FCOB reliability evaluation simulating multiple rework/reflow processes

Abstract

Flip-chip assembly [direct chip attach (DCA) or flip-chip on board (FCOB)] on printed wiring boards (PWBs), in conjunction with conventional leaded device surface mount technology, is beginning to proliferate in compact and portable systems. DCA with conventional controlled collapse chip connection (C4) bumps requires solder coated bond pads to allow joining in typical surface mount technology (SMT) reflow cycles. A flip-chip device on a typical FCOB/SMT board will usually experience no high temperature excursions after the die joining and underfill encapsulant cure unless the board undergoes a rework cycle. FCOB single chip packages and multichip modules are now in development with standard C4 bumps, and a new Motorola E-3 bump which requires no solder on bond pads. These solder interconnects must be stable through multiple heat treatments expected in subsequent system level assembly and repair operations. Flip-chip plastic ball grid arrays (FC-PBGAs) will typically undergo three solder reflow, or reheat, cycles to /spl sim/220/spl deg/C subsequent to initial flip-chip reflow assembly. The multiple reheats are for BGA ball attach, board level BGA SMT assembly, second side BGA SMT assembly, and possible rework operations. In this paper, the effect of multiple reheats on the solder connection microstructure and strength (before, and after, underfill encapsulation), and the integrity of the underfill encapsulant adhesive and cohesive strength is reported, using both FCOB single chip packages and multiple chip modules. The effect of multiple reheats on electrical resistance of daisy chain nets, and die stress (radius of curvature), is also reported. Hot air gun rework (before underfill) is simulated and standard belt furnace reflows are utilized. Cross sections of bump connections and underfill interfaces were studied to assess changes induced by the temperature exposures. The reliability of the FCOB assemblies was assessed via temperature cycle, thermal shock, and autoclave tests.