Comparison in between Good and Oxidized Leadframe: a Case Study in Package Delamination Analysis

The new electrical conductive material, copper alloy, C194 in situ composite, has been developed for the application to leadframe, high field magnet and trolley wire. This material has a low cost, high thermal and electrical conductivity, easy fabrication and joining, and also has a wide range of attainable mechanical properties. In C194, copper is one of the core materials in producing leadframes, interconnection wires, heat sinks and foils for flexible circuits in electronic packaging. Copper oxidation is considered as a serious reliability problem in microelectronic package. Unlike aluminum oxide, the copper oxide layer is not selfprotected which could lead easily to the oxidized condition. This study focused on copper leadframe which consist two types of condition, good and oxidized since good leadframe and oxidized leadframe have a different structure and composition. Both leadframe were applied in Quad Flat No-Lead (QFN) package. It was found that oxidized leadframe has a negative effect on package reliability. It will produce cracks at Cu-Al interface on the copper interconnection wire that will causes delamination between the leadframes die pad and molding compound. It also induces poor adhesion between the copper leadframes and molding compound. INTRODUCTION Copper is widely used as the core material in producing leadframe for microelectronic packaging. The application of copper is popular because it serves primarily in order to mechanically support the chip during the assembly of plastic package and to connect the chip electrically. Copper alloy is an ideal material in producing leadframe because of electrical resistance and thermal conductivity standpoint [1]. Among the choice of material that is used in semiconductor packaging, are argentums (Ag), aluminum (Al), aurum (Au) and copper (Cu). Copper offers low electric resistively compares to Ag that has a resistivity about 5% lower than copper electrical resistance [2]. Furthermore, Copper has high thermal conductivity that can allow faster heat dissipation than a low thermally conductive material. Copper also has a wind range of attainable mechanical properties, easy of deposition, fabrication, and joining. Solid State Science and Technology, Vol. 16, No 2 (2008) 223-231 ISSN 0128-7389 Corresponding Author: [email protected] Unlike an aluminum oxide, the copper oxide layer is not self-protected, so the copper can be easily oxidized even at low temperature storage. Therefore, study on copper oxidation has becoming very important in microelectronic packaging. There are two different mechanisms of copper oxidation. In aqueous environments at ambient temperature, a thin layer of Copper (I) oxide (Cu2O) forms first on the copper surface by the oxidation and reduction partial reactions [3]: 4Cu(s) + 2H2O(l) -> 2Cu2O(s) + 4H(eq) + 4e (anode) O2(s) + 2H2O(l) + 4e -> 4OH (eq) (cathode) A Cu2O is a p-type semiconductor with negatively charged vacancies. The growth of the Cu2O takes place on the top of surface through the mass transport of the Cu ions and electrons in a direction normal to the surface via vacancies. However, Cu2O is hardly detected in experiments because Cu2O is unstable in air, which will immediately change to copper (II) oxide (CuO). The second stage of oxidation, the formation of the CuO from Cu2O is usually a slower process. It is governed by the in-diffusion of oxygen into the oxide [4]. Figure 1 shows the corrosion rate of copper leadframe as the function of autoclave test time. Initially, Cu has higher percentage than CuO, which is 59% and 41%. However, the CuO concentration increases gradually after 384 hours under autoclave test. At 576 hours, the Cu turns into CuO completely, while no Cu is detected [5] Figure 1: Copper oxide concentration and rate of corrosion as a function of exposure time [5]. In this paper, the good and the oxidized leadframe have a different structure and composition was used. Both types of leadframe are mainly used in Quad Flat No-