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Material Characteristic of Electronic Packaging Ag-alloy Bonding Wires under Current Stressing
Ion-migration,Ag alloy wire,Electromigration,Fusing currennt,Water drop test,
|Publication Year :||2017|
離子遷移是採用水滴試驗法進行，研究結果發現銀線受到電解後由晶界開始釋出銀離子，部份銀離子往負極與電子結合還原成銀沉積，連續沉積的銀形成銀鬚受正電場吸引往正極連接造成短路；另一部分銀離子與氫氧鍵結合形成Ag2O於正極沉積。第二部份的研究發現，粗晶與細晶銀合金線的銀離子遷移率分別為7.90μm/s及9.22μm/s，由於銀合金的電解是於晶界進行，粗晶銀合金線的孿晶比例較高且外圍晶粒較大，並因孿晶界的低界面能減緩電解速度；然而當銀合金線的晶粒較小時，代表有較多的晶界可進行分解，因此細晶銀合金線將溶解出大量的銀離子於溶液中，而晶粒細的材料晶界較活性，能提供銀離子還原時成核位置。為瞭解環境對銀合金的影響，我們將去離子水改變為3.5 wt% NaCl 溶液進行離子遷移試驗，結果發現銀鬚完全沒有成長或跨接於正負極兩端，此現象說明由於Ag+與Cl–離子於正極形成AgCl，導致銀離子無法遷移至負極還原沉積；另一方面，若使用矽膠保護銀合金線時，即使線材浸泡於水中達1000小時也完全不會有離子遷移效應發生。
在電遷移研究中Ag-3Pd及Ag-4Pd於1.23x105 A/cm2電流密度下平均壽命分別為78000及73391分鐘；而相同電流密度下銅線及鍍鈀銅線受氧化影響僅有10 ~ 60分鐘的壽命，此結果顯示氧化問題造成銅線的平均壽命比銀合金線低1000倍；為了避免線材受到大氣氧化，當銀合金及銅線覆蓋上矽膠後平均壽命提高至74000分鐘以上，因電遷移所產生的焦耳熱透過載玻片傳導散失，並加上被矽膠覆蓋後減少氧化問題發生，減緩晶粒因焦耳熱效應快速成長。
銀合金線隨線徑增加熔斷電流由0.37A增加至0.60A，主要是因為線材截面積增加所能夠承受的電流密度也隨之提升，而當Pd含量提升1 wt%，各種線徑的熔斷電流承載能力皆下降5 ~ 8%，進一步將Ag-3Pd添加Au 元素後，所有線徑的熔斷電流大幅下降14 ~ 20%，因電阻率隨合金元素的添加而增加，導致線材需承受的總功率上升，而線材受焦耳熱效影響無法快速散熱導致線材熔斷。
The positive characteristics of Ag alloy wire include low resistivity, low hardness, high ductility, and high thermal conductivity, all of which combine to make it one of the most popular bonding wires. However, Ag-ion migration causes electronic devices to short circuit and fail in conditions of high humidity. Therefore, the first part of this study will focus on the mechanism of ion-migration in silver alloy wires. With the understanding of the mechanism, we will discuss the effects of ion-migration on silver alloy wire by varying the alloying elements, grain sizes, and environments. In the last part of study, the electro-migration and fusing current characteristics of silver alloy wires will be discussed.
Ag ion migration has been examined using the water drop test. The results showed that the silver wire is electrolyzed at the grain boundary, releasing silver ions. Some of these Ag+ ions migrate from the anode toward the cathode and are reduced to Ag at localized sites on the cathode. A short circuit occurs between the two Ag alloy wires due to the bridging of the Ag dendrites. Some of the Ag+ ions encounter OH－ ions to create AgOH products, which then decompose on the anode to form Ag2O deposits. The second part of the study will focus on the Ag ion migration rates of coarse-grained and fine-grained Ag alloy wires, 7.90μm/s and 9.22μm/s respectively. Since the silver wire is electrolyzed at the grain boundary, coarse-grained Ag alloy has higher twin percentages and the external grains are larger. The low energy of the twin boundaries can obstruct the electrolyzation at the grain boundary. However, when the grains of the silver alloy wire are fine, the greater number of grain boundaries allows faster decomposition. A large amount of silver ions dissolve in the solution because the fine grain boundaries are more active, providing positions for silver ion reduction nucleation. To understand the effects of the environment on silver alloy, we changed the distilled water to 3.5 wt% NaCl solution for migration tests. The results showed that no short circuit resulted from the Ag+ ions encountering Cl－ ions to create AgCl because the silver ions cannot migrate to be negatively reduced. On the other hand, Ag-alloy wires sealed with silicone gel and stressed in distilled water revealed no dendrites after more than 1,000 hr.
In the study of electromigration, during electrical stressing at a current density of 1.23x105 A/cm2, the mean-times-to-failure (MTTF) of Ag-3Pd and Ag-4Pd were 78,000 min and 73,391 min respectively. In contrast, under the same current density, copper and Pd coated Cu wire have only 10 to 60 minutes of life due to oxidation; in fact, the MTTF of copper wire is 1,000 times lower than that of silver alloy wire. In order to prevent oxidation, Ag-alloy wires and copper wires were sealed with silicone gel and stressed at a current density of 1.23x105 A/cm2. The MTTF increased to 74,000 minutes or more; electromigration caused by heat transmission through glass loss and the use the silicone gel to reduce oxidation can slow the grain growth due to heat.
The fusing current characteristics of silver alloy wires increased from 0.37A to 0.6A with the increased wire diameter, because the larger cross-sectional area of the wire can withstand the increased current density. The current-carrying capacity of each Ag alloy wire is reduced by 5 to 8% by each increase in Pd content of 1 wt%. With the further addition of Au to Ag-3Pd wire, the wire diameter of the fusing current drops by 14 ~ 20%. This increase in resistivity results from the addition of alloying elements, because the Joule thermal effects cannot quickly dissipate by total power rise of ag alloy wire, and cause the wire fuses.
|Appears in Collections:||材料科學與工程學系|
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