Design rules for polyimide solvent bonding

This paper describes a method for bonding polyimide layers to previously patterned polyimide layers and the results of a factorial study on bond quality (the amount of voids present at the void interface). The polyimide-to-polyimide bonding process has been demonstrated for forming microfluidic channels. First, a photosensitive polyimide precursor layer on the first substrate is pattemed with the desired geometry and vent channels, but not cured. Next, a thin layer of solvent with dissolved precursor is used to coat a soft-baked layer of Pyralin® PD PI-2723, a polyimide precursor, on a second substrate. The two halves are placed in contact and cured. Tests on a dozen samples show that several factors are statistically significant for void-free bonds. These factors include soft-bake duration. vent channel spacing, quantity of polyimide precursor in the solvent layer, and spin-coat speed for solvent application. The bonding method has been used to make 50- to 1000-μm-wide, 3- to 30-μm-deep channels for chemical analysis and heat transfer devices.