Chemical and Temperature Resistance of PIB-Based Temporary Adhesives

Temporary bonding adhesives play a critical role in thin substrate handling. Those most suitable for fab processing are those which are inert to chemicals and temperature. One leading temporary bonding adhesive is WaferBond™.[1] Patents for this product describe it as hydrophobic, comprising hydrocarbon resin, ethylene propylene diene monomer (EPDM) rubber, and a carrier solvent. [2]-[3] Rubber is a common strengthening aid for hard and brittle resins. When compounded into an adhesive, the rubber maintains hardness, minimizes brittle character, eliminates cracks, and minimizes shattering when exposed to high stress during wafer thinning. Most raw forms of rubber exists as unsaturated molecules, containing reactive double bonds. These include EPDM and styrene block copolymers such as SBS, SEBS, SBR, and SIS.[4] Unlike these reactive rubbers, polyisobutylene (PIB) is saturated, inert, unreactive and stable, resisting strong oxidizers and corrosives. Inert materials maintain their amorphous nature when exposed to harsh chemicals, allowing them to be easily dissolved and rinsed afterwards. Unsaturated chemistries will react, crosslink, and leave unwanted residue following their removal. To effect cleaning, more aggressive effort is needed, which drives up costs and/or reduces yield. Adhesives compounded with PIB exhibit improved chemical resistance even to the most challenging reagents. PIB is defined by its straight chain, saturated condition, leading to dense packing when cast into a film or coating. Comparing mixtures of EPDM with that of PIB, nearly identical physical properties are observed (i.e. viscosity, coating, and transparency), and appear the same as a cured coating or film. The polymers appear the same until their cleaning following aggressive chemical exposure. PIB offers properties of low permeability to oxygen, carbon dioxide, and other gases as compared to conventional unsaturated rubbers. Choosing PIB rubber in your product provides process latitude for high thermal and chemical resistance. Evaporative cast liquids fix quickly onto surfaces and remain thermally stable due to the elastomer's rise in rheology with solids, resulting in tight molecular packing during cure to give a low TTV. PIB rubber adhesives can be cast from a range of liquids and at varying concentrations as well as be created in the form of dry films that are ready-to-use after being cut to shape. Debond processes may proceed by thermal-slide, chemical diffusion, or peeling.[5] Due to PIB's high solubility, cleaning is possible with simple hydrocarbon solvents. For example, fast cleans is achieved in using aliphatic (straight-chain) hydrocarbons in spray, immersion, and special equipment.