|Two basic imaging approaches are used for inspecting PCBs. |
Defects generated in a surface-mount-device (SMD) printed-circuit-board (PCB) production line can often be attributed to the print process. If such defects can detected immediately after printing, defective PCBs can be removed, cleaned and reprinted, or repaired at this stage. Such inspection can ensure that only properly printed PCBs are populated with components, reducing subsequent repair costs and helping to boost production-line yields. The cost of rework increases as the PCBs progress through the line; with less wasted cost and effort by detecting defects as early in the PCB production process as possible.
There has also been a major push from automotive customers manufacturing safety products, such as air bag sensors and braking systems, to eliminate PCB defects from their products. The cost of failure for such products can be very high. Producers of these products are not allowed to clean PCBs to correct for defective printing. Boards with defective printing must be removed from the line and scrapped. As a result, there has been a considerable demand to monitor and control the print process for such products. Initially, two-dimensional (2D) solder-paste-inspection (SPI) solutions were considered adequate for PCB monitoring; however, experience has shown that three-dimensional (3D) SPI approaches provide more information and better control of the printing process. Many suppliers now offer 3D SPI solutions for PCB printing inspection and control, which can be sorted according to practical considerations.
|The Pi system is an inspection solution designed for PCB production. |
By paying attention to PCB defects, rework costs and the overall costs of PCB production can be reduced. An additional major benefit is gaining control of the solder print process to ensure higher PCB quality and higher production yields. The most basic solder errors to identify include volume, height, and surface area shapes, including 2D and 3D circuit structures. It can also be useful to detect any contamination of paste or pad when inspecting a PCB.
3D SPI Approaches
Many companies providing inspection equipment now offer 3D SPI solutions that meet many if not all customer requirements. The majority of these inspection systems are based on one of two major techniques, either laser scanning or some form of Moiré fringe technique, to develop a 3D image of an inspected PCB.
In most inspection systems, a solder print deposit is viewed with a vertical camera, yielding a 2D measurement of the viewed surface. At the same time, either a laser raster or Moiré fringe is projected onto the surface, facilitating height measurements by monitoring the pattern shift at various positions. A volume measurement is derived by multiplying the 2D measurement by the height measurement, although some may question whether this is a true 3D measurement.
A number of issues arise with conventional inspection solutions, which can lead to some concerns when using these inspection techniques. The first issue relates to the reference point for measuring the paste height. Inspection systems often rely on a local reference point, ideally on the pad, sometimes more than one reference point. Errors can arise from small displacement if, for example, a reference point is not on a pad.
|This is a topographical view of an individual circuit pad. |
The second issue relates to position and angularity corrections to cope with PCB warpage. This can be time-consuming, involving much mechanical movement and causing further inaccuracies in reference measurements.
The third issue relates to the frequency of fringe (or laser raster) that generates the 3D image. The chosen frequency determines both range and resolution for a measurement. To resolve paste heights (in the region 100-120µm), some systems only measure above a fixed height, such as 40µm. The area under this threshold is assumed to be solid. The forth issue relates to positioning of the orthogonal camera, which achieves a 2D view from above and requires some image construction to produce a 3D view.
Fortunately, a novel inspection technique known as 360° Moiré technology can generate a true 3D PCB image and address these issues with existing inspection systems. The technology is the basis for a 3D SPI system developed by Vi Technology known as Pi.
The Pi solution, which is covered by 10 patents, employs the latest techniques in image acquisition, lighting, and algorithms as part of an easy-to-use production tool. It provides large PCB capability (boards to 533 x 600mm2) in true 3D color, within a very small factory floor area (800mm wide), complying with increasing constraints on floor space in modern SMD lines. A number of Pi models are available, for different customer needs and budgets. Each model delivers the same quality of inspection, with different inspection speeds to match the needs of an SMD line.
A Windows-based operating system (OS) was not fast enough for the real-time image processing of this inspection system. Linux, which is less susceptible to virus infection, was chosen as Pi's OS. The Pi inspection system does not use a keyboard or mouse; it operates with a natural user interface (NUI) and a touch-sensitive screen. Many users require as little as 10 minutes to learn how to operate the system.
A web-based software tool called Sigmalink is included with each Pi system to assist with programming, such as converting a PCB's Gerber data into a pad file and adding basic customer data. The software can be programmed to minimize activity on the Pi system during product changeovers, to maximize production throughput. A rotatable carousel with full PCB image view simplifies finding previously inspected products.
