Delaminations at various interfaces are one of the most critical reliability issues in plastic packages. Delamination is formed as a result of the imperfect interface adhesion between the integrated circuit (IC) package components. Adhesion has been identified as one of the key factors to improve delamination related problems. A thorough understanding of substrate, surface coatings, polymeric materials, interface adhesion properties and hygrothermal effects on reliability performance of the packages are necessary in order to minimize the problem of delamination and the associated package cracking issue. Previous research works have focused on dimples to improve the interfacial bond strength without full consideration of the surface coatings and hygrothermal effects on interfacial adhesi...[ Read more ]

Delaminations at various interfaces are one of the most critical reliability issues in plastic packages. Delamination is formed as a result of the imperfect interface adhesion between the integrated circuit (IC) package components. Adhesion has been identified as one of the key factors to improve delamination related problems. A thorough understanding of substrate, surface coatings, polymeric materials, interface adhesion properties and hygrothermal effects on reliability performance of the packages are necessary in order to minimize the problem of delamination and the associated package cracking issue. Previous research works have focused on dimples to improve the interfacial bond strength without full consideration of the surface coatings and hygrothermal effects on interfacial adhesion. In the present investigation of interfacial adhesion improvement between lead frame and epoxy molding compound (EMC), special emphasis has been placed on: i) the effects of dimple and metallic coating and; ii) the effects of temperature and hygrothermal ageing excursions simulating the practical package assembly processes. Moreover, the extension of copper oxide coating technology from printed circuit board (PCB) to IC packages, especially for copper lead frame and the heat sink/substrate for tape ball grid array (TBGA) packages, have necessitated a more detailed study of characterization and optimization of black oxide coating material, effect of debled treatment and hygrothermal effect on interfacial adhesion.

A study is made of the effects of dimple and metallic coatings by evaluating the surface characteristics, such as wettability, surface roughness and elemental compositions, which in turn are correlated with the maximum pull force measured from the lead pull tests. It is found that the dimples enhanced the maximum pull force through improved mechanical interlocking of molding compound, depending on the type of coating. The wettability of metal surface represented by the surface energy played a predominant role in controlling the interfacial adhesion. Hygrothermal ageing at 85℃/85% RH followed by solder reflow process reduced the maximum pull force significantly especially for the hydrophilic and polar metal surfaces. The reduction in maximum pull force for hydrophilic surfaces is higher than hydrophobic surfaces. There was approximately a linear correlation between the reduction in interfacial bond strength and the differential work of adhesion between the dry and wet environments. The mechanical interlocking provided by dimples retained the interfacial adhesion in humid environment and stopped the propagation of delamination.

In addition to the aforementioned metal plating and dimples, black copper oxide is another very useful chemical conversion coating for improved adhesion of copper with various polymers, and has been widely employed since the early days of printed circuit technology. The surface was characterized and it's thickness was optimized based on several state-of-the-art analytical instruments and interfacial adhesion study. The maximum shear force between the black oxide coated substrate and glob-top resin increased rapidly at a low range of oxide thickness, and became saturated at thicknesses greater than about 800nm. There were functionally similar dependence of interfacial bond strength on oxide thickness, surface energy and surface roughness. A treatment for 150 sec is sufficient to produce a coating thickness with maximum adhesion with glob top resins. Mechanical hardening involved in the debleeding process consolidated the fibrillar cupric oxide into a flat surface, which reduced in turn both CuO/Cu₂O ratio and the interfacial bond strength. Analysis of fracture surfaces indicated that cohesive failure occurred within the oxide layer for the black oxide coated substrate (cohesive failure), while fracture tended to occur along the coating-resin interface (adhesive failure) once the coated surface was processed with debleeding treatment. The presence of black oxide coating discouraged the moisture absorption, and encouraged the desorption of moisture within the coating. Autoclave test (or pressure cooker test, PCT), is regarded as the most severe test affecting the adhesion performance.

The major contributions of this thesis are: i) to incorporate dimple and metallic coatings under hygrothermal conditions and to study its interfacial adhesion strength; ii) to report that the improvement in the interfacial adhesion due to dimples was higher for the coatings with inherently weak interfacial adhesion than those with inherently strong adhesion characteristic; iii) to study the correlation between interfacial adhesion and differential work of adhesion at dry and wet condition of metallic coatings; iv) to report the correlation between the work of adhesion difference and polar surface energy of metallic coatings; v) to report that dimples retains the interfacial adhesion under humid environment; and; vi) to compare the surface characteristics and interfacial adhesion of as-received black oxide and debled black oxide under different oxidation treatment time and hygrothermal conditions.