Almost by default, hybrid micro-electronic assembly producers have locked themselves into using expensive liquid nitrogen as a source for their inert gas needs. While seam-seal and some other manufacturing procedures admittedly require the 99.99 percent purity of liquid nitrogen,the largest volume of nitrogen, and thus the largest expense, is for inert gas blanketing of assemblies stored in dry boxes where the required purity can be on the order of 97 percent, as long as the gas is dry.
One new system* extracts a constant, economical supply of dry gaseous nitrogen from ambient air, which can be perfectly adequate for use as an inert atmosphere in dry boxes as well as for burn-in ovens. This air separation system uses a semipermeable membrane to separate compressed air into its component gases. The nitrogen product stream can be adjusted to provide a purity level of 95 to 98 percent N2.
Inert gas is needed in the production of hybrid microelectronic assemblies to exclude moisture, oxygen, and other contaminants. Hybrids are sealed, for example, in a nitrogen atmosphere where the oxygen content can be no greater than 50 ppm. Completed components are stored in an extremely dry nitrogen environment where less than 20 ppm water vapor can be tolerated. Nitrogen also is used as an inert cover gas in the ovens during the burn-in period.
Liquid nitrogen was the single source of all inert gas for manufac- turing hybrid components at Ball Corp.’s Aerospace Systems Div./ Microelectronic Products facility at Huntington Beach, Calif. But liquid nitrogen is very expensive, especially for applications such as the dry boxes that demand a continuous flow of gas. The division was using 637 standard cubic feet per hour (SCFH) of gaseous nitrogen, averaged out over one year. The cost for liquid nitrogen as the source of that inert atmosphere was $3642 per month, or $43,700 annually, including rental of a 10,000 gallon insulated storage tank.
With these costs in mind, and because it was estimated that better than one percent of the liquid nitrogen was wasted daily through evaporation and vaporization, the company began investigating pos- sible lower cost, alternate sources of nitrogen. Obviously liquid nitrogen at 99.99 percent purity would still be needed for the critical demands of seam sealing. However, the company was encouraged to discover the largest single use of nitrogen, the dry boxes, did not require the absolute purity of liquid nitrogen.
This revelation provided the key to considering an alternate nitrogen source. The membrane air separation system, because it provides a continuous supply of dry gaseous nitrogen at an adjustable purity of 95 to 98 percent N2, seemed like an ideal source of nitrogen for the dry boxes. The cost analysis and economic justification were very attractive, showing a payback in only eight to nine months.
Nitrogen from Ambient Air
The Air separation system was introduced in February 1985. Based on the same proprietary hollow fiber technology that made the artificial kidney possible, the system passes compressed air through a semipermeable membrane to separate the air into the component gasses.
The hollow fiber, which is finer than a human hair, is made of a polyolefinic material. Each fiber has a perfectly circular cross section with consistent wall thickness and uniform bore through its center. Because the fibers are so small, an extremely large effective surface area is available for high volume air separation in a relatively small space. The separation module is only nine inches in diameter, yet it contains 10,000 miles of fiber.
Gases permeate the membrane at different rates, depending on the molecule size and characteristics of each specific gas. Oxygen, for example, has a higher permeability rate than nitrogen and passes through the membrane several times faster. On the other hand, since nitrogen has an appreciably lower permeability rate than oxygen, a significantly smaller portion of the nitrogen is able to permeate the membrane.
Thus the oxygen in the air is physically separated from the nitrogen. While all but a small percentage of the oxygen passes through the membrane and into the bore of the hollow fibers, a large portion of the nitrogen never penetrates the membrane but is swept around and past the fibers.
The hollow fibers are assembled parallel to a central perforated feed core, and the bundle is inserted into a pressure case to form the air separation module. Compressed air at normal plant pressure is introduced through the core and passes in intimate contact with the fibers, where oxygen selectively permeates the walls of each fiber and into the hollow core. The oxygen-enriched gas flows through these cores to the end of the module where the gas is discharged.
