Printed circuit boards (PCBs) continue to be at the core of most electronic devices, and their design involves a series of intricate processes. A vital characteristic of PCBs is the slot feature, which plays a pivotal role in electronics engineering. Design engineers tasked with building electronic gadgets must fully comprehend the importance of PCB slots and become knowledgeable about the various types available. The incorporation of diverse slot designs on a circuit board provides numerous advantages, making it crucial to possess an in-depth understanding of this area.
What Is a PCB Slot ?
As PCB design engineers, we understand that slots in circuit boards are essential for accommodating the passage of electrical components, typically with a diameter of 0.5 millimeters. Unlike other types of holes, slots occupy less space and are frequently utilized to connect boards to each other or to a chassis. The inclusion of slots in the circuit board does not significantly increase production costs and can be accomplished at an affordable price.
To avoid misinterpretation, it is crucial to accurately define slots. The first step in defining a slot is to specify its dimensions. To achieve this, computer-aided design (CAD) software is used to determine the hole size, ensuring that the holes do not intersect before defining the slots using X and Y dimensions. Defining the geometry of the PCB slot is another crucial stage in the process.
During PCB construction, careful evaluation should be given to whether or not the slots should be plated. Most PCB design software provides options for hole definition, allowing the geometry of the slot in the design to be determined. Unplated slots can also be incorporated into printed circuit boards.
In addition to dimensions and plating, PCB slot depth must be considered, as it determines the maximum current that can flow through the circuit. While designing a PCB slot, engineers must take into account both the depth and length of the slot, with the size of the copper pad used for the PCB slot determining how deep the slot goes into the circuit board.
Choosing the most suitable slots for your PCB design requires careful consideration. However, if you require further assistance or access to resources, please do not hesitate to contact us.
PCB Slot Fabrication and Assembly Considerations
PCB slot fabrication and assembly considerations include:
● Routing vs. Punching: PCB slots can be fabricated using either routing or punching techniques. Routing involves cutting the slot using a milling tool, while punching involves stamping or punching the slot out of the board. The choice of technique depends on factors such as the slot size, the board material, and the required precision.
● Copper Plating and Mask Application: After the slot is cut or punched, the inner walls of the slot may be plated with copper to improve electrical conductivity. A solder mask may also be applied to protect the copper plating and prevent solder from flowing onto the slot during assembly.
● Board Thickness Consideration: The thickness of the board is a critical consideration in PCB slot fabrication and assembly. The slot should not compromise the structural integrity of the board.
● Slot Edge Clearance: The distance between the edge of the slot and the board edge must be considered to prevent any mechanical stress on the slot.
● PCB Assembly and Testing: PCB slots can be challenging to assemble and may require additional care during the assembly process. Before the final testing, the connections through the slot should be tested to ensure proper functionality.
These are some of the critical fabrication and assembly considerations for PCB slots. The specific considerations may vary based on the requirements of the design and the chosen fabrication and assembly techniques.
Design Guidelines for PCB Slots
● Slot Placement and Orientation: The positioning and orientation of the slot should be carefully considered to avoid any conflicts or interference with other components on the board.
● Slot Edge Clearance: Sufficient clearance between the slot edges and other components should be provided to prevent any mechanical stress on the slot during assembly.
● Slot Shape: The shape of the slot could be circular, rectangular, or any other shape depending on the component package or the function of the slot.
● Avoidance of Slots on High-Speed Signal Traces: Slots on high-speed signal traces could negatively impact signal quality, and as such, should be avoided.
● Slot-to-Trace Spacing: A sufficient amount of space should be provided between the slot and adjacent traces to avoid any signal crosstalk or interference.
● Copper Pour: Copper pour around the slot can help reduce electromagnetic interference (EMI) and protect the signals and traces.
These are some of the essential design guidelines to consider when designing PCB slots. The specific guidelines may vary based on the specific requirements of the design and the chosen fabrication and assembly techniques.
Slots and Cutouts
As a leading PCB manufacturer, JarnisTech specializes in producing PCBs that feature plated and unplated slots and cutouts, as well as offering through-hole assembly services. Our vast experience has exposed us to various types of slots, cuts, footprints, and components. For through-hole components, most footprints are designed with circular holes to accommodate circular or square leads, like those used with the USB 2.0 connector. While this layout is appropriate for most through-hole components, there are instances where rectangular or “blade-style” leads are used, which are not ideally suited for circular or square holes. As a result, a plated slot footprint should be utilized.
