In order to guarantee that a drawing or a procedure is at the most current revision level and that it's the document associated with a particular instruction or bill of materials, the revision date and number should be visible in a predetermined location on each controlled document. (See: 5 Steps to Reducing Design Change Problems.)
A revision date and number in the Header of the document helps identify quickly if the document being reviewed is the same version number as indicated on the Engineering Change Order Release record. Please refer to the Procedures and Guidelines from Components Engineering for a detailed procedure for managing Revision Control.
Again, different companies have varying methods for handling revision control issues. There are systems that have a two placeholder identifier where both alpha and numeric indicators are used. In one company, an alpha in the first position may indicate that a drawing or a procedure is not yet released to manufacturing. When the first placeholder is changed to a number instead of a letter, it is an indication that Manufacturing has received the document and is assuming primary responsibility for informing Engineering of any changes. Documentation Control is showing the drawing as “Released.” The identifier scheme can just as easily be implemented where the first placeholder is a number and not a letter, also indicating “Pre-Release” status.
On procedure documents, there is usually a cover page that shows a table for revision entries. The table will include the “bump” to the next revision level shown as the next Alpha or Numeric in sequence. In other words, 1A may bump to 2A. The next column over in the table may be a brief description of the change. Another column will contain the name of the person entering the change.
The date of the change is shown and the last engineering change notice (ECN) number prior to this most recent change. After a period of time, this table contains the complete list of revision numbers and changes made to the document. Documentation Control retains all archive and earlier revisions, so anyone interested in the history of the document will be able to retrieve it via the document number and revision number.
When an engineering drawing is created, there is a section in the title block called “REV.” This is where the author indicates the first revision of the drawing. Up in the top right corner is where the table for revision tracking is located. It is similar to the procedure revision table in that over time, the table grows as changes are made to the drawing, and the revision letters are recorded for historical reference.
Software and firmware are also maintained under revision control. A bill of materials is often associated with a schematic diagram and a printed circuit board. As an example, a bill of materials at revision 1B might indicate the following:
- 1 = 1st revision of the PCB fab
- B = 2nd revision of the schematic
If on the schematic a resistor value changes, but the package type remains the same, then the schematic revision needs to bump a level, but the bare PCB does not change. Then we have the following change to implement on the PCB(A) assembly bill of materials documentation.
- 1 = 1st revision of the PCB fab (No change)
- C = 3rd revision of the schematic
If the schematic changes, causing a board change, then everything bumps up 1 revision. Hence:
- 2 = 2nd revision of the PCB
- C = 3rd revision of the schematic
A quick assessment of a revision number for a printed circuit board assembly at revision 8E would tell us the PCB has changed seven times and the schematic has changed four times — not necessarily simultaneously. We should be able to go to Documentation Control and ask for documents supporting bills of materials dated from the Release to Manufacturing, inclusive of Schematics 1 through 8, and PCBs A through E. Why would I want this? Because customer service is responsible for repairing every version, and all historical information on a product has to be readily retrievable.
Revision control is absolutely necessary for a multitude of reasons. Don't get caught waiting too long to control your company's documents.