Bare Board ECOs, ECNs, and Design Modifications

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While rework has been historically defined and memorialized by the IPC 7721 board-level repair guidelines as a populated board operation, there are many types of PCB ECOs on bare, unpopulated boards that require changing or modifying the physical aspects of 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 performed 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 design error, or to change the routing due to the use of other components, is to perform physical board modifications to populated and unpopulated PCBs. These ECNs or modifications need to be performed on the bare boards, keeping in mind that subsequent board processing including stencil printing, reflow, wash cycle(s) and wave soldering will be performed on the PCB. In this case, both the “as built” board information and the “needs to be modified” board design information needs to be sent to the rework or remanufacturing location. Boards will then be shipped to the initial fabricator or rework 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 new bare boards arrive sooner than reworked ones, thus eliminating the need for board-level ECO/ECNs. Timing of these changes needs to be such that the supply pipeline does not have any discontinuity.

There are a variety of options available to the assembler or OEM for revising or fixing board errors. One of the options for fixing a layout, design or fabrication error, or to change the routing due to the use of another part configuration, is the respinning of a PCB. This involves the design or layout engineer revising the layout in the design software, redoing the output and re-sending the output files back to the PCB fabricator. It may take a couple of days to a couple of weeks to get the pipeline filled up again with new PCBs using this option.Read the full article here.Editor's Note: This article originally appeared in the November 2014 issue of SMT Magazine.


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