What Is the Smallest PCB Slot ?
In the world of printed circuit board (PCB) design, achieving a slot width of 0.50 mm is considered the smallest possible width for slots in flex circuits or rigid-flex circuits, with the shortest length being 1 mm. This is because rigid-flex and rigid PCBs tend to have greater thickness compared to flexible PCBs. As a result, slots for such boards need to be created using mechanical NC milling for precision purposes. However, due to its relatively low thickness, a laser may be utilized to create slots in the case of a flex circuit.
Proper slot length is also crucial in ensuring a well-designed PCB. In this regard, increasing the length of the slot leads to a straighter configuration, which is an important aspect in maintaining the overall integrity of the circuit board. Moreover, it is necessary to remember that the minimum length of a slot should be twice its width. For instance, if a slot width is 0.60mm, the minimum slot length should be 1.20mm to ensure a properly designed slot.
Why Is the Minimum Annular Slot Not the Minimum Drill Hole ?
During the production of printed circuit boards, annular slots can be created through the milling slot procedure. Milling tools, as opposed to drilling tools, have larger diameters, which enable them to produce longer annular slots and wider drill diameters. In some cases, the minimum dimension for annular slots is 0.8mm, while the minimum drill distance is 0.3mm. Thanks to their capabilities, milling tools can cater to longer slot production and larger drill diameters.
How to Define Slot on PCB ?
Mechanical Layer: Incorporating slots or cutouts into your PCB design requires careful attention to detail. The Gerber mechanical layer is the optimal location for displaying these features and the profile of the PCB, providing a secure and comprehensive method for presentation. There are two options for displaying the slots/cutouts on this layer:
● Utilize draws and/or flashes with appropriate slot/cut-out end sizes.
● Draw the slots/cutouts with a 0.50mm line to aid in visually inspecting the clearance between copper and the PCB board edge. The slot’s edge will be defined as the center of the line.
When combining the definition of slots or cutouts with the PCB’s contour (outline) in the Gerber file, ensure that this layer aligns with the copper layers, but also verify that the copper layer contains the PCB outline.
If a mechanical layer is not available, it may be necessary to adapt another layer. Avoid specifying slots in just a copper layer or in a legend layer to avoid confusion or missing them entirely. Large cut-outs in a copper or legend layer are possible, but ensure that there is a clear outline and the text “CUTOUT” is placed in the center of the layer.
In the drill file, it is possible to define slots in certain CAD systems. These should be characterized as slots with X and Y dimensions, rather than a series of overlapping holes. The X and Y measurements refer to the slot’s width and length, respectively.
When it comes to plating, slots and cuts with copper plating on the top and bottom are considered plated; otherwise, they are unplated. For unplated components, the copper should be pulled back at least the specified distance from the copper to the board’s edge to indicate the change in design files. The finished slot’s size should be provided in inches, and any necessary plating adjustments will be made accordingly. For non-plated slots through copper pads, specify this in the mechanical layer, a separate drill file, or in the file name.
PCB Slot: Types
When it comes to PCB design, the type of slot needed depends on your intended use. There are several types of slots that can be incorporated into your PCB design:
● Plated slots: These types of slots are often plated with copper and are frequently used in multilayer PCBs. They typically have a non-circular shape and are ideal for electrical connections and component packing. They are designed to accommodate round slotted holes that fit into a round or square lead. Plated-through slots are particularly useful for larger blade-style connectors that utilize pins of increasing size.
● Non-plated slots: Unlike plated slots, non-plated slots feature holes that are larger than the copper size of the pad, which means that copper is not always present. However, the copper overlays due to the electroless copper process carried out by the manufacturer before drilling.
● Metallic annular slots: These slots are typically drilled in the copper foil of a PCB during the metalized plating process. In order to add non-metal drill slots to a copper foil, a new mechanical layer must be defined and called “NPTH“.
● Non-metallic annular slots: These slots require the use of a keep-out or mechanical layer and can be created by specifying a 0.8mm line width on the mechanical layer or by marking out a box on the dimension layer. It is recommended not to use the silkscreen layer for this purpose. In complex slot designs, it is essential to use clear markers or images to highlight key locations.
Things to Think About When Making Plated Slots
When it comes to producing plated slots in PCB design, designers must specify certain parameters in EDA software. Most EDA technologies allow designers to manually create plated slots by providing details for each hole instead of installing a typical pad. Creating a plated slot involves considering these three factors:
● The shape of the hole.
