How and When to Use an Engineering Change Order to Revise a Document
Image credit: Bureau of Labor Statistics
The tag-line for a big box hardware store comes to mind: “Never Stop Improving”. We can say that about almost any industry and certainly any high tech electronics company that employs printed circuit board designers. Since words have connotations, I refer to “changes” as “improvements”. New and improved has better optics than new and different.
It all starts with rev 1 for prototypes and rev A for release to production. In any case, we release the most elegant design possible in the limited time we have to hit a market window. That window could be defined by a trade show, shipping customer samples, back-to-school season or, any point in time. The shiny new product has to make a splash at the right moment to be of any relevance.
That kind of pressure is on the entire development team. The software team may be able to release updates long after the product has shipped. The hardware is not so pliable. There are many little pieces that make up the physical product. There are often a fair number of suppliers for each type of component. Those components may not be on the market for the long term. The lead time or price of a part could become unrealistic. Times change.
Image credit: Author - Sometimes, things do not go right the first time.
On the other side of the dial, a competitor may announce an even better version of something with a must-have feature. That feature will soon be part of a product line that will make yours a much harder sell. Regardless of the reason, research and development are evergreen tasks that are not going away; at least not in any successful organization. You have to keep up your end of the bargain.
The Three Fs: Form, Fit and Function
There are many reasons as to why we make improvements. One link in the supply chain is the company’s stock rooms. These locked down cages of inventory have a bin box for every part that goes into the end product. Every bin box has a unique part number on it. The piece parts that match the components description in terms of form, fit and function go into that particular box. The form, fit and function rule applies to all part numbers. The revision on the part number is irrelevant. Bin boxes do not usually include that variable.
Any change that affects the form, fit or function will require a new part number as well as a disposition for all of the previous parts.
The basic rule is that any part going into a stock room bin box has to be completely interchangeable with all other parts in that bin. For components, the part value, tolerance, rating, exterior dimensions, and recommended footprint should all match. The parts color (setting aside LEDs) or the fact that it is on a tape-and-reel or in a waffle package do not constitute form, fit or functional differences. The different transport packaging of the electronic devices may be broken out as dash-options which trigger new stock bins for tracking what are otherwise identical pieces.
Let’s break down a standard part numbering scheme. A part number starts with a two or three digit class code that codifies the general type of the part. The “intelligence” of the part number should go no further than that. Class codes include each component type with bare boards and assembled boards having their own class codes. These prefixes are followed by a sequential number issued by the Document Control entity. For the sake of continuity, the same numbers are often used across the different class codes for the bare board fabrication drawing, the assembly drawing, the schematic and other production files.
Bill of Material: 160-nnnnn-06
Bare board: 232-nnnnn-01
Where nnnnn is the same family number and the suffix at the end indicates any specific options related to that assembly or piece part. I’ve been in companies that try to make the part number more meaningful than using the next available number in the sequence. The idea always seems to run into situations that are not covered by the available categories.
Let’s say that there is a bucket for any length of flat-head screws with an 82-degree countersink and 4-40 UNC-2B threads. What about the captive knurled-head screw with reversed metric threads? Ok, I made that up to illustrate that you can’t think of everything when you set out to create a part number system where all of the digits have a specific meaning. Avoid painting yourself into a corner.
Revision or New Part Number?
When answering the question of whether an improvement constitutes a revision or requires an all-new part number, the simple test is this. Can I put the new one alongside the old one in the same stock bin and kit up the next build with either one without any problems on the assembly line or downstream to the end of the lifespan of the product? If so, then you can use the same part number with a new revision.
Any change that affects the form, fit or function will require a new part number as well as a disposition for all of the previous parts. Those parts might be on order, in stock, on the factory floor or out to the customer(s). The decision of what to do with those parts range from being purged, reworked, returned to the vendor, or hopefully, used as is.
Image credit: GrabCAD - typical ECO form
An Engineering Change Order (ECO) is required to revise an existing document or bring a new one into existence. The ECO is usually written by an Engineer, but a manufacturing delegate may initiate a change with an Engineering Change Request (ECR). The request can be met on an ad-hoc basis with a Temporary Variance Authorization. The TVA is the vehicle for keeping the line moving while the solution is adopted during the ECO lifecycle. Interlocking forms complete the circle of stewardship. It all runs on information; good or bad.
Clarifying a note or adding a missing dimension are examples of where a revision roll is sufficient. Adding or moving a component is going to trigger a new part number. Changing the value of an existing part will affect the revision of the schematic and Bill of Material. While you certainly can, there is no compelling reason to maintain the same revision for every member of a PCB family. It is a common practice for aesthetics and simplicity to roll all of the revisions together but not a requirement.
Keep Your Options Open
Dash options of the B.O.M. are a common way to control the parts list for various circuits based on the same bare board. Revisions of the stuffing options can then be carried out on whatever version(s) get the upgrade. In this way, the fabrication and assembly documents are sheltered from the caprice of the parts used when a PCB has a number of variants.
By the way, keeping these documents stable is important because a revision or, especially, a new part number, may trigger another round of costly and time-consuming re-qualifications. That might not be popular or even possible in every situation. As you get further along with the design, the latitude for making improvements decreases.
When I start out, I like to follow a software model where there are dot-revs in between number revs. Any change of direction or even flipping the calendar could trigger changing the rev from 1.0 to 1.1 and so on. Major rip-ups would accompany a full number-revision. To this point, an ECO is not required and all of the revision data would be cleared for the initial release.
Rounding it Out
Once the product is under ECO control, the ability to churn out new ideas is greatly diminished. The paperwork is only part of the headwind. The number of units in different stages of the build and how to make them whole is the tactical side of the equation. Thinking of every board as a potential production piece puts your head in the right space. The pre-production units may work well enough to go to mass production as-is. That could be great or horrible news depending on how it was approached.
Writing and incorporating ECOs is how a team records their accomplishments. If it’s not on Agile, (or Oracle or whatever), it didn’t happen. The work leading up to the first release is tracked and shared with a less formal method. Once the project is under document control, all improvements require justification and buy-in from a list of stakeholders. Further, all existing stock requires a disposition. The engineering change order is the vehicle for keeping the train on the tracks once it is underway.