Terra Universal article first published in Advanced Applications in Contamination Control’s A2C2 Magazine in April 1999.

Desiccator Storage: The Missing Link in the Manufacturing Chain

Manufacturers understand the need to control show signs of degradation until after a “dwell moisture, static, and particulates at every critical time” that can last for weeks or months, when processing stage. Unfortunately, however, they consequences are much more serious. Imagine an often limit comprehensive environmental aircraft’s high voltage ignition system failing safeguards to discrete manufacturing operations. because humidity exposure during manufacturing The fact is that sensitive materials – integrated compromised a key component, or a circuits (ICs), components for high voltage chemotherapy treatment rendered ineffective systems, pharmaceutical solids, medical products, because moisture has decreased the drug and others – are vulnerable to degradation or potency. destruction at virtually every moment, and especially during extended periods of storage One notorious, and too familiar, example of between these operations. This damage often moisture related damage is the “popcorn” effect results from chemical reactions that can be that occurs during reflow soldering of IC packages avoided entirely if materials remain in the dry, (Figure 1). Although the vast majority of ICs are inert environment provided by a . packaged in plastic encapsulants (because they are cheaper than ceramic ones), manufacturers not only safeguard against defects are often unaware of the consequences of using caused by particles, static, and chemical vapors hygroscopic materials in a solder reflow process. (including moisture), but also offer features – Moisture absorbed into the package vaporizes including data during the rapid management, heating and space efficiency, generates documentation, pressure along and security – metal-to-plastic designed to contact regions. meet critical Differences requirements in between the a wide range of coefficients of industries. thermal expansion of the Humidity two materials can Control cause loss of adhesion, Because its swelling, and effects are not cracking. immediately Figure 1: Fluctuations in humidity levels during storage correlate directly to moisture absorption rates of plastic encapsulant.1 apparent, In the best- moisture is one of the most insidious scenario, a loud, audible “pop” signals the environmental hazards. Sensitive materials stored problem, and testing confirms a deterioration of at an unacceptably high humidity level may not electrical function. In the worst-case, but more

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Terra Universal, Inc. 21 July 2015 Page 2 common, scenario, the damage is not rates among PCB-bound packages into significant immediately detectable, and the “walking failure levels by the time these packages are wounded” ICs pass through several subsequent mounted. As shown on the board yield charts processing steps – and are delivered to the (Figures 3 and 4), assuming a typical complexity customer – before the inevitable electrical failure of 150 parts, 15% of boards will fail and require occurs. Because moisture absorption, and with it costly reworks if only one in a thousand packages the likelihood of part failure, is directly related to (1000 PPM) failed during board assembly. With the duration of exposure, dry storage is an board complexity and competitive yield obvious solution to this problem. thresholds on a sharp increase, manufacturers must control moisture exposure at every turn in Moisture- the processing cycle in order to keep failure levels related below this 1000 PPM standard. damage is also common in package bonding. If the IC packages have been in inventory for more than six months but

Figure 2. Relative oxidation rates, shown inadequately as a function of humidity and temperature protected against moisture, leads will oxidize to a point where the solderability is materially affected and the board yields will drop (Figure 2). Figure 4. Yield chart of a typical (assumed) 15% failure rate.

Worst of all, the harsh mathematical reality of Moisture also poses significant risks in other compounding failure rates in PCB (Printed Circuit industries. High humidity rates facilitate the Board) assembly transforms relatively low failure growth of mold, bacteria, and other microbes, making it an important environmental concern in pharmaceutical, biomedical, medical device manufacturing, and food processing applications. Biotechnology research that requires controlled moisture removal from DNA samples after protein synthesis also requires low-humidity storage facilities.

