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Solder Paste Printing Inspection: An Inside Look
December 31, 1969 |Estimated reading time: 12 minutes
Industry cost control pressures and technology drivers demand more powerful 3-D AOI machines for control of solder paste printing. This article looks at the factors users of these systems should take into consideration.
As surface mount assembly technology evolves, real in-line automated inspection becomes essential to ensuring reliable, cost-effective production. Driven by shrinking component size, reduced lead pitch, BGA density, shortened tact times and ever-growing board complexity, traditional use of human inspection can no longer sustain world-class production performance.
Tough competition in the EMS segment intensified by significant restructuring of operations during the recent economic downturn and the resulting need to lower cost-per-board propels the industry toward higher yields and lower scrap and repair costs. This necessitates that inspection be an integrated part of the process from its earliest stages.
As the use of BGA components becomes prevalent and density increases, inspection after the solder process leaves many joints hidden from view. Problems in BGA components are undetectable on a standard post-reflow inspection system, and X-rays are complicated, slow and costly. Repair costs increase dramatically after the assembly process is complete and closed-loop feedback for process control is less effective. Board repair is not always possible, meaning entire boards may end up on the scrap heap.
Estimates link from 60 to 80 percent of end-of-the-line defects to problems in the printing process. Solder volume is considered by some to be the best predictor of finished board quality.¹
Improving the printing process can reduce some problems in the later stage of the process. A good printing process can prevent small shifts and twists of components from becoming defects.
The printing process is dynamic and, therefore, sensitive and defect-prone due to a variety of factors, including cleaning of and wear and tear on the mask, aging of the solder paste, sensitivity to environmental conditions, and handling and frequent refilling of the paste. Problems in the printing phase typically will affect areas and components all over the board. Detecting such problems just after printing helps reduce rework and "bone pile" costs.
For all these reasons, more companies are seeking optimal options for inspecting SMT boards from the start of the line, especially for BGAs and fine-pitch components.
Paste Printing Inspection
Simpler applications with relatively large pads and components can be managed adequately with standard 2-D inspection (Figure 1). Such processes are more stable; small changes in solder height are of little significance. However, in applications that involve small components like 0402, 0201, CSP and fine pitch components (0.4 mm), 3-D inspection is essential. The dimensions of each solder deposit are of the same magnitude in all directions. Therefore, the height of the deposit (a result of the stencil thickness) is about the same as its width and length. As a result, small changes in the height of the deposit are as important as small changes in its area, possibly reducing solder quality.
Figure 1. (left) is a 2-D image of a BGA. The colored image (top) shows pads in the BGA corner in 3-D. Colors represent the height (blue — low, red — high). (bottom) shows the same place from a different angle. The gray levels are taken from the top image. Bright reflective pads can be detected at the base of each deposit.
Moreover, when components have a large number of pins (IC or BGA), verifying that height variation between the deposits of each of the components is small enough to ensure all joints will be soldered properly is critical. If one of the component legs or balls is not touching its deposit, an opening might occur, or the joint might lose reliability.
The 3-D inspection process should cover the entire board, not just the intended locations of solder, to detect solder inadvertently deposited outside the designated printing area. This is also known as "unexpected paste," which can result in solder balls that might cause severe, unpredictable damage.
The Challenge
To ensure accurate inspection, several challenges must be met:
- Accuracy. Inspection must provide accurate measurements of paste deposit volume, area, shape and location relative to the plane of the contact pads.
- Closed loop process control. The measurements should be accurate enough, not just for defect detection or gate keeping, but for real process control. The data should enable closing the system-printer loop and data exchange between the system and later stages of inspection.
- Speed. The machine should be able to perform 100 percent inspection of the whole board, even at the fastest line speeds.
- Boards. The system should be able to cope with a range of boards despite differences in color, thickness, and type of pads and solder paste. The machine also must handle the entire range of board warpage.
- Simplicity. The machine should require neither long setup time nor highly trained operators.
Each of these aspects can have a tremendous impact on the quality, usability and benefits gained by introducing such a system into the production line.
