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April 2016 • SMT Magazine 31 bEsT-PracTicE ProcEss PrEParaTion for Pcb assEmblY Managing Component rotation After merging the accurate shapes of the components into the data model, still one more crucial issue for SMT placement needs to be ad- dressed—component rotation. Component ro- tation is challenging because it's a relative mea- surement, relative to the original design foot- print rotation of 0°. As a result, a standard defi- nition of rotation in design is rarely achieved successfully, and even more rarely communicat- ed. Even within the same design team and with- in the same shape of components, there can be cases where the rotation of zero degrees of one part number is different to that of another. Just looking at the rotation, it is often impossible to know the actual correct orientation of the com- ponent. Correct orientation is achieved on the board when pin 1 of the component is actually located on the designed pin 1 of the pad stack or component footprint. At the time of importing the design data, automated adjustments can be made. For ex- ample, as a first step, a rule can be applied based on the quadrant that pin 1 or the first electrode of a device appears in. This rule will set consis- tent normalized rotations. Once normalized in this way, the rotations can then be adjusted to a defined standard for that shape using a simple look-up table. Very quickly then all the correct rotations can be neutralized in the product data model. An error in rotation or offset is serious enough, however, that positions and rotations must always be 100% checked for new prod- ucts. The standard approach in the industry for a new product introduction to SMT is to run a special board covered in a film of sticky tape in lieu of solder paste, through the SMT placement machine. People will then study this board through a magnifying glass to see whether in fact all of the rotations and positions of all of the components are correct. Corrections can be made to the program and the process repeated until perfect. This can go on for awhile, con- suming a lot of line time and materials which cannot then be reused. The result of this even cannot be fully trusted because having gone through the machine programming software to make program "tweaks" on the shop floor, com- pensating errors may have been introduced, po- tentially causing serious quality issues later on. Alternatively, the Mentor Graphics Valor Data Preparation has a built-in simulator known as "virtual sticky tape," which shows exactly how each component will look when placed on the surface of the PCB. This is possible only because the actual design and shape data are available. When overlaying these two data lay- ers for each component position, the result is a significant reduction of time and cost. Quality can also further be improved. Often 180° errors on symmetrical polarized parts are missed in vi- sual checks of the physical board because polar- ity marks are sometimes obscured. Stencil Design Stencil design traditionally involves sending out a stencil guidelines document to the sten- cil vendor and a Gerber layer that instructs the stencil vendor how to define the various aper- tures needed for the stencil. They make these changes, typically contacting the manufacturer a number of times, to create a complete stencil. This is then sent for final confirmation to the manufacturer who can approve it before get- ting the stencil and a Gerber file that describes the stencil. This whole process takes time. It is also very manual as each stencil does not learn Figure 6: Automatic rotation normalization is used to account for inconsistencies in design data.