Solder droplet printing is one of the solder bumping technologies which are very important in flip-chip technology of electronic packaging. Fabrication processes of solder bumps using solder droplet printing were investigated in this study. Solder/UBM structure used in this study is eutectic Pb/Sn solder and electroless Ni-P/Au UBM system.
In order to establish the solder bumping process, solder droplet printing system with jetting nozzle with piezoelectric device was equipped. The effects of jet variables on jet status and stable jet conditions were investigated in view of several jetting modes such as satellite droplets, position accuracy, big droplets, solder stream and so on. Under the specific jet conditions, solder droplets were placed on target position of substrate without satellites or any non-uniform droplets and then solder bumps were fabricated through reflow.
By solder droplet printing, Pb/63Sn solder bumps were formed with 50㎛ diameter and 200㎛ pitch. And solder bumps were fabricated on Al/electroless Ni/Au pad with various sizes of 50㎛, 85㎛ and 120㎛ by changing fall time of waveform and the number of jetted droplets. Solder bumps with various sizes were able to be formed the most easily in solder droplet printing than in electroplating or screen-printing.
To compare with Pb/63Sn solder bumps formed by screen-printing, interfacial studies and bump shear test were performed as a function of reflow and aging time. In result, the intermetallic compounds grew and Ni UBM was consumed with increasing of reflow and aging time in Pb/63Sn solder bumps and electroless Ni UBM system. In order to confirm the possibility of real application of solder droplet printing, high frequency characterization of jetted solder-flip chip by functional IC and RF IPD (Integrated Passive Device) was performed. In addition, DRAM chip with 8 x 8 jetted solder bumps was assembled with PCB as the example of flip chip application.
The effects of the size of solder bumps on Ni-Sn interface were investigated using SEM, DSC analysis and solder etching experiment. Firstly, it was observed by SEM that Ni-Sn IMC (Intermetallic compounds) at the UBM/solder interface were created and grew with bulky shape and bigger size as a function of bump sizes. Secondly, the results of DSC (Differential Scanning Calorimetry) and solder etching experiment showed that more Sn atoms of large solder bump were diffused through channels between IMC and reacted with Ni atoms.
