Current density simulations for polymer cored CSP interconnects

Abstract

Polymer cored spheres have been proposed as a more flexible and therefore more reliable alternative to solid solder balls for use in ball grid array (BGA) and chip scale package (CSP) interconnects. However the use of such polymer cored interconnects will result in a significant increase in the average current density within the individual interconnects, due to the much smaller cross sectional area of metal through which conduction may take place. High current densities in metals are well known to lead to electro-migration and this may lead to reductions in their mechanical performance. The soft solders used in electronics manufacturing operate at high homologous temperatures and are therefore particularly vulnerable to electro-migration damage, particularly as miniaturization also leads to increasing current densities.This paper presents results of electrical current density simulations for geometries typical of a polymer cored sphere based CSP interconnect, in comparison with a conventional solder ball based CSP interconnect, and explores the electrical stress raising effects of the presence of tracks feeding into the solder pads on the allowable maximum current before electro-migration becomes a risk.The results show that for a structure where there are no tracks, i.e. for a via in pad design, the allowable current for a polymer cored interconnect may be only half that for a solid solder ball, but where there are surface tracks the polymer cored interconnects may result in a significantly lower stress concentration effect and therefore allow a higher current than for a solder ball.

Effects of high current density on lead-free solder joints of chip-size passive SMD components

“…Elevated temperatures may also present a risk for EM at lower current densities. Kumar et al (2009) and Li et al (2014) showed that local current density at values of 7.5 Â 10 7 A/m 2 , and at values of 2 Â 10 7 A/m 2 (Kumar et al, 2009) Whalley and Kristiansen (2010) also demonstrated the possibility of reliability problems of solid solder ball grid array (BGA) joints with a predicted maximum current density of 4.8 Â 10 6 A/m 2 at a current of 1 A, and polymer core chip scale package (CSP) joints with a maximum current density of 7.45 Â 10 6 A/m 2 . Bradley (2011) described the mechanism of current density increment because of void formation.…”

Advances on high current load effects on lead-free solder joints of SMD chip-size components and BGAs

“…Meinshausen et al (2012) used 100°C and 120°C (ambient) (with 130-150°C working temperature due to Joule heating) to report results of 1.4-1.8·10 7 A/m 2 critical current densities. Whalley and Kristiansen (2010) reported BGA joints with 4.8·10 6 A/m 2 and polymer core CSP joints at 7.45·10 6 A/m 2 critical current densities.…”

“…The figure shows through-hole, blind and microvia configurations, where entrapped gas and the solder paste flux volatiles cause large absences of solder (voids) at the center of the bump. With a similar approach, that is seen in Whalley and Kristiansen (2010), it is necessary to investigate lead-free solder bumps affected by large voids inside their structure.…”