Precision coating and adhesive dispensing for medical devices
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Precision coating and adhesive dispensing for medical devices

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The human body is quite possibly the harshest and most complex environment known for microelectronic / micromechanical assemblies. Medical device assemblies are decreasing in size to provide access ...

The human body is quite possibly the harshest and most complex environment known for microelectronic / micromechanical assemblies. Medical device assemblies are decreasing in size to provide access and comfort for extracorporeal and in situ devices. In addition the scale of electronic components will continue to consistently become smaller and denser with the advances and improvements in technology. Packaging and assembling these devices is requiring a much higher level of precision than ever before, whether conformal coating for environmental protection or dispensing medical adhesive for assembly. Non-contact jetting of conformal coating as well as semiconductor packaging fluids represents a paradigm shift to a higher level of precision in the method of applying coatings and adhesives versus traditional dispensing methods. This paper will cover the advancements in manufacturing methods for high precision environmental coating and adhesive dispensing.

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Precision coating and adhesive dispensing for medical devices Precision coating and adhesive dispensing for medical devices Document Transcript

  • PRECISION COATING AND ADHESIVE DISPENSING FOR MEDICAL DEVICES Brian Schmaltz Asymtek - 2762 Loker Ave West Carlsbad, CA 92010 U.S.A. Applications Engineer / MSME bschmaltz@asymtek.com The human body is quite possibly the harshest and most complex environment known for microelectronic / micromechanical assemblies. Medical device assemblies are decreasing in size to provide access and comfort for extracorporeal and in situ devices. In addition the scale of electronic components will continue to consistently become smaller and denser with the advances and improvements in technology. Packaging and assembling these devices is requiring a much higher level of precision than ever before, whether conformal coating for environmental protection or dispensing medical adhesive for assembly. Non-contact jetting of conformal coating as well as semiconductor packaging fluids represents a paradigm shift to a higher level of precision in the method of applying coatings and adhesives versus traditional dispensing methods. This paper will cover the advancements in manufacturing methods for high precision environmental coating and adhesive dispensing. Dispensing of material for medical devices can be primarily divided into two major sub sections; conformal coating and adhesives. Conformal coating is essential to a medical devices sustainability and compatibility. Typically a bio- compatible material is applied to the device to act as a protection barrier against the environment; if the device is not coated could result in complete failure. One of the oldest and best-known methods of coating is the dip process. In the manual dip operation, operators immerse the PCB in a tank of coating. Components on the PCB that cannot be exposed to coating must first be masked Tape or boot masking is manually applied prior to coating. The masking is then removed after the board is cured. Masking is labor intensive and involves consumables, making the process inherently wasteful. Some dip systems automatically move the board in and out of the tank, allowing for better repeatability. Although dip systems are simple and involve a low capital investment, the variation in coating thickness, contamination issues, viscosity variations, manual masking, cleanliness and operator comfort and exposure make this a crude process with little control. There is also a high cost due to material waste and solvent evaporation since the dip tank is typically subject to environmental effects. Brushing is another method used to coat a PCB. There is little investment in equipment or tooling and the process is simple, but crude. Brushing introduces the same problems associated with the dip process. Although brushing may be adequate for low volume prototype runs, this process is not viable for mass production. Manual air-spraying is another common method for applying a thin film of conformal coating material to a PCB Since air spraying produces a large amount of over-spray, hand masking is required beforehand. An operator sprays the PCB’s with a hand held spray gun similar to those used to spray paint. Once the boards are cured, the masking material is removed. The operator is continuously exposed to the coating during and after the process which can create safety and health issues. The coating thickness and consistency is operator-dependent and not highly repeatable. The introduction of needle dispensing, which can be done manually or robotically, was a marked improvement for conformal coating. The coating material is forced through a needle and is dispensed as a bead. Beads of material are placed in different locations on the board and through flow out produce the desired coverage. The manual needle dispensing equipment cost is low, but is operator-dependent and not highly repeatable. Automated