A Multidisciplinary Approach to PWB Design

Chao-pin Yeh, R. E. Fulton, C. Umeagukwu George W: Woodruff School of Mechanical Engineering

J. L A. Hughes School of Electrical Engineering

The Georgia Institute of Technology

Abstract

Although virtually all electronic products have mechanical requirements, the mechanical design aspects of printed wiring boards (PWBs) are typically considered only at the fmal stage of the design process. Due to recent advances in PWB technology, the mechanical design aspects, especially the thermo-mechanical behavior, have become increasingly important in the PWB design process. Today, the electrical computer-aided design (ECAD) systems alone are no longer sufficient as a design environment to meet the ever-stringent requirements of the commercial and military PWB specifications. In addition to enhanced ECAD capacities (such as electromagnetic modeling and testability analysis), PWB design systems need to incorporate comprehensive mechanical computer-aided design (MCAD) tools to deal with mechanical design issues, such as thermal management and structural vibration.

At Georgia Tech, a research team is collaborating with the electronics industry to develop a prototype integrated E/MCAD PWB design system denoted Thermal Structural Electromagnetic Testability (TSET). TSET, responding to this E/MCAD conapt, proposes an integrated information framework which employs an executive-centered information management approach with a common database approach. This paper addresses TSET development with an emphasis on the mechanical design and information management issues. The TSET database design is based on a systematic, structured methodology which includes the following steps: requirements analysis, information analysis, logical design and physical implementation.

I. Introduction

Traditionally, PWBs are developed using a design-build-and-test method which is very time-consuming and expensive. With the increasing complexity in modern microelectronics, a growing number of boards are too sophisticated to be prototyped in this manner. Instead, computer simulation (electronic prototyping), has become a vital and mandatory design tool for PWB designers. In recent years, a wide range of design tools and support systems have been brought to the PWB design market. These tools and systems have greatly improved engineering accuracy and productivity [I] however, there exist twoproblems.

First, the majority of these tools are ECAD tools, which focus on schematic capture, circuit simulation, component placement and routing [2]. Little effort has made in developing the MCAD tools [3]. Mechanical design aspects, such as thermal design, vibration analysis, design for assembly (DFA) and design for manufacturing (DFM) have been deferred and sometimes ignored in the PWB design process. With the advent of surface mount technology, ultra large scale integrated (UUI) circuits and hybrids microelectronics, the power density in the PWB has increased by a factor of 6 to 60 times [2]. The increasing power density could cause a high failure rate for electronic components which are very vulnerable to elevated temperature. The thermo-mechanical design becomes increasingly crucial to the PWB design process. Existing ECAD systems are also inadequate in the areas of PWB testability and electromagnetic analysis. These existing ECAD systems alone are no longer sufficient as a design environment to meet the ever- stringent requirements both for military and commercial applications.

The second problem is that these tools, developed and implemented with a very narrow focus, have their own internal data representations and formats which are incompatible with each other. Such incompatibility results in an environment composed of what has been called "islands of automation" 191. Piecemeal design automation leaves far too many gaps in the flow of the system from concept to completion and the overall performance is not satisfactory. This unsatisfatory performance arises since CAD/CAM design tools concentrate on specific aspects of the entire PWB design process, without utilizing an underlying integrated information framework [SI, To

tackle these problems, many independent or wperative projects have been initiated and proposed to develop such information frameworks. Due to the enormous development effort and time involved, most these frameworks are research-oriented prototypes which have only limited capability in real world applications [5,6,7J. A few major projects, such as Engineering Information System (EIS) [8,9] and CAD Framework Initiative (CFI) [lo], formed by CAD vendors, electronics manufacturers, and government agencies, are currently developing large-scaled information frameworks which can be used in a wide range of engineering applications. However, they will not be available in the market for several years.

In a separate effort, a research team at Georgia Tech is collaborating with the electronic industry to develop a prototype information framework with an emphasis on the PWB design. This integrated PWB design system, called the Thermal Structural Electromagnetic Testability (TSET) system (Figure l), employs an executive-centered information management approach with a common (global) engineering database. In this paper, incorporating thermo-mechanical and enhanced ECAD tools in the PWB design procesr is addressed in Section 2. The common data exchange formats/languages for PWB design are described in Section 3. Finally, Section 4 illustrates the information management issues along with the proposed TSET system architecture and its database design methodology.

