island is a copper-dipped area that is not connected to the network on its layer, but will gain a network connection when stitched to other copper-coated areas. In Proteus Design Suite, the suppress island check box is used to control copper coating when the power layer is placed.
Once the flooding area on both layers is defined, the user invokes the via stitching command. Typically, the configuration dialog will allow you to select the through-hole style and the minimum gap between the through-holes, and then the software will take care of the through-hole placement. When the configuration is complete, this usually stitches the entire common area of both planes.
While this may sound obvious, one of the most common user mistakes is stitching flat surfaces together too early in a layout package. A fully stitched PCB will make wiring more difficult, so unless there is a good reason, stitching should always be performed after wiring is complete.
4.2 Application Shielding holes
Shielded holes, sometimes called through-hole fences or picket fences, follow the same principles as sewn through-holes, but tend to consist of a single row of peripheral through-holes around a track or copper-clad boundary. These are almost always used to isolate areas of the board that operate at different frequencies and are used for EMI control as described above.
Procedurally, you will apply the mask through holes in almost the same way. If the through-hole is applied to the copper-clad perimeter, you can choose which track or power layer to shield. The via shielding dialog box is then launched from the context menu and configured in the same way as via stitching.
4.3 Precautions for suture hole application
(1) PCB manufacturing
If the board will be very hot when operating at high current, it is best to explore other materials that can withstand higher operating temperatures. Although these materials may be more expensive, they may avoid heat-related problems in the long run, which ultimately saves money. You should also work with your manufacturer to develop the best layer stacking configuration and power plane strategy for high current boards.
(2) Board thickness
By increasing the thickness of the board, you can increase the weight of the copper, thus making the wiring thicker. This may allow for a reduction in the line width, allowing more space for wiring and component placement. As with any manufacturing issue, these changes should be agreed with your manufacturer before incorporating them into the design.
(3) Automatic assembly
As we have seen, higher currents require more metal for both electrical and thermal reasons. But at the same time, the same metal that emits unwanted heat during operation can also cause problems for PCB assembly. Large areas of metal can cause a thermal imbalance on smaller parts, which can affect their welding. To avoid this, be sure to use cooling devices when connecting parts directly to wide wires or large areas of metal.
(4) Component placement
If it can be avoided, components carrying high currents and heat should not be placed on the edge of the board. By placing these parts closer to the center of the board, there is more room for the heat that the board naturally emits.
The above is a brief introduction to the PCB suture hole, I hope it can be useful to you, and welcome your advice.
English
Chinese