The back-end process: Step 8
Flip Chip Underfill
08/01/2002
A guide for successful processes
BY BRUNO MIQUEL
The advantages of the flip chip package over other types of electronic packaging are many, but the most obvious is the reduction in the package size, which delivers savings in board real estate and product thickness. Flip chip packages also offer higher electrical speed and a greater number of I/Os than similar wire bonded packages.
Why Underfill?
When applied to flip chip technology, underfill encapsulates the area between the active side of a flip chip and the substrate upon which it is mounted. The underfill material protects the interconnect area from moisture and other environmental elements, and it reinforces the mechanical connection between the substrate and the die. It also compensates for any difference in the thermal coefficient of expansion (TCE) between the chip and the substrate. Once cured, the underfill material provides additional mechanical support to the interconnections, preventing them from cracking and breaking. The process of underfilling a flip chip typically involves capillary action and often is considered to be a production bottleneck.
Process Requirements
Void-free Underfill: A key requirement of flip chip underfill is that it be void-free. Air trapped beneath the die or around solder bumps will result in reliability problems and possibly early component failures.
 Figure 1. The basic mechanism of a) time/pressure valve pumps, b) rotary positive displacement pumps and c) linear positive displacement pumps. |
No Epoxy on the Die: Dispensing low-viscosity underfill material is a very precise process, which requires tight control of both needle placement and material flow. These attributes are required to ensure that no contamination of the top surface of the die occurs during encapsulation. Even a small amount of material can generate stress and hot spots that can cause cracking and early component failure.
Appropriate Fillet Size: The optimum fillet height depends on the package, but a basic gauge is that the fillet should cover at least 70 percent of the die thickness.
Equipment Heating Systems
Elevating the temperature of the material lowers its viscosity, increases the flow rate under the die and increases the overall production rate. Contact or noncontact heating devices heat the part to 70° to 80°C before and during dispensing.
Contact vacuum chucks provide a faster ramp-up and uniform temperatures regardless of substrate mass and configuration (multi-up carriers). The preferred method is to heat individual parts because the heat transfer is fast and uniform, and the vacuum stage provides a reliable and even surface to hold the substrate.
 Figure 2. Typical dispensing pattern for underfilling perimeter-bumped devices. |
When contact heating systems are not practical to use, or when the bottom side is populated, noncontact heating methods, such as infrared (IR), forced heated convection or a combination of both, are used. IR elements provide a quick temperature ramp-up, but they are difficult to control and can result in uneven heating.
Pump Technology
Three basic pump designs are available for underfill dispensing:
Time/pressure valves (Figure 1a) use air pressure to apply force to the plunger inside the syringe to drive out material. The longer the dwell, or the higher the syringe pressure, the more material dispensed. The major disadvantage of this dispensing technology is that the volume dispensed per given dwell/pressure varies with material viscosity. As the material's viscosity changes over time, the amount dispensed changes as well. Unfortunately with underfill, the volume dispensed needs to remain constant within a set of specific tolerances.
Rotary positive displacement pumps (Figure 1b) are used widely throughout the industry for different dispensing applications because of their versatility and ease of use. Advanced pump de signs optimize the material flow path within the pump to eliminate areas where air pockets could form. Auger pumps have been shown to be reliable and versatile with adequate dis pensing repeatability, despite their sensitivity to material viscosity variations.
 Figure 3. Typical dispensing pattern for underfilling array bumped devices. |
Linear positive displacement pumps (Figure 1c) typically are composed of one or several piston/chamber assemblies driven by a servomotor. They are the least affected by material viscosity changes, and generally the dispensing repeatability is better than other pump designs. A disadvantage of some single piston pump designs is that they require a recharge step to refill the entire chamber once empty. Depending on the viscosity of the material and the pump design, the recharge and purge step can take up to 10 seconds.
Multiple piston pumps do not require a recharging step. While one chamber refills, another is actively dispensing. The challenge with multiple piston pumps is that to meet the dispensing requirement, each set of piston/chamber assemblies has to be identical.
