Soldering without flux is generally regarded as difficult if not impossible. Yet a fluxless process is routinely done with 80Au/Sn ribbon and preforms – how can this be?
简而言之：因为它可以作为钎料进行加工。
Let’s remind ourselves of the role of flux. Flux is a chemical which cleans (deoxidises) surfaces and protects it from re-oxidation during soldering so that the solder will wet and form reliable solder joints. Gold doesn’t tarnish. So, in theory, we don’t need flux. However, in practice, we rarely solder to pure gold versus gold-plated surfaces. Unless it is very thick, the Au plating (for example: flash Au < 0.5 microns thick) rapidly dissolves into the AuSn, and the solder joint is actually made with the base metal which lies beneath the Au. This is almost always nickel. Unfortunately, nickel does oxidize and can do so even when plated with gold. So, we still need protection from re-oxidation during soldering, as well as oxide removal. Protection from re-oxidation is done easily enough using a nitrogen atmosphere. Oxide reduction and removal can be accomplished by adding hydrogen to the atmosphere, otherwise known as forming gas. So, although we haven’t used a liquid chemical, the N2/H2 mix is a “chemical” and behaves as a flux.
在传送带/带式炉中，氢气的比例通常在 5-10% 之间，而在专用设备中，氢气的比例可高达 100%。氢气含量越高，传热效果越好，因为氢气的热容量约为氮气的 14 倍。在较高的氢气百分比下，氧化物的还原速度更快，但此时可能需要使用安全预防措施（火焰捕捉器等）。有趣的是，当氢气浓度达到 75% 左右时，由于没有足够的氧气支持燃烧，氢气就不再具有爆炸性（尽管存在氢脆问题）。氢含量低于 5%的混合物是不易燃的，因为氢过于稀释而无法燃烧。不过，在这种稀释度下，氢气和氧化物的去除速度可能太慢，不实用。
It is also important to note that the rate of oxide reduction is enhanced by higher temperatures. In a production environment with Au/Sn solder, process temperatures are typically 330-350°C for good oxide reduction. With a fluxless soldering process, there is an absolute need for cleanliness as parts can be affected not just by oxide but surface contamination as well. Surface contamination can be introduced by handling during processing of the AuSn preforms as well as in use. Preforms are made in a mechanical process so great care needs to be exercised to minimize contamination. In addition, your solder supplier has to carefully control the thickness tolerance. AuSn “wets in place”; it does not spread when wetting. Any variation in thickness will tend to be present in finished joints. Choose your supplier carefully!
Next time we will look at reflow with fluxes including formic acid (which is not the same as forming gas).
