New generation pre-deposited (no-flow) underfill for low-cost flip chip assembly

The achievement of an acceptable balance between flip chip reliability, process throughput/complexity, final yield and cost is an increasingly difficult task, especially in the competitive hand-held electronics market. A simple process, compatible and integrated with normal SMT processing, is a desirable goal which will be enabled by the substitution of a self-fluxing, pre-deposited underfill for the post-deposited materials currently in use. The pre-deposition process also has the potential to move flip chip assembly into mainstream SMT, especially if reworkability can be built in. The benefits of a 'no flow' process have been well documented. The limitations of the materials in current use have been overcome via a unique chemistry which can be tailored to the application. Room temperature storage, effective fluxing coupled with no outgassing, and a choice of reworkability after reflow or nonrework/full cure, can now be achieved within a single materials technology. This paper describes the properties of the new family of materials compared to conventional post-deposited underfills. The development sequence and procedure for material properties characterisation, including evaluation of the effectiveness of the fluxing action on a range of solder alloys, is documented. A typical application is described, outlining how a minimum of two process steps can be eliminated, and how improvements in materials handling, process robustness, and ultimate yield were realised. A simple rework regime is proposed, and the near drop-in replacement aspect of the new material is discussed.