AIR REFLOW OF LEAD-FREE SOLDERING FOR FINE PITCH BGA
Authors: Wei Wei Chin, Ching Ching Chong, Lian Huat Ng, and Cheng Siew Tay Company: Intel Product (M) Sdn. Bhd. Date Published: 10/11/2007
Abstract: Tin lead to lead-free transition has driven the whole semiconductor industry to evolve tremendously in process development to cater for the lead free soldering challenges. This is due to higher melting temperature of lead-free system which results in increase of oxidation attributed by higher soldering temperature. Air reflow is preferred in lead-free system as lower cost solution. However, this poses challenge to SMT assembly as the lead-free formulation usually relates to lower wettability. To provide a robust solder paste for high volume manufacturing environment in air reflow with varying manufacturing temperature ranging from 25 to 35oC, a comprehensive solder paste and reflow process characterization is essential. This paper describes the assembly challenge in air reflow (BGA non-wet, which is one of the common open joint defect and not providing electrical good contact between BGA package and PCB), solder paste characterization, defect mechanism understanding and SMT process optimization for finepitch BGA products. This study focused on the effect of two solder paste (SAC 305 and SAC 405 from suppliers A and B) properties and stability to SMT assembly defects. The variation and cliff of three manufacturing parameters were included to understand the robustness of the paste, ie aged solder paste, lower paste volume and reflow atmosphere in air or N2 on fine-pitch BGA (0.5mm and 0.8mm ball pitch). The solder joint defects formation mechanism of different solder paste material properties is understood through various lab scale metrologies. The non-wet mechanism was well understood. It was caused by imbalance wetting that resulted from a high surface tension barrier preventing metallurgy contact between solder paste and solder ball during solder melting stage. The assembly results indicated that solder paste A has lower non-wet fallout rate over temperature and time compared to solder paste B. This result was well supported by the material characterization results such as pH, viscosity and wettability. The robustness of solder paste A was again demonstrated in the solder volume cliff evaluation results in air reflow (with respect to BGA nonwet defect), in which solder paste A has relatively larger solder volume margin than solder paste B. This detail work is useful to provide an insight into new solder paste material and reflow process characterization and hence enable the assessment and selection of a robust solder paste materials in assembly with air reflow.
Key words: lead-free solder paste, BGA, non-wet, air reflow