Issue link: https://iconnect007.uberflip.com/i/1523825
JULY 2024 I DESIGN007 MAGAZINE 45 e goal is to ensure a smooth, unencum- bered flow from one system to the next. e 5.0 mm wide "Keep out zone" noted in the detail represents the surface area of the panel reserved for conveyor transfer mechanisms. Tooling holes are to be located near the cor- ners of the panel. Because these holes are reserved for post-assembly processing, they must be specified on the fabrication detail as "non-plated." e non-plated tooling holes may also be used during the electrical testing of the assembled unit(s) and later, during the de-panel operations. Fiducial targets: Most of the systems developed for surface mount assembly have a built-in vision system that can identify key features on the circuit board's surface. ere are "global" fiducial targets and features desig- nated as "local" fiducial targets. Global fiducial targets are to be located in areas outside the component mounting zone. ese target fea- tures are initially used to enable precise align- ment of the solder stencil to the surface mount component's land patterns. Fiducial targets are small-diameter copper features formed during the final circuit etching process. e size of the fiducial target (Figure 3) can range from 0.5 mm to 1.0 mm in diam- eter. To allow unobstructed access for vision- assisted solder paste and component place- ment processes, the fiducial must be clear of solder mask coating. e solder mask opening will be two to three times the diameter of the fiducial target profile to ensure vision system recognition. Following solder paste printing or solder paste deposition, these same global fiducials will serve as the 0-0 reference for the precise X-Y location for robotic SMT component placement. Additional fiducial targets located within the component's mounting zones are oen required to assist in accurate placement of the more complex semiconductor packages. For example, two or more fiducial targets may be needed near the mounting sites for "fine-pitch" semiconductors (BGAs, CSP, flip-chip), par- ticularly those with ball terminal centers less than 0.4 mm. ese local fiducial targets may also be furnished in close proximity to a group- ing of micro-passives (0201s, 01005s, and smaller devices). When possible, review the fiducial target locations with the assembly process engineer and those responsible for assembly system program development. In developing the optimum multiple-unit panel dimensions for assembly processing, the designer must first consider minimizing mate- rial waste while maximizing assembly process efficiency. Medium to large system-level circuit boards can be designed as a single rectangular- shaped unit or panel typical of that shown in Figure 2. is format will enable efficient in- line conveyor transfer of the smaller circuit board as well, but the circuit board's laminate material must be extended away from the basic board outline in order to furnish a more uni- form panel-like format. e uniform panel outline facilitates efficient conveyor transport of the multiple unit panels between assembly systems. To further maximize assembly process effi- ciency for the irregular-shaped circuit boards, the designer can better utilize material by alter- nating the orientation of each board to create a uniform panel format typical of that shown in Figure 4. Figure 3: Fiducial target example.