Tom’s Circuits – Optimize PCB Shapes for Panelization to Lower Cost

December 20, 2016 , in Blog, Tom's Circuits

Non-optimal board shapes lead to scrap. Individual boards are routed from a large rectangular shape called a panel. To reduce scrap, design PCB shapes for panelization and follow the guidelines to reduce waste.

Printed circuit boards are cut out from large panels. A common large panel size is the ‘full panel’ which is 18″ X 24″. Many shops prefer to work with ‘half panels,’ which are 12″ X 18″. Alignment holes in a ring around the board maintain alignment between the layers, and give the machines in the shop a way to hold to the boards. This ring is scrap that won’t be used as part of the boards. The size of the ring can vary, with a typical size of 1″ on each side.

Assuming 2″ of total width and height are lost to the ring, the usable area of a half panel will be 10″ X 16″. The panelization process divides the usable area into individual boards with a scoring process, a routing process, or a combination of the two.

Scoring

Scoring is a good option if all the constraints can be met. There is less scrap due to the kerf of the router bit, and no ugly depanelization marks. Scoring puts a v-shaped groove into the board in the bare-board shop. After the panel is loaded, a pizza-wheel-style blade finishes cutting the board.

Here are the constraints for scoring. Many of these constraints can be lifted by combining scoring and routing:

Scores go in straight lines. The scoring wheel can’t turn corners!

If boards are to be loaded as a panel, there are additional constraints. If the boards are loaded individually, these constraints don’t apply:

Keep fragile components away from the board edges. Surface-mount capacitors are usually the most restrictive. They are brittle and lack toughness. Damage to cracked capacitors is not always obvious. The circuit appears to work, but over time the cracked capacitor absorbs moisture and shorts out.

Components can’t overhang the scored edges of the board. Overhanging parts would block the cutting wheel. The manual work-around is to hand-solder the overhanging parts after board separation, or to combine v-score and routing.

Copper routing and planes need to be kept away from the score line. The copper keepout is the distance from the copper to the center of the score line. Due to the angle of the scoring tool, thicker boards need more clearance.

Board thickness Copper to
Score Keepout
0.031″ 0.008″
0.047″ 0.009″
0.062″ 0.010″
0.093″ 0.013″
0.125″ 0.016″

 For other board thicknesses, use the formula:

keepout width” = 0.005 + 0.09 * thickness”

Routing

Routed boards are common because routing allows complicated board shapes and more processing flexibility. The router bit for the board outline can be as small as 0.078″. For internal routes, the bit can be smaller. Wobble in the router bit limits the board size tolerance to +/-0.003″.

Routed boards require some sort of breakaway structure. The spacing between the breakaways should be no more than 4″ apart.

This structure is a typical board breakaway. If there is not room for five holes, three can be used instead. The holes intrude into the finished board area to prevent a stub from sticking out from the board outline. Stubs would make the board difficult to fit inside of a product. Removing the stubs is inconvenient because it must be done in a shop with excellent dust-handling capabilities that can also work on loaded boards. This is an option if the boards are loaded individually and not as a panel.

The finished board does not have a protrusion, but it looks like it was chewed by a mouse. This remaining structure is called a “mouse bite.”

The mouse bite has a keepout for both copper and components. These keepouts  prevent shorts and component cracking due to board flex during depanelization.

Plan ahead for the breakaways. The breaks should go along the whole panel or subpanel, and should not turn any corners. Breakaways that are not in line cause large stresses on the board, leading to delamination. Design the sequence of breaks for a pair of pliers to grab each mouse bite. This is a puzzle to work out in advance.

Routed boards can tolerate parts that overhang over the edge of the board. However, the parts with overhang should not cover up the breakaway structure.

Combination Routed and Scored

In the picture above, the breaks for edge 1, 2, and 3 are good, but break 4 has the additional challenge that a pair of pliers would have to reach across the board to get to the break. This is possible, but not the first choice. In this case, the combination approach is better, using a score instead of routing for edges 4, 6, 8, and 10.

The gold-plated connector fingers are another constraint in this design. They are an example of a section that can’t tolerate mouse bites. If scoring is used, make sure that the mating connector can tolerate the less-flat edge created by the v-groove shape of the scored edge.

Get Help with Panelization

The contract manufacturer can take care of panelization for you. If your board has constraints where mouse bites can’t be used or v-score is desired, add notes to the drawing such as ‘no mouse bites’ or ‘use v-score.’ If you want to design the panelization yourself, have an early design review with the CM.

Understanding the constraints and limitations of panelization will bring you closer to the goal of a scrapless ‘boards and dust’ design. Even if your CM designs the panelization scheme, plan ahead to prevent delay due to redesign or tooling development to make panelization possible!

 

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