Height, Volume and the Concept of 3-D Inspection
Since deposits are not necessarily uniform on top, attempts to extrapolate the volume of a deposit from just a few samplings does not necessarily yield accurate results. Some typical deposit profiles are pictured in Figure 2.
Figure 2. Typical deposit profiles.
This method becomes increasingly problematic as the number and distribution of measurements per deposit decrease. An alternative approach is to measure many points per deposit in the required tact time, but to inspect only a small number of deposits — only critical points or a small sample — hoping to gather enough information to predict printing problems.
Since printing defects are random, to derive maximum benefit from paste printing inspection, manufacturers need to inspect the entire board using accurate 3-D volumetric measurement.
3-D by Triangulation
Several techniques can be used for measuring height for 3-D mapping, including Moiré and phase shift interferometers, triangulation, time of flight, focusing, and confocal methods. The most commonly used, suitable technique for height measurement of solder paste printing is triangulation. With a simple adaptation, this method can measure a large number of points simultaneously.
The basic triangulation method is demonstrated in Figure 3a. Two lines can intersect at only one point. One of the lines is a projection of a laser dot. The second line is the line between the camera and reflection point at the angle the optics define. This method requires a single line camera and yields a single measurement point.
The same technique is employed to measure a line of points simultaneously. Rather than projecting a single dot, the laser projects a line or "laser screen." The intersection between the laser screen and its reflection on the camera results in a line of height measurements (Figure 3b).
Figure 3. Triangulation of a point and of a line.
The angle between the two lines is important for accuracy of measurement. When the laser is projected from the top, each pixel has a predefined X-Y location. Thus, the pixels are spread evenly throughout the resulting image. When the laser is projected on an angle, only the Y of the pixel is known in advance. Since triangulation deals with both the X and the Z of the target, the pixels recorded by this method are not regularly spaced. As a result, difficulties may arise in calculating volume accurately (Figure 4).
Figure 4. The difference between triangulation when the laser line is projected from the top, to triangulation when the laser line is projected in an angle.
The laser spot (or line) will vary in width and can lose symmetry, making it difficult to find its exact center. The advantage of projecting the laser on an angle is the simplicity of the optical configuration scheme needed to achieve the same resolution. This is why this configuration has been chosen by many of today's inspection system manufacturers.
Nonetheless, to achieve better accuracy and reliability of the data, placing the laser directly above the target clearly is the superior method.
Adding a True 2-D Channel
It is not possible to ensure that the printing is good enough without distinguishing between the paste and the rest of the board, and measuring paste height relative to the height of the pads. Conductors, pads, silk screens etc. are not necessarily of uniform height, and smears of paste can be thin. Therefore, knowing the height of each point is not enough to clearly identify the paste. Only knowing the height (from a 3-D image) and having a gray-level (a 2-D image) of each point on the board enables the detection of true defects (Figure 5). The 2-D image also is essential for finding fiducial marks.
Figure 5. Insufficient solder deposits visible only on a height map image (above) and a short verified only by a gray-level image taken from above (below). (Clockwise from top left) (1) 2-D image taken from 3-D AOI, (2) Height image taken from 3-D AOI, (3) Height image taken from 3-D AOI, (4) 2-D image taken from 3-D AOI
One option would be to use the laser illumination reflected in the triangulation process to create a gray-level image. Unfortunately, this would result in a poor quality image due to limitations of such lighting. Also, problems such as shadows or poor reflection will appear at the same points in both the 2-D and height mapping images. To improve image quality, another channel must be added so that the 2-D image uses a different optical path and a different light source than the height mapping image.
Figure 6. Two different boards each need a different type of light setting for 2-D imaging. (Clockwise from top left) Board A Light setting: 'full slant', Board B Light setting: 'full slant', Board B Light setting: 'full top', Board A Light setting: 'full top'
The addition of this optical channel also creates the option of making the illumination scheme adjustable to different board types; for example, one configuration for boards with tin air-leveled coating on the pads and another for board with golden pads (Figure 6). To maximize the ability to distinguish between pads and paste, illumination should adapt itself automatically in the learning phase for each board type.
