Synthesis and characterization of electrolyte substrate materials based on hyperbranched polyurethane elastomers for anodic bonding

Abstract

A series of hyperbranched polyurethane elastomers (PEO‐HBPUEs) as polymer electrolyte substrate materials was developed for anodic bonding with aluminum (Al) foil in micro‐electro‐mechanical system (MEMS) devices. The PEO‐HBPUEs were prepared by pre‐polymerization method with toluene‐2,4‐diisocyanate(TDI), polypropylene glycol (PPG), 1,4‐butanediol(BDO), trimethylolpropane(TMP), lithium bis(trifluoromethanesulphonyl)imide (LiTFSI), and polyethylene oxide (PEO)‐based electrolyte in varying proportions via solution casting technique at room temperature. All prepared PEO‐HBPUEs exhibited low glass transition temperatures, good thermal stabilities, and suitable mechanical properties. The XRD results showed that PEO‐HBPUEs are amorphous, and LiTFSI was well dissolved in the polymer matrix. The component of PEO‐based electrolyte in PEO‐HBPUEs contributed to increase the ionic conductivity, of which the highest value reached 1.23 × 10−3 S/cm at 75°C for PEO‐HBPUE4. The anodic bonding of PEO‐HBPUE substrate with Al foil was conducted by the coupling action of electric field, temperature field, and pressure field. A clear intermediate bonding layer between the substrate and Al foil was observed and the elements diffusion around bonding layer can be detected by SEM, indicating PEO‐HBPUEs and Al foil have been jointed together successfully. The highest tensile strength of the bonding interface of PEO‐HBPUE4/Al reached 1.88 MPa. All results demonstrated that the prepared PEO‐HBPUEs materials would be promising substrates for flexible MEMS device that can be applied to flexible packaging by anodic bonding technology.

Theoretical Derivation of the Effect of Bonding Current on the Bonding Interface during Anodic Bonding of PEG-Based Encapsulation Materials and Aluminum

“…Anodic bonding is an efficient, clean, and reliable process of joining dissimilar materials and has been used to encapsulate various MEMS (Micro-electro Mechanical Systems) devices [ 1 , 2 , 3 , 4 ]. In anodic bonding, strong electrostatic and temperature fields are required, the surface properties of the material are altered, and an irreversible physicochemical coupling reaction occurs at the contact point to form a permanent connection, with the role of the bonding current being dominant [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. Under different bonding conditions (i.e., materials and process parameters), the current variation characteristics during anodic bonding differ.…”

Flexible anodic bonding for the bonding between elastomer and metal

“…Various mechanical or electrical structures bonded to polymer flexible substrates is an important measure for the preparation of flexible devices 17–20 . In addition, the representative encapsulation means of flexible devices is to connect the polymer substrate and the polymer cover plate through an intermediate bonding layer by anodic bonding 21,22 …”