This paper describes flexible printed circuit boards,used as “Product Boards” or “Interposers”,chip mounting structures. Traditionally,flexible circuit boards are made with polyimide dielectric cores. These are made either by lamination processes of a foil with adhesive,or the build-up of the copper from an “adhesiveless” process

X

Hand-held communication and entertainment products continue to dominate the consumer markets worldwide and,with each generation,offering more and more features and/or capability. And even though the actual functionality of the new product offering expands,the customer is expecting each generation to be smaller and lighter that its predecessor. More functionality typically requires additional or more complex electronics and greater memory capacity. Increasing functional capability,however,can adversely impact the products size as well as manufacturing cost. The challenge manufactures face when competing in the world marketplace is to offer a product that will meet all performance and functionality expectations within budget and without increasing product size. Increased electronic functionality can be achieved through the development of more complex silicon integration (system-on-chip) but that route generally requires a great deal of capital resources and time. With the rapid deployment of new products from an ever growing number of competing companies’,time-to-market can be the difference between leading and following. For that reason,many manufacturers will rely heavily on more innovative IC package solutions,solutions for integrating a number of already proven functional elements within a single-package outline. When adapting multiple die configurations,each package becomes a fully tested subsystem that can be certified by the supplier before board or module level assembly. To achieve system level integration and miniaturization goals,companies’ can now rely on a combination of multiple-die package solutions and high-density flexible film based substrate methodology. For many applications,the multiple-die package is actually proving superior to the system-on-chip alternative because it minimizes risk and economically integrates several different but complementary functions. In the case of memory for example,multiples of the same function can be vertically stacked for increased density. This paper will explore three flexible film based multiple die system level applications developed within Tessera’s Package Engineering Service Laboratories.

Recent advances in chip technologies have prompted a rapid increase in the density of solder joints in electronic components. Further reductions in pitch are likely,leading to joint structures exhibiting sub 0.100mm (100µm) dimensions. EC legislation from mid 2006 bans the use of Pb,for most applications,in solders which means that next generation solder pastes will have to be Pb-free. One low cost assembly solution is stencil printing/wafer bumping of fine particle solder pastes. For ultra fine pitch applications this will present significant challenges and there is a requirement to understand the sub processes in stencil printing at ultra fine pitch. Paste roll; aperture filling/release; post print behavior and paste open time have been examined using fine particle Pb-free solder pastes,and solder paste rheology,particle size distribution,metal content,flux type and stencil aperture attributes have been investigated to provide ultra fine pitch solutions. In this paper we report that solder paste printing has been achieved at sub 100µm pitch using Pb-free solder paste with IPC type-6 (15-5µm) and type-7 (12-2µm) particle size distributions. For the type-6 paste,full array printing was achieved with 50µm deposits at 110µm pitch,and for peripheral printing patterns,60µm sized deposits at 90µm pitch. For type-7 paste sub 100µm pitch printing was achieved for full array patterns. The results satisfy the criterion that paste deposits can be produced at ultra fine pitch. Furthermore,subtle differences in the performance of type-6 and type-7 suggest that each is suitable for different specific application geometries. Reflow trials indicated that solderability of the small volumes depended heavily on reflow profile ramp rates and reflow atmosphere. Process models for introducing inert nitrogen reflow atmospheres are presented.

x

x

A flexible circuit is more than just thin materials made into an interconnecting device. Understanding the characteristics of the materials and their properties versus circuit design type and requirements,are variables that circuit designers and fabricators should understand in order to build the best performing and cost effective alternatives for their customers. Sometimes the requirements of a design are only vaguely identified and the information requested from a materials supplier is not always the most applicable for the desired design and application. Over or under performance is designed in,requiring several iterations of building and testing to get to the most desired cost effective end result. Prototyping with one material type and then going to production with another material type also is cause of concern for the end user. Not all materials are the same,even when they are “generically” lump ed into groups for convenience sake.

Two emerging board technologies,embedded passives and embedded optical waveguides,have the potential to change the way that printed wiring boards operate. No longer will interconnects be relegated to copper,but true passive electrical functionality will be incorporated into the board. Some high speed electrical interconnects may actually be replaced with pulses of light guided through transparent optical materials built into the board. These technologies are reviewed in simplified form,and the implications for board designers are examined.

Embedding resistors right into the printed circuit board substrate is not new,but it is gaining momentum as a rapidly
emerging and pivotal technology for the PCB industry,perhaps preceded only by the plated thru hole in the 50s and
microvias in the 80s. Embedding resistors requires designing resistors. Designing resistors requires understanding the resistor manufacturing process. Resistor materials are available in a wide range of values and technologies. This paper is written from a design for manufacturing perspective and includes guidelines for designing resistors with commercially available materials and manufacturing technologies to embed directly into the PCB substrate.

X