不同金屬的電偶腐蝕

{0>Not all metals play well together.<}0{>並非所有的金屬都能夠相互相容得很好。<0}{0>And this spills over in to the world of electronics and the materials used therein.<}0{>這些涉及到電子品的世界及在那裏使用的材料。<0}{0>A relatively new and relevant example came about with the advent of RoHS and the desire to identify a "drop-in" replacement for SnPb(Ag) solders.<}0{>一個相當新的及相關的例子是RoHS的出現以及“直接”替代元件SnPb(Ag)焊錫膏的願望。<0}{0>A eutectic blend of Sn (tin) and Zn (zinc) was very attractive as a possible replacement.<}0{>共熔混合Sn(錫)和Zn(鋅)作為一種可能替代品很有吸引力。<0}{0>91Sn 9Zn is eutectic, does not contain any RoHS materials, and melts at 199C, only 16 degrees higher than eutectic SnPb solder and ~20 degrees lower than common SAC blends, and does not contain any expensive precious metals.<}0{>91Sn9Zn是共熔的，不含任何的RoHS材料，在199℃下熔融，只比共熔的SnPb焊錫膏高出16℃，比SAC混合物低~20℃，不含任何貴重的金屬。<0}

{0>But the alloy contains Zn.<}0{>但合金含有Zn。<0}{0>And that is a show-stopper for two reasons.<}0{>那樣會停止工作，原因有兩個。<0}{0>Zn is quite reactive, making it very difficult to create a flux-vehicle that would allow for a stable solder paste (possess an appreciable shelf life).<}0{>鋅的反應性活躍，使它很難產生一種適用於穩定焊錫膏(有明顯的使用壽命)的助焊劑載體。<0}{0>The second, the one which we will go into detail in this post, is the electrode potential of Zn.<}0{>其次，我們在本帖子裏詳細研究的是鋅的電極電勢。<0}

{0>The greater the difference in electrode potential between metals, the higher the likelihood that galvanic corrosion will occur.<}0{>金屬之間的電極電勢差異越大，發生電偶腐蝕的可能性越大。<0}{0>The electrode potential of Cu (copper) is +0.334V.<}0{>Cu(銅)的電極電勢是+0.334V。<0}{0>The electrode potential of Sn (tin) and Pb (lead) is -0.140V and -0.126V respectively.<}0{>Sn(錫)和Pb(鉛)的電極電勢分別是-0.140V和-0.126V。<0}{0>Whereas, the electrode potential of Zn is -0.761V.<}0{>其中，鋅的電極電勢是-0.761V。<0}{0>So, incidental contact of Zn to Cu can create a potential difference of 1.095V.<}0{>所以，Zn偶然接觸Cu可產生1.095V的位差。<0}{0>Throw in a little humidity (moisture), and one has a prime scenario for galvanic corrosion to occur.<}0{>增加一點濕度(水分)，假設可能產生電偶腐蝕。<0}{0>It can be further aggravated by the presence of halide ions (salt spray).<}0{>鹵離子（鹽噴霧）的存在又會進一步加重。<0}

{0>So, given all this, would a solder alloy of 91%Sn and 9%Zn be a problem?<}0{>所以，既然如此，由91%Sn和9%Zn組成的焊錫膏不成問題嗎?<0}

{0>I did a simple experiment to find out.<}0{>我做了一個簡單的實驗才找出來。<0}

{0>I took some SnZn solder ribbon and copper coupons, simply laid them on one another and exposed them to 85C/85%RH over the long Labor Day weekend (>72 hours).<}0{>我取一些SnZn焊帶和銅片，只需要將彼此疊放並暴露在85C/85%RH下超過勞動節的週末(大於72小時)。<0}

{0>Below is an image of the SnZn solder ribbon before it was submitted to the conditions mentioned above.<}0{>下面是SnZn焊錫帶在達到上述條件後的圖像。<0}

{0>The next image shows the SnZn solder ribbon after having been exposed to the 85C/85%RH for >72 hours.<}0{>下面是SnZn焊錫帶暴露在85C/85%RH下超過72小時的圖像。<0}{0>This sample of ribbon was not in contact with any other metals.<}0{>焊錫帶的樣品沒有接觸任何其他金屬。<0}{0>I was trying to determine what "normal" oxidation of this alloy, under these conditions, would be.<}0{>我正在測定該合金在這些條件下會是怎樣“正常的”氧化。<0}{0>I likewise did this with the copper coupons.<}0{>同樣，我還利用銅片做了實驗。<0}{0>(Next Image)<}0{>(下圖)<0}

{0>This last image shows the effect of incidental contact of copper with the SnZn solder ribbon.<}0{>下圖顯示銅偶然接觸SnZn焊錫帶的效果。<0}{0>Take note of how much worse the oxidation/corrosion of the copper coupons is.<}0{>請注意銅片的氧化/腐蝕多麼糟糕。<0}

{0>In this simple experiment we can see the effects of galvanic corrosion that happens through incidental contact of SnZn solder alloy to copper.<}0{>在這個簡單的實驗裏，我們SnZn焊錫膏偶然接觸銅發生電偶腐蝕的效果。<0}{0>I say "incidental contact" because the materials are merely touching each other, they have not been soldered together.<}0{>我說"偶然接觸"是因為材料只是相互接觸，它們還沒有被焊接在一起。<0}{0>It would be interesting to see if the effect would be as great if the materials were bonded together with no gap for water to penetrate.<}0{>材料被粘合在一起，使水沒有縫隙滲透，如果看到效果理想則是有趣的。<0}

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