September 1998
eMail the Editor
Fine-Pitch Devices From High-Density Substrates
In the rush to lay out high-density substrate panels with as little as 200-µm spacing between devices, the impact of singulation may have been underestimated.By Doug Feicht, TechSearch International, Austin, Texas
Ball Grid Array (BGA) integrated circuit packaging has emerged as a new standard in semiconductor packaging by providing advantages in density, assembly yield and package height over competing fine-pitch surface mount formats.In the highly competitive telecommunications segment, this accelerating trend is especially evident as low cost, fine-pitch BGA (FBGA) packages begin to proliferate for high-demand products.
At the component level, manufacturing cost savings are being realized through better substrate utilization, which is driving a trend towards high density substrates with more devices per panel and less space between them. Savings associated from squeezing additional functional devices into the same panel area are directly reflected in product cost reductions.
Encapsulation advances have reduced constraints on panel layout, enabling many devices to be contained within a large molded area. As a result, many companies are implementing plastic wirebonded FBGA packages in a high-density substrate (HDS) panel format (Figure 1). Similar economies are also driving flip-chip BGA packages into high density panels.
Separating Fine-Pitch Devices
For equivalent package requirements (for example, number of leads required), the number of devices manufactured on a panel has increased 3-5 times in less than a year. We've come to expect density increases of this magnitude, though perhaps less often than we'd like to see in packaging.The savings have further increased FBGA competitiveness. But in the rush to lay out HDS panels with as little as 200 µm spacing between devices, the process of separating the device from the panel, commonly called "singulation," may have been underestimated.
Figure 1. A: Sample plastic fine-pitch ball grid array (FBGA) device and B: High-Density substrate (HDS) fromat panel
Conventional punching and routing processes can no longer accommodate the tolerances, tight spacing or thin materials, so a new method is needed. New handling issues also arise due to the number of devices which are now transferred to trays.
Dicing Saws
To singulate FBGA development samples and initial production, most companies start with what they already know by using a conventional dicing saw with a 10-12 mil-wide blade to make a single cut between devices (Figure 2). Like a wafer, the panel is mounted to a carrier tape stretched across a wafer-frame fixture, which then may be presented, sawn and rinsed, using available wafer handling systems (Figure 3). To provide the greatest area for tape adherence, the substrate is mounted with the solder balls upward for sawing.Automatic saw alignment systems look directly at the ball-side metal for alignment. Diced devices are manually picked from the tape.
Initial results from saw alignment systems have been very encouraging, enabling most companies to quickly bring sample products to market. Early parts exhibit good, cut-edge quality and reasonable processing rates for initial production. Process optimization is underway to increase cutting speeds and define blade parameters for specific packages. Thicker tapes are being considered to accommodate substrate warpage without cutting through the carrier tape.
A separate piece of equipment is used to mount the substrate to the carrier tape ring, which is loaded into the saw using a tape-ring magazine. To automate transfer to trays, equipment to expose the tape to UV light may be used to release the adhesive prior to transfer using a commercial die picker.
Production Challenges
Figure 2. High-density FBGA panel being singulated,
using a commercially available dicing saw.
As production rates increase, capacity and cost constraints appear. For high volumes of devices, production throughput is gated by the need to pick hundreds of devices from the tape and transfer them to output trays. Picking yield is also affected by the need to contact the ball side while picking from the tape.
A considerable number of saws, handlers and operators are required in the traditional singulation process, driving substantial capital investment. The consumable cost of tape represents a noticeable adder to product cost.
HDS panels present unique design challenges for high-volume singulation:
- Fixturing is extremely difficult.
- Stresses from molded or underfilled encapsulants may warp the panel or shift the array position.
- Some high-density FBGA substrates may carry as many as 15,000 solder spheres, leaving little area for contact on the ball side.
Eliminating Tape
Figure 3. The automatic wafer saw includes carrier tape handler and wash unit. A commercially available die picker is used to flip and transfer FBGA devices
Recently, new handlers have emerged which interface conventional semi-automatic dicing equipment to saw mechanically fixtured substrates with the solder balls downward while providing volume transfer to JEDEC output trays. These systems eliminate the consumable cost of mounting tape while increasing production rates for comparable capital costs.
Motorola recently introduced a family of HDS substrate singulation systems which functions as handlers interfaced with commercially available dicing saws. These systems are available from a number of well-established suppliers, including Disco and TSK (Figure 4).
The semi-automatic and automatic handlers place HDS panels into the saw, initiate sawing of the panel, remove the array of singulated devices, then clean-off saw debris prior to transferring the devices into JEDEC trays.
