M. Koch

Large area embedding for heterogeneous system integration

The constant drive to further miniaturization and heterogeneous system integration leads to a need for new packaging technologies which also allow large area processing with potential for low cost applications. Wafer level embedding technologies and embedding of active components into printed circuit boards (Chip-in-Polymer) are two major packaging trends in this area. This paper describes the use of compression and transfer molding techniques for multi chip embedding in combination with large area and low cost redistribution technology from printed circuit board manufacturing as adapted for Chip-in-Polymer applications. The work presented is part of the German governmental funded project SmartSense. Embedding by transfer molding is a well known process for component embedding that is widely used for high reliable microelectronics encapsulation. However, due to material flow restrictions transfer molding does not allow large area encapsulation, but offers a cost effective technology for embedding on a medium size scale as known e.g. from MAP (molded array packaging) molding (typically with sizes up to 60 × 60 mm2). In contrast, compression molding is a relatively new technology that has been especially developed for large area embedding of single chips but also of multiple chips or heterogeneous systems on wafer scale, typically up to 8” or even up to 12”. Wiring of these embedded components is done using PCB manufacturing technologies, i.e. a resin coated copper (RCC) film is laminated over the embedded components - no matter which shape the embedded components areas are: a compression molded wafer, larger rectangular areas or smaller transfer molded systems (MAP). Typical process flow for RCC redistribution is lamination of RCC, via drilling to die pads by laser, galvanic Cu via filling, conductor line and pad formation by Cu etching, soldermask and solderable surface finish application - all of them standard PCB processes. The feasibility of the technology is demonstrated by the fabrication of a Land Grid Array (LGA) type package with two embedded dies. First step is a high precision die placement on an intermediate carrier. For embedding, both compression molding and transfer molding are used and directly compared with regards to material properties, processing, resulting die shift and warpage after molding. Reliability testing including MSL testing, temperature cycling, and humidity storage has been performed with LGA packages manufactured using the different technologies. The reliability potential and failure modes are intensively discussed and backed by destructive and non destructive failure analysis. Finally, an outlook for the integration of through mold vias into RCC redistribution process flow is given showing also the potential for package stacking.

High temperature potential of flip chip assemblies for automotive applications

Flip chip technology has been widely accepted within microelectronics as a technology for maximum miniaturization. Typical applications today are mobile products as cellular phones or GPS devices. The upper temperature limits for such applications range from 80 /spl deg/C to a maximum of 125 /spl deg/C. To widen the application range of flip chip technology and to address the volume market of automotive and industrial electronics, the development of high temperature capable assemblies is crucial. Typical scenario for the integration of electronics into a car is a control unit in the engine compartment, where ambient temperatures are around 150 /spl deg/C, package junction temperatures may range from 175 /spl deg/C to 200 /spl deg/C and peak temperature may exceed these values. When using flip chip technology under high temperature conditions, major challenges are found in the application of interconnect media and supporting polymers. At elevated temperatures, the intermetallic phase formation of lead-free solders might lead to a reliability decrease, where polymeric materials as substrate and encapsulant do potentially show mismatched thermo-mechanical properties or material degradation and thus reliability is reduced. Literature does typically describe flip chip technologies behavior on organic substrates for consumer applications, but almost no information is available on the performance at temperatures beyond 125 /spl deg/C. Within the European project HOTCAR, dealing with high temperature electronics for automotive use in general, a German consortium consisting of an IC manufacturer (IFX), two technology users (Siemens VDO & Temic) and a research institute (Fraunhofer IZM) have cooperated to evaluate the high temperature potential of lead-free flip chip technology for automotive applications. According to automotive demands, an experimental study on the suitability of advanced Underfill encapsulants for high temperature has been performed. With the outcome of this pre-study, two promising underfill materials were selected and used in a test run with an automotive test vehicle. This comprises an automotive grade /spl mu/Controller mounted on a substrate manufactured according to automotive standards, as the major system components. Solder material used was SnAg with a Ni UBM in combination with two different substrate finishes NiAu and immersion Sn. These test devices were submitted to temperature cycles according to automotive specifications with a maximum temperature of 150 /spl deg/C. Intermetallic phase formation was studied after high temperature storage by cross sections and shear tests. Typical failure modes for flip chip failure have been identified and are described in detail. The experimental reliability investigations were backed by thermo-mechanical simulations. Taking advantage of the so-called submodelling technique, the solder joint behavior could be studied in detail for lead-free solders. Starting stress-free at 150 /spl deg/C, the calculations followed the real thermal cycling regime. As primary results, the accumulated equivalent creep strain and creep strain energy distributions were obtained. Based on Manson-Coffin-coefficients from recent experiments at IZM, mean cycles to failure (MCF) have been estimated for solder joint fatigue and compared with observed failure. In summary, a status of the high temperature potential of lead-free flip chip technology under automotive conditions is given and a first design guideline for high temperature automotive flip chip applications is provided.

