覆晶接點於承載電子流下之失效機制研究

Study of the Failure Mechanisms of Flip Chip Solder Joints under Current Stressing

林彥良(Yen-Liang Lin)

指導教授 ： 高振宏 ； 劉正毓

國立中央大學/工學院/化學工程與材料工程學系/博士(2007年)

摘要

近年來，隨著電子產品微小化且具備更強大功能之需求，封裝方式也有明顯的改變，封裝形態朝著高I/O 密度封裝發展。於新型態封裝技術中，覆晶（flip-chip）封裝技術除了提供更高的I/O 密度外，更具有較佳的訊號傳遞速度(low RC delay)的技術優勢，目前已經成為高階封裝技術最佳選擇。然而在微小化的過程中，銲點將承受較以往更高的電流密度，進而導致銲點內部發生電遷移現象。覆晶封裝之銲點直徑目前約為100μm，未來將縮小到50μm。以直徑50μm 之銲點而言，當通過銲點之電流為0.2 安培（A）時，此時通過銲點之電流密度將高達103A/cm2。相較Cu 或Al interconnect 內之電流密度(105-106 A/cm2)，此一電流密度雖然較小，但是已經達到50 μm 銲料本身的臨界電流密度，足以引發電遷移效應。因此，電遷移效應將成為影響銲點的可靠度要素之一。 本論文著重於覆晶封裝試片在承受電流下所發生的失效機制之研究。文中將針對衍生的失效機制做深入探討，並提出在此一通電條件下，延長覆晶銲點壽命可行性方法。 首先，藉由微結構變化的觀察，我們提出了一個新的失效機制。此一失效機制中，電流叢聚（current crowding）效應與衍生的局部焦耳熱效應（local Joule heating）首先導致位於晶片端電子流入口處的局部Ni UBM 被快速的消耗殆盡。一旦Ni UBM 被消耗殆盡，此一區域將成為一非導電的區域而電子流因而轉向流入鄰近的區域。因此鄰近區域變成電子流入口處，此一區域的Ni UBM 又將被消耗殆盡。通電的過程中，此一現象將不斷的重覆，使得非導電面積不斷擴大。一旦非導電區域面積幾乎橫跨了整個銲點與導線的界面時，銲點內部將成為斷路而失效。 為了延長銲點通電時的壽命，本文針對了覆晶銲點搭配不同NiUBM 厚度與覆晶銲點搭配不同基板表面處理層做進一步研究。在搭配不同的Ni UBM 厚度研究中，選用了0.3，0.5，0.8μm 三種不同的 NiUBM 厚度。結果顯示，覆晶銲點搭配0.8 μm Ni UBM 的平均失效時間最長，超過了1000 小時。微結構變化的觀察也指出了銲點搭配較厚的Ni UBM 將有較長的平均失效時間。另一個銲點搭配不同的基板表面處理層對於覆晶銲點可靠度的影響之研究中， 基板端選用了Au/Nisurface finish 與OSP/Cu 兩種結構。銲點搭配OSP/Cu 的平均壽命是銲點搭配Au/Ni surface finish 的平均壽命的六倍（ 3080 v.s. 530 小時）。此一研究中顯示了決定銲點平均壽命的因素-為Ni 的消耗速率。對於搭配OSP/Cu 基板的銲點，Cu 原子將由基板端釋放出並藉由交互反應減緩晶片端Ni UBM 的消耗，進而延長了銲點的壽命。 此外， 本文也進一步探討覆晶銲點失效時造成銲料重熔（ remelting）的機制。藉由比較實驗結果與3-D 熱電模擬的結果，我們發現到當Ni UBM 幾乎被消耗殆盡且被非導電性的多孔結構取代時，剩餘導電界面的區域電阻驟升，產生了過量的Joule heating 將導致銲料熔化。

關鍵字

失效機制 ； 電遷移 ； 銲點 ； 覆晶

並列摘要

This dissertation investigated the reliability of eutectic PbSn flip chip solder joints under applied current of 0.32 A at 150°C. It includes the discussion of the failure mechanisms and two feasible methods for prolonging the lifetime of solder joints.
We propose a new failure mechanism in this dissertation. Microstructure examinations uncovered that the combined effect of current crowding and the accompanying local Joule heating accelerated the local Ni UBM consumption near the point of electron entrance. Once Ni was depleted at a certain region, this region became non-conductive and the flow of the electrons was diverted to the neighboring region. This neighboring region then became the place where electrons entered the joint, and the local Ni UBM consumption was accelerated. This process repeated itself, and the Ni-depleted region extended further on, creating an ever larger non-conductive region. The solder joint eventually failed when the nonconductive region became too large, making the effective current density very high.
To prolong the lifetime of solder joints under current stressing, two kinds of experimental setups were used. One is to experiment the solder joints with different Ni UBM thickness. In this part, three different Ni thicknesses in the Cu/Ni/Al UBM (0.3, 0.5, and 0.8 μm) were used in order to investigate the effect of the Ni thickness on the reliability. The solder joints with 0.8μm Ni UBM have the longest lifetime more than 1000 hours. The failure model is also able to support the observation that the joints with a thicker Ni tended to have a longer average lifetime.
The other is to experiment the solder joint with different UBM and surface finish combinations. In this part, two substrate surface finishes, Au/Ni and organic solderable preservative (OSP), were used to study the effect of the surface finish on the reliability of flip-chip solder joints under electromigration at 150 oC ambient temperature. It was found that the mean-time-to-failure (MTTF) of the OSP joints was six times better than that of the Au/Ni joints (3080 vs. 530 hrs). The key factor determining the MTTF was the Ni consumption rate. The joints with the OSP surface finish had a longer MTTF because Cu released from the substrate was able to reduce the Ni consumption rate.
Besides the study of the reliability of flip chip solder joints under current stressing, we also discuss the existence of melting solder at the last stage of current stressing. By microstructure examination and 3-D coupled thermoelectric simulation, it was found that due to the fact that Ni UBM has been completely consumed and almost replaced by the non-conductive porous structure that the local resistance abruptly raised at the region where there is almost no conduct area and caused excessive Joule heating generation to melt solder joint.

並列關鍵字

failure mechanism ； electromigration ； solder ； flip chip

參考文獻

[BLA01] J. R. Black, IEEE T. Electron Dev., ED-16, 338 (1969).

[BLE02] I. A. Blech and C. Herring, Appl. Phys. Lett., 29, 131 (1976).

[BLE03] I.A. Blech and K.L. Tai. Appl. Phys. Lett, 30, 387(1977).

[CAL] W. D. Callister, Jr., Materials Science and Engineering an Introduction, 5th ed. Wiley, New York, 2000!, Chap. 6, Table 6.1, and Appendix B, Table B.2.)

[CHU] Y. C. Chuang and C. Y. Liu, Appl. Phys. Lett., 88, 174105 (2006).