Failure and failure characterization of QFP package interconnect structure under random vibration condition

Highlights

• QFP package with the self-testing real-time circuit has been tested to failure under narrow-band random vibration.

• The charging time of external capacitor in the self-testing circuit has been extracted as the monitoring signal.

• The charging time can characterize the failure and failure process of interconnect structure well.

Abstract

Electronics installed in modern complex systems, like automotive and aircraft systems, are mainly subjected to mechanical vibrations in applications. The interconnection structure, as the mechanical fixing and electrical interconnection between the electrical device and the printed circuit board, is a key part in electronics. Presently, there are few researching articles on reliability of QFP interconnect structure under random vibration. In this paper, eight specimens of QFP100 with the self-testing real-time circuit have been tested to failure by subjecting to a narrow-band random vibration at its first natural frequency. The charging time of external capacitor in the self-test circuit has been extracted as the monitoring signal. Failure modes of interconnect structure have been analyzed in statistics. Then, the relationship between the monitoring signal and interconnect failure has been investigated. Finally, failure data, conforms to a two-parameter Weibull distribution, has been used to establish the life distribution of interconnect structure. Results show that there are four failure modes and three failure places in interconnect structure under random vibration, which is consistent with simulation results by FEM, and the charging time can characterize the failure and failure process of interconnect structure well.

Introduction

The electronic device and the printed circuit board are packaged together with tiny solder joints, which provide the mechanical fixing and electrical interconnection between the chip and the circuit board [1,2]. With the advancement of electronic manufacturing technology, electronic devices have been developing in the direction of miniaturization and high packaging density. A circuit board containing a CPU chip may have thousands of tiny solder joints. However, the service environment of electronic equipment has become more and more severe with the diversification of task requirements, especially in the aerospace and military fields. Airborne or ballistic electronic equipment are usually operated in frequent transitions between high and low temperature, or in harsh environments such as vibration and impact. The micro-interconnects on the circuit board are vulnerable to damage under severe external load conditions, what's more, if one of interconnect structures is damaged, the entire electronic device may fail [3]. According to statistics from the United States Air Force, vibration and shock is one of the main factors that cause the failure of electronic devices, and the failure of electronic devices caused by vibration and shock is as high as 20%. Therefore, many studies have focused on the reliability of interconnect structures under vibration and shock conditions.

Che et al. [4] studied the reliability of PQFP assembly with lead free solder joints under random vibration test. They used three different cycle-counting methods and the Miner's law to predict the fatigue life of the copper (Cu) lead and PQFP solder joint. Results show that solder fatigue failure is more critical than Cu lead failure for the PQFP assembly under the random vibration test. Gu et al. [5] researched vibration reliability of SAC305 after thermal or isothermal cycling. The result shows that the vibration reliability is decreased, and the failure mode is changed after thermal or isothermal cycling. Zhang et al. [6] analyzed failure mode of SAC305 solder joints subjected to random vibration loading under different temperatures. Results indicate that cracks exhibit three types of failure modes including crack between IMC and copper pad, crack within bulk solder and mixed crack in IMC and bulk solder. Liu et al. [7] investigated the lead-free solder joints reliability of BGA under random vibration. The results show that the failure mechanisms of solder joints vary as the acceleration PSD amplitude decreases, and the failure location changed from the solder bump body of the PCB side to the solder ball neck, finally to the Ni/intermetallic compound (IMC) interface of the package side. In 2015, they proposed FE model of BGA, and predicted its fatigue life under random vibration loading [8]. Based on the Palmgren-Miner's rule, Xia et al. [9] presented a general methodology and predicted the fatigue life of the Package-on-Package (PoP) under random vibration loading by means of vibration tests and finite element (FE) simulation.

Most of researchers have investigated BGA solder joint reliability under vibration conditions. However, the failure of the Plastic Quad Flat Package (QFP) interconnects under vibration conditions has not been adequately addressed, which is the focus of this study. QFP are commonly used in electronic products at present and in the future. The research object of this paper is the interconnect structure of QFP packaged devices. Fig. 1 shows the schematic diagram of QFP package. The interconnection structure referred in this paper consists of three parts: lead, solder and pad.

Currently, the information of the changes in the electrical and mechanical properties caused by the degradation of interconnect structure is mainly used to characterize its degradation. By attaching the strain gauges on the PCB near or rear to the key solder joints of the chip, Tang et al. [10] used the strain measured to predict the fatigue life of BGA solder joints. While Lall et al. [11] utilized high speed cameras to extract full field strain on the PCB in experiment. However, the requirements for strain measurement are high, and the characterization effect is closely related to the strain position and sensor accuracy. Liu et al. [7] connected solder joints of special BGA package in series to form two daisy-chain loops and measured the voltage change of daisy-chain loops to detect solder joints failure. But this method cannot be used with the package function implemented. Some researchers [[12], [13], [14]] have used FPGA's internal fabric logic to identify its fault. Wang et al. [15,16] proposed online test methods for FPGA single and double solder joint resistance. The result shows that the method can be used for online measurement of solder joints resistance in an FPGA design. Interconnect failure would lead to increase of interconnect impedance before complete failure occurs, so it is an effective way to predict the failure of interconnect by monitoring the impedance. In this paper, the interconnect structure is equivalent to a resistor and a capacitor in parallel, and an online monitoring circuit of QFP interconnect is built to monitor its health status, referring to BJBIST method of FPGA [14].

To investigate the failure and failure characterization of QFP package interconnect, a study is conducted under random vibration loading in this paper, by monitoring the health status of interconnect in real time.