needle dispense systems increase repeatability, but fast coverage and dispenser robustness are difficult to achieve. Although different in purpose the demands for adhesive dispensing in medical devices remain similar to conformal coating. Bio-compatible adhesive dispensing requires a high level of precision and repeatability in the manufacturing process. Typically adhesive dispensing is done with either an automated or manual needle syringe. To this day many conformal coating and adhesive dispensing applications are done using a needle. Asymtek jet dispensing technology offers many advantages over traditional dispensing techniques. Advantages which are particularly applicable to the medical community are the volumetric repeatability as well as the small stream size/dot size. The DispenseJet® uses a pneumatic piston with a ball tip end to push fluid through a narrow
  • orifice at the jet nozzle tip. Air pressure raises the piston, which allows a fluid to flow around the needle and into the nozzle. Spring force returns the piston to the nozzle tip when the air pressure is removed. As the ball tip on the end of the piston engages in a seat at the nozzle, the fluid is energized and shoots a droplet from an orifice at the tip of the jet. A series of hardware and control variables determine the size of the droplet. Because the typical fluids change viscosity with temperature, it is necessary to control the tip temperature to ensure consistent operation. Variations of hardware and software parameters allow for a wide range of fluid dispense optimization. The jet is capable of depositing at a rate of 200 dots per second, far superior to needle dispensing. As a repeatable high precision alterative to needle dispensing the conformal coating jet is ideal for applying coating materials to highly selective areas. This solution is designed for coating small substrates with high-component density, and when there are tight tolerances between coated and uncoated areas. The conformal coating jet can reach locations not accessible by other applicators, using a needle dispensing method coupled with a jetting action. Better control of the dispensed material volume delivers line widths down to 1.5 mm wide. Due to the level of precision the need for masking is eliminated because the flow is so closely controlled. The conformal coating jet can be used with Acrylics, Silicones, Urethanes, UV-Cure, and Water-Based materials with various viscosity ranges of 0 to 850 cps. Due to the harsh conditions presented by the human body, environmental protection presents a critical challenge to medical devices. A biocompatible and biostable coating material must often be applied to provide moisture, chemical, and dielectric protection. During the manufacturing process accurate repeatable coverage and thickness is required in order to meet various FDA standards. In order to provide this repeatable coverage a highly reliable and repeatable process must be obtained for the production cycle. The conformal coating jet provides a solution to this problem by jetting a repeatable volume into these critical coating areas. (Figure 1) Figure 1: Conformal Coating Acrylic– Precision Jet Coated Board The conformal coating jet allows for a higher level of precision than its predecessors methods. This high precision allows for a faster throughput by coating only selective areas as well as virtually eliminating excess material waste. The jet provides a fast, repeatable, reliable, and cost effective solution. Specialized adhesives are currently used in a wide range of medical devices. This market of these adhesives will continue to grow as the technology progresses further demanding new materials in assemblies that contribute to longer life expectancy. Traditionally five classes of medical devices have been assembled with adhesives: Disposable Implantable Reusable Sterile Reusable Resposable These devices are expected to withstand the rigors of sterilization, exposure to fluids, as well as typical abuse over time. One of the primary fluids now being used in medical assembly manufacturing are ultraviolet (UV) curing adhesives. These UV-cure adhesives provide a fast cure, high flexibility, as well as high adhesion and autoclave resistance. Used in disposable medical devices, non-disposables and non-woven hygiene products various UV adhesives are formulated for consistent, reliable performance that meets or surpasses the medical industry’s rigorous validation and qualification procedures. Repeatability of the dispensing process is critical in medical device manufacturing for FDA certification and ensuring each device is identical on a production level. As discussed previously the jet does not use a surface tension to pull material from a needle fluid orifice. Instead a predetermined volume is filled before each jetting cycle; this process ensures a repeatable jetting volume. Typical results when dispensing UV cure adhesives provide a 3σ weight variation of sub ± 5%. A drawback to the conformal coating jet was the ability to dispense high viscosity materials. The DispenseJet uses the same principals and provides reliable and repeatable dispensing of various formulations of UV and alternate high viscosity adhesives used in the medical industry. The DispenseJet can be used with a much wider range of materials than the conformal coating jet with various viscosity ranges up to 250,000cps. The optimized design allows for dot sizes in the 1.0 nano-liter range. Dot diameter size may vary fluid to fluid as it is mainly dependent
