Printed circuit boards have continued to be the core of most electronic devices. The manufacturing of these boards requires some steps. The PCB slot is a vital aspect of the PCB design. Manufacturers must understand the PCB slot before commencing on any device.
The presence of slots in a circuit board has several benefits. The effect of slots in circuit boards can’t be underestimated. PCB slots are available in different types. Therefore, it is important to have detailed knowledge about these slots.
What is a PCB Slot?
A PCB slot is a hole in the circuit board that is too big to be formed by normal drilling methods. Manufacturers need to cut out these holes with a routing bit during the fabrication of the PCB. A slot PCB can either be non-plated or plated.
Types of PCB Slot
The two major types of PCB slots are the plated slot and non-plated slot.
Plated slot
In PCB manufacturing, a plated slot has copper plating. The plated slot is the type that has no circular shape. A PCB slot that features copper on the bottom and top is a plated one. This type of slot is ideal for electrical connections. Plated through hole slots are ideal for component packaging. Multilayer PCB always feature this type of PCB slot.
In PCB manufacturing, there are several component types for circuit boards assembly. The majority of through-hole part outlines feature round holes to take square or round leads. Most through-hole assemblies match this configuration. However, several parts use rectangular leads that don’t fit into square or round holes. Therefore, a plated slot is a better option.
Big blade-style connectors should incorporate plate through slots as the pins’ size increases. When the parts are small, manufacturers can make use of round holes featuring rectangular pins. However, round holes use up more space on the circuit board. Plated slots are more ideal for size-constrained designs.
Non plated slot
The non plated slot features a hole bigger than the pad’s copper size. Sometimes, there might be no copper. In a non-plated slot, the copper of the pad overlays. The manufacturer drills the non-plated slot after the electroless copper process.
Defining PCB Slot on Circuit Boards
If you are including slots in your PCB design, it is better to put them in Gerber mechanical layer. This layer is the safest way that shows the slots and the profile of the circuit board. There are two ways to do this:
- Use flashes or draws featuring the right end size of the slot
- Use a 0.50mm line to draw the slots. The line helps to analyze the clearance of the copper to the circuit board edge.
You can combine the slots’ definition with the PCB contour into the Gerber file. The mechanical layer must be with the copper layers. However, ensure the copper layer also features the PCB outline. If no mechanical layer is available, you may have to use another layer.
Don’t define slots in a legend layer or copper layer only. This is because they are easy to misunderstand. Indicate large cut-outs in a legend or copper layer. However, ensure you put a clear outline. Also, it is very important you indicate the slots in a README file. Do this when you are skeptical about the right one.
Some CAD systems enable PCB manufacturers to define slots in the drill file. However, the manufacturer must define the slots using an X and Y dimension. The X and Y dimension refers to slot width and slot length respectively.
What is the Smallest PCB Slot?
The smallest width for a slot of rigid-flex and flex circuit is 0.50mm. While the smallest length for a slot is 1.0mm. This is because of the thickness of a rigid-flex and rigid PCB. Therefore, the mechanical NC milling should create the slots. The thicknesses of a flex circuit are thin. So, a laser can cut these slots.
If the length of a slot is longer, the slot will be straighter in length. Note that, the smallest length of a slot is 2 times of the width of the slot. For instance, if the width of a slot is 0.60mm, the length of the slot will be 1.20mm.
Slots Milling
The manufacturer mills out the slots from the rigid PCB material. Then, the PCB fabrication makes use of a NC grooving cutter bit. This cutter functions like a CNC machine. Also, the cutouts’ inside corners will feature a round edge to them. Make sure you keep an eye on this in enclosure designs.
The smallest radius for inside corner is 0.50mm. The smallest tool for routing out circuit boards is 1.0mm. Milling slots create air gaps for voltage isolation on the circuit board.
How to Use Eagle to Create Slot PCB
You might wonder if it is possible to create slots in Eagle. Slotted holes are popular in PCBs since components need to manipulate rising current. Many components feature wide pins. This is because a wide pin provides a component with more mechanical integrity.
Manufacturers can create slot PCB. It is important to use a through hole pad that has a diameter that fits your slot. An ideal pad for this operation is Oblong pad.
Draw the slots
Draw the slot outlines to draw your slotted holes. You can do this in a different way. Using the dimension layer is a great way to do this. However, this method has its limitation. The autorouter can’t get to the inside of the pad. Thus, there will be some dimension errors.
Ensure you set the pad’s drill size to fit within the slot PCB outline. This will help prevent any confusion and ensure a better result. The PCB manufacturer will mill the area within the outline from the board.
Get mill data to board manufacturer
Ensure you export a Gerber file to your manufacturer. While exporting the file, include a note. This note will specify that the manufacturer must mill the contents on the Gerber file from the board.
