Improving Solder Paste Printing through Phase Shift Interferometry Print E-mail
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Written by Chrys Shea   
Monday, 01 March 2010 00:00

New measurement technology enables high degrees of verification and confidence.

Since the dawn of SMT, the solder paste printing process has been considered the primary source of assembly defects. The process itself is an enigmatic combination of art and science, and is not easy to control. As surface mount technology evolved, a variety of solder paste inspection tools progressed, often employing the latest advancements in optics, lasers and machine vision systems. But when miniaturization efforts produced 0.5 mm area array devices, the incumbent inspection technologies all fell short, either on accuracy or cycle time, or both. The ability to read 0.010˝ deposits well enough to be trusted and quickly enough to meet production requirements became the Holy Grail in automated paste inspection. That’s when phase shift interferometry, a light and vision-based technology, was introduced as an inspection method, and changed the way we look at solder paste.

How does it work? Simple physics. Imagine sun shining through window blinds. The sunbeams form a pattern of stripes, alternating light and dark. These stripes appear straight on planar surfaces such as floors or tables, but seem to jog back and forth as they cross over furnishings that alter the surfaces’ flat topographies. The amount of perceived zigging and zagging depends on the height of the object and the relative angles of the observation and source light. By taking several measurements at known intervals, the height of the surfaces can be calculated mathematically.

The alternating light and dark stripes are known as a Moiré pattern (Figure 1), and the method of measuring them at known intervals is called phase shift interferometry (Figure 2). The Koh Young solder paste inspection system utilizes a patented, sophisticated application of phase shift interferometry to measure solder paste deposits. It essentially divides each paste deposit into tiny segments as small as 10 µm, and measures each segment’s location and height (Figure 3). It then calculates the individual segments’ volumes, and constructs a 3-D model of the entire solder paste deposit (Figure 4). The measured model’s volume, height and/or location are then compared to a theoretically perfect model. The modeling is unprecedentedly accurate, demonstrating a gage repeatability and reproducibility (GR&R) of less than 10%.

Fig 1

Fig 2

Fig 3

Fig 4

Vicor Corp., a manufacturer of power components, uses the Koh Young inspection equipment on its prototype and production lines. Sometimes referred to as “bricks” by assemblers, Vicor’s power management components are anything but brick-like on the inside. These packages contain some of the smallest and most complex SMT packages available: 0.5 mm area array devices, leadless packages like QFNs and SOT883s, and 0201 passives. The boards on which they are assembled are panelized in 10- to 20-up arrays, and a typical solder paste print consists of over 10,000 deposits, most of them smaller than 0.012˝.

Ten thousand paste deposits in the 0.012˝ range is not an impossible process, but it certainly is a challenging one. Ray Whittier, senior process engineer, is charged with keeping it in control. He uses the phase shift inspection technology extensively, both online and offline. Online process control includes screening prints for quality and optimizing throughput. Offline analysis includes stencil qualification and verification, paste qualification, and solder powder size selection.

The best place to begin employing the technology is in production. Implementation begins with programming a board, which takes about 10 min. The system’s computer creates a virtual “golden board” to which it compares actual print readings. It learns the theoretical stencil aperture sizes and locations from the stencil’s Gerber file; it learns the theoretical Z-height from user input, and it learns the reference plane by reading a bare board. The programmer can set a number of inspection parameters, including allowable deviations from the golden board’s stencil heights, stencil volumes, or aperture positions. Typical starting parameters are 50% minimum and 150% maximum of the stencil height and calculated aperture volume, and 50% center-to-center deviation from paste to pad.

To inspect print quality, the system scans the printed board and creates a second model in its computer. It takes less than 20 sec. for it to read over 10,000 paste deposits, construct the model, and compare the measured solder paste deposit volumes and locations to those on the golden board. If every deposit falls within the programmed specifications, the system passes the board on to the next operation. If one or more deposits do not meet the set criteria, however, the system fails the board and alerts an operator.

Whittier cautions that a failed print may not necessarily be a poor quality print. It may be an indication of variation among bare boards. “Critical features like circuit card dimensions, pad sizes and locations, and solder mask alignment each can vary substantially from lot to lot. A failed inspection may be the result of a bad print, or it may be an indication of a change in the bare boards. To distinguish between the two, operators run a bare board to reset the reference plane and scan the print again. If it passes, it is returned to production. If it fails, the print is scrapped. Many of the scan fails are the result of PWB tolerances, and are resolved by the bare board re-teach operation.”

How many actual print failures are captured? That depends on the complexity of the different products and the mix on the production line. Figure 5 shows the percentage of scrapped prints for top and bottom sides of the boards. The less complex bottom-side averages around a 1 to 2% failure rate, while the more challenging topside runs around 6 to 7% failures for the current product mix, with one monthly spike topping 12%. Had all these bad prints continued down the production line, they would likely have created expensive defects. Reworking BGA and QFN components is costly and risky for most assemblers, but for Vicor, rework is not an option. Failed assemblies are scrapped. With no chance for after-the-fact recovery, it is absolutely essential to catch defects at their point of origin.

How is overall process control instituted? The operating procedures are very specific regarding failures. If three true print failures occur sequentially, the process is paused and support is called out to the line to investigate the source of the failures. Interestingly, nowhere do the procedures call for visual confirmation of the machine-identified defect. Pass/fail decisions are based purely on the output of the machine. Whittier explains that “with a GR&R consistently in the 7 to 8% range, the machine is far more accurate than the human eye. It makes absolutely no sense to override a superior system with an inferior one.”

Boosting Yields and Cutting Costs
What is the end of line impact? For Vicor, overall yields climbed 3% upon implementation of the systems – a considerable and noteworthy boost. The bottom line also was improved by cost reductions and utilization improvements. When Vicor recently changed solder paste formulations, Whittier used the bridge detection feature to dial in the wipe frequency in production. He found he was able to double the wipe interval from every print to every other, halving his cost of wiper paper and the down time associated with changing it. He was also able to identify the pause time at which paste kneading is necessary to ensure good prints, eliminating unnecessary print rejects, kneading operations and board cleaning.

What is Whittier’s favorite feature of the phase shift interferometry equipment? “The combination of speed and accuracy. Every process engineer knows the tradeoffs between the two. In this machine, there are none.” His favorite unadvertised application? “Stencil verification. We now approve stencils for production faster and more accurately than ever before, as we can catch missing or improperly cut apertures in one reading” (Figure 6). His next future app? “DfM feedback to the design teams. There are no off-the-shelf DfM guidelines for the type of product we make. Using this inspection technology on our prototype lines alerts us to potential stencil printing issues while there is still time to design them out of the product.”

As for offline applications, Whittier was able to quickly characterize the transfer efficiencies of solder pastes under multiple print scenarios. The data he’s generated have allowed him to understand the impact of stencil suppliers and manufacturing processes on paste printing, resulting in simultaneous quality improvements and cost reductions. It’s provided him with a solid comparison of current and new solder paste formulations to upgrade his materials set and reduce variation in print quality. And it’s given him guidelines as to which feature sizes will require a switch to Type 4 solder paste so he can plan the timing of its introduction to meet Vicor’s product designs.

Imagine increasing an end-of-line yield by three percentage points. That alone is worth the price of admission, as it directly improves profitability. Cutting costs, improving throughput, qualifying suppliers, preparing for next-generation technology – they all improve profitability also, but less visibly. And, unfortunately, they’re still sometimes forgotten.   CA

Chrys Shea
is founder of Shea Engineering Services (sheaengineering.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Monday, 08 March 2010 22:43
 

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