This set of guidelines will help you design your PCB for manufacturing on an automated assembly line such as the one at Dena Technologies. Some of the specific details may vary with other service providers, but the general principles apply anywhere.
PCB’s are fabricated in large panels, typically 18 x 24 inch sheets (shown as green in the image below), and then routed out either individually (shown as blue in the image below) or into smaller panels (shown as red in the image below). Even a single-up panelization may provide manufacturing benefits over a simple individually routed board by having rails which provide clearance between components and the edge of the board or by providing missing fiducials.
Panels are transported by edge supported conveyor lines. This permits components to be placed on both sides of the board, but no components are recommended to be placed within 3.2mm of the edge even if you are only using one side because some stages have overhang above the board conveyor (see loader/unloader magazine). Panelization will typically add rails along the edge much larger than that, permitting you to place components closer to the edge of your board. Be aware that larger ceramic capacitors may be subject to cracking from the separating forces if placed too close to a v-scored edge.
The separation between the edge conveyors of each piece of equipment in the line must be adjusted to the width of the panel. Each machine has at least one eye for detecting the presence of a board on the conveyor. All of the conveyors at Dena Technologies have a minimum separation of 50mm (2 inches) and can’t carry a PCB narrower than this. Each machine has a slightly different maximum width.
We strongly prefer to run panels with the long axis parallel to the conveyor direction. If your board is shorter than 50mm in one axis and placed it with that axis parallel to the conveyor direction it would fall between the machines, since each machine has its own conveyor and there is a small gap between them. Even though technically the smallest panel is 50 x 50mm, we would strongly prefer the panel be longer than that to avoid falling in the gaps at either end of the reflow oven which has larger diameter chain wheels.
Very wide panels may sag between the conveyor edges, especially in the reflow oven, and they are prone to vibration and bouncing during population in the pick-and-place machine.
|Double||12 x 7in (305 x 178mm)||20 x 10.4in (500 x 264mm)|
|Single||12 x 12in (305 x 305mm)||20 x 13.2in (500 x 335mm)|
The maximum size of a PCB panel is determined mainly by our stencil options. Dena Technologies uses two different frameless stencil systems: QTS and VectorGuard (VG). VG stencils are larger than QTS, but cost about 50% more, so we use QTS most of the time, except for boards longer than 12 inches (305mm). It is not a good idea to panelize a board that is less than 12 inches long causing it to result in a panel which is greater than 12 inches long. The maximum length of a panel on the VectorGuard stencil is 20 inches (500mm).
The maximum width of a PCB depends on whether we would fit both top and bottom sides on a single stencil. If you have SMT on both sides but the size is too large, then it is necessary to get two stencils for twice the cost.
PCB’s larger than this are possible but require some manual processing which increases the cost. For very small runs under 10 boards, this would not be significant, but for larger volume the difference between manual and automation both increases cost and decreases quality.
There is no significant benefit in having a smaller number of boards in a panel with a small-to-medium scale production line. The pick-and-place machine will be the main bottleneck regardless of how many boards are in the panel unless you have multiple series or parallel placers. The solder-paste printer only takes 13 seconds to paste a board. You would have to have less than 25 parts per panel in order for the placer to overtake the printer. The boards spend the most real time in the reflow oven (five to ten minutes), and they will be spaced by the release rate of the conveyor before it, which won’t let the next board in until the previous one is clear. As many as a dozen or more panels could be inside the oven simultaneously. The typical speed of the oven conveyor is around 5mm per second, so a fairly large 12 inch panel will take one minute to load, about as long as it takes to populate 100 components, and there are surely more than 100 components in such a large panel.
The selective solder system is not part of the SMT line and it doesn’t have a conveyor, but it does support panels by the two edges as the SMT line does, so the same basic clearance rules apply. Be particularly aware that since the board is supported by the bottom edges, and the solder bead is applied to the bottom side, through-hole pads must be well clear of the long edges of the board unless the board is panelized with rails providing the required clearance.
The board separator (sometimes referred to as a “pizza cutter”) is only used to separate V-scored boards, and then only if there are no components overhanging or within 20mil (0.5mm) of the edge. For tab-routed boards, a special hand tool is used to bite off the tabs. Due to the manual nature of this process, we prefer boards to be V-scored whenever possible.
V-scoring can only cut vertical or horizontally all the way across a panel, so normally only rectangular boards can be separated using V-scoring, but it is possible to route corners and round edges while still using V-scoring for the major edges. V-scoring provides edges without “rat-bites”, but no copper features can come right to the V-scored edge.
If you have not already done so, please familiarize yourself with the PCB Fiducial Guideline. It is recommended to have fiducials in the panelization even if you have fiducials in your board. In a multi-up panel, fiducials of individual boards may be crossed out with a marker to tell the pick-and-place machine not to populate that unit if it failed electrical testing. As with the individual board, there should be two or three fiducials with the optional third fiducial placed asymmetrically relative to the two diagonal fiducials.