Benefits of a flux-free atmosphere
BY THOMAS TONG, KRISTEN BROWN and PIERRE LEMIEUX
Wafer-level packaging has grown in popularity in recent years because manufacturers can now maintain the cost of the IC packaging as a constant percentage of the total wafer cost.
Wafer bumping, which involves using solder bumps for electromechanical interface for the die, is a key enabling technology for wafer-level packaging. The reflow process is used to form a metallic interconnect phase between under-bump metallization and solder. After the solder is deposited on the wafer, the reflow process is used to form homogeneous solder spheres. The prevention of solder oxides during reflow is essential for strength of the interconnection and the success of downstream processes.
Reflow Process
The prevention of oxide formation during reflow can be accomplished in numerous ways. Currently, the most common approach is to coat the wafer with flux prior to reflow. The coated wafer is then reflowed in a nitrogen atmosphere that further discourages the formation of solder oxides. Nitrogen atmosphere purities can approach two ppm oxygen in the most controlled reflow oven atmospheres.
Another approach in preventing and reducing oxide formation during reflow is the use of a reducing atmosphere. A hydrogen atmosphere is most commonly used, although formic acid has been tried in the past as well. Hydrogen concentrations approaching 100 percent with oxygen and moisture levels as low as two ppm are used for this process. Forming gas with concentrations of only four percent hydrogen has also been tried but without success for a flux-free process.
Costs
One of the main reasons for flux process popularity is that it is perceived as less costly and safer than the hydrogen process. However, anyone who has worked with flux knows what problems are associated with it. It is messy to use in the process, the equipment and the environment. Accumulated flux residue in the equipment can often result in particulate contamination. Often equipment downtime becomes a problem when frequent cleanings are required to maintain process cleanliness.
After reflow, flux residue must be cleaned off surfaces. Any leftover flux residue in the oven, the flux-coater or in the cleaning solution must be cleaned and disposed of as hazardous waste — which can be costly. In addition, appropriate safety precautions must be taken to isolate equipment maintenance personnel and operators from flux exposure because many formulations are carcinogens.
 Figure 1. A typical solder reflow profile |
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The use of flux has other inherent costs that must be considered. The equipment used to coat and clean wafers with flux must be purchased. The equipment consumes valuable floor space and operator resources. Over the line's operational lifetime, this results in a substantial cost of ownership burden. While flux-free reflow has been used for many reflow processes at both board and package levels, flux residue interferes with wafer bump reflow. This has led to increased use of flux-free processing.
Another Way
Solder oxides can be eliminated during wafer bump reflow by using a hydrogen processing atmosphere. The hydrogen process takes oxygen from the system, so solder oxide cannot be produced. Hydrogen combines with the oxygen to form water. The reduction of oxide, a chemical reaction, happens in the presence of hydrogen combined with heat. Heat provides the activation energy and the higher the reflow temperature, the more easily hydrogen can work. For this reason, high lead solders with reflow temperatures in excess of 350°C are most commonly used with flux-free hydrogen reflow processes (Figure 1). In general, alloys that work best for reflow are those with the highest peak temperature. Lead-free lower than 250°C is less widely used, but has proved feasible.
In the reflow process, elimination of solder oxide is important. Processing in hydrogen, therefore, prevents further oxidation of solder. Spiking the heat allows the process to dissolve oxide while not damaging bumped wafers.
In the next few years, bump pitches will fall below 150 µm (Figure 2). As the solder bump size shrinks, the surface-to-volume ratio increases, thus, creating a higher tendency to reduce surface energy by surface oxidation. This will increase the importance and difficulty of solder oxide removal.
Historically, as line widths and pitch sizes decrease, the need for process cleanliness increases. It will be increasingly important to remove any residues completely from the wafers and prevent the introduction of any contaminants into the molten solder.
Hydrogen provides a clean processing atmosphere. No flux residues are created. The furnace system is kept clean due to the reducing atmosphere, creating little opportunity for process contamination.
Two types of reflow furnaces, batch and continuous, are commonly used for the hydrogen reflow process. Batch furnaces typically process many wafers at one time. These wafer lots can contain as many as 50 wafers at once.
 Figure 2. Bump pitch size |
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Although well suited for high-volume production, these batch furnaces are cycle time-limited for small lots and test runs. This is because an entire lot must be loaded to maintain the thermal load of the batch furnace. The user must run an entire load whether or not they need to. The load/unload times for these wafers can be significant, pushing the cycle time for one test wafer to over one and a half hours.
