Andreas Ostmann

Low cost bumping by stencil printing: process qualification for 200 μm pitch

Abstract Area array packages (flip chip, CSP (Chip scale packages) and BGA) require the formation of bumps for the board assembly. Since the established bumping methods need expensive equipment and/or are limited by the throughput, minimal pitch and yield, the industry is currently searching for new and lower cost bumping approaches. The experimental work of stencil printing to create solder bumps for flip chip devices is described in detail in this article. In the first part of this article, a low cost wafer bumping process for flip chip applications will be studied in particular. The process is based on an electroless nickel under bump metallization and solder bumping by stencil printing. The experimental results for this technology will be presented, and the limits concerning pitch, stencil design, reproducibility and bump height will be discussed in detail. In the second part, a comparison of measured standard deviations of bump heights as well as the quality demands for ultrafine pitch flip chip assembly are shown.

Stretchable electronic systems

The variety and applications of electronic systems have undergone a considerable evolution during the past century. Paramount milestones have been the introduction of the printed wiring board, which allowed for a rationalized assembly of electronic systems and the fabrication of integrated circuits with still ongoing increase of functional density due to miniaturization. Additionally sensors, micro electro mechanical systems, displays, and light emitting diodes have greatly added application potentials to electronic systems. Miniaturization is still one of the drivers for development in electronic components. On the other hand, however, conformity of the electronic systems to some give shape or space is increasingly requested for future applications. At present a great number of electronic appliances e.g. in mobile communication and medical electronics are manufactured to be foldable by using flexible printed wiring boards for assembly. The flexible printed wiring board restricts the bending to be only around on axis at a time. For a conformity to a truly three dimensional shape the wiring substrate needs to be stretchable to a certain extend. The development of stretchable wiring boards is the aim of the European project STELLA (stretchable electronics for large area applications). In the present paper process technologies for the efficient fabrication of stretchable wiring substrates using printing technology and/or photo structuring will be described. The conductive wiring which has to be as stretchable as the substrate matrix is designed as a meander and was optimized using FEM modelling. The electric contact formation is effected either by soldering techniques using low melting solder alloys or with anisotropic conductive adhesive. The interface between components assembled on the stretchable board and the wiring is mechanically the most fragile point of the wiring. Approaches for the reinforcement of the vicinity of the assembled components and first results are presented.

Stretchable Circuit Board Technology and Application

An innovative technology for the mass production ofstretchable printed circuit boards (SCBs) will bepresented in this paper. This technology makes itpossible for the first time to really integrate fine pitch,high performance electronic circuits easily into textilesand so may be the building block for a totally newgeneration of wearable electronic systems. Anoverview of the technology will be given andsubsequently a real system using SCB technology ispresented.

Stretchable electronic systems for wearable and textile applications

Assembly of electronic components on rigid and/or flexible printed circuit boards is today the customary way to fabricate electronic systems in stationary, mobile and automotive applications. On the other hand, many of the demands from emerging application fields like wearable and textile electronics cannot be met if with standard technologies. These fields have therefore become mayor drivers for the development of novel technologies. Among these dasiastretchable electronicspsila have attracted much attention recently. Especially for textile applications the potential of the electronic system to comply with the body shape and movement will considerably improve the user comfort. In this paper we will present a cost effective technology for the realization of stretchable systems by common printed circuit board techniques like lamination, lithography, etching and micro via technology with polyurethane as a stretchable matrix/substrate material. Mastering of the adhesion between materials and the transitions region from stretchable to non-stretchable parts of the system are crucial for the mechanical performance and robustness. Technical approaches and the obtained results to tackle these issues will be presented. After a complete embedding of the components/interconnections the systems can be firmly attached to textile or non-woven cloth, which can be subsequently integrated into garments. The described process technology bears the potential for large scale roll to roll processing. Reliability aspects for stretchable electronic systems are so far not standardized and will be discussed briefly. Electrical and mechanical functionality of test vehicles subjected to multiple stretch and mild washing cycles will be presented. A functional electronic demonstrator with embedded passives, a micro controller, and LEDs which was realized with this technology will be shown.