The Pi system automatically configures tolerance settings, which can be replicated for any PCBs in production that might be similar. Different tolerances can be set for different pad geometries, and an area/roof ratio, commonly used for stencil designs, helps establish an optimum tolerance for each pad geometry. To speed setups, tolerances can be set for groups of similarly shaped device pads. An "Expert" mode offers full flexibility to set and modify discrete tolerances, allowing tolerances to be named and saved into look-up tables for future reference.
|This close-up PCB image is zoomable and rotatable for ease of inspection. |
The Pi system, with embedded SPC capability, allows real-time monitoring of process trends, providing warnings when some action might be required, such as a stencil cleaning or replenishment of paste. Each Pi system includes an on-board calibration jig, for periodic calibration of mechanical and optical characteristics to ensure consistent and repeatable performance and smooth transfer of different programs across different inspection platforms. Interface modules can be configured to provide feedback to Stencil printers. Most customers chose to modify only stencil position offset (X and Y position).
In addition, work is under way to determine criteria for initiating (either wet or dry) stencil cleaning based on details from an inspection. This could provide potential savings in cost (such as solvent and paper) and significant improvements in throughput. Many customers currently select a stencil cleaning frequency that experience has shown to minimize problems. For a complex product, this may be after as few as two PCBs. For a print time of 10secs and clean time of 30secs, the time for 2 PCBs may be only 50secs, or a line rate of 25secs/PCB. If the stencil cleaning frequency can be extended to 4 PCBs, the rate becomes 17.5secs/PCB or a throughput increase of 30 percent.
A number of factors should be considered when choosing an SPI system, including: the length of time required to train an operator to use the system, which can be critical where there is high staff turnover; ease of programming the system, since this will determine the skill set required to run the system; the speed of programming and how long is really needed to put the system into production; the cycle time, and whether an SPI solution can match a production line's output rates; inspection validation, and confidence in the results provided by the SPI system; and high reliability, with accurate and repeatable results.
Programming an SPI machine is generally considered fairly simple (compared to an AOI system) and relatively fast. It does, however, involve numerous important steps:
- Gerber-to-pad conversion, which is usually executed with the assistance of third-party software, such as Sigmalink.
- Entering pad data to the inspection system.
- Entering basic customer/product/batch data to the inspection system.
- Supplying stencil thickness input data to the inspection system.
- Optimizing light levels for maximum contrast between solder paste and PCB surface.
- Configuring suitable process settings, which is usually performed by a process engineer who may need to set different tolerances for different PCB pad geometries. and
- Displaying and presenting results.
The Pi 360° Moiré system performs fine-tuning by means of large-area image analysis and process setting using the area/roof ratio to optimize its inspection capabilities for a given PCB product. It uses a programming sequence that requires little time for an operator to learn.
Comparing Cycle Times
Comparing cycle times for inspection systems can be confusing. Practical throughput is based on a number of different steps, including PCB load time, image acquisition, correction for PCB warpage, image analysis and signal processing, and display and presentation of results. Most competitive systems can be configured to match the line production speed required.
It should be noted that laser-based inspection systems typically employ two different lasers or two passes of one laser to eliminate shadow effects. Obviously, a pass with one laser would be faster than passes with two lasers, although using only one laser would result in reduced inspection quality.
Similarly, typical Moiré systems offer two or four projectors to eliminate shadow problems and ensure consistent angular measurement of defects. Lower-cost inspection packages have only two projectors, with the choice of high quality or high speed. The compromise in achieving faster inspection speed is degraded quality performance.
The Pi system maintains high inspection quality with each variant, with the only compromise being in inspection speed. The system can be upgraded for high-speed capability as required.
Conventional PCB inspection systems offer a small field of view (FOV). They define a small number of references within the FOV to determine the zero level for measurement. The position of these reference points is critical as even a slight offset can significantly affect the height measurement. But often, capability is needed to image a larger area of a PCB, especially when performing warpage correction when inspecting multilayer PCBs.
For accuracy, the Pi system can identify multiple reference points in X, Y, and Z axes to overcome the effects of board stretch and determine a zero height for each pad. PCB inspection systems typically show a small area. An alternative solution shows a true 3D image of a larger area for a more representative view of a PCB. With the Pi system, 2D, 3D, and topographic images of large areas are available as needed.
Contact: Vi Technology, Inc., 903 N. Bowser, Suite 202, Richardson, TX 75081 972-235-1170 E-mail: firstname.lastname@example.org Web: http://www.vitechnology.com