By Ron Iscoff, Editor
At the dawn of the IC assembly business decades ago, singulation was a messy, totally thankless job. Throughput in the early years was a combination of the operator's speed and the saw's MTBF. Automation in separating devices? Forget it! The equipment you specified to singulate devices from their substrates looked like it belonged in a machine shop grinding brakepads for airplanes-not in a high-class IC assembly plant. Things do change, don't they! Automation grabbed hold more than a decade ago, affecting every piece of equipment on the assembly floor. In the past several years, integration of process steps has been a goal of users and equipment suppliers. Suppliers of singulation equipment have complied, and many offer singulation combined with handling/offloading, vision inspection and other functions. Once you might have preferred keeping several goats in your living room to maintaining and housing angular singulation equipment. But today's gear is clean, fast, not small-but getting smaller, and often elegant in appearance, as shown in Figures 1-6. Competing Technologies Today, singulation users can select between the competing technologies of sawing or stamping. On the horizon, companies in Asia and Europe are preparing to enter the singulation equipment market with tools that employ laser ablation (see sidebar, page 33). Is it enough to be automated and fast? We checked with the people who should know, the ones who spec assembly gear for their lines. And we'll share their comments with you.
First, we must address the topic of stamping vs. sawing. Both have their place, and much depends on the application. Dr. Gerald K. "Skip" Fehr, a vice president at OSE USA (formerly IPAC), San Jose, notes that stamping tools require separation between the units and an individually molded unit for each package size. The downside of stamping is the tooling costs, which are much higher than those for sawing. The singulating process is typically faster with stamping, however. Sawing allows the user to work with a standard substrate strip or leadframe strip, such as a QFN design. This, Dr. Fehr observes, requires only one mold for numerous package designs. As a result, the saw allows the user to singulate different packages, but requires a different tooling kit for each package. The tooling can be priced at up to $20,000. Dr. Fehr would like to see several improvements. "Most of the saws need to be more user friendly. Changeover from one package size to another requires a tooling kit change-nest, handling mechanism and tray set-up." He would also like saw makers to incorporate a mechanism to "hold down the device when sawing either a very small device, warped substrate or an unmolded flip-chip device, where the vacuum hold-down does not work well. The pick-up tool may also need some modification for this class of devices." Finally, Dr. Fehr wants a longer blade life. "When using slotted blades, we find that the broken blade detector must be turned off-not nice if you actually break a blade!" Whatever the technology, it must be "cost-effective, stable and reliable," points out Andrea Chen, technical marketing manager for Siliconware USA (SPIL), San Jose. Punching Remains the Standard Chen observes that while punching remains the standard, for newer, very small leadframe-based CSPs-such as QFNs-the sawing method "may prove more effective-and definitely more flexible-if sawing costs prove attractive." With substrate-based packages, however, Chen expects singulation to be equated with sawing. "The sawing process is already a mature and popular technique for singulating CSPs and wafer-level packages, and should remain so into the foreseeable future," she says. Despite its maturity, Chen agrees that saw singulation exhibits areas that need improvement. She specifically tags such factors as better cut quality, precision and accuracy. She would also like to see consistent package transfer after sawing into package trays and faster overall throughput. Key Attributes An acceptable cost of ownership, reliability and quality are all key attributes for singulation equipment. Assuming the above exist in every machine, Kelly R. McKendrick Sr., engineering manager for Amkor Technology Inc., Chandler, Ariz., says "the first thing that comes to mind is flexibility." You only need to scan the vast package portfolio that a major packaging foundry, such as Amkor, offers its customers to see why flexibility is so important to people like McKendrick. "With the new, complex packages-and every customer wanting a different flavor-flexibility is key." This means, he adds, flexibility in off-loading methods (tubes, trays, tape and reel or even bulk) and also flexibility of different package sizes and/or pin counts. McKendrick emphatically declares, "We do not want a machine for one particular part, or one that takes five techs three hours to change over. We want a machine that we can change over quickly (in minutes) and inexpensively." For example, he says, "On an MLF (Micro Leadframe) device, we may need to run a 3x3, a 4x4, a 6x5, a 7x7, a 9x9 and even a 12x12 on a single machine. And until volumes are mature, flexibility is key, especially on the new, very dense leadframes and substrates." Flexibility also looms large for Edgardo G. Padrinao, assembly engineer at Advanced Micro Devices, Sunnyvale, Calif. An "immediate requirement," he declares, "is flexibility allowing the machine to handle different types of CSPs (organic substrate, Cu/Alloy-42 leadframe-based packages). While throughput (UPH), cutting accuracy, operator and maintenance friendliness are "musts," he says, "not all of these features compliment each other. As we push for higher UPH, accuracy is affected, and so is yield."