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Figure 1. The System Architecture for TSET System.

2-

p Electronic components a re extremely sensitive to extremes of

temperature [ll]. It has been shown that for every 10 to 20 C 0 rise in temperature, the component failure rate doubles [12,2]. In recent years, PWB packaging density and power consumption have increased rapidly, while packaging size and weight have decreased considerably. In short, the heat flux that must be dissipated from the electronic package has increased. As a result, the thermo-mechanical analysis for PWB design has become more critical in order to achieve acceptable product reliability. Primarily, thermo- mechanical analysis consists of two closely-related analyses: (1) thermal analysis of temperature distributions and heat dissipation on the package under the power cycling and operating conditions, and (2) thermal stress/warpage analysis of board stresses and deformations when subjected to thermal loadings, such as during soldering. Traditionally, thermal analysis is performed by building and testing a prototype board. Nevertheless, it is generally deferred until the end of the design process [13], at which time it k difficult and expensive to resolve problems by modifying component

1090 0569-5503/90/0000-1090 $1 .OO ' IEEE

selections, placement decisions or layout configurations [14,3]. Heat sinks, thermal rails and additional airflow are only available solutions that can be implemented within the allowable time. Unfortunately, these solutions are not normally optimal and, hence, not preferable.

2.2 Thermal Str ess/Waroaee Analysis Usine the Finite Element Method

The warpage and stresses induced by the mismatch of the Coefficients Thermal Expansion (CTE) of PWB heterogeneous materials (components, board composite layers, copper traces, etc.) at an elevated temperature can be signifcant. Such warpage and stresses can cause failures and malfunctions invarious forms. For example, excessive deformation/warpage may cause fractures in solder joints and component pins and excessive stresses may cause delamination between layers and cracking in plated-through-hole (PTH) copper barrel plating. A 1985 DOD study showed that thermal stress caused more failures than any other causes including vibration and humidity 121. Although thermal-induced warpage and stresses are very critical to electronics reliability, they have frequently been ignored and neglected in the PWB design process [ll]. Most thermal- mechanical analysis is done by experimentally using thermal cycling testing, thermal shock testing, infrared thermography testing 1151, piezoresistive strain gauge testing, and photoelasticity testing. Although indispensable in thermo-mechanical analysis, these methods are severely limited (for example, restricted to only the surface of the boards). In addition, building a prototype PWB is time- consuming and expensive. Most importantly perhaps is that testing alone without the support of analytical fundamentals, will not provide the needed understanding and insight to PWB behavior. These fundamentals and inslghts are crucial to the development of enhanced MCAD tools for PWB design. Furthermore, the experimental approach is not easily incorporated into a computerized design system.

For these reasons, the use of computer-based analytical tools has been proposed and implemented to replace or to abate the role of experimental measurements in the PWB design process. Many numerical simulation techniques are available such as the finite difference, finite element and boundary element methods 1161, each with a sound theoretical basis. Although each of these methods has its advantages, it is believed that the fmite element method (FEM) is more suitable for PWB thermo-mechanical analysis. First, the FEM is capable of mathematically simulating an odd, irregular and complex geometry (such as in a PWB) where a closed-form solution is almost impossible to obtain [16]. The development of state-of-the- art finite element techniques as well as the advances in high-performance digital computers allows static/dynamic, linear/non-linear, and steady- state/transient analysis to be done in a rather economical and rapid manner, which is vital to the PWB design process. Furthermore, the FEM is advantageous Since parametric changes in the model (dimensions, material properlies, and loading characteristics, etc.) can be easily incorporated.

Surface mount technology (SMT) has become a major trend in the electronics packaging industry. With SMT, compact and modular surface mount components (SMC) are soldered directly to the surface of a PWB. SMT offers potential cost and weight reduction as well as higher package density. However, at the same time, SMT generates some potential problems in the manufacturing soldering process where the reflow temperature can reach as high as 2700C. The CTE mismatches between the SMC and PWB may rupture the solder joints created during the soldering or subsequent cooling processes. Therefore, thermal analysis for such a process is frequently performed using thermal shock testing. The finite element analysis software must be able to simulate the PWB behavior in the soldering process. The dielectric composite layer material (such as FR4, polyimide, etc.) properties vary considerably with different temperatures, especially when the board temperature surpasses the glass transition temperatures (Tg). This indicates that a complex, computation-intense, non-linear and transient analysis is necessary.

