Anyone involved in surface-mount board manufacturing knows that the performance standards just keep rising. Cutting manufacturing and test costs, meeting impossible deadlines, and turning out high-quality products are all in a day’s work. And you are expected to do all this while facing the challenges of increasing board densities and finer-pitch packaging which make it much harder to get electrical access with in-circuit test (ICT) fixtures.
In this environment, automated optical inspection (AOI) with high defect coverage can make a significant difference. AOI uses high-magnification optical cameras (vertical and/or angled) mounted above the circuit board. An LED lighting dome illuminates the board under inspection from above and from the sides.
Using different combinations of camera angles and lighting, AOI systems inspect for solder joints, solder shorts, and component presence and location. AOI systems can be used in-line with automation equipment and often are integrated with a repair station.
Accurate, real-time feedback from AOI can help monitor and control your assembly process by monitoring different steps during the board assembly process. For example, in a post-reflow application, AOI systems inspect for missing components, misplaced or skewed components, solder shorts or bridges, lifted leads, low solder, misoriented parts, and tombstoned or billboarded components. Focusing on process improvement keeps costs, quality, and schedules under control.
If electrical access is diminishing because of dense boards and fine-pitch SMT packages, AOI provides some of the defect coverage traditionally delivered by ICT. For example, where electrical bed-of-nails access is being lost, AOI inspects for common faults such as missing devices, shorts, and opens. Optical inspection also can check fault classes that cannot be tested electrically, such as low solder, billboarded devices, and missing bypass caps.
AOI vs Human Inspection
Compared to human inspection, AOI offers several benefits. It eliminates the number of human inspectors needed and the associated costs of hiring and training. AOI reduces the number of locations that a human inspector must examine from thousands to a few sites per board. This means the number of human inspectors can be cut to one per line.
AOI also improves inspection consistency and accuracy to increase yields. High-magnification optics and automated inspection provide more reliable, consistent results than human operators can deliver, especially on fine-pitch devices such as 20-mil or 16-mil quad flat packs.
On dense boards, it’s especially hard for human inspectors to do a good job. For the accurate, immediate feedback needed to control and improve the soldering process, AOI systems are superior to people. AOI systems also offer statistical process control tools that extract key trends and defect data. Getting this type of data from human inspectors is much harder, and the data is not always reliable.
AOI and ICT
AOI and ICT are best viewed as complementary technologies for detecting process-induced faults on circuit boards. Figure 1 shows how AOI and ICT address overlapping but different defect spectrums. Each technique expands overall defect coverage by finding defects that the other technique cannot find.
Less obvious but equally important, each technique also supplies coverage on common faults where the other technique does not. For example, ICT can find a lifted lead where visual access renders AOI useless, and AOI can detect a missing device in cases where electrical access is unavailable for ICT fixture access. By combining both techniques, you can eliminate virtually all process defects before functional test, leading to improved yields at final test.
In addition to covering different fault spectrums, AOI and ICT have different strengths and weaknesses that make them complementary rather than competing. AOI does not require a test fixture. Unlike ICT, AOI can be used on partially built boards and can give direct feedback on your solder process. However, AOI repeatability depends on the consistency of your process. It also requires visual access.
By contrast, ICT applies power to the board and tests for faults that are not physically visible. Repeatability in ICT is independent of the consistency of the assembly process. On the other hand, ICT requires electrical-probe access and a unique fixture for each board type.
Case Study: Networking Manufacturer
Here is an example of a company that combined ICT with AOI to achieve higher yields. This manufacturer of networking products has a low-volume, high-mix production environment. A typical board is very large, double-sided, with up to five or six thousand test nodes and many fine-pitch devices and BGAs.
Faced with increasingly complex and costly boards, the company sought a test strategy to identify faults earlier in the assembly process to simplify rework after functional test and reduce the cost of failed boards in the field. A second goal was to reduce the overall test cycle time: the time for the board to pass through inspection or test, the actual inspection or test run time, and the time needed to identify the physical fault.
Because this manufacturer had traditionally depended on end-of-the line functional testing, most boards were designed without ICT access in mind. The limitations on electrical access made optical inspection an obvious starting point.
An AOI system was used to inspect each side of the board when fully loaded, which was consistent with the goal of identifying faults earlier in the assembly process. AOI also offered high fault resolution that would reduce the time and costs associated with rework of bad solder joints and misplaced or misaligned components.
The AOI system was targeted at areas where process defects were most likely to occur. In this case, the company chose to visually inspect all fine-pitch devices below 20 mils. This strategy was designed to get the highest fault coverage in the shortest amount of time.
The AOI system was chosen primarily because the machine used angled rather than just perpendicular cameras. Angled cameras can examine solder joints at different angles, important because of the number of different lead types, such as J-leads and gull wings, used on the company’s boards. The system also was able to accommodate the large boards.
Adding ICT
Despite the advantages of the AOI system, AOI alone could not detect all faults. Shorts under components, for example, could not be detected easily using AOI.
Defect coverage with AOI alone was estimated at around 70%. To achieve fault coverage of close to 100%, ICT was added after the optical inspection.
In selecting a test strategy, the company evaluated different strategies in terms of test cycle time and costs:
AOI and functional test only.
ICT and functional test only.
AOI, ICT, and functional test.
The analysis showed that the third approach produced the optimum results for both test time and costs.
The results of this inspection and test strategy are illustrated most dramatically in the lack of field failures. Board failures in the field due to process defects have been almost completely eliminated. By combining the strengths of two different but complementary technologies, this manufacturer achieved significant yield improvement.
About the Author
Dominic Haigh is a business unit manager for optical inspection at Teradyne’s Assembly Test Division. He h
February 1998