The Signal Distortion and Failure Characteristic Analysis for Intermittent Nanometer-scale Fracture of the Solder Ball in a BGA Package Device

Abstract

Since Ball Grid Array (BGA) package has a small form factor, and high pin density, it is advantageous to build a denser circuit on a limited Printed Circuit Board (PCB) area. In addition, BGA package has low lead parasitics—especially, low inductance-due to its short length of connection. With those advantages, BGA package is widely employed to the high performance electronic system. However, if the package is placed on a PCB through the reflow process, the solder ball fault inspection of the package is very difficult. This dissertation analyzes the solder ball fracture that could be a source of intermittent errors. The electrical characteristics of a momentary fracture (open), which may appear at the very beginning of a progressive solder ball failure, are extensively studied. A solder ball fracture causes an intermittent open circuit on the transmission line. The resulting basic failure mechanism is a drop in signal voltage, due to the capacitance-induced Alternating Current (AC)-coupling effect (which is induced by the fractured solder ball). The distorted signal reduces signal integrity under the fracture and results in increased jitter and reduced eye window. This fracture causes the dropping of the signal voltage level, and this dropping erroneously affects the system when multiple failing conditions are simultaneously satisfied. The two major contributing factors to this error are fracture height and the persisting duration of the consecutive same-logic-value signal. SPICE simulation is conducted to demonstrate the effects of a momentary fracture using the DDR3 memory tester system. In the case of a 10 nm fractured solder ball with a PRBS pattern, the eye height is reduced from 597 mV to 349 mV, and the jitter is increased from 38 ps to 132 ps. The bits that violate the eye-mask window begin to appear with a heavy bit stream and cause intermittent bit errors. The required signal that induces a voltage drop, sufficient to detect nanometer-scale solder ball fractures, can be composed by repetition of certain signal patterns even for the I/O connections with run-length restrictions. The methodology is newly proposed to determine potential ranges of solder ball fractures. Test pattern generation is made by maximally exploiting the compounding effect of various sizes of same data bits to generate effective run-length that is larger than maximum run length for the purpose of detecting intermittent solder ball fractures. In DDR3 memory systems with 5-nm solder ball fractures, at least 29 bits of consecutive logic “1” or “0” signals are required to detect fractures if the system has 1,600 Mbps of data rate. If the system has a maximum run-length of 10, 20, or 30 bits, the test signal—which has the equivalent voltage-dropping effect as 29-consecutive bits—can be generated with six, two, or one repetition of the test pattern, respectively; the test pattern is in the form of concatenated N-1 bits of consecutive logic “1” and 1 bit of logic “0” where N is the maximum run length. In addition, a SPICE simulation was conducted to confirm correlation between calculations and actual results. In the simulation, in order to detect a 5-nm solder ball fracture, at least 37 bits of consecutive signal were required.