Step 11: The back-end process: Singulation step by step
11/01/2000
by Bill Abeyta
Consumer demands for miniaturized portable wireless applications and personal electronic devices have become the driver for reduced manufacturing costs and smaller integrated circuit (IC) packages. Integrated device manufacturers and subcontractors have resolved this challenge by maximizing the population of ICs packaged within the substrate area. Gang sawing or dicing methods using ultra-thin diamond cutting blades solve the challenge of how to singulate highly populated substrates and is a widely accepted method in the packaging industry.
Chip scale packages (CSPs) and ball grid array (BGA) package designs are now being gang sawn or diced rather than punched or panel routed for the singulation step. This separating process has shown to be cost-effective because of the ability to separate each device within narrower cut lanes ("streets"). Street size between packages on the substrate is typically 9 to 10 mils and is easily removed with a diamond blade of equal thickness.
 Figure 1. A 6 x 6 matrix singulated in a single pass using a seven-blad gang assembly. |
Saw singulation is an automated processing step between laser marking and final package inspection and sorting, and is fully integrated to an input/output transport system that uses either conventional tape-mounting frames or advanced vacuum fixtures to hold each substrate during the cutting step. Because of the high cost of ultraviolet (UV) tape mounting materials and lower units per hour (UPH) throughput levels, vacuum holding fixtures, with the associated low cost of ownership, are often chosen for many high-density and high-speed pick-and-place handling applications (Figure 1).
Basic Material Conditions
CSP substrates, or strips, are rectangular in shape and range in size from 50 x 187 mm to 70 x 250 mm. Individual IC packages vary in size from 1 x 2 mm to 17 x 17 mm. The more popular package sizes in volume-production applications include 5 x 5, 5 x 8, 6 x 6 and 8 x 8 mm.
Each strip consists of either a single array of integrated devices throughout the substrate or sections of smaller arrays called panels. These smaller arrays are usually arranged in groups of four or five panels that are equally spaced along the length of the substrate strip. This substrate design is preferable to a single array because it not only affords improved dimensional control within smaller sections along the substrate but also improves the overall accuracy of cut placement. Miniature BGA components, which provide the electrical connection to the die and are not much larger than the die from which they are made, are encapsulated in epoxy mold compounds less than 0.2-mm thick over the die to further reduce the overall package volume.
 Figure 2. Alignment compensation for warpage along the X-cut direction. |
There are three basic types of CSP substrate materials. The first, and most common, is BT resin laminate and is often used for chip arrays (CABGA) and logic and flash memory applications. A second type is tape-based and uses a copper frame to which one or two layers of tape films are applied. The third common type incorporates a copper substrate approximately 8 mils in thickness that functions as both the bonding and electrical connection medium for the final product.
A typical problem associated with sawing accuracy during the singulation step is mechanical deformation or substrate warpage caused by temperature and pressure variations in the molding and solder reflow processes. Based on the severity of the out-of-flat condition of the substrate, the singulation process uses one of two clamping or fixturing methods: tape frame (ring) mounting on porous ceramic vacuum chucks, or vacuum jig or tapeless mounting chucks.
Tape frame (ring) mounting on porous ceramic vacuum chucks: This method offers advantages over dedicated vacuum jigs in that it accommodates warped substrates greater than 0.5 mm and a variety of CSPs as small as 1 x 2 mm in size. The major disadvantages are cost of the UV tape (consumable), and potential issues in cleaning residual contaminants from the cut parts.
Vacuum jig or tapeless mounting chucks: This non-tape and most-common method is a dedicated vacuum jig or fixture, which has a vacuum plenum and elastomeric pads matching the singulated package size. This method is often used instead of tape frame mounting methods because it eliminates tape costs and allows for nesting approaches to hold each singulated device. Another advantage of tapeless vacuum jigs is the ability for singulated devices to be directly picked and placed into JEDEC trays, tube off-loaders or bins after the wash-and-dry sequence of the handling system. The disadvantages of this fixturing method include limitations in the part sizes that can be cut because of the low vacuum or clamping forces that can be obtained on package sizes smaller than 4 x 4, as well as a need for the molded substrate to be sufficiently flat (<0.5 mm) across both the long (x) and short (y) axis of the strip.
Tackling Process Variations
CSP manufacturers are continually driving to increase yields and UPH capability of singulation systems. Therefore, cutting speeds and accurate cut placement relative to the BGA are essential elements of the process. To achieve this in unattended operation, a vision alignment routine is performed using either pattern recognition or edge detection, or a combination thereof, depending on the size and shape of the fiducials or alignment marks. This alignment system is integral with the saw's four-axis motion controller and establishes proper rotation (theta) alignment while optimizing cutting tool placement to fiducials placed coincident with cutting streets within each array on the substrate. Optional mapping routines for warpage compensation are necessitated by dimensional variations on the substrate and mechanical tolerances in all of the components that comprise the singulation process.
