Production Test Verification is part four of our look at New Product Introduction, if you missed the previous articles, get caught up here. In this post we use the prototypes from the Engineering Verification build to start the production test procedures and fixtures.
At this point in our process through the New Product Introduction, we just received our first manufactured boards from the Engineering Verification run and are giddy with excitement I’m sure (or frustrated with hardware bugs!).
Your manufacturer is also excited about the boards as well so they can start the next step in the NPI process: designing and producing the Production Verification Test.
What is Production Test Verification
As we previously hinted at in the Design for Manufacturing and Test phase, the manufacturer has an incentive to make sure every product that goes out their door works to the best of your design. This is where the Production Test Verification comes in.
While the EV build isn’t the final product, it still gives the manufacturer something to work with to figure out how and what to test. This will help drive changes in the next build that would enable more thorough testing.
Production Test != Unit Test/Design Test
A quick note of what Production Test Verification is not. When a manufacturer is looking at testing the product, they aren’t talking about unit tests or design verification. This is what your team does with the EV units and during normal development. You hammer on the edge cases, the raw performance metrics, and error handling.
The manufacturer simply wants to test to make sure the board was built correctly and any firmware was installed. They are looking for as simple of a test that can verify they didn’t screw anything up in assembly or need to adjust parameters in their system.
Types of Production Test Verification
To accomplish this simple verification test, the manufacturer has a few different types of tests to call on. The three most common are listed below, in order of increasing complexity and cost.
- Visual Inspection (optical/x-ray)
- In-Circuit Test (ICT)
- Functional Test (FCT)
Each has its strengths and weaknesses so part of this phase of the NPI process is selecting the which tests will provide the best test coverage balanced with the cost of test development and per-unit test cost. We go in depth on each below.
The simplest and most straightforward method of making sure the product was built correctly, is to simply look at it. This could be as crude as having technicians manually inspecting each piece to impressive pieces of equipment that automate the process.
Optical verification uses cameras to look at each component on a PCB carefully and compares it to a pre-programed image of how it should look. Each image can verify the component was placed in the correct orientation, check the part markings to make sure it is the right one and even look at the solder connections to verify they are correct.X-ray is used on BGA parts since the contacts are located underneath the part. The x-ray looks for solder bridges (shorts between pins) and good contact between the pins and the pads – the two most common failures for this compact parts.
Benefits of Visual Inspections
Visual inspection is typically easy to set up as the machine reads in the placement files from the pick-and-place machines and records known good boards. With these fundamental data points, the machine is ready to go. By far the simplest of setups we will discuss here.
For parts that include part markings, this inspection can be an excellent first check to ensure that parts were placed correctly (correct value positioned in the exact spot and the right orientation). This could solve many ‘weird’ issues that occur in later tests as missing capacitors or resistors would fail the visual inspection.
Downsides of Visual Inspections
The downside of the optical and X-ray verification is time. Both take significant time to look over all the parts on the board, make an image comparison, and move the camera to the next location. X-rays also are also dangerous, so manufacturers require extra care whenever they are being used. Both time and complexity add cost to the production price, and thus your product.
They also only work well for parts that have value markings. Small resistors and capacitors typically don’t have any visual marking so the visual inspection can only verify it is placed, not if the wrong part was placed.
In-Circuit Test (ICT)
The next step up in complexity and evaluation is the In-Circuit Test (ICT). Similar to the visual inspection station, this is a specialized machine. Think of it like a big, complicated multi-meter that can quickly switch between hundreds of pins and check voltage levels, resistances, capacitances, and even inductance.
Your PCB would have a test point exposed on the board for every net that you want to test. The manufacturer creates a bed-of-nails that attach to their ICT machine which has spring-loaded pins. Each pin matches up with the test points on your board. These points could be via, larger pads, or traces with specific areas of solder mask removed.The machine would then be programmed to insert voltage levels at various test points within the design and measure the output at others. The voltage could be DC signals to measure resistance and shorts, or AC signals to measure capacitance and inductance.
Benefits of ICT
The benefit of the ICT is that it can verify most of the components were placed correctly and were the right values. This covers one of the weaknesses of the visual inspection in that exact values of components are measured.
In some ways, the ICT can replace the visual inspection as it can detect missing components or shorted nets. These errors would get detected by invalid measurement values at various nets which would allow the system to narrow down the problem and determine the error.
Downsides of ICT
The ICT has three major downsides:
- Expensive to design the bed of nails
- Significant amount of time to program the correct sequence of tests into the system
- Requires a test point for every net exposed on the board which could drive the size and complexity of the layout up
Functional Test (FCT)
The highest level of complexity and customization is the Functional Test (FCT – yes the C is a bit odd).
The FCT runs the product through its paces, perhaps not as a customer would use it, but rather in a way that all the functionality can be verified. You or the manufacturer would build up a test fixture that would include pogo-pins (spring loaded pins), mounting posts, and some sort of clamp. The board can then be powered up, voltage and current levels checked, and digital communication piped into a computer.
For embedded products, this could include a particular version of firmware, or firmware running in a special test mode. This is the level at which the FCT is differentiated from the ICT in that the board would be active and tests the functionality rather than the raw values of the passives.
Benefits of FCT
The benefit of the FCT is that it tests the product much like your customer would, including testing the firmware, buttons, lights, and communication systems work correctly.
Where the ICT and optical inspection ensure parts are installed correctly, the FCT ensures that they are functional as well. While the ICT checks passive (RCL) values, it cannot verify that silicon components are functional.
The FCT is also a great place to program in serial numbers, calibration offsets, or other device specific values that need to be added to the specific product.
Downsides of FCT
The downside is that special test firmware will be required as well as the test infrastructure. The FCT also takes some time to run, adding additional cost for every facet that you verify.
The test infrastructure may require your company to buy test equipment for each testing station at the manufacturer and maintain your own computers to run the test software.
The End Result
In the end, your team and the manufacturer will decide on the best approach to testing the product which might include a combination of all the steps above – optical inspection of critical components and FCT of the user interface. And while every test adds cost to the product, shipping broken units because a part manufacturer sent the wrong reel, will cost even more.
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