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 a low 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 an RDSON 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.
Now attention is turning to the solder joint as a major contributor to the total RDSON. So, how do we estimate this for each solder joint? Firstly, the basics. Remember that:
Conductivity = (1 / Resistivity)
Indium Corporation has a lot of solder alloy data, including a measure of the bulk alloy conductivity as 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, where current is flowing in the z-axis and the customer is concerned about what contribution to the RDSON is coming from this one solder joint.
Using Indalloy 151 (92.5Pb/5Sn/2.5Ag) solder, the electrical conductivity is 8.6% of (1/1.72micro.Ohm.cm), or 0.086/1.72microOhm.cm, or a resistivity of 20microOhm.cm. Using this value, the resistance in the z axis (Rz) can be easily calculated:
Rz = 20microOhm.cm * (z / x.y)
So, for a solder joint 50microns thick and 2mm x 2mm, the resistance is 2.5microOhms. And, for an RDSON of allowable limit 2mOhm, this equates to only 0.125% of the limit.
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.
As always, comments and corrections are much appreciated.