|A typical production line includes (from left) a solder printer, AOI paste inspection, component mounter, AOI placement inspection, reflow oven and AOI solder inspection. |
Contract electronic manufacturers as well as OEMs need to maintain a good reputation for delivering defect-free printed circuit boards to retain current customers and attract new business, especially in these tough economic times. Most have adopted one form or another of Automated Optical Inspection (AOI) to ensure improperly placed components and defective solder joints never reach the customer.
Manufacturers need to thoroughly evaluate AOI equipment technology in an effort to ensure the highest quality SMT manufacturing process for their products. Here, we will help manufacturers identify the board inspection tools that deliver the most reliable results.
|Leads appear short and partially hidden due to optical distortion. |
Between 70 and 80 percent of circuit board defects are caused by solder problems. The remaining 20 to 30 percent are component-related problems. Based on these numbers, if a manufacturer has only one piece of inspection equipment, it should be to check the results after reflow soldering.
Manufacturers expect the inspection solution they invest in to be 100 percent accurate with zero false calls. That expectation has spurred a range of recent technological advances that allow automated inspection to come closer to delivering those results.
Where to Inspect
When touring manufacturing sites, it helps to know where inspection systems fit into their surface mount operations. The first place to inspect is solder deposition to ensure the solder brick is present and the shape is within tolerances for a reliable solder joint. Not every manufacturer inspects at this stage, but it can be critical to catching and correcting errors before component placement where more value is added to the circuit board. The AOI system is usually an in-line machine located between the screen printer and the component placement machine but can also be handled off-line as well. The second point of inspection in the SMT process can be the pre-reflow step. There are two common opinions at this stage of inspection. There are those who find this the best location for inspection because it is the easiest and least expensive place to repair the board. Others find that most of the placement issues will be self corrected in the reflow oven and that more defects can potentially be created by manually adjusting component positions as a result of disrupting the solder paste.
|A telecentric lens corrects this distortion effect (as shown). |
Inspecting solder joints after reflow is the top challenge in ensuring the board has been properly assembled prior to operational testing. Recent improvements have advanced the reliability of optical inspection systems. The advent of faster X-ray inspection machines has enabled this accurate technique to move in-line. Depending on component types and whether components are mounted on both sides of the board, a complete solder inspection after the reflow oven includes an AOI system and an X-ray system (AXI).
Since solder defects account for most of the faults in assembly, manufacturers try to choose the most accurate and easiest-to-use inspection system they can afford. With different technology types available, it is important to know how to judge the options.
For optical systems, it is important to understand what the camera sees to correctly interpret the results promised by the various techniques. The key specifications for these systems include field of view, optical vs. digital zoom, resolution and number of image-detecting elements.
|(1) Board before inspection. |
The megapixel rating that is so important in choosing a camera for home use makes much less of a difference when inspecting at the micron level. It affects the field of view size that can be inspected in a single image and can contribute to faster tact time by covering more real estate in each view. However, no AOI system should be adopted based only on megapixel ratings.
Optical zoom relates to the resolving power of the camera lens and the density of image information available for analysis. Digital zoom takes the maximum optical capability and uses subpixel calculations to digitally enlarge the size of the pixel causing increasing blurriness rather than adding more image data density.
Optical zoom has the greatest effect on the accuracy of image data going for analysis. A recent advance in image fidelity comes from using a telecentric lens to control for optical distortion at the edges of the field of view. Indications that distortion has been causing false rejects include instances when components appear to be leaning to one side or the leads appear to be shorter on one side than the other when they really are identical.
|(2) Grayscale view provides limited data for inspection. |
Resolution specifications, when related to optical zoom capability (true optical resolution), provide important information about how much image data can be gathered from a designated field of view. What size components and solder joints can be accurately measured at various resolution levels?
- Use 10µ resolution to inspect 01005 components (0.01 x 0.005-in.)
- Use 20µ resolution to inspect typical 0402 and 0603 components.
The 20µ resolution system offers the largest field-of-view and allows a board to be inspected in the fewest snapshots. The 10µ resolution system provides a huge amount of detail on the smallest field of view, and requires a slightly longer time to inspect a whole board. AOI manufacturers offer in-between-size resolutions such as 15µ to maintain a fast tact time from an increased field of view.
How Many Imaging Devices?
How many charge-coupled devices (CCDs) capture the image data? Some lower cost systems use just one, while high-end systems use three. AOI systems that process image data by grayscale generally use just one CCD. With only 256 shades of gray, their image detail is much less. Grayscale systems often have difficulty in differentiating between a solder bridge and silk screen line, resulting in false calls and missed defects. Beware of color systems that use grayscale processing, because they do not retain the color image detail and it defeats the purpose of collecting color image data. The higher end systems use a color highlight system with three CCDs where each one captures data for just one color (red-green-blue).
|(3) Tricolor highlight inspection identifies solder shape and flaws. |
This effectively triples the image data for analysis and allows processing of two-dimensional data into a three-dimensional image where height and shape features are color-coded as on a topographical map. With near human eye color discrimination capability, the image processor can differentiate among the wide variety of colors on a PCB such as the substrate color, component color and silk screen color and accurately image solder fillets.
AOI neatly divides into pattern matching and algorithm-based inspection systems. Pattern matching uses a "golden" model against which to judge passing circuit boards. While perfect for component placement, this simple concept can often miss the wide range of solder faults that are presented. By contrast, algorithm-based systems analyze the pixel data from one or more CCDs to produce a finely detailed image based on measurement data that can indicate shifts in specific locations and identify types of faults.
With the introduction of components where solder joints are formed underneath the component as with J-shaped leads, X-ray technology was applied to solve this inspection challenge. X-ray systems are used to inspect ball grid arrays (BGA), Quad Flat Pack (QFP) heel fillets and connectors. It also provides a way to inspect dual-sided boards with a high degree of accuracy.
The use of computed tomography (CT) technology eliminates the shadowing from bottom-side components while top side components are inspected. Because CT scanning technology alone can be quite slow — such that inspections must be done off-line or handled slowly in-line, it can now be found paired with tomosynthesis scanning to shorten tact time for components mounted on one side.
Root Causes of Faults
All the inspection data collected by AOI and AXI systems at each stage helps prevent specific faults from escaping at a single production stage. Analyzing these data with a data mining software tool has helped some manufacturers identify and correct process problems by finding the root causes of failure. Companies involved in board assembly for automotive subsystems, factory automation, photocopiers and consumer products, have all used this software tool and experienced up to 80 percent reduction in faults compared to using AOI alone. The software reduced waste and re-work while maintaining the improvement more effectively than using AOI alone. Today a higher value is being placed on the quality of goods rather than quantity as demand for finished products has decreased. Making good decisions about circuit board inspection systems helps build a reliable reputation for defect-free production that brings in more business and retains current customers.
Contact: Omron Electronics LLC, 1 Commerce Drive, Schaumburg, IL 60173 866-886-6766 or 847-843-7900 fax: 847-843-8081 Web: http://www.omron247.com