Clean ESD-safe dispensing
05/01/2000
The use of inherently dissipative polymer alloys is the first step to reduce ESD in fluid dispensing.
The demands of advanced electronic technology drive designs with increasingly denser arrays of small components. One growing concern with these miniaturized devices is the problem of electrostatic discharge (ESD). Sudden bursts or events of ESD can damage or destroy devices on semiconductor wafers and finished components.
The Perils of ESD
 Figure 1. UV/visible light transmission of syringe barrels at various wavelengths. |
While many electronic components can withstand ESD events of up to 100 volts without damage, ESD events of as little as 2 volts in the disk drive industry can damage a magnetoresistive head. Although grounding cords, grounded floors and ionization have been used for several years, they do not sufficiently reduce ESD in the manufacture of today's more highly dense, intricate and sensitive components, films and disk drives. Therefore, a new focus on the entire manufacturing process and the materials used is critical.
Dispensing is a key process in all kinds of technological production. Whether the fluids are cyanoacrylates, solder pastes, conductive epoxies or conformal coatings, dispensing plays a major role in electronic products manufacturing. For years, barrels have been used in dispensing without serious consideration of their relationship to ESD. As concern for ESD grew, it became apparent that the dispensing equipment was a causative factor; the plastic syringe barrels actually developed and held a charge that could discharge through a sensitive device, producing catastrophic results. Once discovered, an effort was made to solve the problem by adding conductive materials to the otherwise insulative plastic syringe barrels.
 Figure 2. A grounded microprocessor controller protects sensitive electronics from ESD events. |
The first attempt to create a more conductive syringe used carbon filler mixed into the bulk plastic before molding. These products are referred to as carbon-loaded syringe barrels. Until recently, carbon-loaded barrels were the only improvement to address ESD considerations during dispensing operations. While these products have provided some improvement, the carbon-loaded option introduced new complications.
The Role of Carbon
Carbon is not inherently static dissipative. The carbon conducts the electricity to a ground by having the carbon particles connected to each other. Achieving this requires significant loading of the base polymer or plastic, and this concentration of black carbon produces a black, completely opaque barrel. This can create several problems, including the inability to see the amount of material left in the barrel or contaminants - such as air pockets or foreign materials.
Contamination and degradation are two additional problems that arise when using carbon because carbon is unstable and unpredictable. A manufacturer can produce a dozen lots of ESD barrels and each lot may have varying electrical characteristics. This creates potentially serious issues if the material characteristics drift toward conductive or insulative. The particulate nature of carbon and the requirement for its heavy loading in the polymer provide a perfect opportunity for fluid contamination. Once the fluid is inside the barrel, the carbon can begin sloughing, which results in carbon particle contamination. Depending on the application, this can have catastrophic results if the chemistry of the carbon interferes with the dispensing fluid or process. Early carbon-loaded barrels were infused with so much carbon that it was actually possible to write with the barrel, like a pencil, on paper.
More recently, manufacturers of carbon barrels have attempted to solve this problem by using a low carbon-loaded material to reduce the amount of sloughing. However, this effort has altered the performance of the barrels in relation to ESD specifications. By reducing the carbon loading, the low carbon-load barrels have moved away from the optimum static dissipative region of the electrical conductivity
esistivity spectrum toward the insulative region.
Searching for a Solution
Although ESD is a concern, the complications caused by carbon-loaded barrels make them a less-than-ideal solution. Therefore, a material that can control ESD, but doesn't contain carbon and is safe and compatible with all dispensing applications, was investigated. The material uses inherently dissipative polymer alloys with electrical characteristics in the optimum static dissipative region of the electrical conductivity
esistivity spectrum. It is a new type of polypropylene that is translucent blue, safe and compatible with dispensing system requirements.
The primary benefit of this new material is that even though it has static dissipative properties, it is non-contaminating. Unlike carbon, which contaminates the fluid, this material does not slough or pollute the contents of the barrel.
 Figure 3. Barrell assemblies that provide a carbon-free alternative for ESD-safe dispensing. |
Instead, it has been engineered to allow maximum stability throughout the manufacturing process. The greater consistency during manufacturing allows a much lower margin of variance. Other attributes include ultra-low ionic impurities, a surface resistivity of 3 x 109 ohms/square, a static decay time of less than 0.5 seconds, and no humidity dependence, which makes it suited for use in cleanrooms. The syringe barrels are compatible with industry standard systems and are certified to be silicone- and chloride-free.
One advantage of black barrels (carbon-loaded or standard black plastic) has been that the black color made the barrel opaque, thus providing UV blocking. It was thought that by making a material that was not opaque, the UV blocking properties would be eliminated. However, tests have shown that the translucent blue material does indeed offer UV blocking, which is necessary for the use of adhesives and epoxies that cure under UV light (Figure 1). UV cure materials respond to ultraviolet energy at 365 nanometers. As Figure 1 shows, all four materials have UV blocking capability at this wavelength. Visible light cure materials respond above 400 nm, where only the dark amber and black act as true UV blockers. At this level, the ESD-safe blue barrel is still superior to the industry standard light amber as a visible light screen.
A translucent barrel offers additional benefits for dispensing because it allows a user to see the fluid being used, the amount in the barrel, and any air pockets or foreign materials. These barrels can be used in all dispensing applications, including various cleanroom assembly operations, electronics assembly, medical device assembly, enclosed product manufacturing (e.g., data storage, computer peripherals), as well as chip, wafer, circuit board and semiconductor manufacturing.
ESD is of such concern that it is not enough to just concentrate on the barrels. The entire dispensing system needs to be examined to see the role it plays in generating ESD. With that in mind, manufacturers must ensure that the needle hub, barrel, receiver head assembly and controller are designed to address ESD issues. The receiver head assembly should include a connection to the needle and the barrel that runs to the controller, allowing the whole system to dissipate static to ground (Figure 2).
As the demands for faster, smaller and denser electronic components increase, so does the problem of ESD. Repair and rework of parts and low yield rates are costly, not only in monetary terms, but in dangerous incidents that result from products that don't function properly in critical situations. Every part of the manufacturing process must be assessed to eliminate ESD as much as possible. Yet, in that endeavor, it is important to not trade one problem for another.
SCOTT BREIDENTHAL, director of engineering, and MARC CORTEZ, director or marketing, can be contacted at Techcon Systems Inc., 12151 Monarch Street, Garden Grove, CA 92841; 800-776-0440; Fax: 714-799-6804; E-mail: [email protected] and [email protected].