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Processing

PROCESSING
Source: Charles Heide, "Silicone Rubber for Medical Applications", Medical Device and Diagnostic Industry
Magazine http://www.mddionline.com/article/silicone-rubber-medical-device-applications (November 1999)

Molding
Today, the most commonly used methods of molding liquid silicone rubbers are injection and compression molding. The silicone molding process is different from that of thermoplastics in a few different aspects. To mold a part, thermoplastic resin is heated in the barrel, and injected into a cold mold. Silicone molding of a part requires the liquid rubber to be kept cool in the barrel and injected into a heated mold. The design and fabrication of the silicone tooling is very different from that of thermoplastic tooling as well. Tolerances, venting, and part ejection are examples of tooling design aspects that differ. Although the tooling design is different, optimizing such processing variables such as temperature, pressure, and time are essential in both thermoplastic molding and silicone molding processes.
Here are a few examples of Silicone Molding:

A. Silicone Lost (Disposable) Core
The Y Boot Design requires no flash at the cores intersection therefore a disposable core is used. This type of silicone molding is possible because of the elasticity of silicone.

Evaluation of Removal Rate of Cured Silicone Adhesive from Various Electronic Packaging Substrates by Solvent and Silicone Digesters for Rework Applications
Michelle Velderrain and Marie Valencia
NuSil Technology LLC
1050 Carpinteria, CA 93013
(805) 684-8780, http://www.nusil.com
As presented at the 40th International Symposium on Microelectronics
San Jose Convention Center, San Jose, CA
November 11-15, 2007.

Abstract
Reworking electronic packages is an integral process related to diagnostics and salvaging valuable materials. It is a meticulous and time-consuming procedure that requires some knowledge of the package material composition to determine compatible cleaning solutions and processes. Silicone adhesives are being used more frequently due to their ability to minimize shear stress during temperature cycling. A common method for removing silicone adhesive is by swelling in solvent and removing by mechanical methods taking care not to damage fragile materials and leave minimal residue. Silicone digesters (emulsifiers) are another means of removing cured silicone. They are comprised of weak acids or bases and remove silicone by breaking the siloxane bonds that make up the polymer matrix. They are able to penetrate into areas that are difficult, or impossible to reach, greatly reducing the risk of causing damage due to mechanical removal. The purpose of this study is to evaluate the rate of silicone removal by solvents and silicone digesters on silicones bonded to copper and aluminum. The removal rate was determined by developing a rating system based on time intervals where silicone was observed to delaminate or dissolve. Silicone adhesives and Thermal Interface Materials (TIMs) were used in the evaluation of two commonly used solvents and two commercially available silicone digesters. Copper and aluminum panels were evaluated by using a ~ 0.5 mm thick layer of silicone to bond 2 panels together. The samples were placed in cleaning solution for 24 hours at 40 degree Celsius and evaluated at specific intervals for any changes in appearance of silicone. Based on the performance of combinations of silicone, substrate and cleaner, the engineer can chose which method is best for reworking based on their own assembly configuration and materials.

Key words: Silicone, rework, adhesive, TIM, silicone emulsifier, low modulus

Versatility and Flexibility from Low Outgassing Silicones
Bill Riegler and Michelle Velderrain, NuSil Technology LLC
T.Y.Lim, Sim Yee Engineering Resources
33 Lorong Markisah 12A, Taman Markisah, Bukit, Mertajam, 14000 Penang, Malaysia
Presented as a Poster at the 11th Electronics Packaging Technology Conference, An IEEE Event 9th-11th December 2009, Singapore

Abstract
Silicone materials have unique characteristics allowing use in a broad range of applications and preservation of mechanical properties when exposed to extreme conditions. These mechanical properties absorb stresses incurred during thermal cycling as well as remain stable at temperatures up to 300°C for short intervals. The aerospace industry has utilized silicone adhesives and coatings for over fifty years because of these unique properties. Miniaturization of electronic packages has led to using thinner and more fragile materials. This, in combination with the use of lead-free solder with solder reflow  temperatures up to 260°C, can cause high shear stress during heating and cooling that can damage a device. Subsequently, there is growing interest in silicone adhesives and encapsulants for terrestrial electronic packaging applications. A major concern surrounding use of silicones is the volatile component observed to outgas when silicones are exposed to high temperatures and low pressures (vacuum) for extended periods of time. These volatile components may contaminate sensitive surrounding surfaces and equipment making adhesion or soldering difficult in an upstream process. In extreme cases, such as in Micro Electro Mechanical Systems (MEMS) devices, volatiles can cause catastrophic failures with the device operation itself.

