BY ALAN LEWIS
Devices that use underfill have proliferated both in package type and volume. The need for underfilling a wide variety of packages for reliability is well established. The equipment used to dispense underfill materials has matured, but the technological improvements for needle dispensing have only been incremental in speed and capability. The recent introduction of jetting technology to underfill dispensing is a different approach that can increase speed several times over and can successfully underfill packages not possible with needle dispensing.
Before exploring the use of jetting in underfill applications, a review of the underfill process is in order.
Underfill is used in a wide variety of packages and board-level assemblies. Flip chips, direct chip attach on boards, stacked die packages, and various ball grid array (BGA) assemblies are some of the many permutations that are being used in production today. The “part” to be underfilled can be either a die or the BGA substrate. While these packages often have their own material sets and reliability drivers, the underfill process used for their production has many similarities.
The automated underfill process includes:
- Pre-treatment
- Parts handling
- Heating
- Part location with vision and height sensing
- Dispensing the underfill material with one or several timed dispensing passes
- Material flow out (material flowing by capillary action under the part)
- Dispensing of a seal or fillet pass (optional)
Pre-treatment
For good, reliable underfill, make certain the substrate is moisture-free. The parts must be dried sufficiently before underfilling. This is easy for ceramic substrates, but it can be a challenge for organic and polymer substrates. If there is moisture trapped in the substrate, there is potential for voiding and de-lamination during cure.
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Plasma cleaning is an established process for liquid encapsulation of wire-bonded parts. Work is in progress to show how the use of plasma cleaning improves reliability of the underfill process; published results are pending.
Part Handling
Parts are conveyed into a dispenser generally one of two ways: 1) Packages are usually conveyed on a carrier, such as an Auer boat or a Joint Electronic Device Engineering Council (JEDEC) tray. 2) Board-level assemblies usually do not use carriers; the boards are conveyed using SMT standards. The package and board-level conveyors are similar, but have differences in stop pins, clamps and tooling use for heating.
Heating
Underfilled packages require heat for the fluid to be pulled under the die from capillary action. The ideal temperature for the underfill process depends on many factors, including die size, bump pattern, substrate materials and the underfill fluid itself. The underfill fluid is probably the most important of these factors and the manufacturer of the fluid can be counted on to provide a good working range of temperatures to promote adequate material flow.
In nearly all cases, higher temperatures promote faster underfill. The intermolecular forces that create capillary action increase with temperature. However, the heat-cure epoxies used for underfill create a limitation. If the temperature of the part is too high, the material starts to cure before the underfill flow is completed, which impedes flow. If the speed of the flow-out is critical, experimentation may be required to find the optimal temperature.
Parts can be heated with a variety of methods. These methods include infrared (IR) heating, contact (or conductive) heating and convection heating using heated air. Methods can be mixed.
Contact methods are often preferred when possible due to simplicity, uniformity of heating and robustness in production. Temperature is controlled by feedback at the tooling. There is a temperature offset between the tooling and the part, but this can be measured and compensated easily.
IR and convection methods are good for parts where contact is not possible. With convection heating, the temperature of the air and the airflow are set to get the desired part temperature. Again, the temperature offset must be measured and compensated if precise control is required. When IR heating is used, the feedback comes from measuring the part temperature directly with an IR thermometer. While this method gives direct feedback of part temperature, it requires an IR sensor to be positioned where the heat is applied.
In high-volume production, heating the parts quickly is important to maintain tact time of the production line. However, heating parts quickly can cause thermal stresses that can damage non-underfilled parts, which often limits the rate at which the parts can be heated.
The following are some ways to prevent the heat rate limitation from limiting tact time:
- Use a pre-heat station so that parts are heated while other parts are at the dispensing station.
- Increase the number of parts in a carrier. Since all the parts are heated at the same time, the total units per hour (UPH) can be increased.
- Use a heater that heats the parts from the top and bottom. This reduces the stress caused by thermal mismatch between the top and bottom of the part, allowing for a higher heating rate with lower stress.
 FIGURE 2. Jetting deposits underfill material in a series of dots alongside the die (or package), forming a continuous bead that flows under the die with capillary action. |
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These techniques can be used in combination as needed.
A third heating station that allows the fluid to complete the underfill after dispensing can be added. This third heating station allows the dispenser to begin the underfill process on the next batch of parts and speeds up production. However, it is only an option when a “seal” or “fillet” pass is not required (see “Dispensing Patterns”).
