| 摘要: | 為了因應未來電子產品的高功能性以及便利性，封裝方式朝高I/O數發 展已為必經之路。於新型態封裝技術中，覆晶(flip-chip)技術佔有極大之技 術優勢，勢必成為未來封裝技術主流，如何增加覆晶接點之可靠度，也成 為大家所關心的議題。當覆晶銲點的直徑由現今之 100 μm 縮小至未來之 50 μm，且通過銲點之電流為0.02安培時，此時通過銲點之電流密度將高達 103 A/cm2，雖然此一電流密度仍小於銅導線或鋁導線內之電流密度(105-106 A/cm2 )，但直徑為 50 μm 之銲點的臨界電流密度(Threshold current density) 約為103 A/cm2，因而在此電流密度下，電遷移效應已會對銲點的可靠度造 成影響。因此，電遷移現象對於覆晶銲點可靠度之影響值得深入探討。 本研究利用即時(in-situ)金像觀察之方法，發現覆晶銲點於電子流作用 下，因驅動力之不同，有兩種擴散機制存在於銲錫球內部。第一種為電遷 移(electromigration)擴散機制，其驅動力為電子流撞擊銲錫原子造成原子遷 移，其原子擴散方向與電子流方向相同。第二種為晶格擴散(lattice diffusion) 機制，其驅動力為空孔濃度梯度(vacancy concentration gradient)，此機制中， 原子是由低電流密度之自由表面往高電流密度之電流聚集區域(current crowding region)之方向擴散。 本研究亦探討覆晶接點在通電過程中，因電子流撞擊所導致銲錫原子之 電遷移現象以及因銲錫球內之溫度梯度所造成銲錫原子之熱遷移現象。當電子流造成之驅動力與溫度梯度造成之驅動力方向相同時，因兩種驅動力 作用於相同方向，則此時銲錫原子之遷移量較為顯著。反之，當電子流造 成之驅動力與溫度梯度所造成之驅動力方向相反時，因兩種驅動力作用於 相反方向，則此時銲錫原子之遷移量幾乎為零。此外，在考慮熱遷移對於 銲錫原子於通電過程中之飄移速率(drift velocity)之影響後，得到共晶 Sn37Pb銲料之DZ*值約為-3.4× 10-10 cm2/s。Z*值約為-34。 本論文最後提出一種新的覆晶銲點於通電過程中之失效機制。此機制 為：因電遷移之發生導致銲錫球之局部熔融(local melting)。覆晶銲點在施 以電流下，因電遷移效應，於銲錫球之電流聚集區域(current crowding region) 率先生成一缺陷。此缺陷隨著通電時間增加而成長，進而減少了銲錫球與 UBM(under bump metallization)之接觸面積。UBM與銲錫球局部接觸面積的 減少導致銲錫球局部電阻的升高及散熱量之減少。伴隨著焦耳熱(Joule heating)效應之影響，局部電阻的升高會使得局部溫度因此而上升。溫度的 上升進而加速了原子之電遷移速率、缺陷生長、銲錫球與UBM之接觸面積 減小及電阻再度升高，在此惡性循環之下，直至溫度到達銲錫球熔點，最 後，局部熔融現象因而發生。銲錫球之局部熔融現象一旦發生，覆晶銲點 隨即因銲錫球完全融化而失效。 The flip chip technology has been the dominating packaging solution for high performance chips and will remain so in the foreseeable future due to its shorter electrical connection length between the chip and substrate. As the chip complexity increases, the I/O density on each chip also increases. To accommodate the continuing rise of the I/O density, the diameter of the flip chip solder joints must shrink. At present, the diameter of a solder joint is about 100 μm, and it will be reduced to 50 μm soon. It means that the average current density in such a 50 μm joint is about 103 A/cm2 when a 0.02 A current is applied. Electromigration in flip-chip solder joints has become a serious reliability concern when the current density reaches the 103 A/cm2 level, which is about two orders of magnitude smaller than that in Al and Cu interconnects. The reason for this lower threshold current density to cause electromigration in solders has been pointed out to be the combination of several factors in the “critical product” of electromigration, including the higher resistivity, the smaller Young’s modulus, and the larger effective charge of solders. This lower threshold makes electromigration in solders now one of the major reliability threats to microelectronic devices. This investigation studies how electron flow distribution and vacancy concentration gradient affect the diffusion of solder atoms in a flip-chip solder joint under current stress. The migration of materials was traced by monitoring the positions of 21 Pb grains of the eutectic PbSn solder joint. Experimental results indicate that the displacements of the Pb grains were not uniform along in the electron flow direction. Additionally, certain Pb grains exhibited lateral displacements. The non-uniform material migration is attributable to the combined effect of electromigration and the vacancy concentration gradient, which was caused by electromigration. The combined effects of electromigration and thermomigration on material migration were also examined in this study. When the direction of electron flow is the same with temperature gradient, more solder atoms migrate. When the direction of electron flow is opposite with temperature gradient, less solder atoms migrate. Considering the effect of thermomigration in solder bump, the displacements of the Pb grains were measured, and the DZ* value of Sn in eutectic SnPb solder estimated to be -3.4×10-10 cm2/s. The calculated Z* value is about -34. This study also reported that the solder joints failed by local melting of PbSn eutectic solder bump. The local melting occurred due to a sequence of events induced by the microstructure changes of the flip chip solder joint. The formation of a depression in current crowding region of solder joint induced a local electrical resistance increased. The rising local resistance resulted in a larger Joule heating, which, in turn, raised the local temperature. When the local temperature rose above the eutectic temperature of the PbSn solder, the solder joint melted and consequently failed. This result also shows that several points need to be considered when we face the issues of electromigration on reliability of flip chip solder joints. Firstly, the geometry of flip solder joints should be designed to avoid the formation of current crowding region in solder bump. Secondly, in order to resist the microstructure change, the higher mechanical intensity solder need be chose. Thirdly, increasing heat dissipation of solder joint under current stressing or choosing the solder which has higher melting point in order to prevent the melting phenomenon occurred. |