In order to validate the analytical analysis, an experimental approach has been independently conducted for this research project. A Manhattan- patterned PWB sample is selected since it is easy to build and model and it produces high warpage when subjected to thermal loads. The PWB test sample consists of four signal layers and three dielectric composite layers. The top and bottom signal layers are made using two sets of parallel copper traces that are perpendicular to.each other. The warpage of the sample will be measured at elevated temperatures using the Shadow Moir Method 117. A separate analytical finite element analysis is performed to simulate the PWB's behavior under the identical conditions. The warpage results obtained from both the analysis and the experiment will be correlated to validate the

use of a finte element model and the modeling technique. Figure 2 shows a sensitivity study result (deformations vs. outer copper area percentages) for a Manhattan-patterned PWB using the ANSYS finite element program.

Up to now, the analysis associated with mechanical design (especially for thermo-mechanical analysis) has been deferred to the end of the design process by testing prototypes or employing over-simplified manual or semi- manual calculations [1,14]. These methods are insufficient in modern electronics design where a fast turnaround time and high accuracy are required. To facilitate the PWB design, (finite element analysis software, design for assembly, etc.) are as important as tools. In addition, the MCAD tools should be introduced into the early stage of the d&gn process and should be considered throughout all phases of the design 114,111, to prevent the above-mentioned drawbacks and to increase engineering productivity (181.

2.3 PWB Test-

The electronic (or circuit) design aspects of systems implemented on PWBs have traditionally been viewed as independent of the mechanical and manufacturing design process. However, improved PWB manufacturing technology has dramatically increased the interdependence of mechanical and electrical design. Circuit testability, particularly in-circuit test of PWBs loaded with components, is a prime example.

Existing PWB test methods utilizing bed-of-nails testers rely on a "brute force" approach whose success depends on both a relatively small number of components on the board and easy a m to the interconnection signal paths. Improved manufacturing technology has resulted in r e d u d signal trace width and spacing, an increased number of signal layers, and more advanced methods for component mounting. Surface mount technology allows components to be placed more densely on both sides of the board, making probing prohibitively costly. Pin-grid array and "flipchip" packages cannot be probed from the device side, since the signal connections are physically located between the component and the PWB. New testing methods must be developed that accommodate the changes in the mechanical structure of the PWBs.

The primary emphasis of electronic circuit testing is verifying correct logical behavior of the system under test; i.e., "functional" or "digital" testing. Other types of testing such as timing analysis, signal crosstalk, and parametric characterization are also important in the manufacturing of PWBs. New strategies - which are fully integrated with the mechanical and manufacturing aspects of the design - will be needed for these types of testing to accommodate the increasing PWB complexity.

Figure 2. Sensitivity Study for Manhattan-Patterned PWB Using ANSYS Finite Element Program.

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3. Desim Data Exchange Smcifications for PWB Desim

3 3

Due to the incompatibilities between information representations and user interfaces, most PWB design tools/systems are not able to share design data directly. Since many different tools are employed during various phases and levels of the design process, this inability to communicate with each other creates major difficulties for engineering management. As a result, a translator software for interfacing between any two incompatible tools is needed to bridge the information gap. Vexed by this problem, many electronics organizations, academic societies, and military agencies have joined forces to develop new specifications and standards to facilitate communication and share design data between disparate CAD systems. Among these standards/formats, VHSIC Hardware Desaiption Language (VHDL), (Electronic Design Interchange Format (EDIF), Initial Graphics Exchange Specification (IGES), and Product Definition Exchange Specification (PDES) have been gradually recognized in the PWB design industry. The detailed descriptions for these formats/specifications have been documented in many publications [19,20,21,13,22,23] and will not be discussed in this paper.

