Example Fabrication Drawing (Part 2)
Automated assembly equipment typically requires that the board being assembled move along a track. The actual board sits on two chains on either side of the track. As the chain moves along the assembly line the board is moved along with it from station to station. If components are close to the board edge they may interfere with how the board sits on the chain. Hence the need for rails.
Rails are extensions of the pcb that are attached and later removed after the board is assembled. Rails are added for handling purposes and to provide clearance for components that may be close to the board edge. It is common practice for anyone that assembles printed circuit boards with components to design a pallet optimized for their assembly process. A pallet can be a single pcb with rails. A single pallet may even consist of multiple copies of the same board held together by rails. Below is an example of a pallet drawing that actually goes with the pcb from the previous post.
As with the fabrication drawing the pallet drawing is provided with a zone grid, title block and revision block. A mechanical detail, side detail and notes specific to the pallet are presented on this drawing. Information not relevant to the pallet drawing should be specified on the fabrication drawing.
The drawing title block is identical to the fabrication drawing with the exception of the title and sheet entries. The Title now reads “Pallet Drawing” while the sheet reads “2 of 2″.
The revision block above is also identical with the exception of the sheet numbering. This also reads 2 of 2.
The note section on a pallet drawing deal only with the pallet itself. Adding extra information unrelated to the pallet in the section shall only lead to confusion. If its not related to the pallet drawing then add the information to the fabrication drawing.
The mechanical detail for the pallet specifies dimensional information critical to the pallet. Features already dimensioned on the fabrication drawing are marked as a reference dimension by placing (REF) next to the dimension value. Non-plated tooling holes and registration fiducials are also identified and dimensioned. The pallet break-away interface is dimensioned in a side detail identified as DETAIL “B”.
The side detail above provides the dimensional information for the pallet rail break-aways. A break-away of this design would require the assembler to saw the rail off at the .100″ wide tab. The remaining nub on the edge of the pcb would then be sanded off. To eliminate the sawing operation it is common practice for break-away details to include a line of drilled holes that form a perforation. A .025″ non-plated drilled hole placed on a .050″ pitch is typical for a .062″ thick pcb. Thicker boards may require that the pitch to be decreased to make breaking the thicker material easier. Decreasing the pitch to make the rail snap off too easy may result in the rail breaking apart during the assembly process.