Flexible circuits Design

Frequency as an experimental variable offers additional design flexibility. This approach has several advantages. The most important one is the lack of polarization of the contacts. The second one is the fact that equivalent electrical circuit analysis can be used that aids in elucidation of the transduction mechanisms. Perhaps the most important distinguishing feature of this class of conductometric sensors is the fact that their impedance is measured in the direction normal to their surface. In fact, there may be no requirement on their DC conductivity and their response can be obtained from their capacitive behavior. In the following section, we examine so-called impedance sensors (or impedimetric sensors see Fig. 8.1b). 

This chapter presents an overview of performance plastic polymers in commercial planar and 3-dimensional circuit board products, and describes in detail one approach (two-shot molding) developed as an integrated 3-D circuit manufacturing technology. The distinctions between conventional planar (2-dimensional) circuitry, based on thermoset laminates and "subtractive etching processes, and the enhanced design flexibility afforded by expanded interconnection capacity in three axes are discussed. Specific examples of 3-dimensional interconnect protoypes and products are described and pictured. 

PZT plates, preferable in the thickness range between 100 pm to 250 pm in thickness, are processed to sensor and actuator modules by packaging using flexible circuit board processing. A plenty of designs are known... 

FPGA can be conceived as one application specific integrated circuit (ASIC) in a prototype stage. Two technologies have their own pros and cons, hence have their market share based on application. Major differences between ASIC and FPGA come from costs, tool availability, performance, and design flexibility. The major distinct features have been compared in Table APV/1.3-1. 

This process using Ticona s new LCP grade enables three-dimensional circuit boards, featuring a high degree of design flexibility to be mass-produced. The ability to produce fine conductor patterns in almost any lay-out offers scope for better space utilisation as a way to assist the miniaturisation process. Other applications include mobile phone antenna modules as well as in the development and implementation of new mechatronic systems. 

Coverlays and stiffeners are typical examples of the special components needed to build flexible circuits. Many types of materials have been introduced to accommodate the designs of the end applications. 

Since 1990, many new materials for flexible circuits have been developed in conjunction with design and manufacturing processes to satisfy new requirements in HDI applications. These include ... 

A screen-printing process similar to that used for rigid circuit boards is available for flexible circuits as the low-cost coverlay process. However, specially conditioned liquid solder mask materials are required to produce appropriate flexibility for the circuits. Standard solder mask materials designed for the rigid boards have small cracks and de-laminations when bending. 

New high-density flexible circuits have supplemental structures such as flying leads and micro bump arrays to complete high-density terminations. As volume production is limited, the designers should consider the mamrfacturers process capabilities seriously. 

A double-sided fiexibie circuit can have the same through-hole structures for the traditional designs as rigid circuit boards. However, there are several critical differences in micro via hoies on flexible circuits. Most of the differences come from a thin-film base substrate such as... 

FIGURE 62.18 Design of dynamic flexing part of flexible circuits (a) Unacceptable technique, (b) guide board, (c) loose guide, (d) silicone rubber. 

Because of the thiu and fragile materials used, flexible circuits have lower mechanical rehabihty than rigid circnit boards. They have a low conductor bond strength and low base snbstrate tear strength. Nevertheless, most of the flexible circuits are subjected to more mechanical stresses due to movement. This means that special care is required in the circuit design to gain higher circuit rehabihty. 

FIGURE 62,20 Reliability design tor flexible circuits (a) Unacceptable, (b) Preferred. 

Process elements particular to flexible circuits are required to achieve a high manufacturing yield. Also, an appropriate total process design is required to achieve high productivity with high yield. 

There may be several different processes for each via hole construction in the case of flexible circuits. Suitable processes, including material combination, should be designed according to final requirements (for example, construction, circuit density, reliability, manufacturing volume,... 

The conveyer systems of the wet processes designed for rigid circuit boards do not work properly for thin flexible materials. They can t make a high process yield generating a lot of wrinkles or scratches, even though the flexible sheet is fixed on a leader board or carrier frame. Appropriated conveyer system should be introduced for the volume production hues of the thin flexible circuits. 

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