IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 26, NO. 4, OCTOBER 2003 345 Surface Property of Passivation Layer on Integrated Circuit Chip and Solder Mask Layer on Printed Circuit Board Shijian Luo, Member, IEEE, and C. P. Wong, Fellow, IEEE Abstract—Adhesion of underfill to passivation layer on inte- When the contact angle of a liquid on a solid is 0 , liquid film grated circuit chip and solder mask layer on printed circuit board can be formed by the decreasing of the total free energy is critical to the reliability of an underfilled flip chip package. When is between 0 and 90 , is In this study, the surface properties of solder mask and four passivation materials: benzocyclobutene (BCB), polyimide (PI), smaller than the surface tension of the liquid, thus liquid film silicon dioxide @ PA, and silicon nitride (SiN) were investigated. can not be formed spontaneously. Nevertheless, is still A combination of both wet and dry cleaning processes was very positive, so there is a decrease in free energy on converting the effective to remove contaminants from the surface. The element solid-vapor interface to a solid-liquid interface, thus asperities oxygen, introduced during P plasma treatment or Q treat- on the solid surface can be filled by the advancing liquid. When ment, led to the increase of the base component of surface tension. X-ray photoelectron spectroscopy (XPS) experiments confirmed the contact angle is greater than 90 , is negative, and the increase of oxygen concentration at the surface after Q the surface asperities cannot be filled by the advancing liquid. treatment. Wetting of underfill on passivation and solder mask It was proposed that the surface tension is composed of three was slightly improved at higher temperatures. Although Q components [1], [2] the Lifshitz-van der Waals component cleaning and P plasma treatment significantly improved the wetting of underfill on passivation materials, they did not improve including electromagnetic interaction, oscillating tem- adhesion strength of epoxy underfill to passivation. Therefore, the porary dipoles interaction, and permanent and induced dipoles wetting was not the controlling factor in adhesion of the system interaction; the Lewis acid component ; and the Lewis studied. base component . Their relationship is given by Index Terms—Contact angle, P plasma, passivation, solder mask, surface tension, surface treatment, Q, XPS. (2) I. INTRODUCTION Thermodynamic work of adhesion ( , also called phys- ONTACT angle measurement is the most convenient and ical adhesion) is the reversible work required to separate a unit C rapid method to probe the surface constitution of a solid. It area of two contacting phases. It is composed of LW component can sense the force of monolayer (5–10 Å), thus it is extremely and acid-base component , and it is directly re- surface sensitive. Contact angle is related to the surface tension lated to the surface tension. The nongeometric combining rule (also called surface energy) of the solid vapor interface , was proposed [3] the liquid vapor interface , and solid liquid interface . Their relationship is given by Young’s equation (3) (1) (4) or (5) (1a) Where and are surface tension of component 1 and com- Contact angle value can be divided into three regions: ponent 2 in the medium, respectively, and is the interfacial tension between the two components. It was suggested that sur- face modification can alter the acid-base component rather than LW component, and thus the work of adhesion can be enhanced Manuscript received March 15, 2001; revised February 4, 2003. This work was supported by the National Institute of Standards and Technology (NIST) by surface modification through increasing the acid-base inter- through the Advanced Technology Program (ATP). This paper was presented action [4]. in part at the International Symposium and Exhibition on Advanced Packaging The three-liquid-probe was proposed to measure the surface Materials: Processes, Properties and Interfaces, Braselton, GA, March 2001. S. Luo is with the Assembly Department, Micron Technology, Inc., Boise, ID tension and its three components of a solid surface [1], [3], [5]. 83707 USA (e-mail: [email protected]). Water and ethylene glycol (EG) can be used as two polar liquids, C. P. Wong is with Packaging Research Center, School of Materials Science and diiodomethane is frequently used as the apolar liquid. The and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: [email protected]). surface tension and its three components for these probe liquids Digital Object Identifier 10.1109/TEPM.2003.822996 are shown in Table I. 1523-334X/03$17.00 © 2003 IEEE 346 IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 26, NO. 4, OCTOBER 2003 TABLE I mask and passivation of flip-chip devices, different techniques SURFACE TENSION OF THREE PROBE LIQUIDS can be used to clean or modify the surfaces [9], [10]. It is neces- sary to