Development of high throughput adhesive bonding scheme by wafer-level underfill for 3D die-to-interposer stacking with 30µm-pitch micro interconnections

In 3D integration, die stacking together with underfilling by capillary-type underfill are the principal processes within whole conventional assembly process. How to integrate and shorten the total process steps during assembly and increase the die-stacking yield especially for thin die stack to improve the throughput that can meet the requirement from industry will be a crucial issue. In this investigation, we proposed the high throughput adhesive bonding scheme by using wafer-level underfill material for the die-to-interposer stacking with 30μm-pitch micro interconnections. The reliability characterization of the die-to-interposer stack by such bonding scheme was implemented and confirmed. Die-to-interposer test vehicle was adopted to develop the proposed adhesive bonding scheme. The micro joints of electroplating Cu/Sn solder micro bumps joined with electroplating Cu/Ni/Au micro bumps was selected as the joining structure. There were more than 3000 bumps designed in the test vehicle. Three types of wafer-level underfill material were evaluated and selected to be the suitable processing material. The optimized die-to-interposer boding profile by wafer-level underfill were developed and determined for the purpose of high throughput in this study. After assembly process by the developed adhesive bonding scheme, reliability characterization was conducted on the die-to-interposer modules. Pre-conditioning, temperature cycling test (TCT), thermal & humidity storage test (THST) and die shear test were selected to assess reliability performance of the die-to-interposer module assembled by the proposed adhesive bonding scheme. Under the optimized bonding profile, one-die assembly could be finished less than 20 seconds, which was comparable to the process time of thermocompression bonding only. Also, the wetting and joining abilities of the micro joints were as good as those bonded by thermocompression bonding with flux and no voids were found between dies. By such adhesive bonding scheme, processes of flux cleaning and underfill dispensing and curing were no longer necessary, which could apparently enhance the throughput of die stacking. Results of reliability tests revealed that no electrical-connectivity fail and delamination happened on those die-to-interposer modules with 30μm-pitch micro interconnects after TCT of 1000 cycles and THST of 1000 hours though die shear strength showed a slight degradation less than 20%. In this investigation, the developed high throughput adhesive bonding scheme displayed the high potential that could be suitable and applicable for fine pitch 3D integration and high volume manufacturing requirements.