PCB INNOVATION

- Jun 30, 2018-

You should design your PCB array to secure the benefits of running multiple boards through your processes, while yielding the maximum number of boards from the PCB fabricator’s standard processing panel. Every cut in the array that’s made to facilitate future PCB breakout will weaken the panel somewhat. Therefore, you must also limit the size of the array to prevent PCB-array weakness, which can cause vibration in the pick-and-place machine and sagging in the wave-solder machine.

 

The typical PCB manufacturer runs a standard panel size of his choice, very commonly 18 × 24 in. They will want a 1/2-in. perimeter clearance for handling the panels when processing double-sided boards and a 1-in. clearance when processing multilayer PCBs. On an 18- × 24-in. panel, this translates to 17 × 23 in. of usable panel space for double-sided and 16- × 22-in. inch panel space for multilayer boards. They will need about 0.1 in. of routing space between board arrays, so this is also unusable panel space. A number of “panel calculators” are available for use to help maximize the board count you get from the fabricator’s standard panel; however, if you don’t get 70% usage, you should try harder (Fig. 1).

 

Panelization Methods

The design of the board—namely how much component clearance is provided on the edges, whether sensitive SMT components are close to an edge, and whether or not connectors or other components hang over an edge—will limit the choices of panelization breakout methods from which to choose. Sometimes a combination of methods is appropriate to secure the PCB-array strength, while providing a viable breakout method.

To improve strength, increase the board count on the fabricator’s panel, and facilitate automated depaneling, arrays can be designed with solid tabs between boards in just about any orientation. The depaneling method for this kind of panel is either a depaneling router or a laser-cutting machine. The former creates vast amounts of dust, noise, and vibration, and requires firm holding fixtures. The latter is capital-intensive and limited to board thicknesses of about 1 mm. Still, these methods have their place in the high-volume market and aren’t subject to many of the restrictive guidelines to be outlined later.

The solid tabs between boards can be removed with a hook-shaped blade tool, but this process can be troublesome. Even with very little clearance of the blade in the slot between boards, it can rotate and take a bite out of the good part of the board. If the blade-cutting edge is tapered to prevent board damage, it leaves a little of the tab protruding from the good board. It’s also a very inefficient process.

The two preferred depanelization methods for the low-volume/high-mix arena are V-grooves and perforated tabs. Tabs and V-grooves are pretty well covered in the IPC standards.