Direct SMT Interconnections of Large Low-CTE Interposers to Printed Wiring Board Using Copper Microwire Arrays

Abstract

This paper reports compliant microwire copper arrays, in thin polymer carriers, as an innovative approach for direct surface mount technology (SMT) attach of large silicon, glass, and low coefficient of thermal expansion organic interposers to printed wiring boards (PWBs). The microwire arrays (MWAs) are prefabricated as free-standing ultrathin carriers using standard, low-cost manufacturing processes such as laser vias and copper electroplating. Such wire array carriers are then assembled in between the interposer and the PWB as a stress-relief interlayer. The MWA interconnections show low interconnection stress and strains even without the underfills. The approach is extensible to larger interposer sizes ( $20~\textrm {mm} \times 20~{\rm mm}$ ) and finer pitch (400 $\mu \text{m}$ ), making it suitable for smart mobile systems. The parallel wire arrays that form each joint result in low resistance and inductance, and therefore, do not degrade the electrical performance. The scalability of these structures allows extendibility to finer pitch and larger interposer sizes for high-performance applications. The finite-element method was used to design the MWAs to meet the thermomechanical reliability requirements. Computational models were built in 2.5-D geometries to study the reliability of 400- $\mu \text{m}$ -pitch interconnections with a 100- $\mu \text{m}$ -thick, $20~\textrm {mm} \times 20~{\rm mm}$ silicon interposer that was SMT-assembled onto an organic PWB. The warpage, equivalent plastic strain, and projected fatigue life of the MWA interconnections are compared with those of the ball grid array interconnections. A unique set of materials and processes was used to demonstrate the low-cost fabrication of the MWAs. Copper microwires with $15~\mu \text{m}$ diameter and 50 $\mu \text{m}$ height were fabricated on both sides of a 50- $\mu \text{m}$ -thick thermoplastic polymer carrier using dry-film-based photolithography and bottom-up electrolytic plating. The copper microwire interconnections were assembled between silicon interposer and FR-4 PWB with SMT-compatible processes. Thermomechanical reliability of the interconnections was characterized by thermal cycling test from −40°C to 125 °C. The initial fatigue failure in the interconnections was identified at 700 cycles, consistent with the models.