3.2 Format/Standa rd Inteeration

Since each of the exkting/proposed standards/formats (EDIF, VHDL, IGES, PDES) was developed for a specific set of applications, they are useful for improving expressiveness and reducing communication barriers between various CAE/CAD vendors within the same application. However, there are still some limitations to these standards/formats. Figure 3 shows the various engineering disciplines supported by these formats/languages. IGES and PDES, for example, have only a primitive notion of timing concepts [24] and are not considered to be a suitable languages for simulation. VHDL and EDIF, on the other hand, can only be applied to 2D environment [lo].

Currently, these standards/formats rarely address the representations which are important late in the design process (testing, maintenance, etc.) although newer ones are developing in that direction. a further limitation is that these stands/formats may use the Same key glossary to denote different concepts. This semantic discrepancy may cause unwanted ambiguity. It is believed that no reasonable single specification is broad enough to be applied to the entire design process. Instead, multiple specifications should co-exist with and supplement each other in a coherent manner. To remedy the semantic discrepancy and be able to integrate these exchange formats, an underlying conceptual common data dictionary/directory (Figure 4). along with an integrated information framework, has been proposed and is being implemented in the current research.

4. Information Inteaation Framework

PWB Dtrln Procar

Figure 3. Suitability of Exchange Formats in Various Disciplinary.

A printed wiring board (PWB) design transforms a logical circuit design into a physical realization of component placements and interconnections. The PWB design process starts with a set of specifications and continuously refmes them through a series of abstraction levels, until the layout of the board has been achieved. In order to effectively facilitate and manage the PWB design, the characteristics of the PWB design process need to be identified and studied. The following are some of the important characteristics pertaining to the PWB design process:

Figure 4. Data Evolution for PWB Design.

The PWB design process is complex and time-consuming. The modern PWB is an integration of complex geometry, advanced technology, intricate manufacturing processes and sophisticated business strategies. Advances in miniaturization of electronics have increased the complexity of electronic design by an order of magnitude within the last few years. In addition, the increasingly popular use of multilayer composite materials further complicate the geometric and structural design of the board. As a result, the development time and cost can be enormous [3,1].

The PWB design process involves many design tools. The modern PWB is so complex that performing the design without the use of computers would be virtually impossible 111. Various tools and design support systems such as simulation, schematic capture, layout and routing CAD p~oducts have been developed and are continually being improved to support and facilitate the design process throughout the entire design spectrum. Although these tools and systems provide the capability to address many issues relevant to PWB design, they are not well integrated [25,18,26].

The PWB design process creates a large quantity of data. Due to the complex nature of the PWB design process, a large amount of design information is created. The design information includes the functional, behavioral, and physical descriptions of design objects (layouts, sgematic symbols, interconnections, components), material properties, simulation information, design changes and design documents (specifications, requirements and libraries), finite element analysis results (displacements, temperatures, stresses), etc. [1,27,26].

The PWB design process is iterative, exploratory and hierarchical. The PWB design process often does not follow a precise step-by-step path, using the same algorithm repetitively. For instance, interconnect routing and component placement may require a number of iterations throughout various stages of the process to meet thermal and crosstalk requirements. Electronic design is an exploratory process where abstraction is employed to simplify the design process. The process of abstraction has resulted in what &.known as a design hierarchy [Z].

The PWB design process is multidisciplinary and coordinative. The PWB design process encompasses all activities required to generate the information needed for producing a product; therefore, it covers a broad scope of technical disciplines ranging, for example, from circuit simulation to structural mechanics to thermal science to manufacturing to system engineering. In the PWB design process, various teams are grouped based on the above-mentioned disciplines. Since these teams often perform their design tasks in parallel, numerous interactions among these design teams must occur to deal with design changes and to acquire new information.

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Proper and timely coordination and information exchange are essential to the success of the design project [B].

4.1.3 PWB Desien Process Automation: Although the usefulness of design tools/systems is apparent, there exists few integrated computer-based design systems composed of tool sets that perform all of the steps in the entire PWB design process, i.e., from requirements identification, through specification definition, functional and physical design description, to manufacturing process. In fact, these design systems currently provide automation for only a few design levels and disciplines. Each tool has its own languages and means for representing and manipulating design data which makes sharing the design information with the cooperating tools almost impossible. Unless substantial effort is made to translate and transform information between tools, an environment composed of what has been called "islands of automation" will occur and the overall performance with this approach will not be satisfactory [9,28]. This poor performance arises since the design environment does not provide the designer with uniformity and compatibility and does not focus on the integration issue as a primary goal P I .