Part Location
When automating the underfill process, the die or part location and orientation must be determined in three-dimensional space. The horizontal position and rotation alignment of the dispensing pattern to the part is most often done with an automated vision system. Features on the substrate can be used for positioning, but it is also possible to use the edges and/or corners of the die for alignment.
Direct location of the die (or corner of the BGA package) is often the most robust process. Pattern recognition algorithms that capture position and rotation with one digital picture can improve throughput significantly. The fluid dispense pattern will then be aligned relative to the edge of the die (or BGA package). One challenge is to use a vision system that is insensitive to the variations in die color and surface finish. Die from different wafers or different parts of a singe wafer can have some optical variation. Equipment vendors that specialize in dispensing equipment for underfilling are well versed in solving this problem.
Locating the part vertically can be done either mechanically or with a laser sensor. Either method works well. While laser sensors may cost a little more, they do not physically contact the part and usually have a speed advantage. Optically clear and shiny parts can be a problem, but the technology available for laser height sensors has improved dramatically over the last several years and they can be use with most surface finishes.
Dispensing Patterns
The determination of a dispensing pattern is one of the most significant factors in successful process development. Location, amount and timing of fluid dispensing affect the quality of the part. The size of the die, number of bumps, bump pattern, and underfill gap, etc. all interact with the dispensing pattern to affect quality and speed. Quality can be evaluated by measuring voiding, fillet size and shape and reliability of the package.
Various articles on dispensing have led to these conclusions:
1. Dispensing on one side of the die or part in one pass is the fastest dispensing process.
2. Dispensing on multiple sides of the die or part can reduce the flow-out time, but may increase the chances of voiding.
3. Dispensing the materials in multiple passes with controlled time between the dispense passes can significantly reduce the width of the wet-out area and/or fillet.
To support the variety of dispensing patterns and optimize production speed, the equipment used for dispensing must have a method of timing each dispense relative to the previous dispense. Some packages require a “seal” or fillet pass to get an even fillet around the part to improve the reliability or aesthetics. To prevent voiding, the seal pass may not be applied until the fluid has completely flowed under the die or part.
Trends
The proliferation of flip chip technology in digital cameras, cell phones, disk drives and similar devices has created a large demand for underfilling small die in the range of 1 mm square to 5 mm square. Independent of whether these die are directly attached to flex, circuit boards or some organic interposer, the area around the die available for dispensing underfill materials is getting smaller. In some cases, package designers are trying to push this limit to 250 µm (0.010″).
This creates an enormous challenge for needle dispensing. The tendency is to reduce needle sizes. But even with a 30-gauge needle, the outside edge of the fluid stream is 350 µm from the edge of the die, as shown in Figure 1 (assuming the needle must be kept at least 50 µm from the edge of the die to avoid contact). With a 30-gauge needle, the material flow rate with common underfills will be restricted to less than 1 mg/sec.
Jetting Underfill Materials
Jetting underfill materials enable production of these new package designs. Material is deposited in a series of discrete drops of fluid while the jet is moving horizontally. These drops can be formed at the rate of 100 per second to form a continuous bead of material at the edge of the die. A jet can dispense from above the surface of the die with a stream of fluid as small as 100 µm. The outside edge of the stream is only 150 µm away from the edge of the die, as shown in Figure 1. With the fast firing rate of jetting, material can be delivered in this small stream as fast as 2 mg/sec. Much higher fluid delivery rates are available with different configurations.
 FIGURE 3. Two small (5 mm square) dice are spaced 1 mm apart. Jetting enabled underfill to be dispensed between the two dice. |
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Important Points
When specifying equipment for automating underfill, the important information to have available includes:
1. Size, gap and number of interconnect bumps for the part.
2. Type of underfill fluid or fluids under consideration.
3. Tolerances on fillet sizes and wet out areas or any other tolerances that influence the amount of fluid dispensed.
4. Description of interconnect bump patterns that may affect the flow-out of the underfill.
5. Maximum temperature change rates for heating of the parts.
6. Carrier or transfer method.
7. Tact time, UPH or other production rate requirements.
References
For a complete list of references, please contact the author.
ALAN LEWIS, director of dispensing technology, may be contacted at Asymtek, 2762 Loker Avenue West, Carlsbad, Calif. 92008, (760) 431-1919, e-mail: [email protected].