Summary Chapter One: Flex Circuit Design Guide for Static and Dynamic Flexing

Flexible circuits by their very nature have an inherent ability to deform flexurally. This characteristic permits the roll-to-roll manufacturing processes for large-volume production. Designers have exploited the inherent flexibility to design flex circuits to be flexurally deformed during assembly, adjustment, and repair procedures, to have complex three-dimensional final geometries, and to sustain multiple flexural cycles during functional life.

According to Engelmaier, the items that need attention in the design phase can conveniently be divided into three groups: (1) conductor layout, (2) flex circuit stack up, and (3) material selection.

The factors influencing these design considerations from a flexibility viewpoint are: (1) assembly, adjustment and repair procedures, (2) final circuit geometry, and (3) number of flexures during functional life.

Chapter 1 discusses in detail strain vs. ductility—in particular, applied strain distribution vs. available ductility distribution and the statistical nature of strain and ductility.

Summary Chapter Two: Process Challenges and System Applications in Flex

The ability to produce large-area, fine-pitch flexible interconnect is driven by a number of elements. Those elements are comprised of materials, processes, facilities, equipment, design, and engineering support. The demand for thin, fine-pitch flexible interconnect requires unique considerations that are not possible with traditional printed circuit board (PCB) technologies.

The first part of Chapter 2 deals with requirements for the production of large-area, fine-pitch flexible interconnects, such as clean processing facilities, controlled defect densities, and thin dielectric materials. The second part of the chapter discusses how the use of flex has changed from first-level to second-level interconnect applications—in particular, the recent expansion in the role of flex to include system-level applications. Noting that device I/O counts have increased as semiconductor features have decreased, according to Moore’s Law, Chapter 2 discusses how this disparity in interconnect and semiconductor feature size has created an interconnect “brick wall” that demonstrates the need for advancing the capabilities of interconnect systems. A subsection deals with the field of printed electronics.

Summary Chapter Three: Flexible Circuit Assembly

Flexible circuits offer some unique challenges to the assembly process. The assembly materials and processes for populating and interconnecting components to a flexible circuit range are essentially identical to those used for standard rigid boards, but there are some twists required. The assembly processes range from very simple methods, such as manual component insertion and hand soldering (which requires little or no fixturing), to fully automated methods, which normally require specially developed, design-specific and dedicated fixtures. How does one choose an assembly process and method for flex circuits? It is necessary to consider a number of important factors: What is the flex circuit base material?

What types of components will be used? How many assemblies will be built? This updated chapter provides an assembly overview, with an emphasis on joining interconnecting substrates in a reliable and cost-effective way. Covered topics include flex circuit and component preparation, assembly process fixtures and tools, wave and selective solder fixturing, surface mount fixturing/tooling, component placement, various interconnecting and joining processes, and rework on flexible circuits.

Summary Chapter Four: Solderless Assembly Processes for Flexible Circuits

Because of the European Parliament’s sanction of lead in electronic assembly, there is an ongoing global effort to create lead-free electronic products. In this environment, a new approach to manufacturing all types of electronic assemblies, including flexible circuits, without the use of solder (i.e., solder-free) is now in development. The new process has come to be known as the Occam Process, so named to honor the 14th-century English philosopher and logician William of Occam, whose rigorous thinking and arguments in favor of finding the simplest possible solution served as the inspiration and catalyst for the new approach.

Fjelstad’s chapter addresses, among other topics, solderless assembly process basics, design and routing advantages of solderless assembly structures, and monolithic solderless flexible circuit assembly.