To minimize these problems in the clean manufacturing industries, RH must be monitored and controlled carefully during every period of storage. The specific humidity requirement of the part or material being contained is the single most important consideration when choosing a Figure 3 desiccator system. For example, products

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Terra Universal, Inc. 21 July 2015 Page 3 composed of copper foil, such as batteries, must for contamination-free storage (as well as for be stored at a humidity level of less than 5% RH to many process operations) because of its low dew prevent minute, barely detectable degrees of point, which makes it an ideal storage medium in oxidation that can nonetheless ruin processing moisture-sensitive applications, and because it operations. can be isolated and purified relatively In pharmaceutical applications, the product must inexpensively. In addition, nitrogen is typically be kept dry in order to prevent degrading cleaner than filtered air. By introducing measured chemical reactions. Lab refrigerators can dry to amounts of dry nitrogen into an air-tight only approximately 30% RH, a level insufficient to enclosure, a well-designed control system prevent deterioration. A desiccator, however, can continuously displaces moisture-laden air. automatically maintain the very dry conditions Nitrogen, which has a lower specific gravity than (10% RH or less) to ensure product integrity. air, is introduced into the upper section of the desiccator, and water vapor is purged out of the Nitrogen Purging: An Efficient Alternative bottom through a bleed/check valve. This nitrogen purge must be continuously maintained While regenerating or disposable are in order to counteract the hygroscopic often used to control the level of RH in a characteristics of the plastic typically used in minienvironment, nitrogen purging is the most desiccator fabrication. efficient method of maintaining very low humidity levels. -based dry storage systems – This simple setup works great in theory, but real- often involving dual-tower designs for world circumstances can greatly compromise the simultaneous on-line drying and off-line effectiveness of a flowmeter-based nitrogen regeneration – are effective for low-volume delivery system. The single greatest problem applications, but they cannot provide the confronting storage facility management is the precisely regulated low dew points required in need to access stored components. The moment more and more critical manufacturing an access door is opened, a momentary negative applications. Typically, they are also expensive to pressure draws air laden with moisture and operate because they require built-in heaters to particles into the controlled environment. In a bake the moisture out of the desiccating medium. relatively high ambient-humidity environment, Failure of any system component can lead to a this backfill can expose parts to significant rapid rise in the desiccator humidity level. amounts of moisture, which must be rapidly flushed from the system before it can be Furthermore, these systems typically require absorbed by material surfaces. close monitoring to ensure that the incoming gas flow remains adequate to lower the internal dew This requirement exposes the central dilemma of point as the silica medium nears its maximum a simple flow-regulating setup: if you set the effective adsorption point (the point at which it flowmeter to compensate for this backfill with a has adsorbed between 5-25% of its own weight in high-flow purge, you run the risk of over water). Adsorption effectiveness varies widely pressurizing the system when doors are closed. both with temperature and ambient RH. In Seals eventually fail, and cabinets can even difficult applications, calling for very low humidity explode. Equipping an enclosure with an thresholds at high ambient humidity levels, a adequate number of bleed valves can relieve this desiccant may actually desorb – contributing to, pressure but leads to nitrogen waste when doors rather than solving, the moisture problem. are closed. In a confined or poorly ventilated storage facility, this continuous, high-flow purging A nitrogen-purge system eliminates these can even raise the percentage of nitrogen in the problems. Pure nitrogen is the standard medium

