Originally published on AtlasPCB Blog
Every PCB that enters a surface mount assembly line does so as part of a panel. The individual circuit boards that end up in finished products are too small, too irregularly shaped, or too numerous to be handled individually through pick-and-place machines, reflow ovens, and inspection stations. Panelization, the process of arranging multiple individual PCBs within a larger manufacturing panel along with the rail tooling features needed for machine handling, is a fundamental step that bridges PCB fabrication and assembly. Despite its seemingly straightforward nature, panelization decisions profoundly impact assembly yield, depaneling quality, component reliability, and overall manufacturing cost.
In our fabrication facility, we process over 3,000 unique panel designs per month, and the panelization-related issues we encounter range from minor cosmetic concerns to catastrophic assembly failures. The choice between V-scoring and tab routing, the placement and design of breakaway features, the panel utilization efficiency, and the compatibility of the panel design with the customer's depaneling equipment all contribute to the final cost and quality of the assembled product. This article provides a detailed engineering perspective on panelization strategies, drawing on our direct manufacturing experience to guide designers toward optimal panel designs.
V-Score Fundamentals and Depth Control
V-scoring creates a pre-cut groove on one or both sides of the panel along straight lines that define the boundaries between individual PCBs. A rotating V-shaped carbide blade scores through a controlled depth of the board thickness, leaving a thin web of material that holds the panel together during assembly but allows clean separation afterward by applying bending force along the scored line. The score line extends from edge to edge of the panel, which means V-scoring is applicable only along straight lines that span the full panel width or height.
The depth of the V-score groove is the critical process parameter that balances two competing requirements. A deeper score leaves less residual material, making depaneling easier and reducing the force required for separation. However, a deeper score also reduces the panel's mechanical stiffness during assembly, potentially allowing the panel to flex during handling, solder paste printing, or reflow. A shallower score maintains panel rigidity but requires more force to separate, increasing the risk of board damage during depaneling.
Industry convention expresses V-score depth as the remaining web thickness rather than the groove depth. For a standard 1.6 mm board thickness, a typical remaining web of 0.3 to 0.5 mm provides adequate panel integrity during assembly while allowing clean manual or machine depaneling. The V-score angle, typically 30 to 45 degrees included angle, determines the width of the groove at the board surface and influences the edge quality after separation. A narrower angle produces a smaller surface disruption but requires more precise blade positioning.
In our V-score process, we control depth using precision-ground carbide rotary blades with automated depth setting verified by laser measurement. The depth accuracy we achieve is plus or minus 0.05 mm, which translates to a web thickness variation that is predictable and consistent across the panel. We run verification cuts on scrap material at the beginning of each production lot and measure the actual remaining web thickness with a calibrated micrometer to confirm that the setup matches the specification before processing customer panels.
The Component Proximity Challenge
The most common V-score-related failure we observe is not in the scoring process itself but in the design placement of components relative to score lines. When SMT components are placed too close to a V-score line, the depaneling process can stress the solder joints or the component bodies. The bending action during V-score separation creates a stress field that extends several millimeters from the score line into the board material. Components within this stress zone may experience cracked solder joints, lifted pads, or in the case of ceramic chip capacitors, component fracture from the flexural stress.
The safe distance between a component and a V-score line depends on the board thickness, the remaining web thickness, and the depaneling method used. For manual breakaway of a 1.6 mm board with 0.4 mm remaining web, we recommend a minimum clearance of 1.0 mm from the score line to the nearest component pad edge. For machine depaneling with a pizza-cutter style rotary blade, the clearance can be reduced to 0.5 mm because the machine applies force in a controlled, progressive manner that creates less board flexure than manual breaking.
This clearance requirement has direct implications for panel utilization. The designer must decide whether to increase the spacing between adjacent PCBs in the panel to accommodate the V-score clearance plus component placement, or to modify the component placement to respect the clearance zone. Neither option is free: increased spacing reduces the number of PCBs per panel, while moving components may compromise the circuit layout or require a board size increase.
Tab Routing: Flexibility for Complex Board Shapes
Tab routing, also called route-and-retain, uses precision milling to cut around the perimeter of each individual PCB within the panel, leaving only small connecting tabs that hold the PCBs in place. These tabs are later broken or cut during depaneling. Unlike V-scoring, tab routing can follow any contour, making it suitable for irregularly shaped boards, boards with cutouts, or boards where components are located close to the board edge and cannot tolerate the stress of V-score separation.
The tabs themselves can be designed as solid material bridges or as perforated strips with mouse bites. Mouse bites, technically called perforated breakaway tabs, consist of a row of small unplated drill holes, typically 0.5 to 0.6 mm diameter with 0.75 to 0.8 mm center-to-center spacing, that create a weakened section within the tab. The mouse bite perforation allows the tab to be broken by hand or with simple fixturing, leaving a slightly rough edge at the break point that can be smoothed with light filing if cosmetic appearance matters.
Solid tabs without perforation require mechanical cutting for separation, typically using a depaneling router, a nibbling tool, or a laser. Solid tabs provide greater mechanical strength during assembly, which is advantageous for heavy boards or panels that will undergo aggressive handling during wave soldering or selective soldering where the board is inverted. However, solid tabs leave a tab remnant on the board edge that requires secondary processing if a flush edge profile is specified.
From analyzing over 5,000 panel designs in our production database, we find that the optimal tab design depends primarily on three factors: the weight and mechanical stiffness of the panel, the depaneling method available to the customer, and the edge quality requirements at the tab locations. For lightweight, thin boards below 1.0 mm thickness, three to four mouse-bite tabs per board edge are typically sufficient. For heavier boards at 2.0 mm or above, solid tabs of 2.5 to 3.0 mm width provide the necessary strength to prevent panel fracture during handling.
