March 1998
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Preventing Voids in µBGA® Packages
A new vacuum dispensing process stops the formation of voids during µBGA package encapsulation.
While there are those who argue about how to define chip-scale packages, few dlsagree on their dramatic growth. The increasing popularity of (CSPs has been a phenomenon that is likely to continue well into the new century, and with good reason CSPs offer the size, weight, and performance of a flip chip in a mechanically robust package which is compatible with existing surface mount placement equipment.
In the race for the ultimate CSP, the µBGA® package developed by Tessera is a leading candidate. Keeping company with the design activity are the efforts by manufacturers to develop equipment for producing and assembling µBGA packages.
Cost-EtIective Encapsulation
One of the problems to date has been the need to develop a cost-effective means of encapstllating the die once they have been mounted on the tape substrate. Previous methods, based on a single vacuum chamber concept, have been hindered by a lack of performance and throughput.A new liquid dispensing system developed by Camelot Systems for µBGA manufacturing is discussed in this article.
Dispensing Concerns
A number of different µBGA designs exist through licensing of the technology by Tessera. The primary distinction in these designs, from a dispensing standpoint, concerns the elastomer pad stencil printed on the tape.The elastomer provides an interface between the die and the tape to accommodate the differences in CTE. This interface is needed because during thermal cycling, the material absorbs the mechanical stresses between two mismatched CTEs (silicon die and tape substrate for the package). While this can be accomplished in pad form, some companies, including Tessera, prefer "nubbins" in place of a full pad. For µBGA packages with nubbins, care must be taken while dispensing with conventional systems to ensure a void-free flow of the low modulus silicone material under the die and in and around the elastomer columns.
Dispensing epoxy-type encapsulants so that they flow over and around the wire-bonded leads of a die mounted in a standard BGA package requires selection of the proper material and often the application of heat, either through convection, direct conduction or radiation.
For µBGA packages, however, heat cannot be used with the low modulus silicone material because the viscosity begins to increase with heat due to curing. During thermal cycling, the material is well suited for absorbing the mechanical stresses between the two mismatched CTEs. However, the encapsulant is viscous (85,000 to 95,000 centipoise at 25 °C) and does not flow smoothly and evenly. If the proper dispensing process is not used, voids under the silicon die will result.
To combat this problem, Camelot and Tessera engineers developed a vacuum process which effectively "forces" the silicone encapsulant into the spaces around the leads, nubbins and under the die. This process forms the basis of the CAM/ALOT 3900 system.
System Characteristics
The CAM/ALOT 3900 (Figures 1 and 3) consists of three separate vacuum chambers: pre-dispense, dispense and post-dispense. The three chambers enable the dispense chamber to remain under a vacuum without having to be pumped down each time a µBGA strip (Figure 2) is processed. This approach avoids what may otherwise be a timeconsuming and inefficient process.Each chamber is vacuum-leak tested at 1 x 10-6 torr evacuation using a helium mass spectrometer leak tester.
Figure 2 - Tape p BGA Strip Assembly Format.
Figure 3 - Diagram of CAM/AL0T 3900 system details doors (locks), vacuum chambers. and direction of travel for tape frames.
Table 1-Procedure for µBGA Encapsulation
| Load Magazines | Magazines carrying tape frames are placed in the loader, a system which automatically feeds the frames one at a time into the pre-dispense chamber. (Note: To prevent the silicon encapsulant from flowing through the wire bond access holes in the substrate strip, a piece of manufacturing tape is placed across the entire bottom of the substrate after mounting the strip in the tape frame. After encapsulation, the manufacturing tape is removed.) The process begins by simultaneously pumping down the dispense and post-dispense chambers. |
|---|---|
| Pre-Dispence Chamber | The loader indexes to the first magazine position and pushes the tape frame from the magazine into the pre-dispense chamber, which is at atmospheric pressure. Door #1 (Figure 3), the first of four "doors," closes, and the pre-dispense chamber is pumped down to the programmed vaccum level of approximately 28 inches of Hg. |
| Dispense Chamber | After the pre-dispense chamber reaches the programmed level of vacuum, door #2, separating the pre-dispense and dispense chambers, opens, and the tape frame is conveyed into the dispense chamber. Door #2 then closes, and the pre-dispense chamber is vented to atmosphere to allow anothertape frame to be processed. |
| Tape Frame Secured | In the dispense chamber, a lift table is engaged, lifting the tape frame upward against a clamping fixture and securing it inposition for dispensing. (The lift table is available with an optional heater which features a closed-loop digital controller) |
| Alignment | The dispense process begins with the high resolution CCD camera moving into position to perform the alignment procedure. Using a Cognex vision processor, the system can be programmed to align individual components or reference points. It can process all reference points prior to dispensing. |
| Substrate Measurment | After vision alignment, a touch probe sensor measures the substrate flatness (to+-0.0005 in.) and automatically adjusts the height of the dispensing nozzle. |
| Encapsulate | The selected valve dispenses the silicone encapsulant in a prescribed pattern. (To prevent voids, the sealant must be degassed prior to encapsulation). |
| Post Dispense | When dispensing is complete, the tape frame is released and moved by the conveyor to a dwell station in the dispense chamber. This action provides time for the material to flow and make contact to the external features of the packaged chip. Afterward, it is conveyed to the post dispense chamber. Door #3, seperating the dispense chamber and the post-dispense chamber, is closed. the post-dispense chamber is then slowly vented to atmosphere and, as this ocures, the dispensed encapsulant is drawn under and around the wire-bonded leads and the nubbins. (Nitrogen can be introduced, if desired, to eleminate any moisture from the air.) |
| Curing | After the post-dispense chamber is completely vented to atmosphere, door #4 is opened. The tape frame is moved from the post-dispense chamber through the door opening and is pushed into the first available slot in the magazine, located on the unloader. The ,uBGA packages are then transported to an oven for curing at 120-150°C. |
| Repeat | The above steps are repeated for subsequent tape frames |
Figure 4 - Programmed dispensing path for sealing a pBGA strip.
