Study on the Impact of Lead Sidewall Solder Coverage and Corner Lead Size on Joint Reliability

Solder joint reliability is very important to ensure that an integrated circuit (IC) semiconductor package is functional within its intended life span as the solder joint establishes electrical connection between the IC and the printed circuit board (PCB). Solder fatigue failure or crack under thermal cycling is one of the common problems with board-mounted packages. There are several factors or package characteristics that have impact on solder fatigue life like package size and material properties of the package components. This paper presents a thermo-mechanical modeling of a leadframe-based semiconductor package to study the impact of lead sidewall solder coverage and corner lead size on the solder joint reliability. Finite element analysis (FEA) technique was used to calculate the solder life considering 50% and 100% package lead sidewall solder coverage as well as smaller and larger critical corner leads of the package. The results of the analysis showed that higher lead sidewall solder coverage and larger lead could significantly increase solder life. Therefore, ensuring lead sidewall solder wettability to have higher solder coverage is beneficial. The study also reveals that packages with side wettable flanks are not only enabling high speed automated optical inspection required for the automotive industry, but they are also providing improved solder joint reliability.

A Study on the Effect of Bond Wires Lift-Off on IGBT Thermal Resistance Measurement

Bond wire lift-off will cause an increase of remaining wires’ power dissipation, which usually is ignored for healthy modules. However, only partial wires’ power dissipation transfers through thermal path from junction to case, which will lead to overestimate the whole power dissipation from collector to emitter pole and underestimate the calculated thermal resistance using the proportion of temperature difference to power dissipation. A FEM model is established to show the change of heat flow after bond wires were removed, the temperature of bond wires increases, and the measured thermal resistance decrease after bond wires lift-off. It is validated by experimental results using open package Insulated Gate Bipolar Transistor (IGBT) modules under different current conditions. This conclusion might be helpful to indicate the bond wires lift-off and solder fatigue by comparing the change of measured thermal resistance. Using the Kelvin setup to measure thermal resistance will cause misjudgment of failure mode due to the ignoring of wires’ power dissipation. This paper proposed that the lift-off of bond wires will lead to underestimating the thermal resistance measurement, which will overestimate the lifetime of IGBT module and misjudge its state of health.

An Improved Electro-Thermal Model to Estimate the Junction Temperature of IGBT Module

Junction temperature is a key parameter that influences both the performance and the reliability of the insulated gate bipolar transistor (IGBT) module, while solder fatigue has a significant effect on the accuracy of junction temperature estimates using the electro-thermal model. In this paper, an improved electro-thermal model, which is independent of solder fatigue, is proposed to accurately estimate the junction temperature of IGBT module. Firstly, solder fatigue conditions are monitored in real time with the information of the case temperatures. Secondly, when solder fatigue is found, the update process of the electro-thermal model parameters is performed to match the model parameters with the fatigue device. With the above two-step process, the influence of solder fatigue on the accuracy of temperature estimates can be removed in good time. Experimental results are provided to validate the effectiveness of the proposed method.