純銀與高銀合金帶材對不同基板之超音波接合

Ultrasonic Bonding of Pure Ag and High Silver Alloy Ribbons on Various Substrates

張恆銘(Heng-Ming Chang)

指導教授 ： 莊東漢

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201901136

摘要

電動車發展成為主流，使用之功率電子模組其工作環境高電壓低電流，又需符合車用規範的高可靠度要求。帶材截面積大，具備良好的散熱條件，集膚效應使得帶材電阻更低，為功率封裝模組的潛力材料。銀電阻率低，具有極佳的電熱傳導效率，耐蝕性與抗氧化性佳。

本研究分成線材與帶材兩個部分。線材使用純銀線材進行超音波接合試驗，測試不同基板之接合情形，並觀察界面、拉線強度以及破壞模式。帶材則分別以純銀帶材與高銀合金帶材進行超音波接合試驗，測試DBC基板與墊材，針對可接合者進行界面觀察與帶材拉斷強度測試，並觀察接面印痕與分析破壞模式；不可接合者探討其原因。後續進行高溫儲存實驗，測試溫度150℃，持溫1000小時，比較原始銲點與高溫儲存之銲點界面、拉線強度、破壞模式以及印痕等變化。綜合研究結果，評估純銀帶材與高銀合金帶材於基板與墊材超音波接合可行性。

線材之研究結果純銀線材可接合於銅鎳金基板與銅鎳鈀金基板，形成之銲點外觀完整，接合界面良好。拉線強度以後者較大，但差距不明顯，破壞模式皆集中於界面。接合測試中，第一與第二銲點接合參數不同，與線弧幾何關係有關。

帶材之研究結果在超音波接合參數調整上，基板表面相對硬度越高或者帶材厚度越厚，所需提供的超音波能量越大。不同於線材之楔型接點印痕，帶材之印痕為Dog-bone Structure，且能由印痕之破壞模式佐證強度。因為金與銀的互溶性與潤濕性，基板表面金層有助於改善連續銲點接合良率。

研究發現純銀帶材可接合於銅鎳金基板、銅鎳鈀金基板、鋁墊及鎳墊。接合良率以銅鎳金基板最佳，大約90%。原始銲點外觀與接合界面兩基板及鎳墊皆良好。帶材拉斷強度測試結果兩基板可達700g以上，但以銅鎳鈀金基板較高。經高溫儲存後，銅鎳金基板界面無明顯變化；銅鎳鈀金基板則出現較多裂孔，前者強度變化不明顯，但後者強度略為上升。兩基板高溫儲存可靠度均佳。

高銀合金帶材可接合於銅鎳鈀基板、銅鎳金基板、銅鎳鈀金基板、鋁墊及鎳墊。銅鎳金基板接合良率與銅鎳鈀金基板接近，都能達到70%，顯見其作業性；銅鎳鈀基板則較低。三者之原始與高溫儲存後之銲點外觀與接合界面良好。高溫儲存後，銅鎳金、銅鎳鈀金兩基板都有強度提升的現象，並以後者之原始強度與上升幅度都較高。考慮接合良率、界面品質、強度與高溫儲存可靠度，高銀合金帶材以銅鎳鈀金基板進行超音波接合最具競爭力。

關鍵字

超音波接合 ； 純銀線材 ； 銀合金帶材 ； 高溫儲存試驗 ； 印痕

並列摘要

The electric vehicles has become popular. The power electronic modules have to work in the severe environment, such as high voltage and low current. They must follow the strict reliability requirements of automotive specifications. Ribbons, with large cross-sectional area and superior heat dissipation ability, turn out to be a potential choice to meet the needs for the packaging of power modules. The other advantage of ribbons is the low resistance coming from skin effect. Silver has low resistivity, excellent heat and electrical conduction efficiency. The corrosion resistance and oxidation resistance of silver are well-known.

There are two parts of this study: wire and ribbons. First, in the part of wire, the bondability of pure silver wire on various substrates has been tested with the implement of ultrasonic bonding technique. The interface, the wire pull strength and failure mode were observed. Second, in the part of ribbons, the studied materials are pure silver ribbons and high silver alloy ribbons, respectively. We tested bondability of the ribbons with several kinds of DBC substrates and pads throuugh ultrasonic bonding method. The result can be classified into two parts: the one that can successfully be bonded and the other. Interface observation, ribbons pull strength test, footprints and failure mode were investigated for those that can form bonding. For those cannot form bonding, we must studied what reduce the bondability. Subsequently, high temperature storage test, with 150℃, 1000 hours, were conducted. Then we focused on the variation before and after high temperature storage test.Based on the results, we may evaluate whether it is feasibility to bond pure silver ribbons and high silver alloy ribbons on substrates and pads with ultrasonic bonding method.

For the result of wire, pure silver wire is bondable with copper-nickel-gold substrates and copper-nickel-palladium-gold substrates. The structure of first bond and second bond are both complete in appearance. In addition, the interface is continuous and flat. Wire bonds with copper-nickel-palladium-gold substrates are a little stronger than the other in the wire pull test. However, the difference is not obvious. The failure mode of both locates on the interface. The bonding parameters of first bond and second bond may not be the same since the geometry of the wire loop may needed to take into consideration. The surface contamination plays an important role while conducting continuous wedge bonding.

For the result of ribbons, during ultrasonic bonding parameter adjustment, it is discovered that the higher the relative hardness of the substrate or the thicker of the ribbons, the greater the ultrasonic energy required. Nevertheless, unlike wire, the footprints of ribbons is dog-bone structure. The result of pull strength test can be evidenced by observation failure mode on footprints. At last, since the high solubility and wettability of gold and silver, gold layer on the substrates may improve the continuous wedge bonding yield.

The study found that pure silver ribbons can be bonded with copper-nickel-gold substrates, copper-nickel-palladium-gold substrates, aluminum pads and nickel pads. Pure silver ribbons that bond with copper-nickel gold substrates has the best bonding workability. And both substrates have good bonding quality. The bonds are complete in appearance while the bonding surfaces are flat and continuous. The pull strength of both substrates can reach 700g and even more. However, the copper-nickel-palladium-gold substrate is a little higher. After high temperature storage test, the interface of copper-nickel-gold substrate didn’t change significantly while the copper-nickel-palladium-gold substrate grew more cracks. For the pull strength, both of these substrates didn’t show drastically change after high temperature storage test. But the strength of copper-nickel-palladium-gold substrate did increase slightly. The result of high temperature storage test gave us the evidence that both of them have high long-term reliability.

The high silver alloy ribbons can be bonded to copper-nickel-palladium substrates, copper-nickel-gold substrates, copper-nickel-palladium-gold substrates, aluminum pads, and nickel pads. The workability of the copper-nickel-gold substrate is close to the copper-nickel-palladium-gold substrate, while the copper-nickel-palladium substrate is relatively low. The bond appearance and interface of these three substrates are superb. After high temperature storage test, the copper-nickel-gold substrate and copper-nickel-palladium-gold substrate became stronger. The as-bonded strength of copper-nickel-palladium-gold substrate is greater while the strengthen rate of it is also the greater one. The reliability of both substrates is excellent. To evaluate which of them can be more competitive in the markets, taking workability, interface quality, pull strength and high temperature storage reliability into account, high silver alloy ribbons with copper-nickel-palladium-gold substrates might be the answer.

並列關鍵字

ultrasonic bonding ； pure silver wire ； silver alloy ribbons ； high temperature storage test ； footprints

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