Open Ended Microwave Oven for Packaging

📝 Abstract

A novel open waveguide cavity resonator is presented for the combined variable frequency microwave curing of bumps, underfills and encapsulants, as well as the alignment of devices for fast flip-chip assembly, direct chip attach (DCA) or wafer-scale level packaging (WSLP). This technology achieves radio frequency (RF) curing of adhesives used in microelectronics, optoelectronics and medical devices with potential simultaneous micron-scale alignment accuracy and bonding of devices. In principle, the open oven cavity can be fitted directly onto a flip-chip or wafer scale bonder and, as such, will allow for the bonding of devices through localised heating thus reducing the risk to thermally sensitive devices. Variable frequency microwave (VFM) heating and curing of an idealised polymer load is numerically simulated using a multi-physics approach. Electro-magnetic fields within a novel open ended microwave oven developed for use in micro-electronics manufacturing applications are solved using a de icated Yee scheme finite-difference time-domain (FDTD) solver. Temperature distribution, degree of cure and thermal stresses are analysed using an Unstructured Finite Volume method (UFVM) multi-physics package. The polymer load was meshed for thermophysical analysis, whilst the microwave cavity - encompassing the polymer load - was meshed for microwave irradiation. The two solution domains are linked using a cross-mapping routine. The principle of heating using the evanescent fringing fields within the open-end of the cavity is demonstrated. A closed loop feedback routine is established allowing the temperature within a lossy sample to be controlled. A distribution of the temperature within the lossy sample is obtained by using a thermal imaging camera.

💡 Analysis

A novel open waveguide cavity resonator is presented for the combined variable frequency microwave curing of bumps, underfills and encapsulants, as well as the alignment of devices for fast flip-chip assembly, direct chip attach (DCA) or wafer-scale level packaging (WSLP). This technology achieves radio frequency (RF) curing of adhesives used in microelectronics, optoelectronics and medical devices with potential simultaneous micron-scale alignment accuracy and bonding of devices. In principle, the open oven cavity can be fitted directly onto a flip-chip or wafer scale bonder and, as such, will allow for the bonding of devices through localised heating thus reducing the risk to thermally sensitive devices. Variable frequency microwave (VFM) heating and curing of an idealised polymer load is numerically simulated using a multi-physics approach. Electro-magnetic fields within a novel open ended microwave oven developed for use in micro-electronics manufacturing applications are solved using a de icated Yee scheme finite-difference time-domain (FDTD) solver. Temperature distribution, degree of cure and thermal stresses are analysed using an Unstructured Finite Volume method (UFVM) multi-physics package. The polymer load was meshed for thermophysical analysis, whilst the microwave cavity - encompassing the polymer load - was meshed for microwave irradiation. The two solution domains are linked using a cross-mapping routine. The principle of heating using the evanescent fringing fields within the open-end of the cavity is demonstrated. A closed loop feedback routine is established allowing the temperature within a lossy sample to be controlled. A distribution of the temperature within the lossy sample is obtained by using a thermal imaging camera.

📄 Content

9-11 April 2008 © EDA Publishing/DTIP 2008

ISBN: 978-2-35500-006-5 Open Ended Microwave Oven for Packaging

K. I. Sinclair1, T. Tilford2, M. Y. P. Desmulliez1, G. Goussetis1, C. Bailey2, K. Parrott2 and A. J. Sangster1

1. MicroSystems Engineering Centre (MISEC) School of Engineering & Physical Science Heriot Watt University Edinburgh, EH14 4AS United Kingdom.
2. Centre for Numerical Modelling and Process Analysis (CNMPA) University Of Greenwich London, SE10 9LS United Kingdom

Abstract - A novel open waveguide cavity resonator is presented for the combined variable frequency microwave curing of bumps, underfills and encapsulants, as well as the alignment of devices for fast flip-chip assembly, direct chip attach (DCA) or wafer-scale level packaging (WSLP). This technology achieves radio frequency (RF) curing of adhesives used in microelectronics, optoelectronics and medical devices with potential simultaneous micron-scale alignment accuracy and bonding of devices. In principle, the open oven cavity can be fitted directly onto a flip-chip or wafer scale bonder and, as such, will allow for the bonding of devices through localised heating thus reducing the risk to thermally sensitive devices.
Variable frequency microwave (VFM) heating and curing of an idealised polymer load is numerically simulated using a multi- physics approach. Electro-magnetic fields within a novel open ended microwave oven developed for use in micro-electronics manufacturing applications are solved using a dedicated Yee scheme finite-difference time-domain (FDTD) solver.
Temperature distribution, degree of cure and thermal stresses are analysed using an Unstructured Finite Volume method (UFVM) multi-physics package. The polymer load was meshed for thermophysical analysis, whilst the microwave cavity –
encompassing the polymer load – was meshed for microwave irradiation. The two solution domains are linked using a cross- mapping routine. The principle of heating using the evanescent fringing fields within the open-end of the cavity is demonstrated. A closed loop feedback routine is established allowing the temperature within a lossy sample to be controlled.
A distribution of the temperature within the lossy sample is obtained by using a thermal imaging camera. I. INTRODUCTION Microwave radiation fundamentally accelerates the cure kinetics of polymer adhesives. It provides a route to deposit heat energy primarily into the polymer materials [1]. Therefore, microwave radiation can be used to minimise the temperature increase in the surrounding materials such as the substrate and die during the cure process. This is especially important for devices incorporating either low thermal budget materials or interfaces with large thermal coefficient mismatch. The concentration of heat into the polymer during the cure process promotes its adhesion properties, as the magnitude of residual stress will be low between the polymer and materials, to which it is being bonded. Multi-mode waveguide cavities have been used for the curing thermosetting resins [2], [3]. The ability to couple high power into a cavity oven enables fast polymerisation and large temperature gradients within the epoxy resin [4]. A material within a waveguide cavity oven with lower loss characteristics will remain at a lower temperature compared to the higher loss epoxy materials due to the selective nature of microwave energy absorption. Fundamentally, this characteristic reduces the thermal expansion mismatch between carrier and die allowing curing to be conducted in the presence of thermally sensitive components. As a consequence, it is possible to increase [or a least not decrease] the lifetime of the product [5]. Convention thermal curing heats the substrate first before transferring heat into the polymer while microwave curing process heats the polymer directly due to the volumetric nature of the absorption mechanism [6]. A maturing microwave curing technology is variable frequency microwave (VFM). Current VFM curing methods in electronic packaging bulk ‘heats’ the entire package or substrate [7]. For VFM, the source frequency is swept through a wide bandwidth, resulting in a scintillation of the model fields within the volume of the cavity resulting in a uniform field pattern. As such, a constant heating distribution can be obtained within homogenous materials [8]. In addition, sweeping the source frequency eliminates the likelihood of arcing thus allowing microwave heating of metals and semiconductor without damage due to electro- static discharge (ESD) [9]. Single frequency microwave

Fig. 1 Schematic showing layout of the cavity oven and the comparison between a ‘conventional’ assembly process and the proposed open-ended oven.

Copper wall Ceramic insert

Coaxial feed Polymer encapsulant

Silicon semiconductor Circuit board

Polymer underfill Ball grid arra

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