Fiducial Alignment That means, Padrinao says, that machines need features such as "fiducial alignment" in parallel with the cutting operation. "This alignment should consider all abnormalities a strip may have after exposure to elevated temperatures and repeated handling." These anomalies should include "shrinkage, warpage, damage and/or breakage of strips into a few pieces." Gang saws are not a currently a solution, because "the pick-and-place section of the singulation machine is too slow to cope with gang-blade sawing," he believes. Padrinao also looks for reworking capability or sawing strips "cut into pieces." The density of each strip, he points out, may be greater than 100 units, and "we just can't afford to throw away partially broken strips." He observes that "strip testing" or full integration of the sawing machine to other end-of-line assembly operations or TMP (test, mark and pack) equipment is becoming a trend. "This trend is offering a lot more challenges for the saw singulation machine. Our idea of a fully integrated machine is one which offers strip identification capability. Mapping Defective Assembly Parts "This capability will be able to map the assembly defectives out of molded strips so defective units would not have to be electrically tested and marked. Likewise, mapping would properly place electrically tested (good and bad) parts on assigned JEDEC trays, properly identified to their correct binning." Speed compensation, Padrinao adds, is "going to be a major challenge for the saw, since the tester should be the gating factor for the system's throughput." Discussing gang blades, Padrinao says their use "is good, but troublesome when the blades need replacement, either by schedule or prematurely. The cost of gang blade kits is also too expensive, unless the package size can be standardized to a few selections." Programmable blade settings "could be a good idea, if it can be done," he believes. Equipment Automation Beyond flexibility, Amkor's McKendrick agrees that strip mapping will be crucial, along with equipment automation, in the future. "The future of strip mapping is here. Very few are up to speed on it at this time. There is lot of lip service but little action. In the next five years, mapping and equipment automation will be key. Throw in flexibility, and you will have a winning combination." Michael Cheah, array process engineer- singulation for STATS, Singapore, centers his comments around equipment software. "Many of the parameters in the existing singulation software are laid out for the user," he concedes. However, to understand each parameter, one needs extensive experience with the saw machine itself. "It would be useful," he adds, "to have more structured programming software, with a step-by-step guide provided to users, especially to machine operators."
Cheah says another useful addition would be to capture the eye points (teaching points) after every cut for strip packages. "This would reduce the offset/ misaligned cut and directly increase the quality of the package singulated." For the general singulation saw machine, says Cheah, including the ability to auto- align/auto-teach new packages would be valuable. "The saw must be able to teach and align and singulate the strip without causing offset cuts or damage to the strip. Likewise, the Z-height of the saw should compensate to avoid incomplete cutting."
Conclusion Sawing, stamping or laser singulation? Laser units are likely to face the challenges of newness and market acceptance versus the field-proven sawing and stamping tools. The more immediate question is, how many features can vendors afford to provide-and, conversely, how many features will users pay for-in future singulation tools. For saw singulation equipment, the category that is carving out a growing share of the market, the answer is likely to be decided not only by user demand, but also by advances in saw engines. Since few singulation tool makers manufacture their own saw engines, OEM saw specialists, such as Japanese saw giant Disco, are likely to play a key role in singulation advances. |
Stamping/punching and/or sawing have been the preferred means of singulating substrates for decades. Each method has been proven over the years in dozens of IC packaging assembly lines. Is there a better way?
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