Tab Placement Strategy
The location of breakaway tabs is not arbitrary. Tabs placed at board corners provide maximum panel rigidity but concentrate stress at the corner during depaneling, which can cause corner cracking if the tab is poorly designed. Tabs placed along straight edges distribute the separation force more evenly and reduce corner stress. The ideal tab placement creates a uniform distribution of support points around each PCB perimeter, avoiding long unsupported spans where the board could flex during assembly operations.
A particularly important consideration is tab placement relative to board-edge connectors or other components that extend to the board perimeter. If a connector footprint approaches the board edge, the tab cannot be located within the connector footprint zone. This forces tabs to be placed asymmetrically, which can create differential support that causes panel warpage during reflow. We work with customers during the panelization design review to identify potential conflicts between connector placement and tab locations, often suggesting minor adjustments to connector positioning or board outline that allow symmetric tab placement without compromising the circuit design.
The width and length of each tab determine its mechanical contribution to panel integrity and the force required for depaneling. A wider tab provides more support but requires more force to break, which increases the stress transmitted to the board during depaneling. A narrower tab is easier to separate but may not provide adequate support during assembly. For mouse-bite tabs, we typically recommend a tab width of 2.0 to 3.0 mm with five to seven perforation holes, which provides sufficient assembly strength while allowing manual depaneling without excessive force.
Panel Utilization Optimization
The economic impact of panel utilization is substantial and often underestimated by designers. A standard manufacturing panel size might be 457 mm by 610 mm for our primary process line, with a required border clearance of 5 mm on each edge for tooling and handling. The usable area within this border must accommodate the PCB array, the tab routing clearances, and the rail features required for assembly equipment handling.
Panel rails, the solid border strips along two or four edges of the panel that provide the handling surface for conveyor systems and registration datums for automated equipment, typically consume 5 to 8 mm of width on each edge. Fiducial marks for vision alignment and tooling holes for fixture registration require specific locations within the rail area. Some assembly equipment requires a minimum rail width of 5 mm while others need 8 mm, and the fiducial placement specifications vary between equipment manufacturers.
The array arrangement of PCBs within the panel can be optimized by rotating individual boards, nesting complementary shapes, or combining different board designs on a single panel. Rotation by 90 degrees can sometimes improve utilization dramatically for rectangular boards where one dimension is close to an integer fraction of the panel dimension. Nesting of irregular shapes, while theoretically optimal for material utilization, increases programming complexity for automated routing and may complicate assembly programming if different orientations are present.
In our engineering review process, we analyze panel utilization as a standard part of the DFM report. We have found that many customer-submitted panel designs achieve only 60 to 70 percent utilization of the available panel area, while our optimized arrangements for the same boards often achieve 75 to 85 percent utilization. The improvement comes from adjusting inter-board spacing, optimizing rail dimensions for the specific assembly equipment, rotating the array orientation, and selecting the panel size that best matches the board dimensions.
V-Score and Tab Routing Combined Approaches
Many production panel designs combine V-scoring and tab routing to leverage the advantages of each method. A common hybrid approach uses V-scoring along straight board edges where no components are nearby, and tab routing with mouse bites at locations where components approach the edge or where the board outline is non-linear. This combination provides the clean, low-stress separation of V-scoring where possible while accommodating design constraints with the flexibility of tab routing.
The manufacturing sequence for hybrid panels requires careful process planning. V-scoring is typically performed as one of the final fabrication steps on the finished panel, after all drilling, plating, and imaging operations are complete. Tab routing can be performed either during the initial board outline routing step or as a separate operation after V-scoring. The sequence matters because V-score lines create stress concentrations that can cause panel fracture during subsequent handling if the remaining web is too thin for the mechanical loads imposed by later process steps.
We have developed specific process sequences for hybrid panels that ensure panel integrity through all fabrication and assembly steps. The general principle is to perform V-scoring after all operations that require full panel rigidity, such as plating, where the panel is suspended vertically in chemical baths, and electrical testing, where probe pressure is applied to the board surface. Tab routing, being more forgiving of panel flexibility, can be performed earlier in the sequence without compromising panel integrity during subsequent operations.
Depaneling Quality and Component Reliability
The final consideration in panelization design is the depaneling operation that the customer will perform after assembly. This is often neglected during the design phase, with the assumption that any panel can be separated by any reasonable method. In practice, the depaneling method must be matched to the panel design, and certain combinations of panelization and depaneling produce unacceptable results.
Rotary blade depaneling of V-scored panels requires that the score line be straight, continuous, and free of obstructions above and below the blade path. If components overhang the score line, the blade cannot make contact without damaging the component. If the score line changes direction or is interrupted, the rotary blade cannot follow the path. These constraints must be communicated to the board designer before panelization design is finalized.
Laser depaneling offers the greatest flexibility for tab-routed panels, capable of cutting any tab geometry without mechanical stress to the board. However, laser systems are capital-intensive, and many assembly operations rely on simpler manual breaking or pneumatic nibbling tools. When we know that the customer will use manual depaneling, we design the mouse-bite perforation pattern to ensure that the break force is below 20 Newtons per tab, which is comfortable for an operator to separate by hand without risk of board damage.
The goal of every panelization design is to create a panel that survives the rigors of automated assembly handling while allowing clean, stress-free separation into individual boards at the end of the process. Achieving this goal requires understanding the complete process chain from fabrication through assembly to depaneling, and making panelization decisions that optimize across all of these operations rather than focusing on any single step in isolation.
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