J Fail. Anal. and Preven. (2013)13: 194-201 the coarse topography, we can also indentify crystalline oxide(Fig. 7)whose stoichiometry is Ni2gO7 according to interface cracking as labeled in Fig. 3d. It can also be Chong [4], or as Lee [5] proposed in his work, the nickel estimated that the cracking length is about 3-4 um oxide displayed stoichiometry of NiO2 in the outermost The region marked by a red square on failed pad was part and Nio in the inner part. No matter how different the chosen to be analyzed by EDS. The chemical composition stoichiometry of nickel oxide is, it ultimately acted as an and corresponding weight percentage is given in Fig. 4. As obstacle and blocked off the tin solder from contacting the it illustrates, four detected elements are Ni, Au, P, and Sn. gold layer under it during hot air solder leveling. Since And the weight percentage is 82.33, 13.60, 2.3, and 1.69%0, nickel oxide is a relatively stable substance, the desmear respectively. It is reasonable to find Ni and Au, since the before soldering could hardly play any role. This explains surface finish is ENG. The existence of P is acceptable why de-wetting still could not be avoided even after careful because phosphorus was introduced in during chemical desmear had been implemented before soldering plating of nickel. The little quantity of Sn on failed pad As revealed by the SEM result(Fig. 3d), the existence ould be explained as follows: although the pad was not of crystalline interface cracking acted as a"Grand well-wetted, there was still some tin solder residue on it Canyon", thus aggravated this mechanism by exposing through hot air solder leveling more underlying nickel atoms to oxygen in the environ- In order for comparison, EDS analysis was also utilized ment. Moreover, through"Grand Canyon", the nickel on a section of a normal (well-wetted) pad of the same atoms lied below diffused upward through the canyon to sample as show in Fig. 5. Likewise, the information of the outermost surface, covering the gold layer and forming chemical composition and weight percentage were a sandwich structure finally. Then the external nickel layer revealed. From Fig. 5, we mainly found Sn with little was oxidized and formed a barrier between tin solder and quantity of C and O. This result confirmed us that solder gold layer during hot air solder leveling later. The process used for joint was pure tin and C and o were organic described above was demonstrated in Fig 8. contamination from the surrounding environment which Besides that, organic substance which is highly volatile accidentally adhered to tin bump during storage from the depositary environment was absorbed into the According to the formula, one nickel atom will be canyon and covered the nethermost nickel layer, acting as replaced by two gold atoms. The radius of nickel and gold an isolating layer between solder and nickel and led the atom is 1. 24 and 1.44 A, respectively, thus there is an un-wetting during soldering. Since canyon had a consid- atomic radiuses difference of 16% between Ni and Au. The erable absorption capacity and as proved by Fig. 5 that the synergy of the two factors above left the ENIG finish a quantity of organic contamination in storage circumstance rough bumpy surface full of pits and holes as illustrated in could not be neglected, this mechanism was equally pos- Fig. 6. These pits and holes then exposed the nickel sible for the poor wetting performance. Figure 9 schemed straight to oxygen in the air and caused it to form nickel out the process. However, it is necessary to be put out that c:edax32genesis genmaps spc 08-Jul-2011 15: 33:35 71 KEnt 002.004006.008.0010.001200140016.001800 Energy.kev Element Ni Au P Wt%82.3313.602.381.69 Fig. 4 EDS result of the un-wetting pad (a) The un-wetting pad, (b) chemical composition, and (e) weight percentage of main elements Springthe coarse topography, we can also indentify crystalline interface cracking as labeled in Fig. 3d. It can also be estimated that the cracking length is about 3–4 lm. The region marked by a red square on failed pad was chosen to be analyzed by EDS. The chemical composition and corresponding weight percentage is given in Fig. 4. As it illustrates, four detected elements are Ni, Au, P, and Sn. And the weight percentage is 82.33, 13.60, 2.3, and 1.69%, respectively. It is reasonable to find Ni and Au, since the surface finish is ENIG. The existence of P is acceptable because phosphorus was introduced in during chemical plating of nickel. The little quantity of Sn on failed pad could be explained as follows: although the pad was not well-wetted, there was still some tin solder residue on it after been through hot air solder leveling. In order for comparison, EDS analysis was also utilized on a section of a normal (well-wetted) pad of the same sample as show in Fig. 5. Likewise, the information of chemical composition and weight percentage were revealed. From Fig. 5, we mainly found Sn with little quantity of C and O. This result confirmed us that solder used for joint was pure tin and C and O were organic contamination from the surrounding environment which accidentally adhered to tin bump during storage. According to the formula, one nickel atom will be replaced by two gold atoms. The radius of nickel and gold atom is 1.24 and 1.44 A˚ , respectively, thus there is an atomic radiuses difference of 16% between Ni and Au. The synergy of the two factors above left the ENIG finish a rough bumpy surface full of pits and holes as illustrated in Fig. 6. These pits and holes then exposed the nickel straight to oxygen in the air and caused it to form nickel oxide (Fig. 7) whose stoichiometry is Ni29O71 according to Chong [4], or as Lee [5] proposed in his work, the nickel oxide displayed stoichiometry of NiO2 in the outermost part and NiO in the inner part. No matter how different the stoichiometry of nickel oxide is, it ultimately acted as an obstacle and blocked off the tin solder from contacting the gold layer under it during hot air solder leveling. Since nickel oxide is a relatively stable substance, the desmear before soldering could hardly play any role. This explains why de-wetting still could not be avoided even after careful desmear had been implemented before soldering. As revealed by the SEM result (Fig. 3d), the existence of crystalline interface cracking acted as a ‘‘Grand Canyon’’, thus aggravated this mechanism by exposing more underlying nickel atoms to oxygen in the environ￾ment. Moreover, through ‘‘Grand Canyon’’, the nickel atoms lied below diffused upward through the canyon to the outermost surface, covering the gold layer and forming a sandwich structure finally. Then the external nickel layer was oxidized and formed a barrier between tin solder and gold layer during hot air solder leveling later. The process described above was demonstrated in Fig. 8. Besides that, organic substance which is highly volatile from the depositary environment was absorbed into the canyon and covered the nethermost nickel layer, acting as an isolating layer between solder and nickel and led the un-wetting during soldering. Since canyon had a consid￾erable absorption capacity and as proved by Fig. 5 that the quantity of organic contamination in storage circumstance could not be neglected, this mechanism was equally pos￾sible for the poor wetting performance. Figure 9 schemed out the process. However, it is necessary to be put out that Fig. 4 EDS result of the un-wetting pad. (a) The un-wetting pad, (b) chemical composition, and (c) weight percentage of main elements J Fail. Anal. and Preven. (2013) 13:194–201 197 123