Automated Optical Inspection

There is an inverse relationship between the size of a company and the breadth of an Electronics Engineers responsibilities.  EE’s are schematic and board designers, part-procurement specialists, product testers, firmware programmers, enclosure designers, and often more. Once they export their Gerber-files or IPC-2851 complaint cad data, the next thing an engineer generally wants to do is bring up the board and load firmware a few days later.  Bosses should let them do just that.  Instead, engineers are often asked to be the final quality-control check on their design, a task that is simultaneously tedious, time-consuming, and error-prone for the untrained and inexperienced.  Additionally, any errors require time-consuming corrections or shipping back to the manufacturer.

Engineers are paid far too much per hour to troubleshoot part issues.  As an example, I once spent a full workday and part of the next one investigating a misbehaving new design with an intermittent fault, only to discover the problem was a bad shunt-jumper apparently purchased out of the trunk of a 1974 Ford Pinto from a trenchcoat-clad hustler at “Jim’s Jumper Joint” off of I-90, all for cost- “savings” of around $0.01.  I never even knew that was possible to manufacture a shunt-jumper so poorly it wouldn’t work, and it’s a problem that’s impossible to see with the naked eye, or even with light magnification.  

(If you are wondering how much my former company saved on that one, the answer is “negative one.”  There were so many cost-”saving” measures taken at my coworker’s and my expense over a number of years that they eventually lost me as an employee a few years later when I eventually tired of their cost-”saving” measures.)





So how do you keep your projects on time, your employee’s happy, and your budget acceptable?  One sure option is the Automatic Optical Inspection machine, the AOI.  These machines are tasked with inspecting every part on every board that rolls off of our assembly line and looking for manufacturing faults or errors.  When errors are identified, the boards are immediately returned to the Quality Control department for rework and the boards are repaired before they ever see the inside of a shipping box.

Parts continue to shrink in size.  And where there is one small part, there are usually dozens or hundreds more.  A 0204 (01005 imperial) is 0.2 mm wide by 0.4 mm long.  If the part is shifted off of its pad, billboarded, tombstoned, etc… the error can be due to movement by around ¼ of either of those dimensions.  For reference, a human hair is around 0.1 mm in diameter.  Finding a single billboarded 0204 resistor on a PCB is only marginally easier than staring at a normal human head and searching for a follicle with two hairs coming out of it.

Fallibility & Time

Years ago I taught an electronics class to high-school seniors.  One of the first hand-soldered through-hole projects was a Velleman kit that had blinking lights.  The kit instructions said, “be careful when placing Q3 and Q4, these parts look identical but are different!”  I said, “Be careful when placing Q3 and Q4, they look similar, but are different.”  And I of course wrote on the board “Pay attention to the case markings for Q3 and Q4!”  And invariably, around ⅓ of the students would present me with a non-functioning kit and ask me to help them trouble-shoot it.  I didn’t even have to look, the student had swapped Q3 and Q4 – every single time.

Humans are fallible.  We get tired.  We have bad days.  We can’t sleep at night because we have a new baby in the house, or we are caretakers for our ailing parents, or our knees-ache from bursitis, etc…   In short, some days we make more mistakes than others, and our attention isn’t as keen as it should be.

With that in mind, take a look at the following image of PCB from a production TV set.  It’s got off-board connectors, QFN, TSSOPs, DFP, SOT23, as well some small passives.  How many missing parts can you identify in this picture?  Are you sure?  Are those parts supposed to be DNI or are they in fact supposed to be there?  Are you sure?  Did you ask a coworker? Did you have to double-check? Did their number match yours?  Computers don’t have the same problems that humans do.  An AOI can scan this board in less than a minute.





Now take into consideration that I’ve presented approximately 2 square-inches of a PCB that is double-sided and 7.5 x 9” – leaving approximately 135 square-inches to inspect.  And that’s just for missing parts.  What about solder fillets?  Cold solder joints?  Solder Bridges?

Even if you paid your engineers minimum wage, the break-even when it makes sense to use the AOI process is still less than one-hour of engineer time.





But wait, there’s more!  Computers don’t see the world the way humans do.  They see arrays of numbers that represent pixel values.  And when it is advantageous, they can simply rearrange the numbers to make a part easier to inspect.  As an example, here’s what a board will look like when separated into a variety of color-spaces (one example of each is shown below).  Humans are physically incapable of such feats.

One example of a color-channel separation from each of four color-spaces.

Two-paths

Penny-Wise / Pound-Foolish

Don’t get AOI.  Most of our products come off the line assembled correctly.  And you can gamble that everything went well, that your tiny components didn’t tombstone, that there are no solder bridges, that there are no latent defects that you don’t see when you first bring up your board.

But if there are defects, you’ll need to spend a good amount of time finding them yourself.  Then the boards will have to be packed and shipped back to us, we’ll repair them for free, and ship them back.  You’ll have invested several hours of your engineers’ time, perhaps at the cost of $150, and perhaps another $50 in shipping costs.

Remember — the break-even point for AOI is less than one-hour of engineer time at minimum-wage.  It’s less than 15 minutes at entry-level wages.

Pound-Wise / Penny-Foolish

After a non-recurring engineer setup, whose time depends on the number of line items, our AOI machines can scan each board for assembly errors in about half a minute.  Then operators will verify defects and send them back to the quality assurance team to correct them.  By the time your engineers receive the board, it will be guaranteed to be defect-free.  Then your engineers can get to work doing what they were trained to do — loading firmware, testing, and getting the product into customer hands.

For AOI — customer’s time is valuable.

Repeatability is a concern.  A visual inspector might miss it — a machine will not.

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