NO-FLOW UNDERFILL VOID NUCLEATION STUDY IN HIGH, STABLE YIELD, AND NEAR VOID-FREE ASSEMBLY PROCESS DEVELOPMENT

摘要

The advanced assembly process for a flip chip in packagern(FCIP) using no-flow underfill material presents challengesrnwith high I/O density (over 3000 I/O) and fine-pitch (down torn150 μm) interconnect applications because it has narrowedrnthe feasible assembly process of high electrical interconnectrnyield.rnIn spite of such challenges, a high (>99.99%), reliablern(sample size: 30) yield and nearly void-free (≈ 7.0%)rnassembly process has been achieved using commercial noflowrnunderfill material with a high I/O, fine pitch FCIP in ourrnpast research. The existing 7% void area could cause earlyrnfailures such as solders fatigue cracking or solder bridge inrnthermal reliability. Therefore, this study reviewed a classicalrnbubble nucleation theory to predict the conditions of voidsrnnucleation in assembly process. The heterogeneous nucleationrntheory found the critical point enabling stable voidsrnnucleation. From the result of nucleation theory, systematicrnexperiments were designed to eliminate underfill voidingrnusing parametric studies. The void formation studyrninvestigated the effect of critical temperature on underfillrnvoiding and solders wettability. Then, the study established arnhigh, stable yield and void-free assembly process combinedrnwith former study. The stability of the assembly process wasrnvalidated using a large scale of assemblies.rnThus, the classical bubble nucleation was explored to modelrnvoid nucleation in no-flow underfill process. Besides, thernsystematic studies presented in this paper allowed a high,rnstable yield and void-free flip chip assemblies with high I/O,rnfine pitch using no-flow underfill.