Flip chip pull test

F. Flip Chip Pull (Tensile) Test and Interpretation... 

One of the simplest and most useful tests performed on reflow-attached flip chips to determine the adequacy of the solder joints (i.e., C4) is a tensile pull test (Fig. 31). This is done by adhesively attaching a metal stud to the back of a joined chip and pulling the joints in tension at a slow strain rate (approximately 1.0x10 sec ). The pull force is measured during the test using an appropriate load cell. The pull strength is a useful parameter, but the failure mode is a very important indicator of joint quahty. Planar failure at the solder joint interfaces is indicative of a weak and unacceptable interface condition. 

FIG. 31 Schematic representation depicting the pull test used to determine interfacial integrity in flip chip solder joints, (a) Pull test configuration and conditions, (b) Ductile solder fracture indicating good interfacial strength, (c) Brittle or planar interfacial fracture indicative of unacceptable interfacial strength. 

The pull test was discussed previously as critical to understanding the interfacial strength and the failure mode of flip chip solder joints. As noted, pull testing typical lead-based terminals results in ductile solder fractures. Pull testing tin-based solder joints is more complicated because tin exhibits greater strength and lower ductility than lead (Fig. 26). A correctly manufactured tin-based solder joint can fail at locations other than the bulk solder, e.g., at an interface, or in the metallization and insulation on a chip and chip carrier if the pull test parameters utilized are the same as for lead-based joints. 

---