As a lot of others responded: It depends.
However this is not a new trend. I used the schematic as the root source
of the BOM with a DOS based CAD tool.
And I remember manually checking parts off a hand drawn schematic in the
days of taping.
What has changed is that none of the CAD files are now the controlling
source for the BOM. The MRP system is.
CAD now can be thought of a as a tool to generate data for the MRP/Vault
system. All scheduling, purchasing, stocking, kitting and assembly is
done per the MRP system.
Electronic CAD tools use the schematic to generate BOMs (although why
they don't put them into Excel format is beyond me). Most tools have
some options to create parts that either A: Show up in the layout but
not the BOM, such as a fiducial, or B: Show up in the BOM but not the
layout, such as a screw. Usually some sort of attribute can be added to
do this. The last three places I worked (over a 13 year period) ended
up with someone writing a program to take the CAD BOM into an Excel
format and sort the items based on attributes. You can easily delete
thins like fiducials then and create variants if the CAD tool approach
does not suit your needs. That Excel file is then loaded into the MRP
system. A lot of PCB assembly drawings are done in an outside tool, such
as AutoCAD or SolidWorks, and do not have the intelligence that is built
into the electronic CAD tool, so using the assembly drawing as a start
for the BOM would not work. Note that even Altium's tool does not really
transfer BOM data into into SolidWorks. You would have to build a
SoildWorks library with intelligence to get a BOM from it and there are
issues with Altium's 3D output that would not let this work very well.
The schematic seems to be the best starting point for a BOM and each
company will have to tweak the process to suit their needs.
Greg Smith
On 6/26/2013 11:10 AM, Brooks, William wrote:
> I want to examine and get input on is the trend to use the schematic diagram as a Bill Of Material or BOM/Parts List generation sole source...
>
> This subject is on my mind more and more and I think it needs an in depth discussion.
> What I see happening is engineers who have not had formal drafting training using the schematic as a vehicle to automatically create a complete BOM for a PWA or PCA depending on where and how you term the printed circuit assembly with all the components installed.
>
> The traditional documentation guide book for drafting of electronic systems has its roots in the Department of Defense standards... which were adopted and published by Global Engineering Documents in a Drawing Requirements Manual which is now in its 11th edition authored by JEROME H. LIEBLICH. If you want to review it, you can find it at www.global.ihs.com
>
> On a separate evolution line is the CAD or EDA companies that create the tools that have been used since the computer was starting to become a graphics engine... we can thank them for the creation of drafting tools that do more than just draft a drawing... without them we would not have our boards done in the time constraints we enjoy today...
> My experience predates the computer so I remember using a drafting table and light table to create my boards...
>
> According to historical standards, the controlling document for a Printed circuit assembly is the Bill of Material or Parts List and an Assembly drawing...
> All revision control stems from this document that lists the electrical and mechanical parts and processes including the bare board that make up the PCA. The Assembly drawing is the controlling document by which the part is assembled and inspected.
>
> Traditionally the Assembly Drawing was considered the first sheet of a parts list... often if the list was small it would be documented right on the Assembly drawing... but since more often there were so many parts to list we usually made a separate Parts List (Bill of Materials) but it kept the same part number as the BOM and ASSEMBLY drawing and the revision letters typically tracked... though provision was made in the DOD specs to handle the revs separately too, as long as it was referenced on the BOM. So the BOM was the master top level controlling document for building a PCA/PWA.
>
> The way I am seeing the CAD tools being used nowadays is to drive the BOM from the schematic diagram... and because of this many designers and engineers are creating symbols for non-electrical or mechanical parts on the schematic to get the BOM to generate a complete bill of materials for the PCA.
>
> Should we be doing this... ???
>
> All of the implications may not be so obvious to everyone... I would like to get other professional's opinions and observations regarding this practice ...
>
> Best regards,
>
>
> William Brooks, CID+
> Senior MTS (Contract)
> 2747 Loker Ave West
> Carlsbad, CA 92010-6603
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