Castellated PCBs: Introduction and Design Requirements

With the rapid development of electronic technology, electronic products are moving towards miniaturization, portability, multi-functionality, high integration, and high reliability. This leads printed circuit boards to be often designed to reuse ready-made modules.  For example, IoT Bluetooth modules or NB-IoT modules, which are indispensable communication modules, can be soldered onto PCBs just like chips. These carrier boards are characterized by their small size and a row of metalized castellated holes along some edges, allowing them to be soldered to a main PCB. This PCB assembly process is referred to as the castellated hole process in the industry.

Explanation of Castellated Holes

The following is a close-up photo of a castellated edge on a PCB:

This type of PCB has a row of metalized half-holes along its edge. These holes are relatively small and are mainly used on carrier boards. They serve as a daughter board to a main PCB and are soldered to the main PCB and component pins through these metalized half-holes.

Challenges with Castellated Holes

How to control the product quality effectively after forming the half-metalized holes along the edge of the board, such as preventing copper burrs, peeling, and residues, has always been a challenge in the manufacturing process. If there are copper burrs left inside these half-metalized holes, it can lead to weak solder joints and poor connections during component assembly, and potentially causing short circuits between pins.

In conventional production, castellated holes are first drilled as round holes, and then copper is plated into them. The challenge is to remove the other half of the hole while ensuring that the remaining half's copper wall remains intact and doesn't peel or lift.

Whether it's drilling or milling, the spindle's rotation direction is clockwise. When the tool reaches point A, the copper plating on the hole wall is pushed against the substrate by the tool bit, ensuring that there won't be any copper burrs, peeling, or residues at this point. On the other hand, when the tool reaches point B, there is no support for the copper from the hole wall. As the tool rotates forward, its forces cause the copper to curl in the direction of tool rotation, leading to copper burrs and residues. These factors directly affect the customer's installation and use.

To achieve the above goals, JLCPCB have conducted numerous exploratory experiments and have become proficient in the castellated hole process. After drilling and copper plating, we use a two-step drilling/milling process to ensure that half of the metalized hole (slot) is retained. In simple terms, the castellated hole is cut in half along the board's edge while ensuring the integrity of its plating, allowing customers to solder and use it. At present, the castellated hole process is well-established at JLCPCB.

Reasons for Increased Costs for Castellated Boards: The castellated hole is a special process. To ensure that there is copper inside the hole, we need to mill the board's outline halfway through the drilling process, resulting in extra manufacturing costs. Moreover, castellated boards are generally small in size, making them more expensive than standard PCBs. Special designs require non-standard prices.

Design Requirements

Basic Dimensions

A. Castellated pad edge to board edge distance ≥ 1 mm.

B. Castellated hole diameter size ≥ 0.5 mm.

C. Castellated hole edge to hole edge ≥ 0.5 mm.

D. Castellated hole annular ring 0.25 mm (absolute limit 0.18 mm). Engineering will change any values below this. If there are special requirements for pad-to-pad distance, please specify when ordering and confirm the production files.

Note: If widths smaller than 0.25 mm are required, please leave a note with your order, and select “confirm production files” to later confirm any adjustments made.

Pad Shape and Placement

Castellated holes can be designed as round or oval (with round or square solder pads), but attention should be paid to positioning the solder pads in the internal area of the board outline. For designs with only one-third of the holes inside the board, as shown in Figure 3, this design cannot meet the production process; at least the holes must be positioned on the centerline of the board outline. In addition, oval castellated holes parallel to the board edge cannot be processed, as shown in Figure 6 below.

Panelising Castellated Boards

For castellated boards, single-board dimensions must be ≥ 10 mm (regardless of whether panels are used), and castellated boards can also use the V-cut and mouse bite panelization methods.

One point to emphasize is that V-cut shaping should not be used on castellated edges (because the V-cut tool will pull on the copper, causing it to detach from the hole). Instead, CNC milling should be used for shaping.

V-Cut Panels (Non-Castellated Edges Only)

Mouse Bite Panels

Four-sided castellated hole panel

Ensuring high-quality PCBs is essential to bring your PCB designs to life. At JLCPCB, we are committed to maintaining top-notch production standards by investing in cutting-edge equipment and collaborating with leading raw material suppliers worldwide. With five intelligent self-owned production bases, we are able to lower production costs resulting from large-scale effects on manufacturing so that to provide affordable hardware innovation opportunities to our customers.