Characteristics of ceramic BGA using polymer core solder balls

The thermal reliability of lead-free solder joints between ceramic substrates and FR4 printed circuit boards (PCBs) is a major concern when designing ceramic ball grid arrays (CBGAs), particularly for thicker or larger substrates, since their CTE is much smaller than that of PCB. In order to achieve high reliability for critical applications, polymer core solder balls (PCSBs) are a promising solution. PCSBs enable high reliability without the need for other materials such as underfill resin or modification of package design to ensure reliability. In this study, the characteristics of PCSBs were evaluated using a variety of techniques in order to assess their adaptability to CBGA applications. The solder joint condition and the presence of intermetallic compounds (DVICs) were investigated, and the board-level reliability and electrical performance were compared to those of conventional SAC305 solid solder balls. Thermal cycling tests from -40 to +125 °C were carried out using an alumina CBGA with a 25 × 25 mm² package size and a 1.0 mm ball pitch. As a result of a larger solder bump height and a finer IMC microstructure, PCSBs exhibited a board-level reliability more than three times higher than that for SAC305 solder balls. PCSBs also exhibited stable electrical performance, with a similar initial resistance to that for SAC305 solder balls. Electromigration tests were carried out at a temperature of 150 °C and a current of 1.8 A, and lifetimes of more than 700 hours were found for both types of solder balls. The PCSB failure mode was fatigue cracking in the solder at the interface with DVICs on the ceramic side, which is similar to the case for SAC305. The use of PCSBs in large-sized HITCE CBGAs was also investigated to assess their potential for increasing the demand for larger package applications.