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Terra Universal, Inc. 21 July 2015 Page 4 air to unsafe levels. Other factors, including Teamed with a monitoring system that can fluctuations in ambient humidity levels and maintain a sub ambient internal RH level down to differences in cabinet sizes and conditions, only 0% RH for each chamber of an enclosure complicate the chore of maintain a carefully independently, the flow will be automatically regulated low-moisture environment. activated when an adjustable threshold is exceeded. By improving the efficiency of the Cut Recovery Time, Increase nitrogen purge and moisture removal, a multiplexed gas control system also helps to Fortunately, nitrogen-purge systems are available compensate for the hygroscopic characteristics of today with integrated, automated nitrogen the desiccator material, such as Plexiglas, static- control capabilities that solve these problems, dissipative PVC, and other polymers that allow offering relatively low-cost ways to increase moisture to pass through the enclosure walls. For component yields and boost profitability. These this reason, ambient humidity is another factor to systems make use of advanced gas-flow consider when specifying a storage system, for a engineering to reduce system recovery time (the desiccator in a humid location will require more time necessary to restore an acceptable internal nitrogen purging than one located in a dry humidity level after an access door has be open). environment. Because each chamber of a One system, for example, incorporates a pressure multiplexed system is controlled individually, regulator, gauge, flowmeter, and a variable flow nitrogen usage is minimized and RH setpoint mechanism, allowing it to maintain a low-level, recovery times are greatly reduced. economical positive pressure during normal operation, but automatically increases the Particle Control nitrogen flow when an access door is opened to prevent contamination inflow. When the door is The most widely discussed type of yield-busting closed, the system provides the high flow for a set contamination is, of course, particles. In time, in order to flush out any trapped air, and microelectronics and semiconductor processing then re-established an economical low-level especially, manufacturers have become aware of purge, making this flexible system (Figure 5) the critical need to control smaller and smaller appropriate for storing frequently accessed particle sizes in order to maximize yields of materials. components with submicron geometries.

Particle control inside the minienvironment can be achieved through a combination of strategies. First, close attention must be paid to the component materials and fabrication techniques:  Electropolished stainless steel is the material of choice of metal components subject to heavy use.  Dissipative plastics are preferable to acrylic because they eliminate static charges that attract and hold particles to a desiccator surface.

Figure 5. A “Smart” multiplexed nitrogen control system  Gasket materials are non-corrosive, non- automatically maintains a separate RH level in each chamber, outgassing, and exceptionally resilient. New directing a high-flow purge only where it is needed and only for methods of mechanically attaching gasket as long as needed to restore the %RH set point. materials to stainless steel reinforcement

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frames eliminates the use of adhesives, which can eventually react with the gasket material and compromise sealing effectiveness. High capacity desiccator systems can be specified with integral filter/blower modules, which can maintain cleaner-than-Class 1 particle conditions by recirculating the process gas through HEPA or ULPA filters with efficiencies as high as 99.999% measured at 0.12 m particles.

ESD Protection

While the dry environment of a desiccator protects products from moisture damage, it also Figure 6. Low humidity levels are associated with increased ESD voltages, pointing to the need for effective static-control increases the potential for electrostatic discharge measures inside a desiccator. (ESD). ESD damage occurs when the static charge differential between two materials exceeds the and packaging operations. Similar problems exist field strength of the insulating material. with very fine membranes, foils, thin , and ultra light plastics. As shown in Table 1 and Figure 6, the relative level of ESD voltage generated increases as the RH This is why any nitrogen-purged desiccator should decreases. Thus, the typical amount of voltage incorporate static-control provisions. For the generated in handling a will storage of conductive parts, a grounded chamber increase from 600 volts at 80% RH to over 20,000 with electrically conductive storage shelves is volts at 20% RH. essential. All storage systems should be equipped with a ground continuity indicator/alarm, and Even minute discharges are sufficient to destroy provisions should be made to discharge an ICs and other microelectronic devices. In operator before parts are accessed (either by pharmaceutical applications involving fine means of grounded wrist straps or, more powders, static charges pose a different problem: conveniently, grounded cabinet latches). chemical powders literally cling to a charged surface, compromising the effectiveness of mixing Another method of safeguarding stored materials against ESD makes use of the concept of the “Faraday ,” i.e., that any volume enclosed by a conductor will be completely shielded from electric fields. A Faraday enclosure places the hermetically sealed, nitrogen-purged desiccator environment between stainless steel plates. By keeping the cage grounded and at zero potential, a Faraday desiccator environment safeguards stored products against ESD, even in the presence of strong electric fields (Photo 1).

Table 1

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up charges as they are handled, or charged plastic films that attract and hold contaminants. An effective ionizing system automatically generates a balanced charge of positive and negative ions, which are emitted through nozzles placed inside the desiccator in such a way that incoming nitrogen flow disperses them throughout the storage area. This design requires a plenum wall at the rear of the cabinet, which optimizes the uniformity of nitrogen flow and hence ion distribution.

