Mohamed Lebbai

Surface characteristics and adhesion performance of black oxide coated copper substrates with epoxy resins

Black oxide is a conversion coating applied onto the copper substrate to improve its interfacial adhesion with polymeric adhesives. A comprehensive study is made to characterize the black oxide coating using various characterization techniques, including SEM, XPS, AFM, XRD, Auger electron spectroscopy, TEM, D-SIMS, RBS and contact angle measurements. It was found that the oxide coating consisted of cupric and cuprous oxide layers from the top surface to inside. The cuprous oxide layer was formed on the copper crystal surface, on which densely-packed fibrillar cupric oxide grew continuously until saturation. The cupric oxide had a fibrillar structure with high roughness at the nanoscopic scale, whereas the cuprous oxide was rather flat and granular. There was a continuous change in oxide composition with no distinct boundary between the two oxide layers. The bond strength between the epoxy resin and the oxide coated copper substrate increased rapidly at a low level of oxide thickness, and became saturated at thicknesses greater than about 800 nm. There were similar dependences of bond strength on surface roughness, oxide thickness especially of cupric oxide and surface energy, reflecting the importance of these surface characteristics in controlling the interfacial adhesion.

Effects of moisture and temperature ageing on reliability of interfacial adhesion with black copper oxide substrate

The reliability of adhesion performance of bare Cu, as-deposited and surface-hardened black oxide coatings on Cu substrates was studied. The interfacial adhesion with a polyimide adhesive tape and an epoxy moulding compound was measured using the button shear and tape peel tests after hygrothermal ageing in an autoclave, high temperature ageing and thermal cycles. Moisture adsorption and desorption studies at different aging times suggested that the black oxide coating was effective in reducing the moisture adsorption. The bond strengths for all substrates remained almost unchanged after thermal ageing at 150°C for 8 h. Thermal cycling between −50°C and 150°C for 500 cycles reduced by about 20% the button shear strength of the as-deposited black oxide substrate, but it did change much the bonding performance of the bare Cu substrate. Hygrothermal ageing at 121°C/100% RH in an autoclave was most detrimental to adhesion performance because of the combined effect of elevated temperature and high humidity. The reduction in button shear strength after the initial ageing for 48 h was 50–67%, depending on the type of coating. In all accelerated ageing tests, the residual interfacial bond strengths were consistently much higher for the black-oxide-coated substrates than the bare Cu surface, confirming a higher reliability of black oxide coating. Fracture surfaces analysis of tape-peeled bare copper substrates after 500 cycles of thermal loading revealed a transition in failure mechanism from interfacial to cohesive failure. In contrast, the failure mechanism remained unchanged for black-oxide-coated substrates. The observations made from the button shear and tape peel tests were generally different because of the different fracture modes involved.

Effect of black copper oxide on interface adhesion between copper substrate and glob-top resin

A study is made of the effects of black copper oxide coating and other plating variables on adhesion strength of glob-top resin to copper substrates for Tape Ball Grid Array (TBGA) package applications. The button shear test was performed to measure the interfacial bonding strength. Several surface analytical techniques were employed to establish the correlations between the interface bond strength and various surface parameters, such as surface free energy, chemical element, surface roughness and fracture morphology. The experimental results showed that the presence of black oxide coating (Sample C) resulted in significantly higher interface bond strength than the bare copper (Sample A) or nickel coated (Sample B) surfaces. Further processes (Samples D and E) after the black oxide coating exhibited detrimental effects, to a varying degree, on interface bond strength. The interface bond strength was approximately proportional both to surface free energy and roughness of surface finishes, indicating that wettability and mechanical interlocking both played a significant role in forming adhesion with glob top resins.