Stretchable electronic systems: Realization and applications

Commonplace electronic appliances for consumer or industrial use are still mostly rigid or at maximum flexible entities. The flexibility of foldable units like laptops or cell phones is usually realized through flexible circuit board (FCB) interconnectors. Although flexibility allows for considerably enhanced degrees of freedom in design, it is not compatible with more complex three dimensional curvatures and dynamics thereof. In the past years a number or approaches to realize stretchable electronic circuits in order to reach beyond unidirectional bending or folding of electronics have been reported. In the frame of the European Project STELLA a particular fabrication technology for stretchable electronic systems has been developed at Technische Universitaet Berlin. This technology, termed ?stretchable circuit board? (SCB) technology, is derived from conventional printed circuit board manufacturing. Stretchability of the boards is enabled by (i) using polyurethane instead of FR4 or polyimide as a carrier material of the copper structures and (ii) a meandering design of the Cu interconnects between commercial (rigid) electronic components. Such boards can be (once) extended by up to 300% before fracture of the Cu interconnections. For repeated elongation/relaxation cycles elongations with a few percent are allowable in order reach high cycle numbers. Electronic components are assembled after local application of a solder mask and surface finish for solderability. The electronic interconnection is established using a low temperature solder alloy (SnBi, Tm=142?C). For protection and enhanced system robustness all components are subsequently encapsulated within a polyurethane capping. Systems thus realized can be readily attached to different kinds of surfaces. Most interesting for various application cases is the easy attachment to textile substrates by a simple lamination process. The field use case studies of stretchable systems in the frame of the STELLA are mostly sensor applications in the field of medical electronics like a breathing frequency monitor for babies, a shoe insole pressure sensor for diabetes patients, or a band aid inlay to measure pressure and humidity of an acute wound when pressure therapy is applied. The latter application will be described in more detail since different aspects of bio-medical applications can be explained with this example. Another emerging field of applications is textile electronics, where it has been proven, that stretchable electronics can serve a versatile building blocks for complex electronic systems integrated in textiles.

Highly integrated flexible electronic Circuits and Modules

Within the electronic circuit board industry flexible circuit still cover a small the market share, however, with the fastest growth rate. The technology is increasingly used in automotives and aerospace, in handheld mobile appliances and many medical devices like pace makers or hearing aids [1,2]. Over past years a European consortium of research institutes and industry has explored the future technological potential of flexible printed circuits in the framework of the project SHIFT. One aspect was to investigate the frontiers of flexible circuit fabrication with respect to minimum feasible line width and pitch using different manufacturing methods. Still further beyond today mainstream flex fabrication technologies were the developments to integrate active and passive components into the buildup layers of flex circuits. In this way extremely high integration of electronic systems and highest functional densities can potentially be realized. Techniques and results of these developments will be presented in this paper. Embedded components in order to comply with the thin buildups of flexible circuits should be very thin as well. To this aim components were be mechanically thinned to 20 ptm. A dicing by grinding technique was applied using etched separation grooves on the wafer. Two technologies for embedding of ultra thin components were developed. The first one is thin flip chip assembly on inner layers of the flex and embedding by subsequent lamination of build up layers. The gap between chip and substrate was in the order of a few microns using either low profile solder or anisotropic adhesive.