In addition, thermal profiling of a hydrogen-capable batch system can be cumbersome. Thermocouple wires must feed from an instrumented wafer in the process chamber. This action often necessitates defeating interlocks designed to ensure hydrogen safety. For this reason, many hydrogen batch furnace operators have reduced the frequency of thermal profiles despite the need for process control.
 Figure 3. Oxide-free solder bumps reflowed in a hydrogen atmosphere |
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Continuous belt furnaces (Figure 5), which are common workhorses in many industries, have been designed specifically to meet the needs of flux-free wafer bump reflow.
Despite the fact that a continuous belt furnace has openings at either end, a high-purity atmosphere can be maintained. Gas barriers can be used to create atmosphere isolation. Barriers are essential to ensure a high degree of atmospheric control inside a furnace.
 Figure 4. A wafer bump reflow in a continuous belt furnace |
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A continuous belt furnace using a hydrogen atmosphere affords high repeatability, even with varied loading conditions. One furnace manufacturer reports 99 percent hydrogen purity in a continuous belt furnace, while maintaining a pure atmosphere with only two ppm oxygen and two ppm moisture. The most stringent processes generally require a moisture level of no more than two ppm as a safeguard. Moisture levels can also be kept to a minimum by storing parts in an inert atmosphere to help prevent oxide formation. Because wafer manufacturing is conducted in a controlled cleanroom-type environment, under normal handing conditions, a pre-bake is not necessary.
Ensuring Safety
What about safety? Hydrogen is a relatively safe material for industrial processing, and is commonly used in many applications. From an environmental safety perspective — exposure and disposal — hydrogen is a friendly material, especially when compared to flux.
Despite such affirmation of hydrogen's safety, it does not negate the necessity of treating hydrogen with respect, since it is considered an explosive gas. Explosive is defined as a gas or a mixture of gases where there is a possibility of four percent or greater combustibles. Gases normally included in this list of combustibles, includes hydrogen, methane and others. National Fire Protection Association (NFPA) rules govern the design of furnaces using explosive atmospheres, and furnace manufacturers operate within these restrictions.
 Figure 5. A continuous belt furnace |
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In a furnace environment, hydrogen is quite safe because the system first purges the process chamber of all gases other than that of the purge gas, typically nitrogen, creating a gas barrier on both ends of the furnace. Thereafter, the hydrogen cover gas is introduced to the process chamber. The exhausted hydrogen gets burned by an igniter, and its by-product is water and heat. Extensive use of flow and pressure monitoring devices confirm that there is an adequate supply of gas and constant supervision of the igniters will confirm that spent process gas is not entering the atmosphere.
One common misunderstanding of hydrogen's volatility is the notion that one must deal with a lot of it — a big tank's worth filled with liquid hydrogen or high-pressure hydrogen. However, a thimble full of water contains enough hydrogen for processing, once the oxygen has been separated from it.
It actually takes a considerable effort to get the right amount of oxygen with hydrogen to create a danger of explosion. Without that right amount, all one gets is a flame. Keeping a certain level of air out of the system is not difficult.
Inside the furnace is an oxygen analyzer, which can detect extremely small amounts of oxygen, down to one ppm. With so little oxygen in the process chamber, there is probably more latent oxide on the wafer than that in the furnace. As the wafer comes into the chamber for the wafer bump reflow process, it will give up some of its oxide because of the atmosphere moving across its surface.
Conclusion
Preventing and reducing oxidation of bumped wafers during reflow is essential to the success of wafer-level packaging. Both the flux and the hydrogen reflow process are effective for oxide prevention and reduction. Currently, the flux process has presented its users with environmental risks, cleanliness problems and higher operational costs due to added equipment and personnel, as well as the disposal of hazardous waste. The hydrogen reflow process has been shown to be completely safe, clean and cost-effective. End users who use this process have demonstrated high process yields.
References
For a complete set of references, please contact the authors.
THOMAS TONG, technical representative for Asia Pacific, and KRISTEN BROWN, product marketing manager for semiconductor packaging, and PIERRE LEMIEUX, formerly a senior process engineer at BTU International, may be contacted at e-mail: [email protected] and [email protected].