A low stress alternative for advanced electronic packaging, NuSil Technology's Elec­tronic Packaging Materials (EPMs) allow the designer to choose from a unique line of silicones for various levels of packaging. Packaging options include a variety of products from Flip Chip Under Fill to Glob Top.

Silicone's unique chemical composition compared to organic based polymers applies lower shear stress on components during heating and cooling. Lower applied shear stress can be used in applications where hybrid packages utilize materials with various Coefficients of Thermal Expansion (CTE). Silicones offer stress relief during thermal cycling.

Benefits Of Silicone As Electronic Packaging Material

• Low Moisture Absorption, <0.4%
• Dielectric Strength, > 500V/mil (0.001 inch) or 20 kV/mm
• Thermal Stability, -115 °C to 260 °C
• Low Modulus
• Low Shrinkage, <1%
• Low Ionic Content (Na, K, Cl and NH3)
• Low Outgassing, <1% at 1 hr @ 275°C
• Available as gels, elastomers and greases

Contamination
Silicone adhesives have had a reputation over the years of "contaminating" surrounding areas with silicone oils while the device is in operation. NuSil Technology has addressed this by designing products in our EPM product line to introduce minimal contamination by the following criteria:

• Contain low concentrations of silicone oils: < 1 % Weight Loss when heated 1 hour at 275 °C.
• Low Ion Content: < 20 ppm of individual Na, K and Cl ions per MILSTD 883E.
• Addition Cure Mechanism: This cure mechanism has minimal shrinkage and does not produce chemi­cal by-products, such as acids or alcohols that can affect components in assemblies.

Silicone "oils" are low molecular weight silicones generated during the production of silicone polymers. The polymeriza­tion reaction often begins with polyorganosiloxane rings(A.K.A.cyclics) react­ing with short chain silox­ane species that act as chain terminators in the presence of an acid or base catalyst. The reaction is thermody­namically controlled and the concentration of the various chemical species produced is allowed to reach equilibrium over time. The chemical species produced during this reaction are a range of molecular weights of linear and cyclic compounds. Heat and vacuum are applied and the low molecular weight species can be removed. NuSil Technology has been performing this technique, known as "Wiped Film Evaporation" for the Aerospace industry for 30 years and are experts in this technology.

Stress
Silicones have very low Glass Transition Temperatures (Tg) and low Elastic Modulus compared to other organic polymers. There are organic polymers that may have more than one Tg, which can apply stress on sensitive parts during temperature cycling or possessing. The Tg of a typical dimethylsilicone is —65 °C which allows the silicone to maintain its elasticity over large temperature ranges. Even when filled up to 85% (w/w) with ceramic or metallic fillers, the Tg is < —30 °C.

Silicone does have a high Coefficient of Thermal Expansion (CTE) but has an order of magnitude lower Elastic (Storage) Modulus compared to other organic polymers. The lower modulus allows stress relief during thermal cycling that can prevent warping, bending and shearing of sensitive parts within the device.

Inhibition
EPM Gels, Elastomers and Adhesives are designed to provide excellent optical, thermal and mechanical properties. However, some ingredients commonly found in certain adhesives, plastics and elastomers can adversely affect the cure chemistry in these products. NuSil recommends that adhesives, plastics and elastomers be analyzed for cure inhibition prior to selecting the silicone material for use. This evaluation should include materials used in any transfer containers, dispensing hoses, or utensils that come in direct contact with the gel components.

Processing Highly Filled Silicones
Electrical and thermally conductive silicones typically contain dense ceramic or metallic fillers that can settle over time. Prior to mixing Part A and B together, it is recommended to stir the individual Part A and B that contain the dense fillers to ensure the fillers are homogeneously dispersed prior to use. A vacuum chamber should be used to remove the air introduced during mixing. When working with equipment at reduced pressures, ensure container and chamber are rated to withstand the supplier's recommended operational pressure. Before getting started, reference the Material Certification for "Work Time" or other pot life parameters to determine curing time between mixing and applying to application. Place mixed material into appropriate container and fill approximately one quarter of the container's total volume to allow material to rise. It is recommended to slowly apply vacuum up to approximately 28 inches of Mercury (0.95 Barr). Hold vacuum until bubbles are no longer observed forming. Breaking the seal while pulling vacuum will allow bubbles to burst; expediting the process. It is not recommended to remove air via centrifuging from silicones with fillers having a specific gravity > 1. Centrifuging can be used for silicones that do not contain dense fillers.

Improving Adhesion
Since many substrates are difficult to bond to, the surface of the substrate must be modified to allow good wetting of the adhesive to allow chemical and mechanical interaction. Many substrates need to have the surface "activated" to enhance the adhesive's bond to the substrate. There are, in general, four typical techniques that can be used to improve the adhesive bond; primers, plasma, corona and flame treatment. These surface treatments can greatly enhance the adhesive bond and the type of treatment is dependent on configuration complexity, silicone adhesive and substrates. Adhesion is a very complex subject that incorporates principles from several scientific disciplines that are beyond the scope of this discussion.