Panelization Guidelines from Rush PCB

Rush PCB uses panelization as a popular method of effectively producing small circuit boards in large quantities. Panelization improves manufacturing efficiency and throughput, meaning it substantially reduces manufacturing time and costs per board.

What is Panelization?

PCB panelization is the process of fabricating small circuit boards by making them a part of a single, larger array. Rush PCB uses this technique as our production machines are unable to process boards below a specific size. We solve this problem by a process of step and repeat, placing the smaller boards in an array to form a large panel. After manufacturing the panel, we break the individual boards apart along predetermined fault lines.

Advantages of Panelization

PCB panelization offers high efficiency in mass production. Additional advantages of panelization are reduced shock and vibration to each PCB, and improvement in cost-effectiveness.

As fabricators, Rush PCB uses machines that operate with standard-sized panels. Therefore, creating arrays in these standard sizes lowers our production costs significantly.

For PCB panelization to be successful, designers must consider multiple design specifications. This includes following some best practices in panelization methods, which we will detail in this set of PCB Panelization guidelines.

Important Factors in PCB Panelization

Design: Requirements for products can vary widely, with significant limits on internal circuit board sizes. Some boards may have unusual shapes, using curves rather than the usually popular straight edges. Panelizing such small PCBs can help to reduce waste.

Determining the most appropriate panelization method depends to a large extent on the design of the board. For instance, presence of components hanging over the PCB edge, and the amount of clearance from the edge of the board to the nearest component may make certain panelization methods much less suitable compared to others.

Components: Manufacturing subjects boards to shocks and vibrations, with smaller boards being more susceptible. Designers compensate by using stronger and sturdier components that can endure the shaking.

The larger size of panels minimizes shocks and impacts to individual boards, allowing designers to use smaller parts without compromising the individual board integrity. Although production panel sizes vary, most manufacturers use a standard size of 18 x 24 inches (450 x 600 mm).

Materials: Panelization does allow the designer to use more affordable materials, but as some materials are more prone to splintering during the breakout process, they limit the type of panelization method most appropriate for the board. PCB thickness is also a matter for selecting the type of panelization method, as thin boards have a tendency to crack during assembly, and breaking apart thick boards may prove more problematic.

The above factors limit the choices available to the designer and the fabricator for any one type of PCB. In reality, Rush PCB uses a combination of methods for a PCB array that mitigate issues during breakout while ensuring structural integrity of the array during the manufacturing process.

Guidelines for PCB Panelization

Processing Edge: Most automated production machinery cannot process boards with less than 2 inches (50.8 mm) width. Therefore, for a PCB with less than two inches of width on its longer side, the designer must add processing edges to make it wider, create an array of boards, or make a combination of the two.

A PCB may not have two parallel edges for processing, for instance, when it must fit into an odd-shaped space. The designer must add the processing edges, or create a complex array design. For this, the designer must consider beforehand how to minimize the board cost by fitting them within the standard processing panel of the fabricator and how the fabricator will be depanelizing them.

Dimensions: Design the array to yield the maximum number of boards from the standard processing panel of the fabricator. All cuts made in the panel to facilitate PCB breakout when depanelizing will weaken the panel to some extent. The designer must therefore, limit the size of the array to prevent weakness in the panel. A weak panel may vibrate in pick-and-place machines, while sagging in the wave-soldering machine.

Fabricators using standard panels typically require a 0.5-inch (12.7 mm) perimeter clearance for handling double-sided boards. For multi-layered boards, they prefer a 1-inch (25.4 mm) clearance. Therefore, the usable panel space available to the designer is 17 x 23 inches (432 x 584 mm) for double-sided boards, and 16 x 22 inches (406 x 558 mm) for multi-layered boards. Fabricators also require about 0.1 inches (2.54 mm) space between boards for routing space. With these clearances and routing space, the designer must aim for a minimum usage of 70% of the standard panel.

Panelization Techniques

Panelization breakout techniques depend on the design of the board, such as:

·???????? Amount of component clearance available on the edges

·???????? Presence of sensitive SMT components close to an edge

·???????? Hanging connectors or other components over an edge

Sometimes, the designer must use a combination of panelization techniques for improving strength of the PCB array, while providing a suitable breakout method.

Solid Tabs: Designers can use solid tabs between boards in any orientation for improving the strength. Solid tabs also increase the board count on the fabricator’s standard panel, while facilitating automated depaneling. The depaneling for panels using solid tabs must use a laser-cutting machine or a depaneling router.

A laser-cutting machine is usually expensive and useful for board thicknesses up to about 1 mm. A depaneling router generates large amounts of dust, vibration, and noise, in addition to requiring fixtures for firm holding of the panel.

Although it is possible to use a hook-shaped blade tool for removing the solid tabs between the boards, the process needs careful handling to prevent damage to the boards. After depaneling, often there is a small part of the tab protruding. Fabricators generally consider the process as inefficient.

The PCB industry favors two panelization techniques, especially for low volumes and high-mix requirements—V-grooves and perforated tabs. The IPC standards cover these in detail.

V-Grooves: When the PCB has no components hanging over an edge, fabricators prefer to use v-grooves that produce less surface stress while depanelizing with a properly designed machine. The method is inexpensive and highly efficient. Moreover, the v-groove depaneling machines can be portable.

However, v-groove for panelization comes with many restrictions. As mentioned earlier, it is not possible to use this technique when the PCB has components hanging over its edges and when components are very close to the edges.

Fabricators use circular cutting blades to score the v-groove, and this requires at least a 0.05-inch (1.27 mm) clearance from the center of the v-groove to the component. Taller components, such as radial inductors and capacitors, including power-dissipating radial resistors, require a greater spacing, because there may be variance in their positioning.

Preferably, all SMDs must maintain a distance of at least 0.119 inches (3 mm) from the score line. Designers must orient surface mount MLCCs or multilayer chip capacitors with their long sides parallel to the v-groove if they are less than 0.25 inches (6.35 mm) from the score line. Placing components close to the scoring line can transfer the surface stress from the forced entry of the depaneling blade in the groove to the body of the component, with a possibility of fracturing them. The risk is lower if the component has its length oriented parallel to the scoring line.

Perforated Tabs: Fabricators use perforated tabs where they cannot use v-grooves. These are tabs with perforation holes to allow easy depaneling. Designers must keep traces and SMDs at least 0.125 inches (3 mm) away from the perforation holes. This avoids surface stress and splintering during separation. Likewise, SMD MLCCs must remain at least 0.25 inches (6.35 mm) away from the holes.

Fabricators use standard routers of 3/32 inches (2.5 mm) for separating boards using perforated tabs. Usually, there are five holes in every perforation tab for a breakaway, and three holes for knockouts necessary for filling holes in PCBs undergoing wave-soldering.

Breaking boards with perforated tabs safely requires holding the edge of the PCB with pliers and bending slowly until the tab just cracks, then bending the PCB edge in the opposite direction until it separates completely.

Conclusion

According to Rush PCB, designers must consider PCB panelization with respect to the impact it has on the fabrication and assembly process. PCB geometry and machine capability affect the complex considerations. The array design can affect the PCB cost considerably.