Printed circuit boards remain at the heart of most electronic appliances, and their manufacture involves several steps. A crucial component of PCB design is the PCB slot, and manufacturers must have a thorough understanding of it before starting the production of any device.
The benefits of incorporating grooves in a printed circuit board are numerous and should not be underestimated. There are various types of PCB slots, making it essential to have an in-depth understanding of these slots.
What are PCB Plated Slots?
A slot in a PCB is a larger opening that cannot be created through conventional drilling techniques. It must be cut out using a routing tool during the production of the printed circuit board. These slots can either be plated or unplated, depending on the manufacturer’s specifications.
Types of PCB Plated Slots
The two primary types of printed circuit board grooves are plated & unplated.
· Plated slot
In the production of circuit boards, the plated slot has a layer of copper coating. This slot type does not have a circular shape and features copper on both the top and bottom surfaces, making it best for electrical linkages. The Plated through-hole slots are suitable for component encapsulation and are a common feature in multi-layer circuit boards.
In the manufacture of circuit boards, various components are used for PCBs assembly. Most through-hole components have circular holes to accommodate either round or square leads. This configuration is prevalent in most through-hole assemblies. However, some components use rectangular leads that cannot fit into circular holes. In these cases, a plated slot is a more appropriate choice.
For large blade-style connectors, it is advisable to use plated through slots as the size of the pins increases. Smaller components can use round holes with rectangular pins, but these holes consume extra space. Plated grooves are more suitable for designs with limited space.
· Non plated slot
The unplated slot has a hole that is larger than the size of the copper pad, and in some cases, there may not be any copper. In an unplated slot, the copper of the pad overlaps. This sort of slot is produced by drilling once the electroless copper process has been completed.
Slots in the Printed Circuit Board profile
The most straightforward method of defining the slots is to display the slots/cut-outs and the PCB profile in a Gerber mechanical layer. If your PCB design software does not have a mechanical layer, an additional layer is required. If you are unsure which layer is the correct one, kindly indicate the slots in a README file.
To ensure safety, it is advisable to define the slots in the drill file using X and Y dimensions in specific CAD systems. Defining the slots in only the copper or legend layer is discouraged as it may result in misunderstandings or oversights on the part of PCB manufacturers. For optimal results, kindly inform our engineers or mark any slots or cut-outs on your PCB when placing an order.
What is the Smallest PCB Slot?
The minimum width for a groove on a rigid-flex & flex PCB is 0.50 millimeters, with a minimum length of 1.0 millimeters. It is due to the rigidness of rigid-flex and rigid PCBs, which requires the use of Mechanically controlled numerical control milling for creating the slots. On the other hand, flex circuits are thin, allowing them to be cut using a laser.
A longer slot length results in a straighter slot. Remember that the minimum length of a slot must be two times the width of the groove. For example, if the width of the groove is 0.6milimetre, the length of the slot should be 1.20mm.
Slots Milling
The manufacturer removes the grooves from the rigid printed circuit board material through milling. The process involves the use of a Non-computer Numeric Control grooving tool which operates similarly to a CNC machine. Note that the inside corners of the cutouts will have a rounded edge. It is crucial to consider this aspect in enclosure design.
The minimum radius for an inner corner is 0.5millimeterss, while the smallest device used in routing PCBs is 1.0 millimeters. The milling of slots creates air gaps for the voltage on the PCB.
Plated & Non-plated Slots in Printed Circuit Board design
The printed circuit board fabrication procedure involves etching the copper sheet, which is then placed onto a substrate and drilled with holes. Before plating, these slots are non-conductive. For PCBs with multiple copper layers, manufacturers utilize the electroless deposition process.
Designers should be aware that not all slots require plating. Designers must follow the guidelines provided by manufacturers to determine which slots will be plated. In some cases, designers may have certain expectations for their circuit boards, only to receive a final product that differs significantly, such as with mounting slots.
For example, if your design relies on a specific slot being plated and it is not, this can lead to problems. To avoid such issues, it is essential to distinguish between plated and non-plated slots. This way, you can ensure that your design is executed correctly and avoid surprises upon receiving the final circuit board.
How to consider a PCB Slot as plated?
A slot can only be considered plated if it meets specific criteria. If it does not, it is considered a non-plated slot. In a plated slot, the copper of the pad and the solder stop mask must overlap. The copper of the pad must also be more comprehensive than the slot by at least six miles.
How to consider a PCB slot as non-plated?
Sometimes, a PCB slot may be non-plated for either mechanical/electrical reasons. If a slot does not meet the criteria for a plated slot, it is considered non-plated. For example, a slot is considered non-plated if the hole is larger than the size of the copper pad or if the copper overlays & is larger than the hole, with a clearance of 6 mils between the copper & hole. These conditions signify that the slot meets the requirements for the non-plated slot.
Conclusion
The design of slots in a PCB is a crucial aspect of printed circuit board design. It’s essential to have a solid understanding of how slots operate in a PCB design. Both non-plated & plated slots serve their purpose. However, plated slots are commonly used in the printed circuit board industry.
