Thermo-compression bonding for 2.5D fine pitch copper pillar bump interconnections on TSV interposer

Abstract

The use of portable electronic devices like smart phones and tablets results in high demand for more function, smaller dimensions and reduced power consumption requirements. To meet these challenges, electronic package design uses thinner chips with fine pitch bumping. There is active development in 2.5D and 3D IC packages with through silicon via (TSV). Tighter interconnection in addition to the increased density in the circuit in 2.5D and 3D IC systems provide higher performance with lower power consumption [1]. In addition, there is increased in the demand for fine pitch copper pillar bumping due to the lower silicon node, chip size reduction and TSV technology. Fine pitch interconnections are required in 2.5D and 3DIC integration for the demands of electrical continuity and high performance. In the existing interconnection methods, solder micro bumps have received a great deal of attentions because of its low material and process cost [2]. The major difference of copper pillar FC bonding process comparing to traditional FC bonding process is the reduction of the solder volume on each solder bump. As a result, there is no advantage of self-alignment of the solder during solder reflow process. Flip-chip bonder having accurate chip placement capability is needed to ensure good solder joint formation. Conventional reflow method is still applicable for sizable solder bump of diameter being greater than 100 μm and larger than 150μm pitch [3]. The post underfill processes such as capillary underfill (CUF) and molded underfill (MUF) can be followed after the solder joints are formed. On the other hand, when the pitch of bumps and/or the thickness of the FC go down further, FC with copper pillar bumps bonded by TC process would be one of the solutions for fine-pitch FC applications. It has been shown that the bump pitch can be reduced to as small as 50 μm (inline pitch). This process also allows for better control on the solder squeezed out effect. However this process requires tight control on (i) the planarization between the FC and the bonding substrate and (ii) the stand-off of each solder joint. Good process parameters have to be established to ensure no solder collapse or open joint.