Flip Chip molding - Recent progress in flip chip encapsulation

As the development of microelectronics is still driving towards further miniaturization, Flip Chip technology has been widely accepted as a means for. maximum miniaturization with additional advantages. These are shortest interconnect length for minimum signal disturbance and simultaneous interconnection leading to reduced process times especially for high I/O counts and for RF applications. Flip Chip technology allows for reliabilities required for automotive applications, but to achieve this goal, a plastic encapsulant, the so called underfiller, has to be used. Conventionally a liquid epoxy resin is dispensed near the Flip Chip and is driven by capillary action under the chip. New material developments for transfer molding allow now underfilling and overmolding in one single transfer molding step. Existing standard equipment for encapsulation can be used and no additional process step for underfill dispensing is required. Molded Flip Chips have the potential of high reliability as the low CTE of the flip chip molding compound reduces the thermal mismatch. State of the art in FC molding is the encapsulation of Single Chip Packages as BGA or CSP. Trends of the market driving at SIPs with an integration of different devices as e.g. SMD and FC. Therefore the high reliable encapsulation of these hybrid packages with inhomogeneous topography is the future goal. For the qualification of Flip Chip Molding a test vehicle has been designed at Fraunhofer IZM. This test vehicle for process evaluation allows the encapsulation and underfilling of a single flip chip. Process development is described with a focus on Flip Chip and SIP molding challenges. Here the encapsulation process demands are filling of extremely small gaps without air entrapments, undisturbed bond integrity while molding at temperatures near melting point of the solder and increased pressures and venting of the mold. Device reliability demands are reduced warpage and optimum adhesion of the molding compound to solder mask, solder and die, even in harsh environment. Different materials available on the market are evaluated regarding process behavior and principal applicability for Flip Chip encapsulation. Nondestructive and destructive analysis is used to determine the failures occurring as voids and delaminations in initial state and during reliability investigations. In summary a status of the Flip Chip Molding technology is given.

Flip chip molding - highly reliable flip chip encapsulation

Flip chip technology has the shortest interconnect length for minimum signal disturbance and simultaneous interconnection leading to reduced process times especially for high I/O counts and for RF applications. Flip chip technology allows for reliabilities required for automotive applications, but to achieve this goal, a plastic encapsulant, the so called underfiller, has to be used. New material developments for transfer molding allow underfilling and overmolding in one single transfer molding step. Existing standard equipment for encapsulation can be used and no additional process step for underfill dispensing is required. Molded flip chips have the potential of high reliability as the low CTE of the flip chip molding compound reduces the thermal mismatch. State of the art in FC molding is the encapsulation of single chip packages as BGA or CSP. Trends of the market drive towards SIPs with an integration of different devices as e.g. SMD and FC. Therefore the highly reliable encapsulation of these hybrid packages with inhomogeneous topography is the future goal. For the qualification of flip chip molding a test vehicle has been designed at Fraunhofer IZM. This test vehicle for process evaluation allows the encapsulation and underfilling of a single flip chip.

Advanced flip chip encapsulation: transfer molding process for simultaneous underfilling and postencapsulation

The process development for four flip chip molding compounds was based on material characterization by DSC, DMA and TMA. It was shown that the materials tested do allow reliable flip chip molding. Materials properties concerning processability and reliability are promising. There is strong potential of the technology for the increasing market of flip chip packages as certain types of BGAs and, with further miniaturization, CSPs. As these packages incorporate typically single dies, the transfer mold process can be adapted without major changes to existing equipment. Even for future developments such as one chip flip chip SIPs using advanced IC thinning and assembly methods, the flip chip molding underfill process is a successful vision for reliable encapsulation.