The chambers are isolated from each other, and from the atmospheric conditions through the use of four pneumaticallyactuated valves. Two of the valves are for sealing off the atmosphere at the entrance and exit doors to the system, while two others act to separate the environments between the chambers.
Each chamber has its own vacuum pumping system. The pre- and post-dispense chambers employ a 13 cfm singlestage pump, while the larger dispense chamber is served by a similar but stronger pump with a 38 cfm capacity. The three chambers also incorporate a pressure-sensing transducer, purgevalve assembly (also called a "back-toair" valve), over-pressure valve, pneumatic feedthrough, electric feedthrough, proximity sensor, product stop and segmented transport conveyors.
The chambers offer vacuum access hatches for maintenance and swingaway doors for use when the chamber is at atmospheric pressure. Viewing windows are provided to allow for observation during encapsulation.
The CAM/ALOT 3900 incorporates the company's 680-series rotary positive displacement pump, mod)fied for highvacuum operation. The double-helix auger design of the pump easily accommodates higher viscosity materials.
Table 2-Single vs. Multi-Chamber Process
| Process | Single Chamber Time (sec.) |
Multi-Chamber Time (sec.) |
|---|---|---|
| Conveyor or Manually Load | 4 | * |
| Secure Chamber | 2 | 0 |
| Pump Down Chamber | 60 | N/A |
| Dispense Sealant | 40 | 40 |
| Maintain Dwell Time | 20 | N/A |
| Bring Chamber to Atmosphere | 30 | N/A |
| Open Chamber | 2 | 0 |
| Conveyor or Manually Unload | 4 | * |
| Total Estimated Time | 162 | 40 |
| *Transport time of 4 seconds is applicable for first board in production sequence only. | ||
The Encapsulation Process
The µBGA packages are processed by mounting the tape substrate in a tape frame. Substrates of 35-,48- and 70 mm may be used. Operation of the CAM/ALOT 3900 for encapsulating chips in the µBGA package is shown in Table 1.Multi-Chamber Processing Advantages
Other equipment developed to process µBGA packages employs a single vacuum/dispensing chamber. The disadvantage of this type of design is that the chamber must be pumped down and brought back to atmospheric pressure each time a tape strip is processed. Table 2 compares the typical length of time for each of the steps in single and multi-chamber processes.Once the dispense chamber is pumped down, it remains at vacuum for the entire production run. The dispense chamber consists of two stations, one where the actual dispensing is performed, and a second station, previously mentioned, where the encapsulant is allowed to flow up to and around the edges of the chips.
Such a "dwell" period occurs while encapsulant is being dispensed for another substrate at the first station, and therefore does not add more time to the production sequence. With this type of setup, approximately 2400 chips can be encapsulated per hour.
Conclusion
As Tessera and its licensees continue development of , µBGA design, equipment manufacturers continue to seek ways to maximize yields while achieving cost-effective, high-volume production. In response to such objectives, equipment such as the CAM/ALOT 3900 has been developed to provide void-free encapsulation of µBGA packages.*J. Fjelstad, "Meeting Reliability Requirements for Chip-Scale Packaging Technology," Advancing Microelectronics, November/December 1997.
Mr. Lawing is Sales Support Manager at Camelot Systems. He began his engineering career in 1984 and gained extensive experience at Martin Marietta Aerospace and Quad Systems Corp. prior to joining Camelot Systems. He may be contacted at 508.373.3742, fax 508.521.2105.