Mistake proofing and Poka-Yoke are terms engineers use to refer to implementing devices, tricks, techniques that eliminate defects in the assembly lines; and if defects occur, these tools allow for them to correct them immediately.
Mistake Proofing Key Takeaways
- If you don’t implement mistake-proofing, product returns will happen.
- Having an SOP (Standard operating procedure) is key for mistake-proofing assembly
- Because of current travel restrictions, electronics designers can’t visit China, making it even more important to have a mistake-proofing by design mindset
- Remember, anything that can go wrong will -monkey proof your assembly process.
Electronics manufacturing is a cash flow intensive operation; let things slip and you’ll have a failed project and wasted money and time.
We asked our network what is the no 1 mistake they’ve seen hardware start-ups make and here are the results.
There are real-life examples of companies that gathered lots of money and failed. Juicero.
It becomes then imperative to make sure that, in every stage of product development, you devise a strategy to reduce and keep costs in check without sacrificing quality.
We know this as mistake-proofing or Poka-Yoke.
Table of Contents
TACTICS WE APPLY TO FACILITATE CORRECT ASSEMBLY OF YOUR DEVICE
To achieve zero defects, we implement Poka-Yoke tactics in the design stage, we design devices in a way that it is impossible to assemble them incorrectly.
Here are Custom projects we built for different clients
Here are some examples of Poka Yoke.
1- MISTAKE PROOFING BY DESIGN
Have a look at this case we made for a project. There’s a very tiny part that we added to the design with the only purpose to avoid errors in assembly.
Let me show you what I’m talking about.
That little “Pin” in the green circle makes it impossible for operators to assemble the product the wrong way. This is the beauty of designing a device with assembly mistake proofing in mind – you can monkey proof it.
It doesn’t matter if the operator is a rookie, she or he will know right away if she/he assembled it right or not. Allow me to show you what I’m talking about.
The operator has to put these 2 pieces on top of the case.
The result should look like this
You might think, how difficult can it be to get it right? One piece on one side correct? Well, the thing is that the two pieces aren’t exactly the same. Piece N0 1 has to go (from the image point of view) at the top and piece No 2 at the bottom.
The small “pin” added to the case design makes it impossible to swap them. If you put piece N01 at the bottom it looks uneven and it won’t feel right when you place it there.
The operator will know immediately that it is the wrong piece and correct. Thanks to that little pin in the design, there’s only one piece that fits properly.
This principle of mistake proofing by designing can be applied to every part and component of your electronics.
Here’s another example. Look at this two wires, they might look the same at first, but they’re not.
Have a closer look, when you connect them to a UDB port, the ground wire stays on the right side (we need it on the right side)
But if you use the other end of the wires, see how they fit, see where the ground wire is.
For wire N01 the ground stays on the left (that’s a problem), so how do we make sure operators always connect wire No1 to the USB port so that the ground is on the right? You don’t.
You redesign wire No1, so it doesn’t matter which end you use, the ground is always on the right, that’s what we did and that’s how we made sure operators would always get it right.
We’re in the business of helping companies with the complete process of designing and manufacturing their electronics.
The tactics above are the ones we use for our client’s projects; you can see such devices here.
A zero defects product is something we emphasize from the design stage; what this means for you is that you’ll have a high-quality product reaching the hands of your customers.
If you didn’t take an error-proofing approach and you’re already heading towards mass manufacturing, don’t worry, there are still error-proofing techniques you can implement.
Especially when tackling human errors.
These are effective at eliminating defects, but beware, they take time and money to set up, that’s why we recommend doing mistake-proofing in the design stage, this is one of the six mistake-proofing principles, but more on that later.
HUMAN ERROR IN THE MANUFACTURING PROCESS
- Missing correct orientation of a part
- Incorrect insertion of a component
- Incorrect screws and fasteners use
- Skipping steps
Those are just a few of the human errors that can happen during mass manufacturing, make many of them, and they’ll plague an entire production process with assembly errors.
Imagine having a 20% product return rate on your first product run, just when you’re trying to recover from your NRE expenses; this could put you out of business.
Luckily, there mistake proofing techniques to prevent human errors that truly work.
Throughout this article, we’ll also use the term Poka Yoke, which means mistake-proofing.
MISTAKE PROOFING METHODS
2- BARCODE OR RFID SYSTEM TO PREVENT USE OF WRONG PARTS
Errors occur when operators are assembling devices; especially when dealing with products that have different model variations, which is usually the case for smartphones and laptops.
To prevent a major assembly error, say installing the camera of the iPhone 12 in an iPhone 12 mini, engineers should put in place an automatic verification system.
Installing a barcode or RFID control system for product parts will make for a good mistake-proofing technique.
In large operations, there are many look-alike components, making a manual assembly error possible. There are parts such as ECU modules that look alike but contain different software.
For this situation, a scanner proves to be a great mistake-proofing device, it’ll automatically verify the scanned part against a pre-defined part number.
3- VISUAL CONTROLS AND INSTRUCTIONS BASED ON THE OPERATOR’S EXPERTISE
Each Poka-Yoke technique aims to facilitate correct assembly and set up a reliable process; here’s one that will work wonders in avoiding potential errors.
Interactive assembly instructions for mistake-proofing. When you order furniture from Ikea and receive it, you put it together by following a manual correct? Well, this follows the same logic.
If you give an operator a detailed manual on how to assemble a product; they’re more likely to prevent defects.
Handing out paper manuals isn’t enough, yes this counts as a mistake-proofing tool, but to make a real mistake-proofing device you’ll need something interactive, something with a screen.