Figure 7. Detecting elements of the same height as the covered pads (in this case, vias) helps in setting the reference for paste measurement. Top image (left) and Height image (right) taken from 3-D AOI
As an adjunct to height mapping, accurate 2-D information can help the system significantly reduce false calls. For example, it can detect that an area identified as "too high" is not really paste, but noise generated from reflective surfaces, bright silkscreen or holes. It can find thin smears that extend beyond the designated pad area and threaten other pads, and help identify elements, such as testing points and fiducial marks that are the same height as the pads. Once identified, they can be found in the height mapping image to accurately determine paste volume (Figure 7). Therefore, it clearly is worthwhile to invest in an imaging system that uses both the top and 3-D image to ensure a clear, obvious separation between the key features on the board.
Measurement Accuracy and Process Control
Paste printing inspection systems should not only detect defects, but also help the user improve the overall paste printing process, as well as helping detect trends that might lead to defective boards. This allows the user to correct the process before significant damage is done. They should also accurately measure paste deposit on the board to help calculate registration transformation between the board and the printing mask, a problem that, unchecked, can result in massive defects across the board.
Accurate area measurements aid in preventing defects. An example is detection of clogging of the mask before defects begin to form. This also enables the improvement of mask washing policies.
Precise measurement of height and volume also are important for detecting problems in squeegee speed and pressure, inadequate paste application, or paste aging, all of which typically result in uneven paste surfaces. Problems in the separation between the mask and the board also should be detectable by precise measurement of height profile.
Inspection and measurement systems are usually tested for Gage R&R, a standard procedure for measuring the repeatability and reproducibility of a machine. Gage R&R must be determined for position area coverage, for volume and height measurement, and be less than 10 percent of the process width on typical components and process tolerances.
Warped Boards
Many boards that go through the paste printing process have small curvatures, while others, mainly boards that already have components on the bottom and have already undergone a soldering phase, may have quite significant height variations. The IPC-A-610C standard requires that boards curve no more than 0.75 percent of their diagonals.
This can pose a problem for 3-D systems. Measuring a 10 mm range using a system designed to measure in microns is unreasonable. The system must adjust its measuring range as per the inspection area. This means that either the measuring device must be lowered or the boards must be raised. The necessity to change heights becomes less frequent as the measuring range of the system increases.
The machine should be "smart" enough to find the paste height relative to the pad height in its vicinity to enable accurate measurement of the deposit volume where the board curves.
Speed
Setup for new boards, a central issue in high-mix lines, should be based on standard input files: Gerber, CAD and, eventually, CAM. Setup of a new board should not take more than an hour; changing to an old setup, no more than a few minutes. In addition, setup files should be portable between paste inspection systems ("copy exact").
Solder paste print inspection systems should be used in-line and be able to keep pace with the production line.
Manufacturers looking for new inspection systems should consider not only the line's current speed, but also the maximal capacity of the line and the effects of potential future improvement (Table).
Conclusion
The earlier in the process inspection is performed, the greater the overall benefit to the manufacturer. Paste print inspection has the potential not just to reduce scrap and ease repairs, but also to assist in refining the printing process. To meet current and future manufacturing needs, manufacturing users should select paste inspection systems that offer:
- 3-D inspection at line speed (i.e. at least 40 sq. cm. per second)
- Real 2-D and 3-D simultaneous inspection for maximal fault coverage
- System accuracy and GR&R to meet the inspection requirements of the finest elements
- Quick, simple setup and full coverage.
Manufacturers seeking new inspection systems should consider not only the line's current speed, but also the maximum capacity of the line and the system's adaptability.
References
¹ Amcor, "Surface Mount Requirements for Advanced Packaging Solutions," 2000.
Efrat Litman, system engineer, may be contacted at Orbotech Ltd, Assembly Division, Yavne 81102, Israel, 972-9-7669660 ext. 220; Fax: 972-9-7669664; E-mail: Efrat-l@orbotech.com