Board Level Reliability of Lead-Free Solder Interconnections with Solder Doping Under Harsh Environment

Abstract

The electronics packaging industry has moved from eutectic Sn-Pb solder to the lead-free near-eutectic Sn-Ag-Cu (SAC) solder system in the past few years. Reliability performance for Lead-Free solder joint under “extreme” or “harsh” environments, for example exposition to long-term isothermal aging in elevated temperature and thermal cycling, has been studied and examined. Previous studies show reliability of Ball Grid Array (BGA) packages with SAC105 and SAC305 solder joints can degrade up to 70% after aging at 125℃ for 24 months. People in industry have begun to consider the effect of long-term isothermal aging on the reliability of Lead-Free solder joint. Many solutions targeting applications in harsh environments requiring resistance to both thermo-mechanical (accelerated thermal cycling) and mechanical (vibration, bending, and shock) loading have been proposed. One possible solution is to develop a “next generation” solder alloy with additional elemental dopants metal mixed with SAC lead-free solder family, such as antimony (Sb), magnesium (Mg), nickel (Ni), cobalt (Co), or indium (In). These additives create finer grain boundaries and reduce the intermetallic formations of the tin with silver or copper, resulting in a more reproducible grain as well as a more uniform grain formation in the lead-free alloy. This process is called solder doping. So the purpose for this project is to firstly find a manufactureable solder paste with dopants that can mitigate the effects of aging. A secondary goal is to find a solder material for solder-sphere replacement to enhance individual package reliability. This dissertation presents harsh environment reliability test results for lead-free, principally Tin-Silver-Copper (SAC) based solder pastes with and without the presence of a variety of dopant elements (Bi, Sb, Ni, Mn, Nd, and In). SAC105 and SAC305 solder alloy spheres are mixed with 14 different doped solder pastes and subsequently tested under long-term isothermal aging and thermal cycling condition in order to evaluate the ability of solder doping to mitigate the effect of aging and to enhance the solder joint reliability. In addition to BGAs with package size ranging from 15mm (0.8mm pitch) to 6mm (0.8mm pitch), the test vehicle also incorporates no-lead packages such as Quad Flat No-leads (QFNs) and 2512 Surface Mount Resistors (SMRs). Three surface platings are tested: Organic Solderability Preservative (OSP), Immersion Silver (ImAg), and Electroless Nickel Immersion Gold (ENIG). All test vehicles were subjected to isothermal aging at temperatures of 125°C with aging time of either 0 or 12 months, followed by accelerated thermal cycle (ATC) testing from -40°C to 125°C. This dissertation presents results for the first 2700 thermal cycles. Current reliability data indicates that 8 doped solder pastes demonstrate superior reliability, which can help improving the reliability of lead-free solder under aging, while 4 pastes have somewhat lower performance. Solder pastes with good performance tend to have high Ag content (>3.0%), Bi content (>3.0%), and Sb content (>1.5%). Failure analysis shows continuous growth of Intermetallic Compounds (IMC) at solder joint-pad interface and within the solder bulk. For solder pastes that contain high Silver content, large plate-like 〖"Ag" 〗_"3" "Sn" bulk IMC can usually be noticed, together with particle-like Cu_6 Sn_5 bulk IMC, which can sometimes block the propagation of cracks through solder bumps, thus pastes with high silver content tends to have higher reliability. IMC thickness analysis shows that during thermal aging and cycling, the IMC layer in both the component- and board-side interfaces thicken continuously, consuming material from the copper pads and solder joint. ENIG finish can limit the IMC layer thickness, because it has an additional nickel layer between solder joint and Printed Circuit Board (PCB) copper pad, which acts as a diffusion barrier preventing copper dissolution into the solder. Solder pastes with high bismuth (>3%) and silver (>3%), can help limit the growth of the IMC layer thickness, as well as prevent copper diffusion from the PCB copper pad into solder joint, hence help stabilize and strengthen the interfacial IMC layer and improve reliability performance. Crack propagation is observed in the component-side, near interfacial regions in some materials and through the bulk solder in other materials. 2512 Surface Mount Resistors sampled thus far demonstrate typical fatigue crack propagation mechanics from the inside out through the main fillet. Polarized Light Microscope (PLM) images show recrystallization phenomenon in the solder bulk.