This paper presents stress measurements in leadframe based QFN components due to fabrication and assembly processes, characterized by substitution with stress measurement chips. The results are based on a method to determine stresses within electronic components by use of on-chip CMOS stress measurement technology, realized during the BMBF funded project iForceSens. All relevant production steps have been investigated. Four different chip and package sizes have been studied. Typical stress states will be compared with warpage and delamination analysis. The stress sensor is based on a CMOS chip. Although it is subdivided into 60 measurement cells (300 m grid) the sensor needs only four electrical connections. This alignment allows the determination of the stress distribution on the surface of the whole silicon chip. We are able to measure the shear and both main stresses in-plane of the chip surface with a resolution of < 10 MPa. The stress value of the measurement cell can be interrogated subsequently within 16 ms time steps. We used the main potential of this stress measurement chip by replacing the original electronic chip and investigating all production related loads acting on leadframe based QFN products. With the stress measurement chip we investigated wafer thinning as well as typical packaging processes like transfer molding. The measurement of stresses during soldering of QFN parts on printed circuit boards stretches the view towards the consumer product. We investigated application driven questions like the thickness of QFN packages and whether the QFN or chip size determines stresses inside the component

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Abstract-In this paper, the influence of microwave curing on the reliability of a representative electronic package is examined by reliability testing and measurement of residual stresses. A LM358 voltage regulator die was mounted to an open Quad Flat No-leads package (QFN) for reliability testing. For the stress measurement, a specifically designed stress measurement die was mounted to the QFN package. The chips were encapsulated with_Hysol EO1080 thermosetting polymer material. Curing was performed using an open-ended microwave oven system equipped with in situ temperature control. Three different temperature profiles for microwave curing were selected according to the requested degree of cure and chemical composition of the cured material. A convection cure profile was selected for the control group samples. Thermal cycling and HAST tests were performed on a total number of 80 chips. 95 QFN packages with stress measurement chips were also manufactured. Increased lifetime expectancy of the microwave cured packaged chips was experimentally demonstrated and measured between 62% to 149% increased lifetime expectancy after Temperature Cycling Test (TCT), and between 63% and 331% after highly Accelerated Ageing Test (HAST) and TCT compared to conventionally cured packages. Analysis of specifically designed stress test chips showed significantly lower residual stresses ranging from 26 MPa to 58.3 MPa within the microwave cured packages compared to conventionally cured packaged chips which displayed residual stresses ranging from 54 MPa to 80.5 MPa. This article therefore provides additional confidence in the industrial relevance of the microwave curing system and its advantages compared to traditional convection oven systems.

Packaging and curing induced stresses are the starting points for all further electronic reliability estimations [1]. Strain measurement in epoxy polymers during molding and curing is presented by use of embedded fiber optic Bragg grating (FBG) sensors. By mechanical coupling between polymer and fiber the FBG sensor enables monitoring of encapsulation induced strains and forces acting on the embedded electrical components. Determination of gel point and material shrinkage is possible directly in the mechanical environment of the product and production line as well as structural health monitoring of the encapsulated electronic components. It is shown that the grating is able to detect delaminations within the polymer during curing and post curing processes. We successfully tested fiber optic strain sensors in industrial transfer molding processes.

This paper investigates stress and strain within electronic systems during fabrication and reliability testing. The paper presents results of a new on-chip CMOS stress measurement technology. We investigated stresses during microelectronic packaging and reliability testing. Special focus is on transfer molding, stress during temperature change and during reliability tests. Experimental results are compared with numerical simulations. The stress sensor is based on a CMOS chip, subdivided into 60 measurement cells (300m grid) and needs only four electrical connections. We are able to measure the shear and both main stresses in-plane of the chip surface. The stress value of the measurement cell can be interrogated subsequently within 16 ms time steps. The main potential of this stress measurement chip is to replace selected electronic parts and to investigate all production and lifetime related loads acting on the system. The presented measurement technique enables a more accurate characterization of manufacturing processes and polymers' behavior. Finally the sensor enables the choice of optimized materials minimizing production related stresses and leading to more reliable products