Water diffusion in micro- and nano-particle filled encapsulants

Polymer materials - mainly epoxy resins - are widely used in microelectronics packaging. They are established in printed circuit board manufacturing, for adhesives as die attach glues or for encapsulants as molding compounds, glob tops or underfill materials. Low cost and mass production capabilities are the main advantages of these materials. But like all polymers they cannot provide a hermetical sealing due to their permeability properties. The susceptibility to water diffusion through the polymer and along the interfaces is a drawback for polymer materials in general, as water inside a microelectronic package might lead to softening of the material and to a decreasing adhesive strength and resulting delaminations close to solder bumps or wire bonds reducing package reliability by decreasing the package structural integrity. During package reflow, the incorporated humidity might lead to popcorning, i.e. abrupt evaporation of humidity during reflow soldering. This effect is one major problem during plastic package assembly. The introduction of high temperature lead-free soldering processes has even increased this issue. Therefore, plastic packaging materials with enhanced humidity resistance would increase package reliability during assembly and lifetime ideally without cost increase and with no changes in processing.

Precision material deposition for SiP manufacturing using jetting processes

During the last years, jetting processes for higher viscosity materials have gained widespread interest in microelectronics manufacturing. Main reasons for this interest are high throughput/productivity of jetting, contactless material deposition, high volume precision and freely designable deposition patterns. Especially the higher viscosity materials are of interest for the integration of a variety of heterogeneous components as needed for the assembly of System in Package. As knowledge on jetting behavior of these materials is not generally available, a study combining material analysis and process development has been conducted with the aim to demonstrate the limits of jet dispensing for higher viscosity materials. Summarized this paper gives a detailed insight into jet process develop for higher viscosity materials necessary for SiP assembly and describes process design rules and limitations and thus allows the optimized use of advanced jetting technology for microelectronics assembly.

Nano-particle enhanced encapsulants for improved humidity resistance

Polymer materials - mainly epoxy resins - are widely used in microelectronics packaging. They are established in printed circuit board manufacturing, for adhesives as die attach glues or for encapsulants as molding compounds, glob tops or underfill materials. Low cost and mass production capabilities are the main advantages of these materials. But like all polymers they can not provide a hermetical sealing due to their permeability properties. The susceptibility to water diffusion through the polymer and along the interfaces is a drawback for polymer materials in general. Water inside a microelectronic package might lead to softening of the material and to a decreasing adhesive strength and resulting delaminations close to solder bumps or wire bonds reducing package reliability by decreasing the package structural integrity. During package reflow, the incorporated humidity might lead to popcorning, i.e. abrupt evaporation of humidity during reflow soldering, is one major problem during plastic package assembly. The introduction of high temperature lead- free soldering processes has even increased this issue. Therefore, plastic packaging materials with enhanced humidity resistance would increase package reliability during assembly and lifetime without cost increase and with no changes in processing. The incorporation of nano-particles into plastic packaging materials is discussed as one potential solution for improved humidity resistance as it is a rather low effort approach to material modification opposed to chemical modification of the matrix. To evaluate the potential of such additives concerning moisture resistance the effect of nano-particles mixed with a microelectronic grade epoxy resin is studied. From the large variety of fillers available this work mainly focuses on three different types: nano-sized silica, modified bentonite and zeolites. Working principles of these particles range from large surface impact of nano-particles, barrier functionality due to stacked layer formation and molecular catcher function. Formulations with different particle concentrations and surface modifications are characterized regarding their influence on humidity diffusion, absorption and desorption behavior as well as their influence on other material properties as reaction kinetics, viscosity and thermo- mechanical properties. Additionally the combination of nano- and standard micro-particles needed for thermo-mechanical adjustment of the polymer properties is studied. Experimental work is accompanied by simulations, in order to provide further qualitative understanding on effects of particle form, size and surface properties. In summary this paper describes the potential of different nano-particles as additives for plastic packaging materials for enhanced humidity resistance/barrier enhancement within microelectronic packages. This topic is gaining increased importance when considering the trend towards system in package, where a multitude of components is encapsulated to form one SiP that incorporates a large number of different material interfaces and interconnects. All these interfaces and interconnects need to be protected from degradation caused by moisture ingress, without allowing much increased package volume or package cost. Polymers with improved moisture resistance can be one building block of future moisture resistant packages - the results of this study show their large potential for this field of application.

