Warpage of embossed thermoplastic substrates and the effects on solvent bonding

Abstract

Warpage is a common problem in the injection molding, rolling printing, and hot embossing of thermoplastic microfluidic chips. Warpage can greatly reduce bond quality and can even lead to chip failure. This study adopted an experimental methodology to investigate the influence of warpage on bond quality. We created thermoplastic microfluidic chips made of PMMA with different degrees of warpage embossed by three brass mold inserts with various microfeatures which were fabricated using a micromilling machine. The micro features were used to vary the embossing pressure and thereby induce warpage of various degrees. The warped microfluidic chips (25 mm in width and 35 mm in length) were bonded to pristine PMMA substrates using ethanol solvent. The bonded chips were subjected to leakage tests and burst pressure tests in order to characterize bond quality. Our experiment results clearly showed that chips with smaller warpage (14.5 μm with a STD of 4.3 μm) could withstand burst pressure of 5.99 bars with 100 % bonding success. The chips with larger warpage (18.3 μm with a STD of 3.7 μm) and (23.2 μm with a STD of 4.8 μm) were able to withstand burst pressure of only 2–3 bars and presented a only 33 % likelihood of forming a successful bond. In this study, warpage was shown to be a crucial factor in bonding quality, wherein pronounced warpage deteriorated bonding strength and reduced the likelihood of bonding successfully to the substrate to form a thermoplastic microfluidic chip.