I've been a PCB Designer/ECAD Administrator/ECAD Librarian for over 30 years and I've never seen cells built as shown in the image.
This company required the placement outline to encompass the cell's pads as shown in the image.
I see no benefit to creating cells this way, yet I do see significant drawbacks.
The ECAD system has separate DRC checks for physical items (i.e. placement outline-to-placement outline) and electrical items (pad-to-trace, trace-to-trace, etc.)
With the Placement Outline encompassing the pads, the electrical checks are trumped by the mechanical checks or vice versa depending on the actual design rules. This is an unnecessary limiting of the software's DRC capabilities.
For QFPs (Quad Flat Packs) the corner areas between groups of leads are unavailable for placing other devices unless DRC is disabled. There are false DRC error messages that are generated for devices placed in these otherwise acceptable locations.
Another drawback is with 3D rendering of the PCB when mechanical models are not yet available for mapping. In this case the placement outline is extruded in the +Z axis to the height specified in the cell (1.75mm in the example shown), which looks strange and unfamiliar.
I think this procedure was originally defined by an electrical engineer working as ECAD administrator, someone who had little or no PCB design experience.
This is why electrical engineers make lousy PCB designers or ECAD administrators. Yes, it involves electronics, and the electrical engineer is the only customer I need to satisfy in regard to the PCB layout, but It's a totally different discipline and methodology than electrical engineering.
For this reason, I advocate for a separate ECAD department, call it Engineering Services. There should be representation on the ECO committee for Engineering Services in addition to Engineering. This would enable some advocacy for PCB designers and excellence in ECAD operations.
Although I often open my mouth only to change feet, I might as well voice some other pet peeves that still irk after all this time in industry.
Please read with an implied "In my humble opinion..." and feel free to respond.
- Electrical engineers should stick with electrical engineering and let experienced PCB design staff define how best to satisfy the engineers' requirements. Companies would be well-advised to bring in ECAD consultants in addition to EDA vendors if the in-house staff does not have expertise in ECAD administration or intimate familiarity with EDA software.
- Symbol, padstack, cell, and part creation should be completely documented in unambiguous detail and as concisely as possible. This documentation should be ECO-controlled and is critical when multiple design sites contribute to the corpoarte library.
- Companies typically use their less experienced employees as ECAD Librarian. This could work if excellent documentation and training are provided, but it's a train wreck otherwise. The CAD libraries are the foundation of all electronic development. A poor implementation here will soon get out of hand to the point where ECAD administrators are too busy putting out fires to have any time available to fix the problems causing those fires!
- ECAD administrators should have a broad and deep working knowledge of the software used, otherwise there is no hope of changing or improving a poorly-implemented ECAD system. Any change or proposal not fully understood by the ECAD administrator will be deemed too risky.
- PCB designers are better suited for the position of ECAD administrator than are electrical engineers, but even so may lack an adequate understanding of other parts of the tool such as schematic capture, simulation, MCAD-ECAD data sharing, ECAD library, etc.