Introduction
New silicone materials designed to reduce the potential for contamination while maintaining essential physical and chemical characteristics have begun to emerge. These materials are ideal for use in specialized electronics applications such as Surface Acoustic Wave (SAW) Guides, hermitically sealed packages, MEMS, and optoelectronic applications. Several agencies, including NASA, historically recommend < 1.0 % Total Mass Loss (TML) and < 0.1% Collected Volatile Condensable Material (CVCM) as a screening level for the acceptance or rejection of a material for space applications, as tested per ASTM E-595. ASTM E-1559 is an additional method used to characterize materials by monitoring the outgassing kinetics and identifying the volatile components of the material. In this paper, we examine silicones with different levels of outgassing, comparing cured physical properties, out gassing profiles, and cost of each material.

Silicone Chemistry
The basis of virtually all silicone systems, including fluids, gels, elastomers, and adhesives, is the silicone polymer. The proper name for silicone polymers is actually polyorganosiloxanes, and the diagram in Fig 1 shows their typical structure.

Ring Opening Polymerization (ROP) is commonly used for commercial production of silicone polymers. The process begins with polyorganosiloxane cyclic units to provide the body of the chain and end blocker units to control chain length and participate in the crosslinking reaction. The reaction occurs in the presence of reactive acid or base initiators. Figure 2 shows the reaction components for the most widely used polymer, polydimethylsiloxane (PDMS). Here, octamethylcyclotetrasiloxane (D4) is reacting with a vinyl functional chain terminating species, Divinyltetramethyl-disiloxane, also known as “end blockers.”
The ROP reaction is thermodynamically controlled and allowed to reach equilibrium. When a polymerization is allowed to reach equilibrium conditions, the concentration of total cyclics and linear polymers in the reaction mixture remain constant over time. When ROP is complete, the divinyltetramethyldisiloxane end blockers are ultimately responsible for controlling the molecular weight distribution of the polymer, also known as Degree of Polymerization (DP). The ROP initiator is deactivated when the polymerization reaches thermodynamic equilibrium. What remains is a stable mixture of various molecular weights of cyclics, short chained linear polymers, and higher molecular weight polymers where the concentrations of each species are based on their thermodynamic equilibrium (Fig. 3). The oily substance and fogging associated with silicones are primarily caused by the low molecular weight species; however these species can be eliminated by heat and vacuum in order to prevent contamination.
Low Outgas Material Choices
Different electronic applications may require different maximum levels of outgassing based on risk assesment. Therefore, NuSil Technology has developed four different groups of materials based on outgas levels: Standard (R/CF) materials that do not have weight loss requirements; Electronic Packaging Materials (EPM) with low outgassing for typical electronic applications, Controlled Volatility (CV) materials for extreme electronic and space applications; and Ultra Low OutgassingTM (SCV) materials for the most demanding applications.

Material Cost
A very important aspect of deciding which material to use in an application is material cost. The four types of materials described above have a defined cost structure to allow this factor to be properly assessed (see Fig.4).


Test Method E-595
ASTM E 595 is a widely accepted test standard used to screen materials for volatile content that may outgas from a material in a vacuum or space environment. NASA and the European Space Agency (ESA) recommend testing low outgassing materials per ASTM E 595 prior to use in space. A maximum TML of 1% and CVCM of 0.1% are base requirements set out by these agencies. Each material sample is preconditioned at 50% relative humidity and ambient atmosphere for 24 hours. The sample is weighed and loaded into the test chamber within the ASTM E 595 test stand, as shown in Fig. 6. The sample is then heated to 125°C at less than 5×10-torr for 24 hours. The volatiles that outgas under these conditions escape through an exit port and condense on a collector plate maintained at 25 C. Once the test is complete, the samples are removed from the chamber and the collector plate and weighed.

