Energy Boundary
Impact resistance testing measures the mechanical durability of area-array electronic packages when subjected to physical shock. The parameter solder ball drop height represents the specific elevation from which a test assembly is released to simulate a standardized drop. This test determines the maximum impact energy a joint can withstand before failure.
Impact Stress
Kinetic energy transferred during impact generates high transient stresses at the solder joints of the package. As the test board strikes the base plate, a rapid deceleration wave propagates through the assembly, creating a bending moment. For a given solder ball drop height, the stress is greatest at the corner joints where the shear forces are concentrated.
These transient forces can initiate microcracks at the interface between the solder and the copper pad.
Board Flexure
Flexural behavior of the printed circuit board during impact determines the distribution of tension across the array. The board bends dynamically, creating a mixture of tension and shear stresses that are transferred directly to the solder balls. If the board is too rigid, the shock wave is transmitted with minimal attenuation, increasing the peak force on the joints.
In contrast, a more compliant board design can help distribute this energy, though it may increase the total displacement.
Joint Rupture
Cumulative damage from repeated impacts leads to the propagation of microcracks and eventual open-circuit failure. The primary failure mode observed is intermetallic compound cracking, where the solder separates from the boundary layer of the copper pad. Higher solder ball drop height levels accelerate this fatigue process, shortening the number of drops a package can survive.
Using ductile solder alloys and optimizing the board assembly process can increase this threshold, ensuring that the finished product can survive accidental drops in the field without losing electrical connectivity. This parameter is verified by continuous resistance monitoring of the circuit during the drop sequence to capture transient electrical interruptions.