4.1.4 Inteeration of the PWB Desiun Process: The multidisciplinary nature of the PWB design process results in heterogeneous, complex, variable data structures and representations throughout the design phases, making the task of managing and controlling design and managerial information among these disciplines very painstaking. These data types are grouped into different design views ranging from geometric definitions to thermal structural descriptions to electronic symbolic denotations. Figure 5 gives an example of the various data types and views that may exist in different design activities. Some information is shared between disciplines while some is unique to a single task. For example, PWB layout concerns physical board configuration (pad location, race width, etc.) while circuit simulation concerns a logic description (port connectivity and schematic) in a time-based domain. Furihermore, it is observed that even within the same discipline, the data requirements may vary. For example, in thermo- mechanical design for plated-through-holes (PTHs), the conceptual design phase concerns the only one dimensional (out-of-plane) manual calculation and linear mechanical/thermal characteristics [MI, while in detailed design stage, the 3-D non-linear time-dependent finite element model and temperature-dependent composite material properties are needed. For most complex cases, data related to fatigue and fracture are also included (Figure 6). Information management is a crucial aspect of a PWB design system [7,4]. Maintaining design information which is consistent, accurate and complete is a long-term goal. Information integration is a key to achieve such a goal [31,41.

Dirplinuy Data T m

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Figure 5b.

4.2 Inteerated Information Framework for TSET Svstem

Various Data Types for Various Engineering Disciplines.

4.2.1 TSET Svstem Architecture: In order to better manage the PWB design process and increase engineering productivity, an integrated information framework is necessary and should provide powerful data models to represent various data types and views [W. At Georgia Tech, an integrated PWB E/MCAD design system denoted Thermal Structural Electro-magnetic Testability (TSET) system is being developed and prototyped. TSET responds to the E/MCAD concepts and will be implemented based on an executive-centered information management approach using a common database. The system architecture and implementation for TSET is shown in Figure 1 and Figure 7 and its major components are described below.

Figure 6. Example for Data Types Evolution in One Discipline.

Figure 5a. Various Data Types for Various Engineering Disciplines.

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Figure 7. Implementation of TSET System.

Design Information Management System (DIMS): DIMS acts as the executive for TSET system . DIMS controls access to the common database, supports design versions and view management, and enforces design integrity constraints. DIMS also supplies tool adapters that map design information into the common database. In addition, DIMS is responsible for maintaining information (meta- information) about the design process and providing an user interface mechanism for the designer to interact with TSET.

Database Management System (DBMS): The initial prototype TSET is built around Orade, a comprehensive, commercial SQLbased relational DBMS. The Oracle has been widely used in electronic applications such as PWB design and VLSI design. The Oracle also supplies many facilities such as SQL*Menu, SQL’Net, SQL*Forms, etc. which make the database design rather easy and efficient.

Common Database: The common, so-called global, database contains design information about the different levels of abstraction of a PWB. There are a number of advantages to the use of a common database. approach [4]. First, it provides full benefits of database management technology by using a state-of-the-art commercial DBMS. Second, it provides a flexible design environment since the database can be easily extended to accommodate and support additional application tools. Third, data integrity and consistency can be achieved without too much difficulty.

TSET T d Sets: Although many tools. are required in the PWB design process, the initial TSET prototype will not include all of them. One of the primary goals for this research is to evaluate the E/MCAD concepts and to study the information management issues relevant to PWB design, The foUoWing tools are included in the initial TSET system tool set:

ECAD System: The ECAD system is initially viewed as comparable to Mentor Graphics’ Electronic Design Automation (EDA) software system. EDA consists of three modules: IDEA Station, Board Station, and Package Station. The primary functions of IDEA Station include schematic capture and simulation. Board Station performs gate assignment, component placement, and routing. Package Station is responsible for thermal analysis and packaging design. Mentor’s EDA is widely accepted in electronic design arena and provides comprehensive capabilities for PWB electronic design

Finite Element Analysis: For PWB thermal-mechanical analysis, finite element software is needed. The ANSYS finite element computer program offers appropriate functionality for PWB mechanical design by providing linear/non-linear, steady-state/transient, and thermal/structural analysis capabilities. ANSYS determines the thermally induced stresses and displacements of PWBs by applying the temperature distributions obtained from Mentor’s Package Station. Through the use of finite element simulation, the mechanical behavior of a PWB can be better understood, and the development time can be significantly reduced compared to the build- and-test scenario widely used in PWB industry 13,321.

