The European Union is banning the use of Pb in electronic products starting July 1st, 2006. Printed circuit board assemblies or “motherboards” require that planned CPU sockets and BGA chipsets use lead-free solder ball compositions at the second level interconnections (SLI) to attach to a printed circuit board (PCB) and survive various assembly and reliability test conditions for end-use deployment. Intel is pro-actively preparing for this anticipated Pb ban, by evaluating a new lead free (LF) solder alloy in the ternary Tin- Silver-Copper (Sn4.0Ag0.5Cu) system and developing higher temperature board assembly processes. This will be pursued with a focus on achieving the lowest process temperature required to avoid deleterious higher temperature effects and still achieve a metallurgically compatible solder joint. One primary factor is the elevated peak reflow temperature required for surface mount technology (SMT) LF assembly, which is approximately 250 °C compared to present eutectic tin/lead (Sn37Pb) reflow temperatures of around 220 °C. In addition, extended SMT time-above-liquidus (TAL) and subsequent cooling rates are also a concern not only for the critical BGA chipsets and CPU BGA sockets but to other components similarly attached to the same PCB substrate. PCBs used were conventional FR-4 substrates with organic solder preservative on the copper pads and mechanical daisychanged FCBGA components with direct immersion gold surface finish on their copper pads. However, a materials analysis method and approach is also required to characterize and evaluate the effect of low peak temperature LF SMT processing on the PBA SLI to identify the absolute limits or “cliffs” and determine if the minimum processing temperature and TAL could be further lowered. The SLI system is characterized using various microanalytical techniques, such as, conventional optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and microhardness testing. In addition, the SLI is further characterized using macroanalytical techniques such as dye penetrant testing (DPT) with controlled tensile testing for mechanical strength in addition to disbond and crack area mapping to complete the analysis.

In this study, the interface reactions between eutectic SnPb solder and two Ni-based UBM systems are reported, namely the sputtered Cu/Ni(V)/Al and the electroless Au/Ni(P) systems. Comparisons are made to the conventional Au/Al ball bonding system in terms of microstructure evolution, and metallurgical stability. TEM sample preparation is critical in this analysis. The capability of TEM in UBM microstructure studies is demonstrated.

The presence of gold within a Sn/Pb solder joint accelerates diffusion between the Sn and Ni of the Ni- V/Cu underlying bump metallurgy (UBM), generating early failures. A concentration of 1.2 wt% gold in the solder joint can accelerate time to failure by a factor of 400 [3]. The EDS x-ray microanalysis detection limit for gold in tin / lead solder of about 1.2 wt% gold was found to be above the concentration range of interest (0.1 – 0.5 wt% gold). Due to the reliability impact that even a low concentration of gold can have on solder joints employing Ni-V/Cu UBM, a method using differential scanning calorimetry (DSC) was developed to accurately measure gold concentrations of less than 1 wt% in solder bumps.

Mechanical strength, integrity, and reliability of solder connections used in the microelectronics industry are important factors in overall quality and reliability of the finished product. In most cases tin (Sn) rich solders are attached to a base metal plated with nickel (Ni) and then with gold (Au). Formation of AuSn4 intermetallics in the solder may result in loss of more than 80% of the initial impact toughness, resulting in loss of reliability of the connection. Gold (Au) embrittlement is a major concern in tin/lead (Sn/Pb) soldering or any other joining process with Au and Sn as major constituents. Noncompliance to Au plating-thickness specifications by vendors or insufficient Sn wicking of Au surfaces can result in embrittled joints and unreliable parts.

Very highly purified water such as De-ionized (DI) water tends to become very corrosive once exposed to the atmosphere. This “Hungry Water” as known in the water purification world is known to be a major source of corrosion [1]. The DI water was responsible for corrosion of tin during autoclave (pressure cooker) testing of Integrated Circuit (IC) devices assembled in plastic Quad Flat Package (QFP) with fine pitch leads. The copper leads of these packages are plated with solder. The copper leads of the packages are plated with solder composed of Lead and Tin. Due to the effect of corrosive water, Tin from solder corroded during the autoclave testing and formed thin whiskers of solder. These whiskers created a leakage path between the leads causing the devices to fail for pin to pin leakage.