Evolution of Mechanical Properties and Microstructure in SAC Bulk Solder and Solder Joints During Thermal Cycling Exposures

Electronic packages are frequently exposed to thermal cycling during their service life between low to high temperature extreme. Similar phenomena can be observed in solder joints during the characterization of thermal-mechanical fatigue behavior. This variation in temperature causes the evolution of mechanical and microstructural behavior of solder joints. Also, dwelling at high temperature extreme causes the mechanical properties reduction of solder joints due to thermal aging phenomena which eventually leads to the change in microstructure. In literature, the effect of thermal aging on the mechanical behavior evolution has been reported by several research groups, but the evolution of mechanical and microstructural properties under different thermal cycling exposure is limited. In our prior study, reduction of mechanical properties of SAC305 lead-free solder material under different thermal cycling exposures have been reported for up to 5 days of thermal cycling. It was found that thermal cycling with long ramp period and dwell time has severe effect on mechanical properties reduction. In our present study, previous study has been extended up to 100 days along with the mechanical behavior evolution of solder joints under stress free condition at different thermal cyclic loading. Particularly, the evolutions of mechanical behavior in both bulk SAC305 miniature solder bar samples and small SAC305 solder balls under stress free condition have been investigated for several thermal cycling profiles, and then the results were compared. Reflow solidification technique with a controlled temperature profile has been used to prepare bulk solder specimens for uniaxial tensile testing. Optical microscopy has been used to figure out the single grain BGA solder balls after grounding and polishing to avoid grain orientation effect during nanoindentation technique. Then, both bulk solder bars and solder balls were thermally cycled between −40 C to +125 °C under a stress-free condition (no load) in a thermal chamber. Several thermal loading were adopted such as (1) 150 minutes cycles with 45 minutes ramps and 30 minutes dwells, (2) air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps, (3) 90 minute cycles with 45 minutes ramps and 0 minutes dwells (thermal ramp only), and (4) Isothermal aging at high temperature extreme (no cycle). After each thermal cycling exposure, mechanical properties evolution of both solder bars and solder balls were recorded in terms of effective elastic modulus (E), hardness (H), yield strength (YS), and ultimate tensile strength (UTS). For the BGA solder balls, the evolution of mechanical properties was measured using nanoindentation. Moreover, mechanical properties evolution of both specimens was compared in terms of normalized properties with respect to elapsed time under different thermal cycling exposures. Finally, the microstructural evolution of bulk solder bars was observed under slow thermal cycling exposures with elapsed time.

The Effect of Thermal Annealing on the Microstructure and Mechanical Properties of Sn-0.7Cu-xZn Solder Joint

The microstructural properties of a Pb-free solder joint significantly affect its mechanical behaviours. This paper details a systematic study of the effect of the annealing process on the microstructure and shear strength of a Zn-added Sn-0.7Cu solder joint. The results indicated that the IMC layer’s thickness at the solder/Cu interface increases with annealing time. The interfacial IMC layer of the Sn-0.7Cu solder joint gradually thickened with increasing annealing time and annealing temperature, while the interfacial IMC layer’s morphology changed from scallop-type to layer-type after the annealing process. However, the addition of 1.0 wt.% and 1.5 wt.% Zn in the Sn-0.7Cu effectively altered the interfacial IMC phase to Cu-Zn and suppressed the growth of Cu3Sn during the annealing process. The single-lap shear tests results confirmed that the addition of Zn decreased the shear strength of Sn-0.7Cu. The interfacial IMC of the Cu6Sn5 phase in Sn-0.7Cu changed to Cu-Zn due to the addition of Zn. The shear fractures in the annealed solder joint were ductile within the bulk solder instead of the interfacial IMC layer. Increased annealing time resulted in the increased presence of the Cu-Zn phase, which decreased the hardness and shear strength of the Sn-0.7Cu solder joint.

Investigation of Solder Joint Encapsulant Materials for Defect Mitigation

As Cisco’s next-generation products continue to push the trends of higher signal speeds and increased functional density, the need for advanced PCB structures, such as Via-in-Pad Plated Over (VIPPO) and backdrill, and high-speed memory is becoming more mainstream across product platforms. Furthermore, as these high-speed memory technologies are being driven by consumer applications, the form factor and interconnect pitches continue to shrink to meet the demands of the mobile device market. The use of these advanced PCB structures, like VIPPO and VIPPO with backdrill, within the BGA footprints, particularly for the fine pitch patterns, have been found to result in BGA solder separation defects at the bulk solder to IMC interface upon a 2nd reflow, e.g. during top-side reflow for bottom-side components or during rework of an adjacent BGA.1 In some cases, this solder separation failure mode has also been identified with buried vias under the BGA pad or even without the presence of VIPPO or any vias under the BGA pad. 2.3 Additionally, these small memory components have been experiencing high occurrences of head-in-pillow (HIP) defects even though the overall package warpage over the reflow profile is < ~3mils. This paper will therefore focus on the mitigation of these solder joint defects resulting from SMT assembly with the use of solder joint encapsulant materials to provide enhanced adhesion strength for the solder joints. Leveraging existing test vehicles that are known to induce the aforementioned solder joint defects, 2 different solder joint encapsulant or epoxy flux materials are evaluated in terms of the application process, assembly integrity and compatibility with Cisco’s production solder paste materials and SMT processes.

