The “Black pad” Phenomenon referring to the blackening of electrolessly plated nickel-phosphorus(Ni(P)) films after the immersion gold(IG) process was reproduced using pure chemicals. In this work, it was conducted on various under bump metallurgy (UBMs) samples, in this case sputtered Ag, Au, Ni and Co. It was found that Ni-P films had dissimilar features in accordance with each type of UBM. Like Cu UBM, which undergoes variation of the Phosphorus content and nodular shape across nodules, a large-area black pad was reproduced identically on sputtered Ag and Au samples after the IG process. However, in the case of the Ni and Co UBM samples, no corrosion was noted, and the nodules of Ni-P had a uniform concen-tration of phosphorus across all nodules because both metals were catalytically active so increased nucleation rate made initial phase was not affected by the condition of the surface. The reactivity of the UBM samples determine the morphology of the Ni-P films, hence the Ni-P layers on the Ag and Au UBM samples which were electronegative compared to nickel had the irregularly shaped nodules because of initial electrolytic deposition process. Therefore, the roughness of Ni-P films was higher than the roughness levels of the Ni and Co UBM samples. Ni/Cu double-layered UBMs and Co/Cu double-layered UBMs were used to verify that the characteristic of the top layer of UBMs is crucial in determining the morphology of Ni-P thin films, leading to the black pads.
Furthermore, Black pad phenomena was influenced by the roughness of Cu substrate, so it was con-ducted on various Cu block samples which each one has different surface finishes such as SiC 80 grit finish, SiC 400 grit finish, SiC 2000 grit finish, and Alumina powder 0.3um finish. Topography of each Cu blocks influenced upon Ni-P films, which leads to different fraction of corroded area.
On top of that, before IG process, annealing process was conducted on Ni-P films. It was found that the morphology of Ni-P films was not varied by the increased temperature, but Ni-P films undergo phase transformation, which the microstructure was varied from amorphous to crystals. The result showed that phase transformation diminishes the potential difference between nodules, leads to mitigate Black pad for-mation.
The phosphorus content of the films was measured by EDS. The images of the surface of the corrod-ed films were obtained by SEM, and the property of the cross-section of the films before and after corrosion was analyzed by FIB and TEM. The roughness of Ni-P films was measured by AFM.
무전해 니켈 치환/금도금 시 발생하는 corrosion 현상인 Black pad 현상을 pure chemicals을 통해 재현하였으며, Ag, Au, Ni 그리고 Co의 다양한 metals을 under layer로 사용하였다. Ag와 Au의 경우 이전의 Cu와 동일한 결과인 불규칙한 Nodule의 형태와 Phosphorus content 차이를 보였다. 하지만 Ni, Co를 UBM(Under bump metallurgy)로 사용시 Uniform한 nodule의 형태와 P content 함량이 Uniform하게 나타났으며, 이에 따라 Corrosion도 일어나지 않았다. 이는 catalytically active한 metal인 Ni, Co의 경우 initial phase 생성시 nucleation rate가 증가 되어 substrate의 surface condition에 영향을 받지 않았기 때문이다. 즉, UBM의 reactivity 가 Nickel-Phosphorus film의 morphology를 결정한다. 또한 Ni/Cu, Co/Cu double-layered UBMs의 실험결과는 UBMs의 top layer의 metal type 이 Ni-P film의 morphology를 결정하며, 즉 Black pad 생성 유무를 결정하게 된다. 게다가, Cu substrate를 사용시에 Substrate의 Roughness 에 따라 Ni-P film의 morphology가 영향을 받았다. Surface finish를 SiC 80 grit, SiC 400 grit, SiC 2000grit, Alumina powder 0.3um 단계적으로 변화시킴에 따라 Ni-P film의 morphology가 변화하며 이에 따라 부식영역의 변화도 보였다. Ni-P films을 열처리 할 경우, 고온에서 Phase transition의 유도됨에 따라 Amorphous 에서 crystals로 transformation 됨에 따라 Black pad의 생성이 줄어들었다. P content의 함량은 EDS를 통해 측정하였으며, 표면 분석을 SEM, 단면과 특성분석은 FIB, TEM 그리고 Roughness의 경우 AFM을 통해 측정하였다.
