First approach to cost-efficient fine pitch NCA flip-chip assembly on thermoplastic polyurethane printed circuit boards

Today's demand on high density interconnects and fast packaging technologies enhance flip-chip assembly to an enabling technology. However, bonding of flip-chips to organic substrates is still a challenge regarding cost-efficiency and assembly yield. Limits of soldering like shearing stress due to CTE differences and high process temperatures can be overcome with adhesives. Simultaneously, the reliability requirements result in high demands to adhesives and organic substrates. Compared to expensive isotropic/anisotropic conductive adhesives (ICA/ACA) the use of thermoplastic adhesive without any conductive fillers offers fast and cost-efficient assembly with rework option. In this work a thermo-mechanical bonding process interconnects thinned flip-chips to thermoplastic polyurethane circuit boards without additional adhesive. The substrate itself acts as stretchable PCB and adhesive/underfiller. The technology achieves very high assembly yield and is comparable to NCA bonding on common organic substrates like FR4 or polyimide (PI). TMA and DMA investigations uncover the entropy-elastic behavior of thermoplastic polyurethane. FIB investigations show the contact interface. Thermal cycling and humidity tests characterize reliability of the interconnection.