This is more engaging because it’s interactive.
A screen mounted on a wall or rugged tablet can show a step-by-step process, highlighting the critical parts of it and even allowing the operator to check for the most common mistakes done while assembling a specific part.
YOU CAN SET UP MESSAGES AND WARNINGS
- Avoid mins connecting wire harnesses
- These are the correct replacement substitutes.
- Look out for part defect
- This is the correct way, number, and part of this process step
You can show a video or illustration of how to make a self-inspection.
We consider this one of the best mistake-proofing devices, especially for new operators.
4- APPLYING MISTAKE-PROOFING THROUGH OPERATOR CONTROL
We can also approach mistake-proofing from an accountability perspective; if operators know we can trace any mistakes back to them; they’ll be more cautious and avoid an error.
For big assembly lines, having a system in place that controls the access of operators to an assembly station based on their certifications is key to ensure the qualified personnel is dealing with the correct task.
As manufacturing operations become more dynamic and advanced, ensuring the right operators engage in the right task is a real challenge manufacturers face.
5- TOOL INTERLOCK FOR MISTAKE PROOFING
Screws and fasteners, electronic devices have them. Using the right tools to tighten them is essential to avoid having incorrectly assembled devices.
You don’t want products falling apart when they reach the user’s hands.
Ensuring that each assembly station only carries specific tooling for the intended assembly goes a long way in creating an error-proof assembly line.
MISTAKE PROOFING PRINCIPLES
Of the six mistake-proofing principles, this is our favorite, the one that makes most sense honestly.
ELIMINATION BY DESIGN.
This is all about designing a product/part that doesn’t allow for the wrong assembly, just like we showed you in the examples above.
The change in wires we did avoids incorrect insertion, it simply won’t allow for mistakes in assembly because you can only assemble it the correct way.
This is the best way to eliminate any potential error and deliver on product quality.
Poka Yoke Example 1
- Process description: A shaft is pressed into a press hole in a control arm
- Before Improvement: The two ends of the shaft had the same diameter, and either end could be pressed into the hole. Shafts were often pressed into the hole backwards.
- After Improvement: The diameter of the press hole and the end of the shaft to be pressed were made smaller so that the other end does not fit the press hole. The danger of backward press-fitting is completely eliminated.
Illustration source: Poka-Yoke, Improving Product Quality by Preventing Defects, Edited by NKS/Factory Magazine
Poka Yoke Example 2
- Process Description: Variable capacitators (with attached potentiometers) are mounted onto circuit boards. The potentiometer terminal is located at the top of the capacitor, and it is important to prevent other items form touching the terminal.
- Before Improvement: Plastic tubing was glued over the terminal connection to prevent other items from touching it. However, force had to be applied tot he terminal to attach the tubing , and the terminal sometimes broke under the pressure.
- After Improvement: A new method of attaching and insulating the terminal was developed. Additional work becomes unnecessary, and the defects caused by the tubing are completely eliminated. The standard for assembly is also reduced.
Illustration source: Poka-Yoke, Improving Product Quality by Preventing Defects, Edited by NKS/Factory Magazine
6- MISTAKE PROOFING BY REPLACEMENT
Humans make errors, we already discussed that, so, if you can automate some steps in the assembly line with automatic/robotic processes, you’ll be one step closer to eliminating incorrectly assembled products from your operation.
Perhaps having an automatic dispenser for the correct amount of adhesive needed to stick two parts together.
Color coding parts, instead of having one color, are a simple way to correct a problem fast.
7- MISTAKE PROOFING BY PREVENTION
Limit switches, an electronic sensor, vision systems, or any other means that will prevent mistakes in a product to make it to the next step of assembly and go further processing.
Poka Yoke Example 3
- Process Description: A series of holes are drilled in a plate
- Before Improvement: The operator’s skill was relied on to tell whether the hole was drilled tot he correct depth. However, the drill was sometimes retracted before it had gone in all the way, resulting in faulty drilling. Troubles resulted during assembly.
- After Improvement: Two limit switches were mounted on the drill press. Faulty drilling is indicated if limit switch 1 is released before limit switch 2 has been tripped. A buzzer is sounded to alert the operator.
Illustration source: Poka-Yoke, Improving Product Quality by Preventing Defects, Edited by NKS/Factory Magazine
8- MISTAKE PROOFING BY FACILITATION
Monkey proof the assembly process, create video tutorials on how to assemble a device correctly and have LCD screens on the assembly stations for operators to know exactly what to do.
Combining steps in the assembly line is also another way to facilitate make the work of the operators easier.
Adding marks that will guide an operator to the correct way of assembly.
9- MISTAKE PROOFING BY DETECTION
Have detection systems in place with warning functions that will alert the operator when something is about to go wrong.
Poka Yoke Example 4
- Process Description: Two similar parts are molded with a single die, then sorted and separated by hand during deburring.
- Before Improvement: Because of inadvertent mistakes while sorting, the two parts were sometimes mixed together. Customers complained when they received mixed up parts in their deliveries.
- After Improvement: A photoelectric detector was installed on the jig used in the deburring process. The parts are rotated, and if any light is detected, it is part A. If no light is detected, it is part B. The operator sorts the parts correctly with this device.
Illustration source: Poka-Yoke, Improving Product Quality by Preventing Defects, Edited by NKS/Factory Magazine
10- MISTAKE PROOFING BY MITIGATION
This seeks to minimize the effects of the mistake. This includes mechanisms that reduce the impact of an error and defect; products designed with low-cost, simple rework procedures when discovering an error; extra design margin or redundancy in products to compensate for the effects of errors.
