近年來，為因應電子產品市場朝高密度、高功能及輕、薄、短、小之趨勢發展，一種不需填注底膠(Underfill)之晶圓級構裝(Wafer Level Package, WLP)扮演著關鍵性角色。然由於晶圓級構裝不填注底膠，因此當晶圓級構裝於組裝PCB基板後承受熱循環負載時，焊錫接點可靠度已成為構裝結構主要可靠度問題之一。如何提昇焊錫接點可靠度，亦成為晶圓級構裝設計之關鍵所在。另一方面，錫球剪力測試已被電子業界廣泛採用來評估錫球黏附於先進構裝元件之強度。而低剪力強度的錫球經常於構裝可靠度測試中屬較差之焊錫接點，因此近年來已逐漸要求增加錫球剪力強度。
為解決上述問題，本研究以銅凸塊(Cu Stud)形成於焊錫墊片表面中心位置之設計概念，發展一新型晶圓級構裝結構。同時利用現有半導體製程條件成功的製作出具圓形銅凸塊之焊錫墊片結構，因此所提之新型銅凸塊於製程技術上具可行性。本研究分別建構三維非線性有限單元推球及加速熱循環分析模型，來探討銅凸塊設計對錫球剪力強度及焊錫接點熱疲勞壽命之影響。在推球分析方面，探討銅凸塊之幾何尺寸、形狀和材料性質對錫球剪力強度之影響，並將計算分析所得之剪力與位移曲線圖與實驗結果比較來驗證有限單元推球分析模型之精確性。在加速熱循環分析方面，探討銅凸塊之幾何尺寸、形狀和材料性質與晶片和PCB基板厚度對焊錫接點可靠度之影響，並將分析結果與實驗結果和文獻中之實驗資料比較來驗證有限單元加速熱循環分析模型之精確性。

由有限單元分析與實驗結果比較可知，本研究之模擬分析具一定之可信度。研究結果指出錫球包含適當尺寸之銅凸塊可有效提昇錫球剪力強度。而建構較大尺寸之銅凸塊至晶圓級構裝及PCB基板之焊錫墊片上，可有效提昇焊錫接點熱疲勞壽命，若再減少晶片厚度可進一步提昇焊錫接點可靠度。

此外，本研究也探討共晶錫球與具Au/Ni金屬層之焊錫墊片其接觸面經迴焊及定溫烘烤後之金屬界層成長情形，並探討金屬界層成長對錫球剪力強度之影響。實驗之研究參數包括錫球體積、焊錫墊片直徑及金厚度。金屬界層之成長機制屬於擴散控制(Diffusion-Controlled)，其擴散機構中主要的擴散方式為空位擴散(Vacancy Diffusion)，空位能一個接一個不停地與原子交換位置。本研究之實驗結果指出錫球與焊錫墊片間之Au0.5Ni0.5Sn4金屬界層成長使得接觸界面過於脆弱，造成焊錫接點之剪力強度降低而影響構裝結構可靠度。適當減少金層厚度能降低錫球含金量(Au Weight)及Au0.5Ni0.5Sn4金屬界層厚度，有助於防止錫球剪力強度之衰減。

本研究之結果可作為多種先進球柵陣列型態電子構裝元件之設計參考，以期有助於提昇電子構裝之可靠度。

A wafer level package (WLP) without the underfill layer was introduced in recent years to address the demand of the electronic packaging industry for increased density and performance, as well as lightness, thinness, small size and cost-effectiveness. However, the solder joint reliability under thermal cycling conditions becomes a critical problem when mounting the WLP onto a PCB when the underfill layer has been eliminated. Consequently, enhancing the board level reliability is of primary concern in current WLP design. At the same time, the solder ball shear test has been widely adopted in the electronics industry to estimate the strength of solder ball attachment of advanced electronic packages. A solder ball with low shear strength is usually considered a weak solder joint in package reliability testing. Consequently, demands for increasing the solder ball shear strength have risen in recent years.
In order to solve these problems, this research proposed a new WLP design, based on forming a Cu stud on the center of the surface of the solder pad. The solder pad with a round Cu stud was made using a semiconductor manufacturing process. Therefore, this novel Cu stud design technology is workable. To investigate the impact of a Cu stud on the solder ball shear strength and solder joint reliability, 3-D non-linear finite element models were used for the Cu stud design. In shear analysis, this investigation explored the effects of various parameters including the Cu stud’s dimension, shape, and material properties on the solder ball shear strength. Furthermore, the shear force-displacement curves, obtained by computational analysis, were compared with the experimental results to demonstrate the accuracy of the finite element models. In thermal cycling analysis, this research investigated the effects of various parameters, including the Cu stud’s dimension, shape, material properties and the die and PCB thicknesses on the solder joint’s reliability. To demonstrate the accuracy of the finite element models, the analytical results were compared with the experimental results and the experimental data reported in the literature.

Comparing the experimental data with the results from the finite element analysis revealed that the finite element analysis was reliable. The analytical results established that a suitable size of Cu stud in a solder ball could effectively enhance the ball’s shear strength. Moreover, the solder joint reliability could be significantly improved by forming large Cu studs on the surfaces of the solder pads of WLP and PCB substrate, and could be further enhanced by combining large Cu studs with thin die.

In addition, this research also explored the growth of intermetallic compounds (IMC) under aging for eutectic Sn-Pb solder reflowed on a Cu pad with an Au/Ni surface finish. The effects of the intermetallic layer on the solder ball shear strength were examined for various solder ball sizes, Cu pad sizes and Au layer thicknesses. The IMC growth is dominated by the diffusion-controlled mechanism, in which the vacancy diffusion is the main diffusion mode. The vacancies and atoms can interchange locations continuously. Experimental results indicated that the degradation of the solder ball shear strength was found to be mainly caused by brittle interfacial fracture, due to the formation and growth of the Au0.5Ni0.5Sn4 intermetallic layer. Decreasing the Au layer thickness can reduce the Au weight in the solder and the Au0.5Ni0.5Sn4 thickness, and so avoid the degradation of the solder ball shear strength.

The findings of this research can offer designers and manufacturers an index to adjust the design for advanced ball grid array package to enhance their package reliability.

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