工程师在设计激光二极管叠层中的金属TIM时，不希望听到蠕变这个词。但蠕变真的是一个问题吗？我们在谈论铟时真的了解蠕变吗？答案是相对的。附图是我制作的典型二极管堆叠图。这可能是激光应用，也可能是LED应用。芯片连接层通常是高温焊料或填充银的环氧树脂。铟可以用在这个层面上吗？答案是肯定的，但前提是结点的温度必须远离纯铟的熔点（156 摄氏度）。软焊料就是软焊料，在模具附件中，一些客户已经在这个水平上使用了铟，但它并不像其他高温焊料（如 AuSn、Sac 合金或 SnAg）那样常见。填充银的环氧树脂确实越来越好，但在导电性和工艺方面还存在一些问题。当使用焊料作为芯片附件时，芯片本身可能会在回流过程中浮动或移动。在这种情况下，可以使用某种机制在回流过程中将其固定到位，以确保其对齐完美。因此，要回答这个问题，在这种应用中可以使用焊料作为芯片连接件，但所选择的合金将真正决定它的有效性和可靠性。
In the case of TIM2 (thermal interface material level 2), there are a few more considerations here. Let us first assume that we are going to reflow at this level. Copper at the heat-spreader level will not be a problem, but Nickel at the spreader/sink level will be a problem. Aluminum will also be a problem here. The problem is that these materials are hard to solder to, but it can be done. A high activity flux such as Indiums RSA or Flux number 3 can be used to break the oxide layer that will be present. However a layer of gold on the surface will help assure soldering will be effective. Indium recommends not to exceed 50 micro inches of gold, and recommends that the thinner the better, usually 10 micro inches will do it. Indium the element will actually dissolve the gold or other wise known that the gold will diffuse into the indium. During soldering an Indium/Gold inter-metallic will form. This is a brittle layer and if too much gold is used can induce reliability issues and cracking of the joint. So back to our original question; why would you use indium here and can you use indium here? This is the most common area where creep of indium can be an issue. Creep can be acceptable however. Indium will not creep to the degree that it will come out like pump out or like play-doe. The degree of creep is related to the pressure that is put on it, ie: CTE movement or direct pressure from clamping, as well as the temperature that the interface sees. If the junction temp is less that 20 degrees of the melting point and some movement is allowable, such as in an LED application, this is acceptable. However in a laser application, Indium Corporation usually advises that we do not go with pure indium at this level and rather choose an alloy such as Indium Silver or Tin Silver. The more Silver you add to the alloy the harder the material will become. Consider this. Pure Indium has a conductivity of 86W/mk, mp=156C Eutectic and a tinsel strength of 273 psi. Add 3% silver and the MP goes to 143C Eutectic, the conductivity goes down to 73 but the tinsel goes up to 800 psi. Better yet, add 10% silver and the MP is now plastic from 143 to 237, conductivity goes down to 67 but the tensil goes up to 1650 psi. Then consider SnAg which has a melt point of 221C a conductivity of 33W/mk and a tensile of 5800 psi. What is the best for your application? The answer lies in what is acceptable to you, if CTE is an issue go with less silver, if temp is an issue look at no indium, if conductivity is an issue look at high indium.
在考虑 TIM2 级可压缩金属时，现在的问题是压力的大小、连接处的温度以及表面的平面度。显然，有了可压缩界面，就不再需要任何金，铟与镍直接接触也不会有任何问题。不过，随着时间的推移，铜和铟可能会形成金属间化合物；但铜上的氧化层通常会阻止这种情况的发生。事实上，在我们的热实验室中，只有在 125C 以上温度下实际烘烤组件超过 1000 小时时，才会出现这种情况。即便如此，这种现象也只是名义上的。如果你不相信你会在模块制造 4-5 年后重新加工接口，我认为这不是一个问题，实际上它会提高热性能和可靠性。在 1-2 年内返工也不是问题。我们的许多客户已经将这种级别的铟用作可压缩接口，但很少有人知道，如果改用热弹簧 ™，他们实际上可以提高性能。热弹簧是一种专利工艺，如果压力至少达到 50 psi，我们就能降低金属的接触电阻。这使得叠层可以使用更薄的粘合线厚度，并提高金属热界面的热性能。那么，该界面的蠕变情况又如何呢？同样，改变合金可以消除发生这种情况的可能性，但从标准铟扁平箔转换为热弹簧会进一步降低这种可能性，因为如果使用热弹簧，结合线通常会大大减少。(平均约为 0.003 英寸）。
总之，使用金属热界面或贴片焊料时应考虑以下事项：
- 接口的工作温度是多少？
- 该温度是否太接近金属热界面的熔点？
- 设备能否承受更高温度焊料（如金锡或锡银
）的回流焊温度 - 接口的热性能是否存在问题，如果通过降低铟含量来改变金属热接口的导电性，是否会损害整个堆叠的导电性？例如，从 86w/mK 降到 67w/mK。
- 蠕变真的是一个问题吗？如果您的设备可以接受微小程度的移动，那么使用铟就没有问题。如果即使是微小的问题也会造成问题，建议使用铟合金而不是纯铟。
In the end, Indium Corporation is here to help you. Please see our web site for additional information including our e-list of alloys to help you choose the best Metal Thermal Interface Material.
