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Problems With PCB Micro Voiding

Question: We are having certain batches of PC boards exhibiting micro voiding at the copper-lead free intermetallic region after thermal stress. Can you give me some background on this issue, as we are trying to determine the root cause of this problem?

Answer: IMC Microvoids, as defined in IPC-7095C Design and Assembly Process Implementation for BGAs, on page103. In the section 7.5.3.5 on IMC Microvoids it states:

    “… IMC Microvoids occur within the Intermetallic Compound (IMC) formed between copper and high tin solders, including SAC and tin/lead solders. These IMC Microvoids do not form immediately after the soldering process, but after aging at high temperatures of during temperature cycling of the solder joints. The true root cause is still under investigation, but a Kirkendall voiding mechanism may play a part. These voids can affect solder joint reliability, particularly in instances when brittle fracture is initiated within the IMC during drop or mechanical hock to the solder joint. Doping the solder with certain elements such as zinc is one way to diminish the amount of these IMC microvoids.”

The following was adapted from The Discovery and Acceptance of the Kirkendall Effect: The Result of a Short Research Career, by Hideo Nakajima, and found at the following website:

http://www.tms.org/pubs/journals/JOM/9706/Nakajima-9706.html

    “In 1947, Ernest Kirkendall reported the results of experiments on the interdiffusion between copper and zinc in brass and observed the movement of the interface between the
    different phases due to high-temperature interdiffusion, now called the Kirkendall Effect. This phenomenon supported the idea that atomic diffusion occurs through vacancy
    exchange. Since its discovery, the Kirkendall Effect has been found in various alloy systems, and studies on lattice defects and diffusion developed significantly. The Kirkendall Effect is important in connection with bonding between different materials and, in particular, raises the practical concern of controlling and suppressing the voids that are produced in the boundary region at a bonding interface. Today, the effect has been taken into account in various fields in materials science and technology such as structural materials welding, metals and ceramics powders sintering, thin films, and large-scale integration.”

Here is another reference to these types of voids:

http://ecadigitallibrary.com/pdf/56thECTC/s06p7lr.pdf

Effect of Intermetallic and Kirkendall Voids Growth on Board Level Drop Reliability
for SnAgCu Lead-free BGA Solder Joint
Luhua Xu and John H.L. Pang
School of Mechanical & Aerospace Engineering,
Nanyang Technological University