Gerard Kelly

The importance of molding compound chemical shrinkage in the stress and warpage analysis of PQFPs

This paper addresses the use of finite element (FE) techniques to predict residual warpage in plastic quad flat packs (PQFPs) after encapsulation. Experimental measurements of package warpage are used to validate FE models of the packages. Failure to incorporate mold compound chemical shrinkage into the FE analysis leads to erroneous predictions of package warpage. The warpage sensitivity of different packages to changes in downset is presented. The validated FE package models predict stress levels in packages which are 70% greater than those with temperature coefficient of expansion (TCE) shrinkage alone and questions the accuracy of previous simulations which do not include molding compound chemical shrinkage.

Accurate prediction of PQFP warpage

This paper addresses the use of finite el ement techniques to predict warpage in plastic encap sulated ICs. A basic modeling assumption adopted in such analyses is that after ejection from the mold, warpage occurs due to the contraction of the molding compound as the package cools to room temperature. It is shown that this basic starting assumption can lead to incorrect predictions of the package warpage. Mea surements of the warpage of a plastic power package with temperature indicate that the package is signifi cantly deformed at the molding temperature. This is attributed to chemical shrinkage of the molding com pound. A new F.E. model of package warpage after en capsulation is proposed which considers both the TCE shrinkage and chemical shrinkage of the molding com pound. This leads to accurate predictions of warpage, particularly where heat spreaders make the package asymmetric. Measurements of a 208 lead power PQFP are used to verify this improved model.

The Simulation of Thermomechanically Induced Stress in Plastic Encapsulated IC Packages

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Investigation of thermo-mechanically induced stress in a PQFP 160 using finite element techniques

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3-D packaging methodologies for microsystems

The authors use finite element (FE) techniques to illustrate how thermomechanical stresses are built up within a plastic IC package, due to encapsulation. Plane strain is chosen as the most suitable 2D FE model for the thermomechanical stress analysis of plastic packages. The compressive stress on the die is separated into direct and bending components of stress. This is used to show that the molding compound induces over half of the encapsulation stress into the die. An explanation of the compressive stress distribution throughout the die is presented. The encapsulation stress on the die can be eliminated by isolating the die from the package with a side buffer of soft material.<<ETX>>

Selection of materials for reduced stress packaging of a microsystem

Issues associated with the packaging of microsystems in plastic and three-dimensional (3-D) body styles are discussed. The integration of a microsystem incorporating a micromachined silicon membrane pump, into a 3-D plastic encapsulated vertical multichip module package (MCM-V) is described. Finite element techniques are used to analyze the encapsulation stress in the structure of the package. Cracks develop in the chip carrier due to thermomechanical stress. Based on the results of a finite element design study, the structures of the chip carriers are modified to reduce their risk of cracking. Alternative low stress 3-D packaging methodologies based on chip on board and plastic leadless chip carriers are discussed.

3D packaging of a microfluidic system with sensory applications

Miniaturisation of many types of sensors and actuators has been realised by the advances in micromachining and microfabrication. This has led to a wide range of applications including microfluidic systems, where developments have resulted in much research in the area of μTAS (micro total analysis systems), used especially in analytical chemistry and chromatography. Among the main benefits of microsystem technology are its contributions to cost reduction, reliability and improved performance. However, the packaging of microsystems, especially microsensors, is one of the biggest limitations to their commercialisation as it can be the most costly part of sensor fabrication. This is because microsystems place extra demands on packaging techniques. For example, most microsystems need access to the outside world, other than electrical connection, in order to interact with the medium being measured or monitored. To reduce costs, a microsystem may be packaged in plastic but because of TCE (thermal coefficient of expansion) mismatches between different materials within the microsystem, the packaging process may generate high levels of stress which can negatively affect the systems operation and reliability. It is clear that conventional packaging approaches and materials are inapplicable to microsystems. Three-dimensional packaging techniques have great potential for microsystem integration. This paper will discuss the selection of materials applicable to the 3D packaging of any microsystem, including those containing extremely delicate micromachined structures such as membranes for micropumps and pressure sensors.

Low-stress 3d packaging of a microsystem

Among the main benefits of microsystem technology are its contributions to cost reductio, reliability and improved performance. however, the packaging of microsystems, and particularly microsensor, has proven to be one of the biggest limitations to their commercialization and the packaging of silicon sensor devices can be the most costly part of their fabrication. This paper describes the integration of 3D packaging of a microsystem. Central to the operation of the 3D demonstrator is a micromachined silicon membrane pump to supply fluids to a sensing chamber constructed about the active area of a sensor chip. This chip carries ISFET based chemical sensors, pressure sensors and thermal sensors. The electronics required for controlling and regulating the activity of the various sensors ar also available on this chip and as other chips in the 3D assembly. The demonstrator also contains a power supply module with optical fiber interconnections. All of these modules are integrated into a single plastic- encapsulated 3D vertical multichip module. The reliability of such a structure, initially proposed by Val was demonstrated by Barrett et al. An additional module available for inclusion in some of our assemblies is a test chip capable of measuring the packaging-induced stress experienced during and after assembly. The packaging process described produces a module with very high density and utilizes standard off-the-shelf components to minimize costs. As the sensor chip and micropump include micromachined silicon membranes and microvalves, the packaging of such structures has to allow consideration for the minimization of the packaging-induced stresses. With this in mind, low stress techniques, including the use of soft glob-top materials, were employed.

Microsystem packaging in 3D

Abstract The field of microsystems has in recent years attracted huge interest. Advances in micromachining have allowed the development of microfluidic components such as pumps, valves and flow channels. However, the packaging of microsystems, particularly microsensors, has proven one of the biggest limitations to their commercialization. This paper describes the 3D packaging of a microsystem demonstrator with sensor applications from the Esprit project BARMINT. Central to the operation of the 3D demonstrator is a micromachined silicon membrane pump to supply fluids to a sensor chamber over a universal sensor chip carrying ISFET-based chemical sensors, pressure and thermal sensors. The electronics required for controlling and regulating the activity of the sensors is also available on this chip and as other chips within the assembly, which also contains a power-supply module. Costs are minimized by use of standard off-the-shelf components. In addition, consideration is given to the need to minimize packaging-induced stresses on the delicate membranes and structures involved in the system, and the packaging approach described herein is compared, from a stress-minimization viewpoint, with previous and alternative approaches from within the project.

Thermomechanical Stress in a PQFP

Packaging influences the reliability and performance of microsystems. A brief history of developments in packaging is presented along with an overview of 3D packaging philosophy. An example of the integration of a micromachined silicon membrane pump into a 3D vertical multichip module package is presented. Finite element techniques are used to analyze the encapsulation stress in the assembled structure to improve the integrity of the packaged microsystem.