The sole function of vision alignment and mapping routines for warpage compensation along the substrate is to determine proper cutting tool placement. The first step in the alignment process elicits the necessary alignment information before the cutting step in what is sometimes referred to as the "row direction," or the long axis of the part parallel with the saw's cutting axis. This initial alignment routine first corrects for theta Θ error by finding fiducials at opposite ends of the length of the strip, determining the rise over run (triangulation) and rotating the part until the rise is within acceptable limits. After theta Θ alignment, the correct cutting tool placement is used based on the location of the cutting fiducial at one end of the rotationally Θ aligned substrate.
Because of varying degrees of substrate warpage found on larger substrate formats, it is sometimes necessary to include a corrective measure for bow within the row direction alignment routine. This "row bow" alignment step involves locating a third alignment fiducial in the middle area of the substrate immediately after the row direction alignment function. A measurement of this third fiducial location is taken and compared with the location for the cutting tool placement value. Dividing the difference of the two data points by two and then adding this to the initial cutting tool placement value achieves optimal cut placement of the gang (Figure 2).
 Figure 3. Array pitch compensation for shrinkage along the Y-cut direction. |
The substrate is then rotated 90° from the row cut theta Θ alignment position and is now in position for the second alignment step, sometimes referred to as "column direction." It is not necessary to perform another theta alignment both because having aligned in the row direction of the cutting step has improved the resolution, and because the theta table of the saw is inherently accurate.
Column direction alignment routines typically include corrective measures for pitch errors from panel to panel or array to array. Correcting for array pitch error to achieve optimal cut placement of the gang requires measuring the location of one fiducial in each array. Determining the pitch between arrays is simply a matter of arithmetic - subtracting each array location from the previous array location (Figure 3).
Gang Sawing
An emerging trend to UPH improvement is the use of multiple blade or gang sawing techniques. These high-throughput methodologies require a rigid saw and dual-spindle capability to accommodate from five to ten precisely pitched blade assemblies per package size, and the flexibility to saw rectangular and square package formats in five or fewer passes of the gang.
Gang sawing involves the use of multiple diamond blades cutting at table speeds commensurate with cutting speeds used in single blade singulation saws. Its objective is to reduce the total number of cutting passes required to singulate a complete IC package substrate. Single blade singulation, or dicing, is a conventional wafer-dicing saw process that uses a single blade to cut or separate each row and column of devices from the substrate.
The number and type of blade used in gang sawing is dependent on the cutting loads expected during singulation; the lower the cutting load or forces per blade, the higher the number of blades per spindle on the saw. Another critical factor in gang sawing is the type of bond selected. Bond types can be categorized as nickel, metal or resinoid. For high table speeds and minimal blade-wear characteristics on BT laminate products, nickel bonds offer the most economical solution, while resinoid bonds offer the least desirable wear characteristics but the highest cut quality for copper lead frames. Metal bond is a compromise between the two bonds, but also helps to reduce cutting forces on higher populated gangs.
Each blade on the gang is responsible for controlling several quality characteristics: cut edge placement relative to the solder ball grid array, overmold edge and corner chipping, copper lead smearing and burring, and part size. The selected diamond blade not only must be compatible with the substrate being processed, but also able to withstand the high radial loads generated by high cutting feed rates, exhibit minimal blade wear, maintain its cross-sectional shape, remain free cutting and be easily reconditioned for longer blade life.
The diamond blade cutting tool system for gang sawing is referred to as an HMA, or hub module assembly, and is designed for single-pass cutting modes. Each HMA is fabricated with precision spacers between each blade position that are equal to the nominal size of the package device. For example: if the array pattern is four devices up by four devices across, a total of five blades are required to singulate the array in each cut direction. A typical tolerance held across the HMA is +10 microns and is based on achieving a minimal process capability level of 1.67 Cpk and higher for package sizes specified at +0.100 mm or less.
 Figure 4. Hub module assembly (HMA) radial view. |
Because the distance between each blade position along the HMA is designed to match the nominal size of the package, the HMA is often referred to as a fixed tolerance. This fixed tolerance is highly reliable and repeatable, and typically achieves Cpk values of 3.0 and higher.
The concept of gang saw singulation can be explained as the ability of both the saw and its vision alignment system to accurately place the fixed tolerance of the gang relative to each grid array on the strip (Figure 4).
Challenges Facing Singulation Makers
As CSP device densities increase because of smaller package sizes and larger strip formats, unattended operation at high-velocity processing speeds rely heavily on the interdependencies of both the singulation system and the original equipment manufacturer's ability to continually develop cutting blades and processes consistent with changing technology and materials. Equipment design considerations should not only include minimal manufacturing lead times but also the ability to automatically adjust for array bow, warped substrates and cutting forces above preset control limits æ all without the benefit of operator intervention.
BILL ABEYTA, sales and marketing manager, can be contacted at Manufacturing Technology Inc., 2226 Goodyear Avenue, Ventura, CA 93003; 805-644-9681; Fax: 805-644-3541; E-mail: [email protected].