Contract Manufacturers Can Do More With AOI

As an OEM, your business needs to produce high-quality boards that sustain fewer rejections and warranty returns. It should come as no surprise that contract manufacturers (CMs) want the same thing.

At the same time, your business also wants to keep prices low and lead times short. How can companies keep all these competing challenges in order? One method is automated optical inspection (AOI), which can help your CM deliver on all points, if they use the method to its full advantage.

Testing and inspecting for quality
CMs use a combination of visual inspection and electrical tests — flying probe, in-circuit, functional tests — to prevent defective boards from shipping. Now, with modern vision systems and increasingly sophisticated processing algorithms, AOI systems can reduce the need for electrical tests.

The advantages of AOI over “manual” inspection with magnifiers or microscopes include:

  • Higher consistency: Manual inspection results vary with the temperament, training, and alertness of each inspector.
  • Higher throughput: AOI enables faster inspection of the entire board. It also delivers orders of magnitude faster for our larger boards.
  • More thoroughness: AOI checks every single part, every time, no matter how many or how small.

Humans, on the other hand, tend to miss things. If you were given a board with hundreds of millimeter-sized parts and told that one was missing, could you find the empty pad?

Give it a try: Find the “missing part” defect on this photo of a Z-AXIS board.

As always, quality improvements depend on how well you use the system. Here are five questions to ask your CM about their use of AOI:

1. Is it used fully?
Many CMs use AOI to check for missing parts, tombstoning, and billboarding, and then proceed to electrical tests. It’s more effective to use the full capabilities of AOI instead. A well set-up AOI can check for misaligned parts including wrong polarity, defective solder joints — including opens — and solder bridges. With optical character recognition (OCR), it can read part markings to check for correct parts as well.

2. Is it programmed effectively?
Careful set-up and programming means fewer false positives. This is important since a high rate of false positives can train operators to quickly hit “accept” and override the AOI. This usually means that more defects are mistakenly accepted.

3. Are operators empowered to program?
Typically, a manufacturing engineer programs the AOI for a board, validates it, and walks away. We’ve found it’s more effective to train operators to a higher level and empower them to make changes during inspection. For example, we recently received a batch of components with the part number in a new location, not where the AOI had been programmed to look. Rather than override the “missing P/N” defect on every board, the operator reprogrammed the AOI to find the marking in its new location, and kept the job going.

4. Are defects tracked?
Paperless defect tracking speeds rework. We place a 2D barcode on every board, and save all AOI inspection data and images to our network. Rework techs scan the barcode on defective boards to bring up the AOI data and zoom right to the defect.

5. Do you learn from the data?
Many CMs underutilize the vast amount of data that AOI provides about each job. Analyzing this data can reveal trends and identify opportunities for process improvements. In some cases, it gives us information we can use to recommend design changes for improved manufacturability.

Since adopting AOI five years ago at Z-AXIS, we have increased quality, maintained high throughput, and dramatically reduced labor costs. We have found that with 100 percent AOI, combined with electrically testing passives before placement, we can easily achieve less than 0.1 percent warranty returns. That’s good for us and for our customers.

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3 comments on “Contract Manufacturers Can Do More With AOI

  1. _hm
    May 15, 2013

    Very informative and interesting post. I like AOI and it is very effective tool. However, it does not substitute electric ICT and FCT. Electronics assemblies are much more complex and it needs these additional tests.


  2. prabhakar_deosthali
    May 16, 2013

    AOI is really a very effective and non-destructive  tool for board inspection. It reduces time for inspection, mishandling of the assembled boards on the inspection table and possible damage to the board while testing .

    Only thing this kind of inspection method may not be able to detect is failure of parts during the assembly process ( either because of static charge or infant failure) for which some functional testing may be necessary after the AOI is done, in my opinion.

  3. elctrnx_lyf
    May 16, 2013

    For any PCB assembly manufacturer AOI is an essential step for identification of defects that could be easily identified and can be corrected with very less cost. The defects that actually go into further stages could be very expensive to rework. In addition to AOI, Xray also will be good tool to identify any solder related defects of small pitch high density BGA packages.

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