| Objectives | Background | Approach |
To assess the potential for intermetallic growth between the
Pb free solder attach and the metallization on the chip, lead,
or substrate surface. To determine the effect of intermetallic
growth on the adhesion strength at the attach/substrate interface,
or lead/attach interface and relate it to time-to-failure in
assemblies.
Most of the Pb free solders candidates being considered for moderate to high temperature applications consist of small amounts of alloying elements added to tin. Pb free solders recommended as the most promising candidates for replacing eutectic Sn-Pb by the National Center for the Manufacturing Sciences and the National Electronics Manufacturing Initiative are Sn96.5Ag3.5, Sn95.5Ag3.9Cu0.6, and Sn99.3Cu0.7. These solders have a combination of a moderate melting point, good shear strength, fatigue resistance, and good surface wetting to Cu leads and pads.
The good surface wetting properties come from the abundance of tin, which easily adheres to the lead or pad surfaces plated with copper, silver, tin or gold/nickel by forming a thin layer of M-Sn (Cu-Sn, Ag-Sn, Au-Sn, Ni-Sn) intermetallic. Other binary systems in the solder-substrate system do not form intermetallics. However, it is this same tendency of tin to form intermetallics that raises a long term reliability concern with high tin solders. The intermetallics will continue to grow particularly at high temperature, resulting in the formation over time of a thick layer of brittle intermetallic material. This brittle intermetallic is prone to fracture, especially in the presence of voids created by asymmetric interdiffusion. Fracture of the intermetallic leads to an adhesive failure of the solder joint at the lead interface. Intermetallic reaction between copper and tin is particularly rapid. While the growth of tin-copper intermetallics has been reported for leads covered with Pb-Sn eutectic solder, the higher concentrations of tin in the new solder make it more of a concern.
This study is a companion to projects C01-01 and C01-02 in the Lead Free Solder thrust area. Those projects strive to determine constitutive and fatigue properties for the lead free solders that are important in addressing cohesive solder failure. This study will complement those efforts by focusing on the equally critical concern of adhesive solder failure.
10 bare copper, 10 silver plated (0.2~0.3 mm), 10 tin plated (1.0 mm) and 10 gold (0.1mm) over nickel (2 mm) plated 2" x 2" plastic coupons will each be coated with 10 solder balls of the lead free solder (Sn95.5Ag3.9Cu0.6 solder) under primary consideration by NEMI and NCMS as a replacement for eutectic tin-lead solder. In addition, 5 bare copper plastic coupons will each be coated with 10 eutectic tin-lead solder balls as a control. Care will be taken to ensure that the metal thickness and volume have a similar relation to solder geometry as in real joints and standard soldering process recommended by NEMI will be adopted. The solder coated coupons will then be exposed to temperatures meant to accelerate the effects of time under operating or storage conditions. Temperatures to be investigated will include 0.8Tm, 0.85Tm, and 0.9Tm for each solder. 2 coupons for the lead free solder and 1 coupon for the eutectic tin-lead solder will be removed simultaneously at intervals of 10, 100, 500 and 1000 hours. 2 solder balls on each coupon will be cross-sectioned, and examined by SEM for intermetallic composition and thickness, 4 solder balls will be subjected to shear testing and the remaining 4 solder balls will be subjected to tensile testing. The final result of the work will be plots of adhesion strength vs. intermetallic thickness vs. time at temperature. Finally, the relationship between the adhesion strength and fatigue life will be investigated for flip chip packages. Correlations of adhesion strength to product lifetime will be addressed.