Magnetic Force-Based Measurement Technique to Investigate the Effect of Lead-Free Solder Intermetallic Compounds (IMC) on Interconnect Reliability

This paper presents the results of the continued development of a magnetically actuated mechanical measurement technique and extends its application to measure the mechanical strength of intermetallic compounds (IMC) of lead-free solders. Beneficial characteristics of the technique are that it is fixtureless, low-cost, and compact, and it has the ability to perform monotonic and cyclic loading. The technique can be utilized for a wide range of mechanical tests of electronic packages including delamination and interconnect pull-off. In this current study, the ability of the magnetically actuated mechanical measurement technique to measure the strength of lead-free solder with an IMC layer was demonstrated utilizing SAC405 solder balls on a die. These solder balls had a diameter of 250
$\mu \mathrm{m}$
, a pitch of 400
$\mu \mathrm{m}$
, and were bonded to copper pads on a silicon chip. The IMC layer thickness in the solder balls near the copper pad was measured for three isothermal aging conditions: room temperature for several days, 120°C for 10 days, and 150 °C for 10 days. The average measured thicknesses of the IMC layers for the three different isothermal aging conditions were 2.60, 3.07, and 4.70
$\mu \mathrm{m}$
respectively. The IMC layer thickness increased with increasing isothermal aging temperatures and durations. The isothermally aged solder balls were then subjected to monotonic loading to determine their strength using the magnetically actuated mechanical measurement technique. In this method, a miniature permanent magnet was attached to the solder ball and then subjected to an external electromagnetic field. The force applied to the solder ball was gradually increased by decreasing the distance between the permanent magnet and the electromagnet. The force was increased until the solder ball was pulled off the test chip. The maximum force at failure for the three isothermal aging conditions of orginal solder at room temperature, 120°C for 10 days, and 150 °C for 10 days was measured to be 0.59, 0.31, and 0.28 N respectively. These experimental results showed that the maximum force at failure decreased with IMC layer thickness in the solder ball. This agrees with current literature regarding the strength of IMC layers in lead-free solders and demonstrates the magnetic force-based test method's ability to be applied to future studies of lead-free solder mechanical strength and reliability. In addition to this demonstration of applying the test to different material systems than our earlier studies, improvements to the method were also made. These improvements include a new design to reduce the time to secure and hold the test chip during testing as well as utilizing an offset-levered arrangement of the permanent magnet as compared to previous studies utilizing an aligned arrangement.