Laser Sealing for Vacuum Plate Glass with PbO-TiO2-SiO2-RxOy Solder

Laser sealing for vacuum plate glass is a key step in developing the cost-effective smart vacuum-glass window for the drive towards net-zero energy buildings. In this paper, the pores, cracks, and interface with laser welding are analyzed in depth using PbO-TiO2-SiO2-RxOy system sealing solder to prepare vacuum flat glass. The microstructure of the sealing layer was analyzed by a BX41M-LED metallographic microscope, and the interfacial bonding characteristics were observed by thermal field emission scanning electron microscopy (SEM). The solder was analyzed by an energy spectrometer, and the influence of laser power, sealing temperature, and sealing speed on the gas holes and the crack sand interface separation of the sealing layer are reported. The results show that when the laser power reached 80 W at the welding speed of 2 mm/s, the bulk solder disappeared to most of the quantity and the sealing surface density was higher, due to which negligible pores and micro cracks were found. Thus, the sealing quality of the sealing layer is considered to be suitable when the temperature of 470 °C was maintained and the solder has 68.93% of Pb and 3.04% Si in the atom fraction to achieve the wet the glass substrate surface whilst improving the bonding quality.

Board-level vapor phase soldering (VPS) with different temperature and vacuum conditions

Purpose Vapor phase soldering (VPS), also known as condense soldering, is capable of improving the mechanical reliability of solder joints in electronic packaging structures. The paper aims to discuss this issue. Design/methodology/approach In the present study, VPS is utilized to assemble two typical packaging types (i.e. ceramic column grid array (CCGA) and BGA) for electronic devices with lead-containing and lead-free solders. By applying the peak soldering temperatures of 215°C and 235°C with and without vacuum condition, the void formation and intermetallic compound (IMC) thickness are compared for different packaging structures with lead-containing and lead-free solder alloys. Findings It is found that at the soldering temperature of 215°C, CCGA under a vacuum condition has fewer voids but BGA without vacuum environment has fewer voids despite of the existence of lead in solder alloy. In light of contradictory phenomenon about void formation at 215°C, a similar CCGA device is soldered via VPS at the temperature of 235°C. Compared with the size of voids formed at 215°C, no obvious void is found for CCGA with vacuum at the soldering temperature of 235°C. No matter what soldering temperature and vacuum condition are applied, the IMC thickness of CCGA and BGA can satisfy the requirement of 1.0–3.0 µm. Therefore, it can be concluded that the soldering temperature of 235°C in vacuum is the optimal VPS condition for void elimination. In addition, shear tests at the rate of 10 mm/min are performed to examine the load resistance and potential failure mode. In terms of failure mode observed in shear tests, interfacial shear failure occurs between PCB and bulk solder and also within bulk solder for CCGA soldered at temperatures of 215°C and 235°C. This means that an acceptable thicker IMC thickness between CCGA solder and device provides greater interfacial strength between CCGA and device. Originality/value Due to its high I/O capacity and satisfactory reliability in electrical and thermal performance, CCGA electronic devices have been widely adopted in the military and aerospace fields. In the present study, the authors utilized VPS to assemble a typical type of CCGA with the control package of conventional BGA to investigate the relation between essential condition (i.e. soldering temperature and vacuum) to void formation.

Critical Review of Size Effects on Microstructure and Mechanical Properties of Solder Joints for Electronic Packaging

With the miniaturization of electronic packaging and devices, the size of solder joints in electronics is also decreasing from bulk solder joints to micro-bumps. Both the microstructure and mechanical properties of the solder joints are also evolving with the decreasing size, which brings great concern for the reliability of different sizes of solder interconnections. In this paper, the effect of solder size on the microstructure (i.e., interfacial intermetallic compound (IMC) growth, precipitation in the solder matrix, dendrite arms, and undercooling) and mechanical properties (i.e., tensile property, shear and compression strength, fracture toughness, and creep deformation) are reviewed from the mechanical point of view. In addition, some areas for further researches about size effects on solder joints are discussed.