Electronics Protection - November/December 2013 - (Page 6)

Feature Silicones for High Reliability and Yield in Electronic Applications Bob Umland, Marketing & Sales Director, Electronics and Engineering, NuSil Technology Silicones have been used for decades in electronics, aerospace and other applications wherein harsh environments with temperature extremes are common. These siloxane-based polymeric systems are unique polymers compared to standard organic-based materials due to their atomic composition. The low modulus of their crosslinked networks allows them to absorb stresses during thermal cycling as well as to resist degradation at continuous operating temperatures up to 250°C and greater. Silicones have low glass transition temperatures (Tg) ranging from approximately -115°C to -60°C, which keep their elastomeric systems flexible in cold environments and when experiencing vibration. Thermally conductive silicones provide protection to sensitive electronic components and systems. The silicone matrix is an essential polymer compatible with a variety of fillers due to its unique chemistry, making silicones excellent materials for use as the binder for a variety of thermally conductive fillers where high level loadings can be achieved without dramatically increasing the shear stress. As electronics are becoming smaller, thinner, vertically stacked and require more power, silicone becomes more desirable to increase reliability. The history and prolific success of silicone speaks to its capacity for reliability. Silicone is typically non-hazardous in its "neat" state once cured and complies with the restricted levels of the regulated chemicals listed in the ROHS and WEEE directives. In medical devices, silicones have proven they can be manufactured to have high purity for robustness in high-risk applications. Quantifiably, and most relevant for electronics applications, silicones can be processed to have low D4/D5 (< 50 ppm) content as well as to comply with the specifications outlined in NASA SPR0022A and ESA PSS-014-702, which require a maximum allowable total mass loss (TML) of 1.0 percent and Collected Volatile Condensable Material (CVCM) of 0.1%1,2. This reduces risk of fogging, delamination and other failure-inducing occurrences, which volatile species of impure material can cause. Silicones for electronics can also be optimized to exhibit high purity with regard to ionic content <20 ppm of Na, K and Cl, and their permeability to moisture, a most brutal contaminant, can be adjusted as needed for a given electronic device. Because water is detrimental to many components, extremely low water vapor transmission rates (WVTR) are often imperative of encapsulating materials. Understanding the opportunities and limitations of silicone allows the formulator or engineer to choose the best options available for maximum performance and protection of components in the harsh environments of electronic applications. Silicone in Electronics For their ability to withstand exposure to high temperatures such as in lead-free solder reflow and for longer durations when compared to other polymeric materials, silicone encapsulants are used to protect the components against shock/vibration, moisture, dust and other environmental hazards3. Although naturally insulating with dielectric strengths typically greater than 400 V/mil (15.6 kV/mm), dielectric constant at ~2.5 and volume resistivity at >1X1012 ohm-cm, silicone can be made to be conductive when needed. For instance, as the processing capability of semiconductor devices increases and chip size decreases for compact electronic modules, the need for thermal management increases. Even with a heat sink directly in contact with a chip, heat may not transfer efficiently if the mating surfaces are at all rough or irregular. 6 To enhance the thermal contact between the heat sink and the die, Thermal Interface Materials (TIM) are utilized between the two surfaces. Under mechanical pressure, the soft TIMs conform to the microscopic surface contours of the adjacent solid surfaces and increase the microscopic area of contact between them. The thermal conductivity of the material then assists to reduce the temperature drop across this contact. Lead-free solder reflow temperatures of >260°C and high heat created during operation (>100°C) cause greater temperature extremes during the thermal cycling in the electronic package. Stress within the electronic assembly will increase when it is comprised of a myriad of materials with various Coefficients of Friction (CTE). This stress induces metal fatigue in the solder and can cause cracking of the solder joint. An adhesive or encapsulant in the gap between the printed circuit board and chip helps minimize the shear stress by mechanically coupling the board to the die and restricting the relative lateral motion. This coupling reduces the stress on the solder joints and converts the in-plane stress to a bending stress. The electronic packaging industry has used epoxy adhesives and encapsulants for years as thermal management materials such as TIMs and underfills4 for their strong adhesion and low CTEs. Silicones may have high CTE relative to the organic-based thermosets, but compared to epoxies, silicones have very low modulus. This helps absorb stress incurred from thermal cycling when in hybrid devices, reducing to little or no significance the high CTE they may possess relative to organic thermosets such as epoxies. Historically, silicone greases have been popular thermal management components for electronic applications since they are easy to use and impart minimal stress. These materials are excellent in applications involving flat surfaces or in which risk of shear stress is high. However, greases are mobile and can "pump out" of the device after extensive thermal cycling. This complication can be preemptively combated by crosslinking the silicone covalently. This links all the polymers Table 1. Molecular and Property comparitogether into a threeson of silicone versus epoxy. dimensional network, forming an elastomer and greatly sidestepping creep and mobility. The more bonds created between vinyl and hydride, the higher the crosslink density of the silicone and, generally speaking, the greater the hardness of the elastomer. Theoretically, the resulting tradeoff is an increase in modulus, but this can be avoided with a platinum addition cure silicone material. Addition cure silicones are optimized to contain specific amounts of hydrides on the crosslinker and vinyls on the polymer network. These react in the siloxane system to form a three-dimensional network in which there is no generation of small molecules (Table 2) and generally less than 2 percent shrink- November/December 2013 www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - November/December 2013

New Features Make F-Series TeraFrame Gen 3 One of CPI’s Most Advanced Cabinets
Silicones for High Reliability and Yield in Electronic Applications
Surge and Transient Protection for Telephone, CATV & Satellite Services
Thermal Management of LEDs: Looking Beyond Thermal Conductivity Values
Understanding NEMA Ratings for Electrical Enclosures
Silent Air Cooling: A New Approach to Thermal Management
VadaTech Releases Rugged Conduction-Cooled MicroTCA Ecosystem
Directable Inverted Blowers Deliver High Volume Air Flow
Cima NanoTech Launches Ultra Low Resistance Sante EMI Shielding Film
Littelfuse Introduces Surge-Tolerant Fuses
ProTek Devices’ TVS Array Provides Circuit Protection in Computing Applications
Reell’s PolyTorq Technology Expands Capabilities For Hinge and Torque Insert Applications
Industry News

Electronics Protection - November/December 2013