Printing Solder Paste in Dry Film - A Low Cost Fine-Pitch Bumping Technique

There is a steady trend to decrease bump pitch and the total cost for the bumping process. Several techniques for bumping are available such as screen printing of solder paste through metal stencils, electroplating of bumps, ball placement or C4NP. Initially there appears to be a number of choices for bumping, however after looking at the details the choices are significantly reduced. The use of ball placement or screen printing is limited to larger sized bumps and coarser pitches. Bumping by electroplating which is popular for fine-pitch bumping is restricted to single or double component systems. Electroplating of triple component systems like SnAgCu which are interesting for lead free bumping requires a very complex process monitoring. Additionally, electroplating of ternary metal systems is not yet suitable for manufacturing. In this paper the basic process steps for a different bumping technology are discussed in detail. This technology uses a solder paste which is printed into photo-structured dry film. Test wafers with Ni + flash-Au final metallisation were provided. A dry film photo resist was laminated onto the wafers, exposed and developed. Next, solder paste was printed into the structured resist. The wafers were then passed through a Pb-free reflow profile. After the reflow, the dry film was successfully stripped. A variation of pad size and resist openings were investigated. Correlation between design and results are discussed.

A 50 kW IGBT power module for automotive applications with extremely low DC-link inductance

A power module for hybrid electric vehicles is designed and constructed applying innovative packaging technologies. Instead of a DCB (direct copper bond) substrate and bond wire connections PCB (printed circuit board) technologies with embedded power semiconductors are used to build a 50kW automotive drive train inverter (Power PCB). The PCB technologies enable the design of very flat, compact and low-inductive power modules. The thermal performance of these modules is comparable to conventional DCB solutions. The built Power PCB is proven to be far superior compared to an equivalent bond wire solution in terms of switching performance. In fact, the parasitic dc-link inductance can be drastically reduced from 15:4nH (standard) down to 2:8nH (Power PCB). Thereby, the turn-off overvoltages are decreased significantly. Consequently, the switching losses are reduced and also conduction losses can be potentially reduced by an optional increase of the dc-link voltage. Thereby, the energy efficiency of electric vehicles can be substantially enhanced.

Alternative UBM for Lead Free Solder Bumping using C4NP

This paper analyzes two alternative under bump metallurgy (UBM) structures: sputtered TiW/Ni and electroless Ni/immersion Au (ENIG), with and without Pd. Wafers were fabricated with these UBM structures, solder applied with C4NP, and chip level stressing performed to determine the robustness of these alternative stack-ups. Microelectronic packaging continues the migration from wire bond to flip chip first level interconnect (FLI) to meet aggressive requirements for improved electrical performance, reduced size and weight. Analysis of these structures following multiple reflows and thermal cycling is presented.

Stretchable electronics manufacturing and application

In the recent years the fabrication of electronic systems that can to a certain extent be stretched has attracted increasing attention. As general motivation such electronics will in contrast to conventional electronics be compliant with free form shapes, as for example the human body surface. A number of different successful approaches to realize such systems have been demonstrated. Thus far various aspects of stretchable electronics have been addressed ranging from stretchable Si detector arrays to large scale, low cost stretchable systems with integrated commercial components. Within the EU project STELLA the latter types of fabrication technologies have been developed. These technologies are presently further developed with focus on optical applications in the project PLACE-it. The rationale of both projects is the use of conventional printed circuit board technologies for the fabrication of stretchable wiring substrates onto which components are assembled and embedded. Stretchability is accounted for by a meandering layout of the Cu-interconnects between components and the rubber like polymeric substrate polyurethane, instead of polyimide which is used in conventional flex-prints. In the present paper basic features of the fabrication process for stretchable substrates will be discussed. Selected materials, process parameters and design aspects will be addressed with respect to selected target applications. Reliability requirements of stretchable systems are very much dependent on the application scenario. Besides obvious stretch-to-failure, cyclic stretching and bending tests, also robustness with respect to cleaning and washing is required. Comprehensive reliability standards are yet to be defined. In the present paper a brief overview of reliability assessment will be given. A few of the realized applications using stretchable electronics will be presented and discussed. The versatility of the fabrication technology and its products will be emphasized. The potential impact on different application fields will be highlighted, as for example electronic integration into textiles and in automotive applications.