Strength of Soldered Joints Formed under Microgravity Conditions

Abstract

Experiments have been carried out to compare the mechanical strength of joints soldered with Sn-Ag-Cu under different gravitational conditions. Joints soldered under microgravity (produced during a parabolic flight) have lower strength (by 32% in this case) than similar joints formed under normal gravity. Electron microscopy has shown that this is due to a larger volume of residual porosity (14%) in the joints formed in microgravity compared with <1% for joints formed in normal gravity. The residual porosity in joints formed in microgravity is mainly within the bulk of the solder, with some microporosity in the Cu 6 Sn 5 intermetallic layer near the copper interfaces. The porosity not only weakens the joint, but also biases the failure path away from the intermetallic layer and into the bulk of the solder. These observations show that gravitational buoyancy is important for the expulsion of flux and flux residues from soldered joints.

Improving Solder Joints Formed in Microgravity by Use of Magnetic Soldering Paste Additives

“…Imperial College London flew an experiment onboard a parabolic aircraft flight designed to test the strength of microgravity solder joints made under microgravity conditions. This experiment found that interior porosity of joints increased from 1% to 14% and that the strength of the connection decreased by 32% for joints formed during the flight as compared to those formed under normal gravity conditions [7].…”

“…The joined copper plates can be inserted into an Arcan disc [25], so that both the shear and tensile strength can be tested using an available tensile testing machine (see Figure 32). A test of this nature was previously performed by Imperial College London [7] to quantify the reduction of strength of microgravity solder joints. WVU's MRT conducted a similar experiment on its most recent flight to verify these results and compare strength data for solder joints using the magnetic solder paste to those formed under nonmagnetic conditions, both in microgravity and normal gravity environments.…”

“…For this reason, the crosssectioned solder joints were analyzed to determine if areas around voids had higher concentrations of iron particles than areas with fewer voids.EDS was also used to analyze the species of atoms on the interior surface of voids, to compare these areas of the joint cross-sections with areas that were comprised of void-free solder. It is expected that the interior surfaces of the voids contain higher concentrations of carbon and oxygen, which are the major components of the organic fluxes found in the solders used for this experiment.Therefore, finding higher carbon and oxygen in voids within the solder would indicate that the voids were created by trapping vaporized flux within the molten solder, as previous researchers have speculated [7,8,9,10]. 3.4.4 Image ProcessingMicroscopy images of solder joints were digitally processed to calculate the percent voidage.…”