Intermetallics are a necessary evil in the metal-to-metal bonding world, which definitely includes soldering. There are two basic ways that metal will "chemically"bond to another metal: 1) solid solution 2) intermetallic. We will focus just on intermetallics for the moment as that is the most pertinent to the soldering world.
Many people confuse or interchange "wetting" for intermetallic formation (bonding). Wetting is just wetting. Just because a solder "wets" to a surface does not mean that an intermetallic "bond"has been formed. For example, and Ihave done this myself, 55.5Bi 44.5Pb can be melted onto a piece of copper. The molten BiPb will flow and "wet"to the surface of the copper. However, upon solidification (cooling)of the alloy, the BiPb can be peeled off. Why?… because no intermetallic was formed between the BiPb and the copper surface.
In order for an intermetallic to form, some amount of the surface metallization must dissolve into the molten solder. For this reason, Sn (tin) has long been a critical component of solder alloys. Molten Sn (tin) is an excellent solvent of many other metals. And, conveniently for us, those "many other metals" include elements like copper, gold, silver and, to a lesser degree, nickel. The rates at which these other metals dissolve into molten tin (solder) will differ. Gold dissolves readily into solder; whereas nickel does so slowly. So, because the rate of dissolution is different for each metal, the rate of intermetallic formation is also different. Ihave dealt with companies that have a long history of soldering to copper, and, for whatever reason, they are forced to switch to an ENIG (Electroless Nickel / Immersion Gold ) surface. (It is important to note that the gold layer is very thin and only applied to protect the nickel from oxidation. This gold layer readily dissolves completelyinto the molten solder and the "bond" is actually made to the nickel surface). When they make the change they sometimes encounter a number of issues such as incomplete wetting, poor bond strength, etc. and do not know why. Theyare not aware that the same reflow profile (time and temperature) that yielded a good (intermetallic)bond to copper is not sufficient to get the same intermetallic bond to nickel. Once they adjust their profile (more time and/or higher temperature) to allow for sufficient intermetallic formation, they are able to achieve acceptable solder joints. Keep in mind that dissolution, thephenomenon ofa solid dissolving into a liquid, is effected by both time and temperature. Generally speaking, more time and more temperature allows for more dissolutionand, hence, more intermetallic formation.
As mentioned in my opening line, intermetallics are a necessary evil.Why "evil"?Because they tend to be the most brittle part of the solder joint. Some intermetallics are more brittle than others.(This should be taken into consideration when choosing a solder alloy for a particular metallization). For example, intermetallics that form between Sn and Au are often extremely brittle.Being brittle, they can be subject to fracture, etc. This is a case where more is not always better. Yes, you need an intermetallic to get a "bond". Too thin of an intermetallic layercan be bad; but too thick of an intermetallic layer can be just as bad, if not worse. Believe it or not, the solder may not adhere well to its own intermetallic layer. Intermetallics are generally crystalline and chemically-stable structures….they do not really react with anything else once they have formed. If you have ever looked at a fractured solder joint, you may have noticed that the fracture likelytook place right at the interface between the intermetallic layer and the bulk solder.
One other possible outcome of an excessively thick intermetallic layer is "voiding"at the interface. Why? Well, we first need to look at the reaction products. There are two basic types of reaction products that form the intermetallic layer between Sn and Cu. They are Cu3Sn and Cu6Sn5. In the f
第一种情况是每 3 个锡原子对应 3 个铜原子，第二种情况是每 5 个锡原子对应 6 个铜原子。在这两种情况下，铜原子的消耗速度都快于锡原子。由于反应中的这种差异，在夸张的情况下，铜表面会形成小孔或空位（"空洞"）。
金属间的形成不仅限于焊接过程。即使在固态下，金属原子也会发生扩散。这种运动会导致金属原子相互作用、反应并形成金属间化合物，或导致现有的金属间化合物层变厚。通常会进行 "老化 "实验，以测量金属间层的变化程度及其对焊点机械性能的影响。
详尽讨论金属间化合物远远超出了本博文的范围和目的。关于这个主题，可以写成整本书。因此，我还远远没有对金属间化合物这一主题做出公正的评价。我只能希望对这一主题略作说明。
欢迎提出意见或问题。
