# Intermetallic Growth Rate is Strongly Temperature Dependent

Folks,

In a previous post, I discussed that, contrary to popular belief, intermetallic compounds (IMCs) formed in soldering processes are not necessarily brittle. I reviewed some literature that indicated that failure modes are usually at interfaces between the IMCs themselves, the IMCs and the copper or solder and often in the bulk solder itself. The perspective that IMC growth may not significantly affect reliability is also supported by work performed by Lee, et al. Figure 1, from Lee’s paper, shows that aging for 250 hrs at 150°C does not significantly affect characteristic life in thermal cycle testing.

Figure 1. Aging for up to 250 hours at 150°C did not significantly affect characteristic life in thermal cycle testing in Lee’s referenced paper.

However, it would be prudent to minimize the thickness of IMCs. So this raises the question: how quickly do IMCs grow at any given temperature? Work performed by Siewert, et al[i] holds the answer. In this paper, Siewert supported past work that the thickness of IMCs grows as X=(kt)0.5 and added new data to support modeling using this equation. In this equation, X is IMC growth distance, k is a constant dependent on temperature, and t is the time. One might expect that X is strongly dependent on temperature (T) and it is. Using data from Siewert’s paper, I was able to generate values of k as a function of T and plot them in an Arrhenius plot. See Figure 2.

Figure 2. An Arrhenius plot for k.

I next used Figure 2 to obtain a value of k at 70°C and plotted the IMC growth X in microns as a function of time in hours. The result is in Figure 3.

Figure 3. IMC grow as a function of time at 70°C.

Note that about 40 years are required to obtain a little over 10 microns of growth. Figure 4 shows the results for IMC growth at 200°C. In this case, only 100 hours are required to obtain about 10 microns of growth. So going from 70 to 200°C produces an acceleration factor of over 30,000 in the effective IMC growth rate to 10 microns.

Figure 4. IMC growth as a function of time at 200°C.

These are theoretical calculations from data collected at different temperatures. Let’s see if the formulas work in real life. In another paper [ii]by Ma, et al. his team aged some solder joints at 125°C for 120 hours. The equations used above would predict IMC growth of 2.2 microns under these conditions. From Figure 5, we see about 2 microns of growth consistent with the calculation estimate.

Figure 5. Images from Ma’s paper of IMC growth at 125°C for 120 hours.

So although IMCs are not that brittle, it is wise to limit their growth. Hence, limiting exposure to very high temperature aging is wise, but certainly minimizing solder rework is advisable, as the molten solder enables very fast IMC growth.

Cheers,

Dr. Ron

[i] Siewert, T. A., et al, Formation and Growth of IMs at the Interface Between Lead Free Solders and Copper Interfaces, IPC Apex, 1994.

[ii] X. Ma, et al Materials Letters 57 (2003) 3361-3365.

# Intermetallics and Kirkendall Voids Continue to Grow at Room Temperature

Folks,

In my last post, I discussed intermetallic compounds (IMCs) and what I referred to as the “miracle of soldering.” I also mentioned that research focused on the brittle nature of IMCs suggests that failures in stress tests are more likely due to failures between the interfaces of the IMCs and the solder, the IMCs and the copper, or the IMCs (Cu6Sn5 with Cu3Sn) themselves and are not related to any perceived brittle nature of the IMCs.

Another weakening mechanism in soldering and thermal aging of solder joints is Kirkendall voids. Kirkendall voids form when one metal diffuses more rapidly into another metal than vice versa. A copper-tin interface displays such a mechanism. Copper diffuses into the tin more rapidly than the tin into the copper. This mechanism can result in actual voids in the copper at the metal interface. See the image below. In addition to causing a possible weakness at the interface, the excess copper that diffuses into the tin creates compressive stresses than can result in tin whiskers.

Kirkendall voids

IMCs and Kirkendall voids are formed quite quickly at soldering temperatures. However, even at room temperature IMCs and Kirkendall voids continue to grow, albeit at a much reduced rate. The reason for this continued growth is that on the absolute temperature or Kelvin scale, room temperature is a considerable fraction of the melting temperature of solders. As an example, the melting temperature of SAC is about 219°C, this temperature is equal to 492K (219+273), whereas room temperature is 295°K, so room temperature is 60% of the way to the melting point of SAC solder (295/492 = 0.60). Compare this situation to steel, which melts at about 1480°C. The steel would be red hot at 60% (780°C) of its melting point on the absolute scale. So, since room temperature is 60% of the way to melting, the IMC and Kirkendall forming processes don’t stop at room temperature. Hence, IMCs and Kirkendall voids continue to grow, as do related effects such as tin whiskers.

Stay tuned. Next time we will discuss IMC growth rates and resulting effects in stress testing as we wrap up this series on IMCs.

Cheers,

Dr. Ron