Effect Of Solder Volume On Interfacial Reactions Between Lead-free Solder Balls And Different Pads

Nowadays, with electronic products tending to become shorter, smaller, lighter, and thinner, the miniaturizations of electronic packaging have been widely developed and applied for consumer electronic products. As a result, not only the integrated circuit (IC) chips but also solder balls used to join chips and substrates are also downsizing to meet the trend of the miniaturization. But the reduction in the size of solder balls will take an adverse effect on the reliability of solder joints, which makes the effect of solder volume on the interfacial reaction become a new issue. This study was focused on the volume effect on interfacial reactions between lead-free solders and different under bump metallizations (UBMs) after various reflow soldering. In this study, Sn-3.5Ag,Sn-3.0Ag-0.5Cu and Sn-3.5Ag-0.75Cu solder balls with diameters of 200,300,400 and 500μm were employed to react with Cu and ENEPIG pads with an openning diameter of 250μm for various reflows of 1,2,3,4 and 5 times, respectively. The expermental conclusions are shown as follows:The interfacial reaction between lead-free solder balls with four diameters and Cu pad involved the dissolution of Cu pad into molten solder and the growth of interfacial intermetallic compound (IMC). There was a continuous scallop-type Cu6Sn5 layer formed at the interface during the solid-liquid reaction and an extra Cu3Sn layer would be formed at the interface between the Cu6Sn5 layer and Cu pad as the reaction time was prolonged. The volume effect was discovered in the interfacial reaction. The average diameter of IMC grains increased as the solder ball volume decreased as well as the thickness of the IMC layer after the same reflow time, but the Cu consumption for the smaller solder ball was lower than that of the bigger one. The average diameter of IMC grains, the thickness of IMC layer and the Cu consumption all increased with increasing reflow time when the solder ball volume and the solder composition was identical. The higher the initial Cu concentration in the bulk solder was, the higher the average diameter of IMC grains and the thickness of IMC layer were, the lower the Cu consumption was. Therefore the higher Cu concentration in the bulk solder could be introduced to suppress the excessive dissolution of Cu pad. The Cu concentration in the smaller solder ball rose faster than that of the bigger one which decreased the Cu concentration gradient so that the IMC grains of the smaller solder ball would grow in size rather than in number. On the other hand, the excessive growth of IMC layer would take adverse effect on the reliability of solder joint. There was only chunk and needle-type Ni3Sn4 IMC formed at the interface during the interfacial reaction between Sn-3.5Ag solder balls and ENEPIG pads. It was still noted that the IMC grains of the smaller solder ball were larger than that of the bigger one if only chunk-type IMC grains were taken into account. Also, a P-rich (Ni3P) layer was observed on the Ni-P layer due to the participation of Ni atoms into the Ni3Sn4 formation. A small addition of Cu resulted in differences in the type, morphology and growth kinetics of interfacial IMCs. In the case of Sn-3.0Ag-0.5Cu/ENEPIG reaction, only (Ni,Cu)3Sn4 IMCs with morphologys of needle and chunk shape were observed at the interface of 200μm solder ball. Furthermore (Ni,Cu)3Sn4 IMCs co-existed in the shape of chunk type with the octahedron-type (Cu,Ni)6Sn5 IMCs at the interface between Ni-P layer and 300μm solder ball. In the case of 400μm and 500μm solder balls, only (Cu,Ni)6Sn5 IMCs in the needle-type shape were formed at the interface due to higher absolute Cu content in the bulk solder. The branch-type (Cu,Ni)6Sn5 IMCs would form on the main needle as the reaction time was prolonged. The size of (Cu,Ni)6Sn5 grains of 400μm solder ball was larger than that of 500μm solder ball after the same reflow. The consumed Ni-P and the thickness of Ni3P both increased as the solder ball volume increased after the same reflow. Meanwhile the consumed Ni-P and the thickness of Ni3P both increased with increasing reflow time for the same volume of solder ball. An addition of 0.5wt% Cu into the bulk solder decreased the dissolution of Ni-P layer into liquid solder.Due to the existence of Ag in the solder, besides small Ag3Sn particles, large Ag3Sn plates formed at the interface of the solder joints during the solidification of the solders, which would deteriorate the integrity of solder joints. The differences in the size of Cu6Sn5 grains and in the thickness of IMC layers between the smaller solder balls and the bigger ones decreased with increasing reflow time and with increasing initial Cu concentration in the bulk solders.