Physical Property Comparison
A common practice used by end users is baking out the volatile substances once the silicone is already cured into its final state. This process is costly because it adds additional processing time and can greatly reduce the mechanical properties of the cured silicone. Tables 2, 3 & 4 compare the four types of materials and their physical properties. Note that removing the volatile species early in the process does not greatly affect the elastomeric properties between the standard and low outgas materials.

Test Method E1559
ASTM E 1559 is an additional test method used to characterize materials by monitoring the outgassing kinetics and identifying the volatile components of the material. The isothermal outgassing test apparatus is explained in detail by Garret et al, and will only be discussed here briefly. A schematic of the ASTM E 1559 test stand is shown in Fig. 7. The material sample can range from 0.5 g to 10 g and is placed in a temperature-controlled effusion cell in a vacuum chamber. All samples are preconditioned in accordance with ASTM E 595, unless otherwise specified.
Outgassing flux leaving the effusion cell orifice condenses on four Quartz Crystal Microbalances (QCMs) that are controlled at selected temperatures. The QCMs and effusion cell are surrounded by liquid nitrogen shrouds to ensure the molecular flux impinging on the QCMs is due only to the sample in the effusion cell. The TML and outgassing rate from the sample are determined as functions of time from the mass deposited on an 80 K QCM and normalized with respect to the initial mass of the sample. The amount of outgassing species that are condensable, VCM, is measured as a function of time from the mass collected on the 298 K QCM. After the outgassing test is complete, the QCMs are then heated to 398 K at a rate of 1K/min. As the QCM heats, the deposited material evaporates. The species that evaporate can be analyzed by a mass spectrometer to quantitatively determine the species observed.

Conclusion
As devices and processes become more advanced and sensitive to molecular contamination, more characterization of the construction materials must be obtained. Ultra low outgassing specification requirements of ≤0.1% TML and ≤ 0.01% CVCM can be useful in the overall management of outgassing species. The results from kinetic outgassing data allow engineers to better predict the levels of contamination, migration, and deposition of the material. Achieving these lower levels does not show to compromise physical properties and thus a broad range of silicone materials with unique and specific properties and cost are available.

REFERENCES
1. S.L Sivas, B. Riegler, B. Burkitt, and R.Thomaier. “Testing Ultra Low Outgassing SiliconeTM Materials,” SAMPE Journal, Jan/Feb 2008.
2. ASTM E-595, “Standard Test Method for Total Mass Loss and Collected Condensable Materials from Outgassing in a Vacuum Environment.”
3.ASTM E 1559, "Standard Test Method for Contamination Outgassing Characteristics of Spacecraft Materials."
4. J.W. Garrett, A.P.M. Glassford, and J. M. Steakley, "ASTM E1559 Method for Measuring Material Outgassing /Deposition Kinetics", Journal of the IEST, pp. 19-28, Jan/Feb 1995
5. A.P.M.Glassford and J.W.Garrett, "Characterization of Contamination Generation Characteristics of Satellite Materials", Final Report WRDC-TR-89-4114, Jun 82 – Aug 89
6. Urayama, F. et al., “Modeling of Material Outgassing and Deposition Phenomena” Proc. of SPIE, 5526, 137-146, 2004.
7. Banks, B.A., de Groh, K.K., Rutledge S.K., Haytas, C.A. “Consequences of Atomic Oxygen Interaction with silicone contamination on Surfaces in Low Earth Orbit,” Proc of SPIE, 8784, 62-71, 1999.

Biography
Bill Riegler is the General Manager Asia for NuSil Technology LLC. NuSil, established in 1979, is a 400 employee, private silicone manufacturer, headquartered in the United States. Bill has a BS in Chemistry and a Masters in Business and has been in the silicone industry for almost twenty five years. Michelle Velderrain is Technical Specialist for NuSil’s Electronics/Optoelectronics materials. She has a BS in Biochemistry and has spent 13 years at NuSil in various technical positions. T.Y. Lim of Sim Yee Engineering Resources is the exclusive representative for NuSil in S. East Asia. He has spent over a decade in the electronics industry in various technical roles.

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