Injection molding an IC into a connector or consumable item
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The temperature values above are compiled from data on the RTP Company11 website. Each plastic type usually has several variations with different processing and mold temperatures. Other vendors may offer the same plastic type with slightly different processing requirements. Additives such as reinforcement fibers, pigment, flammability reduction compounds, or conductivity enhancers can also impact the processing conditions. As a guideline, one should select the plastic based on the nozzle temperature or the front zone temperature as specified in the processing conditions for injection molding of plastics. For injection molding at 183°C, the choice of plastic is very limited. At 217°C, most of the materials in Table 1 are feasible. Materials such as nylon (PA) or polyethylene terephthalate (PET) can be used only if the IC is not soldered to a substrate. The plastic cools off rapidly inside the mold cavity. In summary, one should make sure that when the plastic enters the mold, it does not exceed the critical temperature.
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Sterilization compatibility
Products for medical applications typically undergo sterilization, at least once or even multiple times during their lifetime. Popular methods are autoclave steam sterilization, ethylene oxide (ETO) sterilization, chlorine dioxide (CD) sterilization, vaporized hydrogen peroxide (VHP) sterilization, hydrogen peroxide plasma sterilization, and irradiation with gamma rays or electron beams. The sterilization compatibility information in Table 1 is compiled from various publications. A report on hydrogen peroxide gas-plasma compatibility was not found. In any case, caution is advised since the test methods and details are not fully disclosed. For objects with an embedded IC the preferred sterilization methods are ETO and CD. Autoclave and VHP may also be acceptable, although the high temperature or vacuum required by these methods can affect embedded batteries or floating-gate memory cells. Hydrogen peroxide gas-plasma sterilization and irradiation (gamma ray or electron beam) are not recommended since they can damage ICs.
Exceeding the critical temperature
If the temperature of the plastic is too high when entering the mold, the solder that holds the IC in place softens or liquefies. As this hot plastic enters the mold cavity it can push the IC away from the pads on the substrate. Solder can protrude from the mold or short IC terminals, making the end product unusable or unreliable. Therefore, choosing the right temperature is a very critical step.
Nevertheless, there could be situations that require a plastic with a processing temperature above the solder melting point. In that case, one can consider two molding steps: first, use a plastic that stays below the critical temperature; second, use the plastic that requires the otherwise unacceptable temperature. Thus, with properly chosen dimensions and a well-timed temperature profile, the thermal inertia of the first layer protects the IC from the excess heat of the second molding step.
Real-world example
Application note 47021 features a DB9 connector with an embedded TO92-packaged 1-Wire® device. In this case, two of the DB9 pins are crimped to the IO and GND leads of the TO92 package. The NC (not connected) lead is cut off. This construction maximizes the choice of plastics since no solder is involved. Figure 2 shows the same connector before molding. The TO92 package is easily recognized.
Figure 2. DB9 connector with embedded TO92 1-Wire device before injection molding.
Conclusion
Injection molding is the method of choice to embed ICs in medical sensors and consumables. Special care is required when selecting the plastic material since its processing temperature must be low enough not to soften or liquefy the solder that attaches the IC to a substrate. Plastic materials are not equally suited for the sterilization required by medical applications. Consequently, only few of the sterilization methods are compatible with both the plastic material and ICs.
References
- See Maxim application note 4702, Easily add memory, security, monitoring, and control to medical sensors and consumables, and application note 4623, Smart cable aids quality control and authentication.
- Injection molding, http://en.wikipedia.org/wiki/Injection_molding
- For further reading, see Maxim application note 4132, Attachment methods for the electro-mechanical SFN package.
- For examples of polymers best suited for the process, go t http://en.wikipedia.org/wiki/Injection_molding.
- For a discussion of the Characteristics of Various Plastic Materials, go t http://www.fcs.com.tw/webtop/FAQ_EN/index.phtml?action=browseid=170StoreID=9, Fu Chun Shin Machinery Manufacture Co. Ltd.
- Choosing the Correct Thermoplastic Composite, RTP Company, at: http://www.thomasnet.com/white-papers/abstract/100711/choosing-the-correct-thermoplastic-composite.html.
- Selection of Materials for Medical Applications, RTP Company, at: http://www.thomasnet.com/white-papers/abstract/100710/selection-of-materials-for-medical-applications.html.
- Sterilization of Plastics, Zeus Industrial Products, at: http://www.thomasnet.com/white-papers/abstract/101488/sterilization-of-plastics.html.
- Gaseous Chlorine Dioxide and the Myth of Corrosion at: http://www.clordisys.com/MythOfCorrosion.pdf.
- Material Compatibility with Vaporized Hydrogen Peroxide (VHP®) Sterilization, http://www.steris.com/media/PDF/spacedecon/pharmaSolutions/point%20of%20manufacture%20mat%20comp%20pharma%20case%20study%202.pdf.
- Injection molding Processing Conditions for RTP Specialty Compounds, RTP Company, at: http://www.rtpcompany.com/info/processing/index.htm.
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