An alternative to the previous designs is to recharge the pump after each dispense. For underfill applications, the volume dispensed generally is low (5 to 30 mg) and the refill could be accomplished while the head is moving to the next dispensing location. Such a pump is likely to satisfy process and production requirements but remain easy to use and maintain. The number of wetted parts would be minimized to reduce the maintenance/cleaning time and eliminate any wear/aging issues.
Other Equipment Features
Process Control Systems: Process control systems help set up and monitor the amount of underfill material dispensed and compensate for such factors as pump inconsistency or material viscosity variations. They measure the volume, the flow rate or the weight of a calibrated shot of material using a vision system or a weight scale. They are used in conjunction with both rotary displacement and current linear piston pumps.
Conveyor Systems: As previously described, the underfill process relies on the capillary action of the material to encapsulate the bottom side of the flip chip. Depending on the size of the chip, bump height and underfill material selected, the process can take several minutes to complete. Dual-lane conveyor processing can accelerate throughput, allowing multiple parts to be loaded in the dispense area so that process steps can be performed in parallel. Some dual-lane systems require additional equipment to shuttle the substrate in and out of a single lane loading/unloading system. This, however, can increase the overall cost and footprint of the system.
 Figure 4. Typical needle place for flip chip underfill processes. |
Height Sensors: Mechanical height sensors, using a rod/proximity sensor assembly, are commonly used to detect substrate height relative to the gantry system. However, to provide more productivity and accelerate the process, noncontact devices that use laser technology are replacing mechanical height sensors. Charged couple devices are selected on most dispensers because of their ability to sense substrate height in half the time required by a mechanical device and without contamination or damage that could be caused by the probe.
Vision Systems: A vision system is a critical component of any dispensing system, and it is particularly critical to underfill processes because the dispense nozzle must be positioned very close to the die during dispensing. Any misalignment, even by a few thousands of an inch, could result in the part becoming scrap. The vision system must be able to detect and compensate for die shape and color variations. Speed also is an important parameter because vision alignment is performed for every flip chip.
Dispensing Patterns
A key factor in successful void-free underfilling of a flip chip is the pattern used to apply the epoxy. Because the material flow rate is greater when it migrates from bump to bump than in any other area, air could be trapped if an inappropriate dispense pattern is used. Speed, again, is important. Many different dispense patterns can be used to achieve void-free underfill in the required time. The patterns in Figure 2 show the most commonly used pattern for devices with bumps only at the perimeter. For such devices, the optimum dispense pattern for minimizing air entrapment involves dispensing a bead of underfill material along the edge of the longest side of the die. Several passes might be required to complete underfilling the flip chip.
For array bumped devices, the optimum dispense pattern involves dispensing an L-shaped line along two adjacent edges of the die (Figure 3). A fillet is dispensed on the opposite sides after the material flows out, to complete the underfill process.
Successful Dispensing Tips
The side of the needle should be located 0.003 to 0.005" away from the die and 0.005 to 0.008" above the substrate to allow the material to wet the side of the flip chip immediately and initiate capillary action (Figure 4). When the volume is large, for larger chips, both distances should be increased to prevent the material from flowing on top of the die.
The actual distance from the die typically is related directly to gantry accuracy and dispenser repeatability. Gantry accuracy and repeatability often is overlooked, but it is an important parameter to consider in the underfill process. When the material is dispensed too far from the optimum location, the flow time required to encapsulate the flip chip increases, slowing down the process. Similarly, variations in the dispensing location generate inconsistency and impact the yield.
Conclusion
Void-free, successful flip chip underfill is a reliable process that depends on the technology used, appropriate process parameters, attention to accuracy, equipment design, and appropriate dispense materials and patterns. Proper application process development is key. Once the process is optimized, throughput can be enhanced (with corresponding high yields) through dual-lane processing and the use of appropriate pump technology to maximize yields. AP
Bruno Miquel, product manager, can be contacted at MRSI, 101 Billerica Avenue, Building 3, North Billerica, MA 01862; (978) 667-9449 x323; E-mail: [email protected].