Optimization of black oxide coating thickness as adhesion promoter for copper substrate

Black oxide is a conversion coating applied to a copper surface to improve the interface adhesion with polymeric adhesives and moulding compounds. State-of-the-art analytical instruments, including SEM, TEM, XPS, AFM, XRD, goniometry, dynamic SIMS and RBS were employed to characterize the coated surface and interfaces with glob top resins. It was found that the copper oxide layer consists of cupric and cuprous oxides with a continuous change of oxide composition from the top surface to the inside without a distinct boundary in between. Crystallinity of the oxides was barely detected directly from the black oxide coated copper. The cupric oxide exists in the form of a long, fibril structure on nanoscopic scale. The interface bond strength between the copper oxide and glob-top resin increased rapidly in the low range of oxide thickness and became almost constant with further increase in oxide thickness. A functionally similar dependence of oxide thickness and interface adhesion on treatment time was also revealed. Fracture occurred mainly within the oxide layer for black oxide coated substrates (i.e. cohesive failure of black oxide), while fracture tended to occur along the coating-resin interface (i.e. adhesive failure) once the coated surface is debleeded by sandblasting.

Effects of moisture and temperature ageing on reliability of interfacial adhesion with black copper oxide surface

This paper reports the recent studies on adhesion performance of copper alloy substrate with bare surface, black oxide coating and black oxide with debleeding treatment. The interfacial adhesion of substrate with polyimide tape and glob top resin were measured after autoclave test (or pressure cooker test, PCT), temperature ageing and thermal cycles based on the button shear and tape peel tests. The failure mechanisms were studied from the fracture surface analysis. Moisture absorption and desorption studies at different aging time were carried out to understand the property changes due to moisture. The results show that the black oxide coating improved significantly the interfacial bond strength in the dry condition. The mechanical interlocking mechanism provided by the fibrillar copper oxide was mainly responsible for it. The interfacial bond strengths for all substrates remained almost unchanged after thermal ageing at 150/spl deg/C for 8 h. Thermal cycles between -50/spl deg/C and 150/spl deg/C for 500 and 1000 thermal cycles exhibited generally a negligible influence or decreased slightly the interfacial adhesion. The hygrothermal ageing at 121/spl deg/C/100% RH in an autoclave (i.e. PCT) resulted in large reductions in interfacial bond strength after the initial 48 h ageing and tended to level off with further ageing. Fracture analysis of tape peeled bare copper substrates after 500 cycles of thermal loading revealed a transition of failure mechanism from adhesive to cohesive failure. In contrast, the failure mechanism remained unchanged for oxide-coated substrates. The implications arising from the button shear and tape peel tests are different because of the different fracture modes involved. According to the available data discussed above, the PCT is regarded as the most severe test affecting the adhesion performance. Presence of oxide coating minimized the moisture absorption rate when compared to the bare copper surface.

Effects of hygrothermal ageing and thermal shock on reliability of interfacial adhesion between black oxide coated copper substrate and epoxy resin

Black oxide is a conversion coating applied onto the Cu substrate to improve the interfacial adhesion with polymeric adhesives. A comprehensive study was made to characterize the black oxide coating and the corresponding interfacial adhesion with various types of polymeric resin, aiming to optimize the oxide processing conditions. The reliability of adhesion performance of the coating was evaluated before and after accelerated hygrothermal ageing, such as temperature cycling, pressure cooker test, and moisture sensitivity test followed by thermal shock. The moisture resistance of the substrate with black oxide coating was much higher than the bare Cu substrate, during both the moisture absorption and desorption processes. Thermal cycling alone did not change significantly the adhesion performance of any of the substrates studied. Pressure cooker test was detrimental to adhesion performance of oxide coated Cu substrates. Nevertheless, the residual interfacial bond strengths were consistently much higher for the black oxide coated substrates than the bare Cu surface. Significant delamination occurred between the bare Cu and the moulding compound after the moisture sensitivity test followed by thermal shock, whereas there was virtually no delamination on the black oxide coated samples under the same ageing condition, confirming the higher reliability of interfacial adhesion performance for the latter.