Evaluation of the Surface Free Energy of Spin-Coated Photodefinable Epoxy MARKUS P. K. TURUNEN, TOMI LAURILA, JORMA K. KIVILAHTI Laboratory of Electronics Production Technology, Department of Electrical and Communications Engineering, Helsinki University of Technology, P.O. Box 3000, HUT FIN-02015, Finland Received 8 March 2002; revised 13 June 2002; accepted 26 June 2002 Published online 00 Month 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/polb.10274 ABSTRACT: The surface free energy of crosslinked photodefinable epoxy (PDE) was evaluated from the advancing contact angles measured by the sessile drop method. Poly(tetrafluoroethylene) (PTFE) was used as a reference material in the evaluation of the surface free energies by various models. Pure water, diiodomethane, formamide, ethylene glycol, diethylene glycol, glycerol, 1-bromonaphthalene, decane, and tetrade- cane were used as the probing liquids. The surface free energies for PDE and PTFE were calculated to be 43.6 and 21.2 mJ/m 2 , respectively. The contact-angle measure- ments indicated the isotropy of the PDE surface with respect to the surface free energy. The PDE coating was further characterized with scanning electron microscopy and atomic force microscopy. The PDE surface was treated chemically and by reactive ion etching (RIE) to determine their impact on the wettability and adhesion. The treat- ments resulted in decreased contact angles between the crosslinked PDE surface and water as the polarity of the surface increased from about 9% to 18 and 43% by the chemical and RIE treatments, respectively. On the contrary, the surface free energy of the treated PDEs, as calculated by the geometric mean model, did not change markedly (to 47.4 and 41.8 mJ/m 2 by the chemical and RIE treatments, respectively). Conse- quently, the contact angles of diiodomethane and the PDE solution on the treated surfaces did not decrease noticeably. The stud-pull test showed improved adhesion strength for PDE that was left less crosslinked and, therefore, had residual affinity against the sequential PDE layer. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2137–2149, 2002 Keywords: photoresists; sessile drop; printed wiring board; adhesion; surfaces INTRODUCTION In many applications, the performance of a poly- mer product is related to its surface properties. An example is a multilayer printed wiring board (PWB), in which dissimilar materials, including various polymers, are sequentially built up to en- able high-density interconnection substrates. 1–10 For adequate adhesion between the layers, the surface free energy of a solid polymer has to be high enough to ensure proper wettability. 11,12 However, the topmost layer should prevent the adherence of dust and other contaminants. More- over, it should serve as a diffusion barrier against liquids [especially water (H 2 O)] and gases. Over- all, the manipulation of the surface free energy at various interfaces during the sequential build-up process is essential for the reliable fabrication of PWBs. The evaluation of the surface free energy has been used to predict adhesion and wettability. 13 Although the direct measurement of the surface free energy of a solid is impossible, indirect meth- Correspondence to: J. K. Kivilahti (E-mail: jorma. kivilahti@hut.fi) Journal of Polymer Science: Part B: Polymer Physics, Vol. 40, 2137–2149 (2002) © 2002 Wiley Periodicals, Inc. 2137