Defining Plated and Non-plated Slots in PCB design
The PCB fabrication process involves etching a copper sheet. This sheet is then plated on a substrate and holes are drilled on it. Before the plating process, these slots are non-conductive. Manufacturers use the electroless deposition process for PCBs with over one copper layer.
However, designers must know that not every slot will need plating. Therefore, designers need to abide by the rules of manufacturers to know which slot will be plated. Sometimes, designers expect some features in the circuit boards. However, they get an entirely different board in the end. Mounting slots are examples of such.
For instance, there will be issues if your design depends on a certain slot being plated. Unfortunately, you realize that the slot hasn’t been after receiving the board. Therefore, we will point out the difference between plated slots and non-plated slots.
How to consider a slot as plated
Before a slot is considered plated, it must meet certain conditions. Otherwise, it is a non-plated slot. In a plated slot, the copper of the pad alongside the solder stop mask must overlay. The pad’s copper must be more than the slot with at least 6 mil in width.
How to consider a slot as non-plated
Sometimes, some PCB slots are non-plated due to a mechanical or electrical reason. Any slot that features properties that don’t meet the plated slot conditions is a non plated slot. A slot PCB is non-plated, when the hole is bigger than the pad’s copper size. Furthermore, when the copper of the pad overlays and is bigger than the hole. However, there is a space clearance of 6 mil between the hole and the copper. Such conditions meet the requirements of a non-plated slot.
Applications of Non-plated and Plated Slots
- Milling slots create air gaps for isolation of voltage on circuit boards
There can be temporary electrical arcs between traces in a circuit board featuring high voltages. Repetition of electrical arcs can result in PCB carbonation. This can cause a short circuit in the long run. As a result of this, PCB designers include a milling PCB slot between suspect traces.
- Plated slots are ideal for parts featuring square or rectangular leads
These slots fit in for parts with square/ rectangular leads rather than the rounded ones. The footprint for slots is better than for large holes. This is because the space between the wall of the hole and the lead must contain enough solder.
Best Practices for Plated Slots
For a PCB assembly, there are several component types and footprints. Designers use circular holes to design most through-hole part footprints. This is because the circular holes can accommodate square or round leads. This is an ideal configuration for through-hole components. However, many parts use rectangular leads. These leads don’t fit into square or round holes. It is better to incorporate a plated slot footprint.
When the footprint leaves more space around the component leads, the circular hole configuration will be vulnerable to some defects. This can lead to solder joint voiding. This type of defect occurs when more solder fills the holes. Manufacturers become more concerned as the pins’ size increases. Therefore, large rectangular connectors should incorporate plated slots instead of circular holes.
Some manufacturers can work around circular holes for rectangular pins. Also, the circular holes end up taking more space on the circuit board itself. Using big circular holes is not ideal for layout demanding dense component populations. Incorporating plated slots can provide solutions in size-restricted designs.
PCB designers need to consider a few things while designing non-circular holes. However, the design process of non-circular holes is straightforward. Plated slots are commonly utilized in component footprints. Most programs for PCB layout feature an option to define a hole as oval or circular. In addition, some clients identify slot holes on the design layer of their Gerber files.
Designers can also design non-plated slot PCB in a similar way. They can then designate it as Mechanical or NPTH in the CAD software. They can also design them on the Board Outline Gerber layer. It is important that manufacturers pay attention to the design rules of PCB slot.
What to Consider When Creating Plated Slots
Designers need to define some things in the EDA tools to create plated slots. Most EDA tools allow designers to create plated slots through a manual process. Here, the designer identifies all the aspects of the hole rather than placing a standard pad. The three most important factors to consider when creating a plated slot are;
- The shape of the hole
- The copper’s shape on the top layer
- The copper’s shape on the bottom layer
Plated slots differ from other holes in PCB designs. This is because they are drawn on the board outline layer. When PCB manufacturers receive manufacturing files, they interpret the board outline as cutting information. The manufacturer will route the board inside the already drawn shape. This will be done if there is a shape on the board outline layer.
A drawn hole inside the pads on the bottom and top of a PCB is interpreted as a plated slot. Therefore, designers need to draw the intended holes in the footprint on the board outline layer. The slot will display in the board outline file when producing the manufacturing files. Plated slots enable designers to make use of parts with non-circular pins and leads.
Frequently Asked Questions
What is the size of the plated slot on PCB?
0.5mm is the minimum size for a plated slot. If manufacturers realize the through-plating within the PCB, it is a plated-through slot. While through-plating on the outer edge of the PCB is sideplating.
What is the benefit of a PCB slot?
A PCB slot helps to connect the circuit board to other PCBs. It also helps to connect the board to the chassis of a device.
Can slot PCB increase the price of the overall circuit board?
No, slots in a PCB will not increase the price of the overall board. It will not also increase the lead time of your project.
Conclusion
Slot PCB is a vital aspect in PCB design. It is very important to understand how these slots work in PCB designs. The plated and non-plated slots have their function. However, plated slots are more popular in the PCB industry.
