Streamlining new product introduction pressure to make savings on components.
Component shape development Given the use of AVLs and mBOMs and the fact that process engineers typically have no visibility to part procurement, it is essential to ensure that the actual parts used in an AVL will work on the fabricated PCB. The only way to achieve this is to be able to model the precise package dimensions, pin positions and sizes to ensure that the specified parts on the mBOM and AVL fit on the footprint of the corresponding location on the PCB. Likewise, these components need to be validated to ensure they do not conflict with each other or the machines placing them on the PCB. This capability is therefore a critical element of world-class design for assembly (DFA) solutions for PCB assemblers. Being able to accurately model
Figure 2. Pin-to-toe print illustration.
optimum footprint that matches the engineering bill of materials (eBOM). The PCB assembler will use a manufacturing bill of materials (mBOM) to build the PCB based upon locally available parts. These parts will be electrically equivalent to the design specifications but may be packaged differently than their eBOM equivalents. It is this fundamental difference between BOMs where many of the difficulties arise in the PCB assembly process. The AVL provides another method
for PCB assemblers to minimize the component costs by selection from multiple manufacturers for each unique part number on the mBOM. However, this introduces a significant complexity issue, as electrically equivalent parts are not necessarily physically equivalent. There is a high likelihood that the same component from different vendors will have physical differences in the actual package. Each vendor will have a standard package size and tolerance for each component they manufacture. However the same part, even if it is nominally the same shape, may be delivered in a slightly different packaging from various component vendors. In other words, a nominal 1206 chip from Vendor A will likely be different from a nominal 1206 chip packaged by Vendor B. Being able to manage the impact of these differences between packages and packaging allows the component procurement process to find the lowest cost components without sacrificing manufacturability, product reliability and
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quality. Further complicating the management
of the mBOM is the typical use of a manufacturing AVL. The AVL documents the alternative parts from different manufacturers that can be interchanged based on the purchasing department’s ability to source them. If one part has a long lead time or higher cost, an alternative can be used in its place to meet scheduling or cost targets. Remember the earlier point: the component cost is the single largest cost of the PCB, so the component procurement group is under significant
the manufacturer’s specific packages provides the basis for all aspects of the NPI process. Package bodies must be accurately modeled as these are used for component-to-component spacing checks on the PCB as well as ensuring that test points close to components are accessible for test purposes. Accurate component pin information is also critical. This is the point where the rubber hits the road so to speak. A clear understanding of the pin to land pattern relationship is essential so that the optimum stencil pattern can be developed. Proper stencil apertures ensure that a reliable solder joint between the component pin and the PCB can be achieved. Understanding the variation between alternative component parts may indicate that one of the packages may be the cause of solder defects if its pin layout
Figure 3. Dynamic build order management.
Global SMT & Packaging – Celebrating 10 Years – December 2010 – 19
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