Substrates are sawn, solder balls downward, without wafer taping. A transfer arm places the substrate onto the fixture and upon completion of the saw process, the entire array of separated devices is removed. The HDS panel is held in place by vacuum using a product-specific metal fixture with a compliant top surface that helps accommodate substrate variations. The fixture remains in the saw as separated devices are removed, so multiple, in-process pallets for each device outline are not required.
Two configurations of cleaners are available to remove sawn substrate debris from the panel. Devices in the fixture are rinsed with DI water just prior to being removed from the saw. A cleaning system, which uses DI water spray and heated drying to clean the bottom and edges of the devices, is available with the automatic system.
The entire array of separated devices is transported to a hold-down vacuum plate, from which a second head transfers the devices to a JEDEC tray. In the semi-automatic system, a single nozzle is used to transfer devices one at a time. During transfer, the device may be rotated in 90-degree increments to ensure correct orientation in the output tray.
To achieve high transfer rates, multiple nozzles are used to transfer a row of parts to the JEDEC tray simultaneously. During transfer, the nozzle pitch changes to compensate for the difference in device pitch between the substrate and JEDEC output tray.
The HDS systems shown in Figure 4 incorporate quick-change product tooling so that they may be rapidly converted to a different product outline by replacing a few drop-in fixtures and selecting the appropriate menu sequence. Two multi-nozzle pickup assemblies accommodate a wide range [6 to 35 mm] of device outlines.
Which Choice is Best?
The answer, of course, depends on where a company is in the product cycle. When time to market is of the highest importance, sawing using carrier tape provides good results in the least time. For high volume, the cost of tape represents a significant addition to product cost. If new equipment must be purchased, carrier tape-based systems and higher volume mechanically fixtured systems have comparable capital requirements. More operators are required to operate the separate tape mounting, saw and device picking systems.Often it is desirable to have identical processes performed in development and volume manufacturing, though the production scale may be different. A different process may drive re-qualification of a process, which may take six months or more. For development and pilot level production, the entry-level handler performs the same process as the high volume system.
Cutting Methods
Figure 4. A: Motorola's HDS substrate singulation handler interfaces a dicing saw to a DI water cleaner and multiple-nozzle arm from high-throughput device transfer. B: The entry level configuration uses a single arm to directly transfer FBGA devices to output trays.
Aside from sawing, lasers and water jets have been used experimentally to cut HDS panels. These methods are of interest since they can simplify fixturing by employing a data-driven cutting process.
A water jet focuses a stream of extremely high pressure water at the workpiece, which may be sandwiched between disposable support plates to achieve sharp cut definition with enough precision to meet the requirements for singulation. Water jets can accommodate a wide variety of materials, including glass and hardened steel.
Unfortunately, electronic packages are very sensitive to moisture. A water jet-based cutting process must address perceived uncertainty about the effects of water being driven into the laminate substrate.
Lasers
Nd:YAG lasers produce good cuts on some encapsulation materials, but they currently operate at rates too slow for production. CO2 lasers cut quickly, but may leave a rough, charred edge. Removing embedded metal within the saw streets is extremely difficult with a laser, since profoundly different energy levels are required to ablate organic dielectric layers and metal circuitry.Both CO2 and YAG lasers show promise for singulation, if only to indicate that the right answer may lie somewhere between them. As lasers evolve toward an acceptable solution for cutting organic substrates and encapsulation, the resulting production improvements may justify eliminating embedded metal from product designs.
Conclusion
As FBGA production requirements grow, singulation processes and equipment will become available to serve customers' pilot level and high volume requirements. Process throughputs, currently gated by sawing feed rates, will most likely continue to increase. Additional gains may be realized from multiple-blade gang sawing, along with multiple nozzle transfer.The implementation of HDS panel formats has helped drive FBGA costs down to where significant market share increases are attainable compared to conventional surface mount packages. High-volume singulation, capable of separating FBGA devices from high-density substrates without consuming expensive carrier tape, will help promote these savings.
Mr. Feicht is an industry consultant working as an ana-lyst with TechSearch International. Prior to joining TechSearch, he was Marketing Manager for BGA Products at Motorola Manufacturing Systems. He has 15 years of semiconductor interconnection and packaging experience with Motorola and IBM, primarily in the development of equipment and processes for flip chip placement, laser ablation and ball grid arrays. He earned a bachelor's degree in mechanical engineering from the Georgia Institute of Technology and a master's degree in mechanical engineering from Rensselaer Polytechnic Institute. He may be reached at 512.301.7312 or by e-mail at mailto:dfeicht@texas.net.