SMT007 Magazine

SMT-Nov2014

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36 SMT Magazine • November 2014 feaTure by bob wettermann beST iNC. Bare Board ECOs, ECNs and Design Modifications abstract While rework has been historically defined and memorialized by the IPC 7721 board-lev- el repair guidelines as a populated board op- eration, there are many types of PCB ECOs on bare, unpopulated boards that require changing or modifying the physical aspects to the PCB. These cases are most commonly ones in which there has been an error in the board layout, a PCB fabrication error, or a design oversight. The most common types of physical board rework includes the adding or cutting of traces and pads, the select removal of solder mask, and the modification of board dimensions. The types of physical rework that are per- formed most commonly on bare and populated PCBs, the most common options for reworking boards, and the methods used for performing this type of board-level rework will be discussed here. Case studies will be examined, along with photos of the board-level changes that were performed in order to evaluate the different methods. One of the options for fixing a layout or de- sign error, or to change the routing due to the use of other components, is to perform physical board modifications to populated and unpopu- lated PCBs. These ECNs or modifications need to be performed on the bare boards, keeping in mind that subsequent board processing includ- ing stencil printing, reflow, wash cycle(s) and wave soldering will be performed on the PCB. In this case, both the "as built" board infor- mation and the "needs to be modified" board design information needs to be sent to the re- work or remanufacturing location. Boards will then be shipped to the initial fabricator or re- work services provider with a sample lot being shipped back as a first-article inspection prior to the remainder of the lot being modified. In some cases, boards can be simply respun so that

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