底膠材料熱機械性質對低介電覆晶球狀陣列封裝體之熱變形行為研究

Abstract

隨著半導體製造技術的發展，積體電路公司引進低介電材料與銅製程用以減輕電阻電容延遲效應。但是，低介電材料較之傳統的二氧化矽介電質有著較差的材料機械性質與附著性。這將使得低介電材料層發生破裂的風險大為增加。此外，由於環保意識的抬頭，無鉛焊料已逐漸取代傳統的錫鉛合金。然而，無鉛焊料需要更高的迴流溫度（Reflow temperature）以及容易與銅墊產生界金屬化合物（inter-metallic compounds, IMCs）。這些因素都將降低無鉛焊料覆晶封裝製程產品的可靠度。因此，針對具有低介電材料的覆晶球狀陣列封裝體，如何調配其底膠的機械性質以增加其可靠度是目前研發工作的主要課題。 在本研究中，吾人利用高解析雲紋干涉儀去量測並比較二種不同底膠材料的熱機械變形。該儀器解析度可達26nm，足以充分觀測底膠與錫球間的熱變形行為。根據量測的結果，具有較高彈性模數的底膠材料會造成較大的晶片翹曲與較低的錫球剪應變。此外，我們亦藉由熱循環測試（Thermal Cycling Test, TCT）評估了包含不同底膠與焊料合金的六種試片之可靠度。測試的結果指出了高鉛焊料（Sn95Pb）及無鉛焊料（Sn0.7Cu）較傳統的錫鉛共晶焊料（Sn37Pb）可靠度來的差。因此需要機械性質較強的底膠材料來提供錫球足夠的保護。 另外，我們利用ANSYSTM商用軟體建立了一個簡化的三維有限元素模型。此模型預測的晶片翹曲度與雲紋干涉儀所量測的數據差異小於5%。而且此模型針對上述六種試片所預測的應力趨勢亦與熱循環測試實驗有著極佳的相關性。 最後，我們以有限元素分析法來找出應用於低介電覆晶封裝體的最佳底膠機械性質。底膠的熱膨脹係數、彈性模數與玻態轉換溫度是影響封裝體可靠度的三大主要因素。根據模擬的結果，較低的底膠熱膨脹係數能同時降低錫球與低介電層產生裂縫的風險。倘若底膠具有較高的彈性模數，雖然可以提供錫球較佳的保護，但是將會導致低介電層的應力升高。由於上述二個底膠的材料參數在環境溫度超過了玻態轉換溫度後會有急遽的變化，因此玻態轉換溫度在底膠的材料性質中亦扮演一個關鍵的角色。綜上所述，針對低介電覆晶封裝體我們建議使用具有適中的彈性模數、低熱膨脹係數以及高玻態轉換溫度的底膠材料。

As the advancement of semi-conductor manufacturing technology, the IC manufacturers introduced Cu and low-K dielectric to relieve the resistance-capacitance delay (RC delay) issue. Unfortunately, the low-K dielectric material possessed weaker mechanical strength and adhesion than the traditional SiO2 dielectric, leading to increased delamination potential for low-K layer. In addition, due to the rise of environmental awareness, the conventional eutectic solder alloy Sn37Pb was gradually replaced by lead-free alloys. However, lead-free alloys required higher reflow temperature and the alloys would form Cu-Sn inter-metallic compounds with Cu pads readily, which may degrade the reliability of lead-free packages. Therefore, how to formulate the thermal and mechanical properties of underfill materials to meet the reliability requirements for low-K flip-chip ball grid array (FC-BGA) packages is one of the critical tasks in current research and development of flip-chip technology. In this study, a high resolution Moiré interferometry (resolution up to 26 nm) was employed to measure and compare the thermo-mechanical deformation of two types of underfill materials. It could provide sufficient sensitivity to observe the thermal deformation behavior in bump/underfill layer. Based on the measurement results, the underfill material with higher elastic modulus induced larger die warpage and lower bump shear strain. In addition, we also evaluated the reliability of six FC-BGA package samples involving different underfill and solder alloys by thermal cycling tests (TCT). The TCT results indicated the package samples with high lead (Sn95Pb) and lead-free bump (Sn0.7Cu) had worse reliability than conventional Sn37Pb bump. Thus, the underfill with more rigid mechanical properties is required in order to protect solder bumps. Furthermore, a simplified three-dimensional finite element model by ANSYSTM was also established. The die warpage difference between Moiré interferometry measurement and simulation was less than 5%. The stress simulation results by the finite element model also correlated well with aforementioned TCT results. Finally, finite element analysis (FEA) was employed to find out the optimal mechanical properties of underfill material for low-K FC-BGA packages. The coefficient of thermal expansion (CTE), elastic modulus (E) and glass transition temperature (Tg) of underfill were the three major material properties which directly affected the reliability of FC-BGA packages. Based on FEA results, the underfill with lower CTE would decrease both stress on bumps and low-K layer. For the underfill with higher elastic modulus, it would enhance the bump protection, but induced higher stress in low-K layer. Since CTE and elastic modulus of underfill material would change drastically while environmental temperature exceeded its Tg temperature, Tg temperature was a critical materials property of underfill material. In summary, the underfill material with moderate elastic modulus, low CTE and high Tg temperature was recommended for low-K FC-BGA packages.