*2.1. Test Vehicle*

The test vehicle is a coupon of PCB made of FR-4 glass epoxy with 10 mm × 10 mm × 1 mm dimensions, as shown in Figure 1. Each testing coupon consists of a full array of nine SAC305 solder joints. Each 30 mil-diameter joint is reflowed after being attached to a 22 mil SMD Cu pad. The pitch between joints is 3 mm to assure enough space among joints during testing. OSP is the only surface finish utilized in this work.

**Figure 1.** Test Vehicle.

Typical SAC305 solder paste reflow profile is utilized for reflowing the solder joints. The profile starts with a preheating phase to 150–225 ◦C/200 s, followed by an increase to a peak temperature of 245 ◦C with less above liquid period than typical SAC305 paste. This is due to the direct exposure of joints to convection heat, unlike assemblies with components. Finally, the temperature is cooled down with a 4 ◦C/s rate. The whole reflow process is performed within a nitrogen environment. A detailed reflow profile is shown in Figure 2.

## *2.2. Experimental Set-Up*

Instron 5948 micromechanical tester engaged with a customized fixture is used to implement an accelerated shear fatigue testing on individual solder joints, as shown in Figure 3. The fixture is a circular tip 1 mm in diameter utilized to cycle the joints between stress extremes. The X–Y stage, with the help of a load cell, is used to fit the joints in the middle of the tip and assure the proper tip stand-off range according to the JEDEC standard. A schematic diagram is shown in Figure 4. The displacement and the load are measured by the machine and load cell, respectively. These measurements are recorded continuously by the data acquisition system.

**Figure 3.** The Instron 5948 Micro-Tester.

**Figure 4.** Schematic of a customized testing fixture.