The remaining gas, which is essentially nitrogen, passes along the outside of the fiber and is discharged as the product gas stream. Water vapor also permeates the membrane along with the oxygen, leaving a substantially dry nitrogen product stream.
The new air separation system, as a nitrogen source for the dry boxes, was purchased as a complete package from a distributor in southern California **. The system was installed and went on-line in April 1987. Although the system package includes the distributor’s compressor, an existing rotary screw type compressor is the primary feed air source, and the new compressor is reserved as a backup unit.
The system is installed outdoors where ambient temperature can range between 40 °F at night to a high of about 95 °F on a hot summer day. Since the air separation modules require a fairly constant feed air temperature to maintain consistent nitrogen purity and flow rate, a chiller/dryer holds this temperature between 60 °F and 70 °F, which is an ideal operating parameter for the system’s modules. In addition, the dryer maintains a feed air dew point into the modules of 33 °F to 39 °F which is the first step in excluding water vapor from the nitrogen supply.
Feed air enters the separation modules at a pressure of 110 psig. There are two air separation modules in Ball’s Huntington Beach system, each rated at a flow of 228 SCFH 97 percent N2. The system is kept at 97 percent N, purity as indicated by an integral oxygen analyzer (3 percent O2). The nitrogen product stream goes to a dual tower desiccant dryer where the dew point is reduced to -135 °F, which means the final moisture content of the nitrogen stream is less than 0.7 ppm..
Present consumption of nitrogen from the system is well below the approximately 450 SCFH capability of the equipment and allows for increased needs in the future. Although this figure appears to indicate the use of nitrogen has dropped below the 637 SCFH which was used when liquid nitrogen was the only source, part of this difference can be accounted for by the evaporation and vaporization loss suffered with liquid nitrogen. In addition, liquid nitrogen now stored in a small 2-160 liter dual tank is still used for the high purity seam-sealing requirement.
The air separation system runs continuously because the dry boxes require a constant flow of nitrogen. Continuous operation is actually better for the air separation membranes and helps extend their lives. Furthermore, the screw type compressor, the primary compressed air source, operates more efficiently when fully loaded and continuously run. Finally, continuous operation makes the system more economically attractive, even factoring in the additional power consumption, than it would be if the system were used intermittently.
Since the nitrogen flow into the dry boxes is fairly constant, the system can be considered self-regulating. If nitrogen demand were to increase sharply, purity could be affected. Although there is no need at the present time for a nitrogen surge tank to compensate for this condition, one could be added to the system if demand fluctuates or increases. A further way to meet increased demand at minimum cost would be simply to add another module which can increase system capacity by another 228 SCFH.
The only component with moving parts in the entire nitrogen supply chain is the compressor. Should the primary screw type compressor fail, the back-up compressor which was part of the original system package would be switched over immediately. The back-up compressor also is operating continuously, although it is not loaded, to facilitate rapid emergency switch over immediately. The back-up compressor also is operating continuously, although it is not loaded, to facilitate rapid emergency switch-over. This action would be important to protect components in the dry boxes from moisture contamination.
To date, the air separation system has met or exceeded all its published specifications and has more than lived up to Ball’s expectations. The nitrogen supply it is producing meets all applicable military standards including Mil-Std 1772 requirements. The equipment paid for itself by the end of 1987, and over the next five years cost savings are projected to be somewhere between $175,000 and $200,000.
The key to success with air separation as an alternate source of nitrogen is investing time and effort to define the required purity for each inert atmosphere application. For some applications, the ultimate purity of liquid nitrogen is essential. But for others, paying a premium price for a premium product is not truly necessary.
Unfortunately, some engineers in the hybrid microelectronics industry are unaware of the actual required purity for each different nitrogen application. There are no established industry standards, and many who are responsible for production processes do not have the time or financial resources to conduct their own research. Also there may be some people who are not familiar with the present state and practical application of air separation technology. Perhaps knowledge can help fill some of these gaps.
Membrane air separation, in the form of this system, is a vital source of inerting nitrogen for some important hybrid microelectronic manufacturing applications. This system in no way jeopardizes a company’s quality standards, it may save upward of a quarter of a million dollars over the next several years.