The back-end process: Step 9
QFN Singulation
09/01/2002
Two processes compared
BY TERRY DAVIS
Quad flat no-lead (QFN) packages are lead frame-based packages that are similar to several other package families (e.g., TQFPs and SOICs) in terms of design capabilities and the materials set. The primary exception is that in the other package types, the lead comes out of the centerline of the package body and is formed in a gullwing shape to create solderable "feet" for mounting to printed circuit boards (PCB). The QFN lead, on the other hand, resides on the bottom of the molded body. Figure 1 shows TQFP and QFN cross-sections to illustrate the differences and similarities. Although QFN is standard industry terminology, the package type also is known by such marketing identifiers as MLF, MLP, LPCC, QLP, HVQFN and LFCSP, just to name a few.
Market acceptance of QFN packages has been rapid and is one of the few semiconductor market segments experiencing growth over the past 18 months. One industry data source puts worldwide demand for QFNs in 2002 at a conservative 900 million units with a 20 to 30 percent growth rate over the next few years. Market sectors adopting QFNs are wireless, power, PC, wired communication, low pincount analog and logic, and automotive. As the package matures, QFN volumes in analog and logic are expected to increase significantly, and QFNs are expected to displace QFP, SOIC and SOT style packages in both new devices and legacy designs.
QFN Features
QFN packages have numerous advantages over conventional lead frame-based packages. QFNs are small, lightweight, have solid electrical performance and excellent thermal performance compared to non-exposed pad lead frame options.
 Figure 1. A comparison of QFN and TSFP package structures. |
Size and Weight: An 8-lead SOIC has a footprint of roughly 5 x 6 mm or 30 mm2. An 8-lead QFN can be as small as 2 x 2 mm or as large as required for the application. Assuming that the die that fits in an 8-lead SOIC could fit in a 2 x 2 mm QFN, the QFN would have a footprint 86 percent smaller than the SOIC. The QFN is also 90 percent lighter than the SOIC at 0.077 g for the SOIC compared to 0.0073 g for the QFN.
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Electrical Performance: As operating frequencies increase, package inductance (lead plus wire) can be a concern and can exclude some packages from use at higher frequencies. Figure 1 shows the cross-section view of a TQFP and a QFN. For the same die and body size, a QFN will have significantly lower total pin inductance due to much shorter lead length. (The bond wire inductance contribution is relatively constant.) Because the QFN package has lower pin inductance, it is suitable in higher frequency applications such as the RF portion of wireless devices. Table 1 shows bond wire, lead and total pin inductance for a 7 x 7 mm 48-lead QFN and a 7 x 7 mm 48-lead TQFP. The QFN has a much lower total pin inductance.
 Figure 2. Molded strips for shear and saw singulated QFNs. |
Thermal Performance: Most QFNs have an exposed die paddle that, in concert with a conductive die-attach adhesive and proper PCB design, can produce a thermally efficient combination. A 3 x 3 mm 8-lead QFN with proper material set, board design and mounting can achieve a thermal resistance less than 60°C/W, while a standard 8-lead SOIC mounted on a common board is about 125°C/W. The 3 x 3 mm QFN package is less than one third the footprint of the SOIC package but can have significantly better thermal performance.
Manufacturing Methods
QFNs are singulated either by shear or saw processes, and the package configurations are designed for one approach or the other. Shear singulated QFN lead frames are designed such that each unit will be individually molded, and the space between units must be adequate for wire bond and singulate clamping. Saw singulated lead frames are designed such that a large matrix of packages is molded in a block, and removing the connecting bar region between adjacent units separates the subsequent packages. Figure 2 shows examples of molded strips for shear and saw singulation.
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As can be seen in Figure 2, saw singulation processing allows for significantly more units per strip and can be quite attractive when the cost advantages of the increased strip density outweigh the cost disadvantage of the saw singulation operation. Table 2 gives a relative cost comparison between shear and saw singulation. With today's technology for saw singulation, the shear singulation manufacturing method generally is lower cost in body sizes 4 x 4 mm and larger. As saw singulation process technology advances with improved blade life and faster cutting speeds, and as the need for mold flash and singulation tape dissipate, saw singulation will likely become competitive up to 7 x 7 mm body sizes.
Shear Singulation
The design and manufacturing process of shear singulation is close to conventional lead frame manufacturing practice. Die attach, wire bond, mold, topside mark and plating all can be performed on existing equipment that manufactures TQFPs or SOICs, but the singulation operation is unique to QFNs. Most shear singulated QFNs are designed with a clamping lip and have mold compound resident between the leads.
 Figure 3. A typical QFN singulated by a shear process. |
Figure 3 depicts a typical shear singulate QFN showing the mold compound between leads. Mold compound is brittle and can fracture in the area between leads on the finished package. Compound fracture between leads does not pose a reliability risk and most users consider it cosmetic. Tooling design, shear direction, clamping design, shearing sequence, plating type and plating thickness are variables that must be controlled to limit mold compound cracking between leads. Figure 4 shows a typical clamp and singulate design. Because the lead and the package body are in the same plane, burrs on the lead end can be a concern for proper PCB mounting. Shearing from the bottom (burr up trim) of the lead to the top of it can eliminate burrs on the lead bottom. By shearing bottom to top, there is an added benefit of dragging and depositing lead plating over the bottom half of the exposed base metal (generally a copper alloy) on the end of the lead. This generally will promote a solder fillet during PCB mounting.
Saw Singulation
Saw singulation poses some unique manufacturing challenges. The die-attach process is similar to that of conventional lead frame packages, except that with smaller body sizes and high strip counts the time between paste dispense and die placement can be longer than the paste's dwell time. With densely packed lead frames, either the material dwell time must be increased or the machine sequencing must be changed to reduce the time between dispense and place.
 Figure 4. Wire bonding configuration of saw singulated QFNs. |
Wire bonding poses some special problems due to the mold flash tape residing between the lead frame and wire bond heater block, causing poor heat transfer from heater block to lead frame. Additionally, the wire bond clamp contacts the lead frame at the perimeter of the array, so special wire bond parameters are required to control the ultrasonic energy from the bonder. Figure 4 shows the saw singulation wire bonding configuration in detail. The mold, mark and plating operations essentially are the same as for conventional lead frame packages.
Conclusion
QFN packages provide many advantages over other lead frame package configurations, and their use is expected to grow significantly. Two options for singulated QFN packages are shear and saw process. The singulation approach affects design and process variables in other parts of the process flow, so many factors should enter into the decision about which singulation approach to use for QFN packages. AP
Terry Davis, product director, MLF package, may be contacted at Amkor, 1900 S. Price Rd., Chandler, AZ 85248; (480) 821-5000; Fax: (480) 821-6937; E-mail: [email protected]; Web site: www.amkor.com.