Enhancement of barrier properties of encapsulants for harsh environment applications

Medical devices with embedded highly miniaturized microsystems are used as implants in the human body or as non-invasive devices for sensor applications outside the body. Those devices bear quite a lot of economic opportunities but they also do offer unique challenges compared to consumer or automotive applications. Medical applications need to provide biocompatibility, highest miniaturization, rough treatment, autoclave sterilization and harsh environment e.g. humidity, wax, dust, blood or urine to be applicable. And microelectronics packaging needs to protect the functional elements of the microsystem against these rigid conditions. And, with a different set of media, packaging needs to fulfill the same task for automotive applications, where a growing number of control units and sensor systems under the hood in the transmission oil or petrol can be found. For both markets low cost packaging concepts with high media resistivity is needed. Polymer materials - mainly epoxy resins - are widely used in microelectronics packaging. They are established in microsystem manufacturing, for adhesives as die attach glues or for encapsulants as molding compounds, glob tops or underfill materials. Low cost and mass production capabilities are the main advantages of these materials. But like all polymers they cannot provide a hermetical sealing due to their permeability properties. The susceptibility to diffusion of liquids and gases through the polymer and along the interfaces is a drawback for polymer materials in general, as water or other media inside a microelectronic package might lead to softening of the material and to a decreasing adhesive strength and resulting delaminations close to solder bumps or wire bonds reducing package reliability by decreasing the package structural integrity. Therefore, plastic packaging materials with enhanced humidity resistance allowing the manufacturing of miniaturized microsystems for demanding applications as e.g. medical devices would increase package reliability during assembly and lifetime ideally without cost increase and with no changes in processing. As filler particles have an important influence on the final material properties of microelectronic encapsulants, they are well suited for material modifications. Typically micro-sized silica particles are incorporated into the polymer matrix as the thermo-mechanical properties could be well adapted to reliable packaging demands. However, there are a lot of nano-and micro-sized filler particles with potential to enhance the humidity barrier properties of encapsulants. Working principles of these particles may range from large surface impact of nano-particles, barrier functionality due to stacked layer formation (nano-clays), highly hydrophobic particle surface and molecular water catcher function. Micro- and nano-sized SiO2, bentonite, zeolites, Al2O3, carbon black and carbon nano tubes have been selected for a systematic study. To evaluate the potential of such additives concerning moisture resistance particles are mixed with a microelectronic grade epoxy resin. Neat particles as well as formulations are characterized regarding their water absorption, diffusion and barrier properties. Additionally multi-layer encapsulants with highest humidity barrier properties are introduced. Here, the mechanical or thermo-mechanical functionality is separated from humidity barrier characteristic. Polymer layers are processed wet in wet resulting in a homogeneous encapsulation with gradient material properties. Different methods for characterization of the diffusion properties close to microelectronics application have been developed and applied for material analysis. The pros and cons of simple weight measuring for absorption testing, sorption analysis, TGA desorption measuring, dielectric spectroscopy and encapsulated humidity sensors are presented and discussed along testing results with formulations with the different filler particles. The results of the measurement allow a modeling of the diffusion behavior of the characterized encapsulants and therewith a forecast on the later reliability of the overall system.

Nano- und micro sized filler particles for improved humidity resistance of encapsulants

Polymer materials - mainly epoxy resins - are widely used in microelectronics packaging. They are established in printed circuit board manufacturing, for adhesives as die attach glues or for encapsulants as molding compounds, glob tops or underfill materials. Low cost and mass production capabilities are the main advantages of these materials. But like all polymers they cannot provide a hermetical sealing due to their permeability properties. The susceptibility to water diffusion through the polymer and along the interfaces is a drawback for polymer materials in general, as water inside a microelectronic package might lead to softening of the material and to a decreasing adhesive strength and resulting delaminations close to solder bumps or wire bonds reducing package reliability by decreasing the package structural integrity. During package reflow, the incorporated humidity might lead to popcorning, i.e. abrupt evaporation of humidity during reflow soldering. This effect is one major problem during plastic package assembly. The introduction of high temperature lead-free soldering processes has even increased this issue. Therefore, plastic packaging materials with enhanced humidity resistance would increase package reliability during assembly and lifetime ideally without cost increase and with no changes in processing.