Interesting question from a Chinese die-attach customer this week asking about volume resistivity of solder. My friend, Eric Bastow, suggested that Indium make this available to all through a blog post.
It is well understood by Power Semiconductor engineers that a key figure of merit for alow power consumption device is the RDSON (the source-drain resistance in the "on", or forward-biased, state). Since this resistance merely results in energy(Joule-Thompson) losses as heat, the lower the RDSON, the better. Most devices have anRDSON that must balance the pragmatic limitations of cost,system electrical design, and DFM. This target RDSON must also be stable over the projected lifetime of the part; something that is increasingly becoming worrisome for high lead (high-Pb) solders at junction temperatures approaching 200°C.
For clip-bonded devices,with the elimination of wirebonds,the die, itself, is becoming the majority contributor to the overall component resistance. This drives die thinner and thinner, often with unusual results such as increased voiding due to die (+) bending entrapping flux volatiles, and increasing concerns about current leakage due to alpha particles.
现在，我们将注意力转向焊点，因为它是影响总 RDSON 的主要因素。那么，我们如何对每个焊点进行估算呢？首先是基础知识。请记住
电导率 = (1 / 电阻率)
Indium Corporation has a lot of solder alloy data, including a measure of the bulk alloy conductivityas a percent of the IACS standard of 1/1.72microOhm.cm, which is, therefore, an inverse volumetric measure of resistivity. How is it volumetric? Let me show you how this works, using the example of a die-attach solder joint, wherecurrent is flowing in the z-axis and the customer is concerned about what contribution to the RDSON is coming from this one solder joint.
使用 Indalloy 151（92.5Pb/5Sn/2.5Ag）焊料，导电率为（1/1.72micro.Ohm.cm）的 8.6%，或 0.086/1.72microOhm.cm，或电阻率为 20microOhm.cm。利用这个数值，可以很容易地计算出 Z 轴的电阻 (Rz)：
Rz = 20microOhm.cm * (z / x.y)
因此，对于 50 微米厚、2 毫米 x 2 毫米的焊点，电阻为 2.5 微欧。而对于允许极限为 2mOhm 的 RDSON 来说，这只相当于极限的 0.125%。
Note that a clip-bonding application may use three or more layers of solder (LF-Die / Die-clip / clip-LF and so on), so the contribution to the RDSON (simple sum of the resistances) may not be negligible.
我们将一如既往地感谢您的意见和指正。
干杯安迪