understand the surface properties before and after treat- ment processes, and their effect on adhesion. In this study, the surface properties of solder mask and four passivation materials: polyimide (PI), benzocyclobutene (BCB), silicon oxide , and silicon nitride (SiN) for integrated cir- cuit chips were characterized by measurement of contact angles of the three standard liquids on these substrates after different The work of adhesion between a solid and a liquid can also preparation procedures. The surface atomic composition before be expressed by the following equation, deduced from (1) and and after cleaning was also investigated through X-ray (3) photoelectron spectroscopy (XPS). In addition, the wetting be- (6) havior of underfill on the passivation and solder mask was also studied at elevated temperature. The effect of and Thus, the three components of surface tension of a solid ( , plasma treatment on the adhesion of epoxy underfill to passiva- surface energy of the interface between solid and air) can be tion was also evaluated. obtained by measuring the contact angles ( , , ) of the three liquids (with known surface of , , and in air) on the solid II. EXPERIMENTAL surface and solving the following three equations deduced from A. Materials (3)–(6) [6] Deionized water, diiodomethane (99%, from Aldrich Chem- icals), and ethylene glycol (99 %, from Aldrich Chemicals) were used as standard liquids for contact angle measurement. (7) The BCB passivated silicon die was supplied by Dow Chem- ical. The PI passivated dies were supplied by Boeing Company. and SiN wafers were supplied by Silicon Valley Micro- (8) electronics. Solder mask from Taiyo was coated on an FR-4 board and cured according to the suggested curing profile from the supplier. (9) An epoxy underfill (G25) was prepared with one equivalent of cycloaliphatic epoxy resin ERL4221 from Union Carbide, Among solids, there are hard inorganic materials such as 0.8 equivalent of hardener 4-methylhexahydrophthalic anhy- metal, glass, and ceramic, and soft organic materials. Hard dride (MHHPA) from Aldrich Chemicals, and catalyst cobalt inorganic materials, in contrast to the soft organic materials, (II) acetylacetonate (equal to 0.4% of total weight of epoxy have high surface energy, and thus can spread all liquids except resin and hardener) from Aldrich Chemicals. mercury. However, contamination can occur, which can change the surface property dramatically. Contamination of a high B. Surface Preparation Procedures energy surface with low energy organic contaminants will lead Wet Cleaning: The steps were as follows: 5 min soak in ter- to the decrease of surface energy and raise the contact angle; pene; 5 min soak in terpene with ultrasonic cleaning; 5 min soak while, contamination of a low energy surface with high energy in isopropyl alcohol; 5 min soak in isopropyl alcohol with ultra- contaminants will lead to the increase of surface energy and sonic cleaning; three rinses in deionized water for 2 min each; lower the contact angle. dry in a vacuum oven at 120 C for 30 min with a pressure below In a flip chip package, underfill is used to fill the gap be- 30 mmHg. tween the integrated circuit (IC) chip and substrate or printed UV/Ozone Cleaning: UV/ozone treatment of the sur- circuit board (PCB) to increase the solder joint fatigue lifetime. faces was performed at 50 C for 5 min in an UV and ozone The adhesion of underfill to passivation layer on IC chip and to dry stripper (Samco, Model UV-1). solder mask layer on PCB is very important for the reliability Plasma Cleaning: plasma treatment was performed of flip chip package. However, delamination (total loss of adhe- with a plasma reactor (Technics Micro-PD series 95) at room sion) between die and underfill is still a major concern for yield temperature for 10 min. The pressure was 150 mtorr. The power loss and reliability [7], [8]. Delamination at the underfill/die or was 100 W. underfill/substrate interface can lead to cracking of the solder joint interconnection. In a humid environment, water borne con- C. Contact Angle Measurement taminants can enter the package through the delaminated area, A goniometer (Model 102-00, from Rame-hart, Inc.) was and cause corrosion of metal pad, joint, and metal line. One of used to measure the contact angle. A substrate was placed on the many possible reasons for poor adhesion is contamination of the sample stage of the goniometer, and a micro syringe was passivation or solder mask. In order to improve the wetting and used to deposit a liquid drop of 2–3 on the surface of the thus possibly the adhesion of underfill material with the solder substrate. The steady-state contact angle was recorded after the LUO AND WONG: SURFACE PROPERTY OF PASSIVATION LAYER 347 TABLE II CONTACT ANGLES (DEGREE) OF THREE PROBE LIQUIDS AND UNDERFILL ON SURFACES AT DIFFERENT CONDITIONS formation of the sessile drop. Five readings were taken for each III.