Issue link: https://iconnect007.uberflip.com/i/1379105
82 SMT007 MAGAZINE I JUNE 2021 anisms so that similar looking PCBAs do not get installed in a wrong backplane or assem- bly by mistake. e premise here is that tool- ing holes have a multitude of uses and vary in size/shape. My main point of contention with identify- ing a tooling hole as a type of mounting hole is the ambiguity involved in such a classifica- tion. When designing the mechanical features of a PCB, how do you differentiate mounting holes from tooling holes? Is it implied by their locations? Do you have callouts in the fabri- cation data? Do you export a step file of your design for use in a 3D CAD tool? Is it based on the tolerances assigned to different drill holes in the design? is leads to miscommunication of where tooling holes need to be placed, usu- ally because assumptions are made. e typical result is that a re-spin is in order. Early in my career I was responsible for a board design that involved a right-angle, high- density connector with over 500 pins. Each pin had an insertion force of one-eighth pound and an extraction force of one-fourth pound. It took a little over 62.5 pounds to insert the board and over 125 pounds to remove the board. ese force specifications were valid for a narrow angle of entry/exit of the right-angle connector to the backplane connector. Two sets of tool- ing holes were used to accomplish appropri- ate insertion/extraction. Round tooling holes served the dual functions of insertion/extrac- tion load points and PCBA assembly/clam- shell test alignment. Slotted tooling holes were used to minimize rocking at an angle during insertion/extraction of the PCBA. ese tool- ing holes were placed and treated as mount- ing holes in the design. When these PCBs were built, the fabricator asked for relaxation of the requirements of the mounting holes in the design. ey were given permission by our manufacturing engineer and the round tool- ing hole placement was adjusted on one side of the PCB only. is introduced a forced rocking angle that was not caught during development and certification of the product. When the product entered mainstream production with higher quantities and part variations, the back- plane connectors started ripping out of the backplane when board extractions were per- formed. It turns out the uneven placement of the tooling holes was responsible for two con- nectors binding, and the press fit backplane connector ended up being the loser in the tug of war that would ensue. e root cause of the problem was identified as a lack of specifica- tions surrounding specific mounting/tooling holes causing the manufacturing engineer to make an incorrect assumption. e result was three re-spins: my board, the backplane board, and the insertion/extraction tool, to specifi- cally avoid this issue in the future. How can we solve the issue of ambiguity sur- rounding tooling holes? CAD tool vendors have and are implementing cross-platform integra- tion between PCB and 3D CAD tools. is is progress in a good direction but does not solve the issue at hand. PCB and 3D CAD designers still need to communicate the specifics about what they are co-designing. Direct integration between CAD platforms do not differentiate between tooling and mounting holes because they are defined as the same thing: a mounting hole. What I am proposing is a specific desig- nation for tooling holes within the CAD tools themselves—a separately searchable, report- able, and identified item defined explicitly as a tooling hole. is type of unique treatment is like a fiducial, test point, and/or mount- ing hole. e idea is to explicitly identify tool- ing holes within the CAD tool environment to avoid assumptions and miscommunications which inevitably lead to redesigns. I would like to start a new tooling hole motto, "ere is no 'M' in Tooling Hole." SMT007 Chris Young is owner/lead engineer of Young Engineer- ing Services LLC and chief hardware engineer at The Goebel Company. To read past columns, or contact Young, click here.