  • upon surface tension and viscosity. It is also possible to use the jet for high precision conformal coating applications where the conformal coating jet cannot succeed. In a competitive medical market where product size is often a key sales driver, packing high-density devices into a system-in-package (SiP) module is essential. 0201 components were only released a few years ago and are already being integrated in medical electronics. Now 01005 components are presenting themselves as being ready for complete production integration and a higher level of precision across production is demanded. Although these two packages are only beginning to emerge, they are expected to claim an increasing market share, with the International Technology Roadmap for Semiconductors (ITRS) predicting that 01005 types will remain the minimum size until at least 2012. Small and highly repeatable dispensed volumes of material are required in order to keep up with the microelectronics industry. Besides dots the jet also has the ability to create lines for gaskets and other purposes. In order to create lines and various other shapes dots can be dispensed on-the-fly (or while the jet moves). Spacing between the dots can be varied in order to change the characteristics of the line. A larger spacing between each dot can provide less volume of material than using a needle line dispense. Yet a smaller spacing can create a larger width and aspect ratio lines with a repeatable volumetric precision not found in needle dispensing. Figure 2: Jetting UV medical adhesive onto a part 500µm in width. Due to the design of the jet, ease of hardware and software configuration changes provides dispensing solutions for a large variation of medical fluids; Conductive, Non-Conductive, and UV-Cure Adhesives, Underfills, Silicones, Ethanol, Water, and additional fluids used in the medical industry. Challenges that commonly present themselves are the ability to create small dots (Sub 10mil) with fast throughput and high volumetric repeatability. One application for jet dispensing is a silicone based adhesive. The primary concern for this application was consistent fluid dispensing considering the volatility of the solvents inherent in the fluid. A test procedure was conducted in order to test the ability to produce lines 1mm width and 0.2 mm thick with repeatable weight on a flat substrate. Figure 4: Silicone Adhesive 1mm width line/0.8mm dots Due to the low viscosity, rapid evaporation of the solvents, and accumulation of silicone on the needle tip during the dispensing there would be difficulty with dispensing repeatable amounts of weight using traditional methods. By using a jet in place of a pressurized vessel ensures that each shot contains the same amount of volume. Repeatability was the primary concern of the end user and it could not have been met by traditional needle dispensing techniques.
  • The second application for jet dispensing was the ability to dispense 1µL of mixed slurry containing Controlled Pore Glass (CPG). Similar to the first application, the primary concern was consistent fluid dispensing considering the volatility of the solvents inherent in the fluid. The jetting solution was able to maintain a stable dot weight for a production environment. A high repeatability was essential to this application to ensure the proper amount of slurry was dispensed. The third application for jet dispensing was the ability to dispense a UV-cure adhesive for gasket purposes. A narrower bond line than previously created by needle dispensing is required. Figure 5: Needle vs. Jet UV-Cure Gasket The DispenseJet provided a fast, highly repeatable, and accurate solution for dispensing the UV adhesive. Spacing of the dots was varied as shown in Figures 5&6 to provide less volume and a narrower bond line that is far more repeatable than traditional needle dispensing. Figure 6: 250µm dots compressed to form a gasket. The final application for jet dispensing was the ability to dispense various low viscosity materials de-ionized water, p-xylene, ethanol, heptane, acetone, and glucose. The primary concern was consistent fluid volume and considering the volatility of the solvent based materials as well as dot location accuracy. Figure 7: Glucose Oxidase Enzyme (GOx) - 26nl dots The piezoelectric-jet technology has hardware specifically tailored to provide an excellent jetting solution for low viscosity fluids. Typical dispensing would result in leakage virtually eliminating any chance for repeatability. During the last twenty years, dispensing of epoxies and other materials has primarily been dispensed using needle dispensing. Conformal Coating has employed similar primitive techniques such as dip, brush, and needle coating. Now it seems evident dispensing technology has moved from such primitive processes to the era of selective non- contact jet dispensing. Jetting will allow denser circuitry to be developed at the package and board level, while offering a greater degree of process control at faster speed than its predecessor dispensers. The ability to provide a high yield production solution coupled with high precision makes jetting the perfect solution for medical device manufacturing.