應用於輕量智慧化結構材之材料複合化技術

Abstract

在現今物聯網(IoT)蓬勃發展之際，其應用領域也延伸至汽車產業，產生所謂「車聯網 (Connected Vehicle, CV)」之新概念。為確保駕駛及乘客之安全，各式各樣的電子裝置，如：感應器、致動器…等，需要與車體之輕量化結構材做整合，以感測人體之健康狀況(心跳，呼吸頻率)，並將這些電子信號以無線方式傳送至雲端系統，以便即時作出回應。由於作為電子材料基板與車體結構材的材料種類多樣且跨足有機與無機材料，如：鋁合金，聚醯胺…等；所以學界或產業界皆開始重視並開發將電子裝置與輕量化結構材整合之技術；然而，目前之異質接合技術需要以高溫或高真空環境以達成接合目的，將導致電子裝置之熱機損壞，也增加了製程複雜度。因此，在一般大氣環境且固態狀態下就達成異質接合技術實屬需要。除了異質材料間之整合技術，微電子封裝體在實際工作環境時，考慮到微接點之微結構進展影響電及機械性質等可靠度甚鉅，故影響微接點微結構發展之因素需加以被釐清。 敝人為台大與日本國立物質・材料研究機構(NIMS)共同指導之博士生，在日本兩年間，於重藤博士指導下，利用真空紫外光將乙醇氣體分子分解之手法，成功在大氣壓環境下達成異質材料之固態接合。材料選擇上，吾人選澤廣泛應用於輕量化材料及封裝體內的材料：鋁及聚醯銨做接合，實驗結果顯示受到真空紫外光照射之乙醇分子會在鋁及聚醯胺基板表面，進行分子的重新改組以形成架橋層。本人結合古典熱力學與氣體動力論建構了腔體內的氣體模型，並釐清對於架橋層成長行為相當重要的參數：曝露量(=乙醇密度與照射時間之乘積)。對於鋁，隨著曝露量的增加，鋁表面會產生以雙齒配體之羧酸鹽連結鋁表面之帶有氫氧基之鏈狀烷類；對於聚醯胺，其表面分子以非成對電子吸引受真空紫外光照射轉化而成之未飽和之鏈狀烷類與氫氧基，並進而產生飽和鍵結以形成接枝。吾人在130~150°C下，引發在異種材料表面這些氫氧基產生脫水縮合與加速介面間交互擴散，在數分鐘內完成接合。以穿透式電子顯微鏡(TEM)結合電子能量損失能譜(EELS)觀察接合後介面，實驗結果顯示界面處確實發生界面間擴散而形成具有濃度梯度之界面區域，並在當中形成有機無機之網狀結構之奈米級晶粒。在接合性評估方面，吾人以非對稱懸臂梁測試進行評估，測試結果顯示界面強度為 (2.40 0.36) 103 (J/m2)，此值高於兩邊母材本體破壞能，顯示吾人提出之方法創製出高強度且高韌性之異質介面。 本論文第二部分則敘述在台大材料高振宏教授實驗室所完成之研究成果，旨探討Ni/Sn/Ni微小接點在表面擴散與界面反應引致體積收縮影響下接點微結構與表面形貌之進展。為了釐清兩效應分別對接點微結構的影響，吾人以電鍍方式製作不同高寬比之微接點：其中高寬比最小之微接點其高寬比接近1以模擬實際應用之微接點的尺寸比例。本實驗釐清出受表面擴散與體積收縮效應影響下，焊點微結構進展對於高寬比的依存性：從上下Ni/Sn界面長出之Ni3Sn4層互相碰觸前，影響微結構進展主要因素為錫之表面擴散，其造成初期孔洞形成於錫層最外圍區域，此區域深受表面擴散影響。隨著高寬比縮小，受表面影響區域也逐漸向內收縮，使得最後出現一個臨界高寬比(於本實驗~ 6)，此時錫層內所有區域皆受到表面的影響，孔洞可在錫層中心區域生成。然而，此尺寸相依性只能適用於兩端Ni3Sn4彼此碰觸以前；Ni3Sn4碰觸後，影響微結構進展之主因素轉為反應引致之體積收縮。連續的錫層逐漸被Ni3Sn4層隔離層許多沿著錫層中央線之島狀錫包，而孔洞在這些錫包內生成。此外，本實驗利用電子微探儀(EPMA)與能量散佈光譜儀(TEM-EDS)觀察並鑑定出目前Ni-Sn相圖未標示的相：NiSn4－此相剛鍍好時就已存在於錫層中，板狀形貌，大致垂直於Ni/Sn界面成長。NiSn4持續存在直到上述之島狀錫包被完全消耗，才相轉變為破爛板狀之Ni3Sn4。