● The shape of the copper on the top layer.
● The shape of the copper on the bottom layer.
Plated slots differ from other types of holes found in PCB designs because they are drawn on the layer that includes the board outline. This is crucial because when PCB manufacturers receive the manufacturing files, they interpret the board outline as cutting information and proceed accordingly. The manufacturer will route the board to fit within the previously defined shape.
Plated slots are essentially read as holes drawn inside the pads on the top and bottom layers of a printed circuit board. Therefore, designers must create the intended holes in the footprint on the board outline layer. When manufacturing files are generated, the slot will appear in the board outline file. This capability allows designers to use components with non-circular pins and leads during the design process.
Non-Plated and Plated Slots: Applications
In the design of electronic circuits, electroplating slots are commonly utilized, with copper plating present on the circuit layer of such slots. Conversely, non-plated slots lack copper plating on their circuit layer and require two copper solder pads for connection, as the solder resist layer does not feature openings for non-plated slots. For the mechanical layer, non-plated slots are typically milled. To assist designers in determining copper tolerances for both plated and non-plated slots, a 0.50mm thick line is added around the edges of such slots, with the center of the line representing the slot’s edge.
Plated baths are costlier and complex to construct than unplated tubs, although non-plated slots are more user-friendly and less expensive due to the absence of copper covering. Plated slots, albeit more expensive and challenging to produce, provide stronger and more durable connections, making them a preferred choice in applications with higher density. Non-plated slots, on the other hand, are commonly used in motherboards, mobile phones, defense and aviation electronics.
Ultimately, when comparing plated and non-plated slots, the main difference is that non-plated slots offer greater user-friendliness and long-term stability.
Slot PCB: How to Use Eagle Software to Design ?
If you’re using Eagle for PCB design, you may be wondering whether the software allows the creation of slots. Slotted holes are common in PCBs as some components require the ability to manipulate rising current. Components with wide pins, in particular, have a higher level of mechanical integrity, making them a preferred choice in various applications.
To create a slot PCB, manufacturers need to work with a through-hole pad that has a diameter that corresponds to the width of the intended slot. An oval-shaped pad is ideal for this operation.
To draw slotted holes, designers can use various methods, including utilizing the dimension layer. However, this method may have some limitations, as the autorouter may not access the inside of the pad, leading to dimension errors. To ensure accurate results, it’s important to make sure that the drill size for the pad fits inside the PCB slot’s outline.
Once the design is complete, designers should send the Gerber file to the manufacturer, along with a note specifying that the contents of the file must be milled from the board. This will help ensure that the final product aligns with the intended design.
Applications of PCB Slots in Different Industries
PCB slots have various applications in different industries. Some of these applications are:
● Consumer Electronics: PCB slots are used in consumer electronics such as smartphones, tablets, and laptops for integrating different components like SIM cards, memory cards or other accessories.
● Medical Devices: PCB slots are widely used in medical device circuitry to integrate sensors, probes, and electrodes for diagnostic or therapeutic purposes.
● Industrial Automation: PCB slots are used in industrial automation for interfacing and connecting various types of sensors, actuators, or other control components used for monitoring and controlling different processes.
● Communication Systems: PCB slots are used in communication systems such as routers, servers, and switches, for integrating various connectors such as Ethernet cables, optical fibers or other interfaces.
● Automotive Industry: PCB slots are used in the automotive industry for integrating different circuit components like sensors, cameras, and other electronic gadgets used for monitoring and control of various systems.
These are just a few examples of the many applications of PCB slots in different industries, and the actual applications may differ depending on the specific requirements of each industry.
Slot PCBs are a crucial component in the design of printed circuit boards (PCBs). To create high-quality PCBs, designers must possess a comprehensive knowledge of how slots function. Both plated slots and non-plated spaces serve a purpose, but plated slots are the more widely used type in the PCB industry.
At JarnisTech, we provide full-service solutions for printed circuit board (PCB) manufacturing and assembly, catering to the needs of customers with prototyping orders as well as large-scale production needs. Our strategically located factory network across China enables us to provide capacity where and when it is needed the most by our valued customers.
With our innovative digital supply chain solutions and cutting-edge technology platform, JarnisTech unlocks the future of EMS manufacturing. We guarantee faster, more efficient, and logistically streamlined electronics manufacturing than ever before, ensuring customer satisfaction with high-quality PCBs.