一般认为，无助焊剂焊接即使不是不可能，也是很困难的。然而，80Au/Sn 焊带和预型件通常都采用无助焊剂工艺--这怎么可能呢？
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。
还需要注意的是，氧化物的还原率会随着温度的升高而提高。在使用 Au/Sn 焊料的生产环境中，工艺温度通常为 330-350°C，以实现良好的氧化物还原。无助焊剂焊接工艺绝对需要清洁，因为零件不仅会受到氧化物的影响，还会受到表面污染的影响。AuSn 预型件加工和使用过程中的操作都会造成表面污染。预型件是在机械过程中制造的，因此需要非常小心，以尽量减少污染。此外，焊料供应商必须仔细控制厚度公差。AuSn 可 "就地润湿"；润湿时不会扩散。厚度上的任何变化都会出现在成品焊点中。请谨慎选择供应商！
Next time we will look at reflow with fluxes including formic acid (which is not the same as forming gas).