The Internet on Things (IoT) is becoming one of the inevitabilities of automotive industry, which creates the concepts of connected vehicles. In this, an unavoidable task for connected vehicles is to ensure human safety. Thus, numerous electronic packages such as sensors and actuators have to be attached to lightweight structural materials to detect interfacial failure promptly. Given that there are a diversity of materials common both to lightweight structural materials and flexible electronic devices, hybrid integration among these materials must be achieved to realize seamless signal transmission via the vehicles. Existing bonding strategies employ high vacuums or high temperatures, which, however, causes process complication and material deterioration. Therefore, a method to achieve hybrid bonding at solid state and ambient atmosphere is necessary. Other than the bonding between electronic packages and structural materials, joint reliability of solders in electronic packages must be considered in practical applications. In particular, solder surface diffusion and reaction-induced volume shrinkage are important factors in microstructural evolutions in micro joints. These two factors should be comprehensively studied. In the first part, a hybrid bonding method at low temperature and ambient atmosphere was proposed for aluminum (Al)-polyimide (PI) bonding. A vacuum ultraviolet (VUV) with wavelength of 172 nm was employed to dissolve ethanol vapor contained in nitrogen atmosphere to create radical species for multiple surface modification effects simultaneously. This method has been named ethanol-assisted vacuum ultraviolet (E-VUV) irradiation. By combining the thermodynamics and kinetic theories of gases, we established a calculation model to estimate the amount of exposure of ethanol (), which was a key parameter to govern the growth behavior of the E-VUV-induced bridging layers. The E-VUV process developed hydroxyl-carrying alkyl chains through coordinated-bonded carboxylate onto Al, whereas created numerous hydroxyl-carrying alkyls onto PI. Triggering dehydration through those hydroxyls by heating merely at 130-150°C for a few of minutes produced organic-inorganic reticulated complexes within the Al/PI interface. Finally, the interfacial toughness was evaluated by using an asymmetric double cantilever beam (ADCB) method. The results revealed that the fracture energy of the Al/PI interface averaged (2.40 0.36) 103 (J/m2) and was much higher than that of the Al and PI matrix, which implied that the E-VUV process was able to create a robust and tough hybrid interface. As for micro joints, we studied the microstructural evolution in Ni/Sn/Ni micro joints and revealed a conformable tendency among different joint sizes under the influence of volume shrinkage and surface solder diffusion. Before the Ni3Sn4 growth in opposing directions impinged on each other, the main factor contributing to the microstructures evolution was the surface Sn diffusion, resulting in the necking of the Sn layer and the initiation of voids near the periphery of the Sn layer. With the joint size decreasing, the surface-affected region shrunk. Finally, there was a critical size (30 μm in this study) in which all of the Sn was located in this region, and the voids directly formed in the central region of the Sn layer. However, the joint-size dependency remained tenable until the Ni3Sn4 growing from opposite interfaces started to impinge on each other. After the Ni3Sn4 impingement, the main factor shifted to the volume shrinkage induced by the interfacial reaction. At this stage, the continuous Sn layer was isolated into small pockets, where voids initiated and gradually forming along the centerline of the joints. With the help of the joint-size dependency proposed in this study, it is expected that the dimensional design of micro joints can be optimized for high reliability.