Additional Benefits

Remote/centralized monitoring and control. While desiccators minimize the danger of moisture, particle, and static contamination, a number of innovations, including remote data Photo 1. The Faraccator™ uses the principle of the Faraday Cage to monitoring, address other concerns specific to shield components against ESD. clean manufacturing facilities. Such remote Static-dissipative polymers provide further ESD monitoring, display, and processing capabilities protection. Although topical anti-static include the ability to receive and then transfer treatments are available, such coatings offer humidity, temperature, and particle data over a neither the durability nor the uniform surface serial interface for processing or documentation resistivity of plastics featuring solvent-welded of storage conditions at a number of locations, dissipative layers. Static-dissipative PVC is and even to provide remote warning alarms if set especially effective in this regard because it offers points for humidity, temperature, or flow are full visibility of stored components and resists a exceeded. broad range of chemicals. Static-dissipative PVC Optimization of space. Desiccators enhance also makes it easy to wipe down a desiccator production efficiency in a number of other ways. cabinet, since it eliminates the attractive surface Strategic installation of a pass-through desiccator charges that seize air-borne particles. Both can optimize the use of floor space by dividing a external and internal walls of a static-dissipative working area or allowing pass-through access desiccator should have a surface resistivity in the through a cleanroom wall (Photo 2). range of 106 to 108 ohms/sq cm, even when Security. Desiccators providing security from exposed to light abrasion. tampering or theft are also available, with a number of options available to suit different These static-control measures are effective at security requirements. One such unit houses a shielding stored parts from external fields or nitrogen-purged desiccator inside a locked dissipating charges on conductive materials. enclosure. System software documents each Nonconductive parts, however, require the entry into the inner shell of the desiccator. The neutralizing effect of ionizing equipment. Static outer shell, constructed of steel, resembles a neutralization is especially important in processes standard filing cabinet, with a full-length, lockbar involving plastic component packages that build mechanism.

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The Bottom Line: Low-Cost Versatility

The rapidly escalating cost of microcircuit manufacturing, not to mention the often astronomical expenses associated with reworking or scrapping failed packages, makes the initial investment in a desiccator storage system absolutely crucial if low-humidity storage correlates to appreciable increase in yield. When the competitive stakes are high, the consequences of unreliable humidity control become prohibitively costly. Any cost-benefit analysis must also take into account the savings in nitrogen provided by more efficient systems, as well as the savings in labor costs associated with monitoring and adjusting less sophisticated storage systems, or of managing the alternatives to low-humidity storage (vacuum sealing and resealing, oven baking to remove moisture, etc.).

For such applications, these innovations in Photo 2. Pass-through desiccators optimize floor space, allowing storage system design clearly overcome parts access into a cleanroom or division of a workspace. significant drawbacks of flowmeter-regulated or desiccant-charged cabinets while preserving the The highest level of security is provided by economy and convenience of a fully visible, easy- placing a desiccator within a safe, surrounded by access storage cabinet. Best of all, the newest a 4-in. steel wall and locked by a combination lock systems can extend the shelf lives of stored and triple-action security boltwork. When locked, materials – and thereby improve yields to steel rods secure the top, bottom and both sides profitable levels – much more effectively than of the dual swing-action, and reinforced access other storage methods. In short, the desiccator, doors making it impossible to open even if the while still a relatively simple storage concept, has outer hinges are removed. The combination lock gone high tech and manufacturers in many can be reset on site to ensure maximum security. industries would do well to take notice. Boeing engineer Jitu Gaglani attests to the benefits of using desiccators. More than 100 are used at Boeing’s facilities to protect microelectronics against ionic contamination, moisture, and corrosion. Gaglani notes that the use of desiccators also prevents components from becoming lost or incorrectly identified; components that look alike, but have different functions, are stored in separate, labeled drawers within the desiccator.

Reference: 1. Nicolaou, Chris. “Plastic Package Failure Modes,” Surface Mount Technology, November 1992, pp. 45-50.

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