Enhanced Capabilities: One of the research project goals is to develop enhanced capabilities for the TSET system. Tools such as crosstalk analysis and circuit testability software are currently being developed. TSET is to be designed as a platform for additional capabilities.

4.2.2 The TSET Database Desien: Database design is the major cornerstone of the integrated information framework of the TSET system. Although there is no general consensus on the phases of database design, the following four stages should be identified (Figure 8):

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Figure 8. Stages for TSET Database Design.

(1) Requirements analysis The requirement analysis stage begins with a high-level analysis of the

functions that must be performed to meet the requirements that the information framework needs. The analysis defmes these requirements and helps limit the scope of the work. Many information requirements are addressed in publications [28,33,34]. The following are identified to support TSET as a multidisciplinary PWB design system.

Dynamk Schema: A schema defmes the framework of representations for the design object and associated relations. A schema consists of structures and constraints. Due to the highly iterative and dynamic nature of the PWB design process, the data models must support the evolving charaderistic of the data schemas. This implies the data model should be flexible enough to support schema modification and evolution. The data models must also be sufticiently rigorous to allow the mapping of information from each stage of the design process.

Complex Model Representation: In PWB design, the design objects may consists of complex sub- assemblies, which themselves contains more subassemblies. They all have various complex structural characteristics, constraints, f’unctionalities and behaviors. The information framework should be able to accommodate such great expressive power both for data structures and fundionalities.

Handling Heterogeneous Data m/vims: As stated before, a PWB is an integration of many disciplines. Each discipline has its own information representations and Views. Each view holds a particular set of information which is relevant to that representation. For instance, there are 10 different view types for a cell defined in EDIF. The integrated information must support view d e f ~ t i o n and view management in a flexible manner.

Data Independence: There are numerous off-the-shelf design tools offering design support at various levels and/or disciplines within the entire design process [XI. The information framework should not only adapt, manage and integrate these existing tools in an uniform and harmonious manner, but also provide a development environment so that any new tools can be easily developed and gracefully integrated into the design system (351. In order to achieve such a goal, i t is very important to maintain data independence, i.e., to have a constant logical view of the information representation independent of its realization on a physical storage medium.

Supporting Data Exchange Standards/Fonnats: As described earlier, there are a number of data exchange standards/formats for transferring design information and providing common means of communication between heterogeneous systems and disciplines. These exchange formats have been gaining tremendous momentum in the engineering design community. Sooner or later, these formats will become the standards and all tool vendors will adopt them into their tool development. The information framework, hence, should support these standards.

Other requirements for engineering databases (or design databases) such as supporting top-down hierarchical design style, long transactions, versioning, concurrent management, etc., are addressed in many publications should also be considered and possibly included.

(2) Information Analysis The information analysis stage involves the development of two

separate models: a process model and a data model. For the TSET system, IDEM and IDEFlX modeling methodologies were used to develop these models. Detailed illustrations and descriptions of these methodologies have been documented [4,36]. A brief summary of these methodologies is as follows:

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IDEFO Methodology: The IDEFO method uses a diagrammatic technique which hierarchically decomposes the entire system in a systematic and logical manner into its simplest levels in terms of their functions. IDEFU is a process model which identifies information and information flow through functional relationships. IDEM is most suitable for modeling the design process for engineering applications. Its top-down approach allows the system to be decomposed into units of manageable size, thus encouraging teamwork while still maintaining consistency in terms of the f d system integration. The PWB design process adapted in the TSET system has been tentatively modeled using IDEM modeling method with assistance from Motorola, Inc. A typical IDEFU example is shown in F w e 9.

L A I Figure 9. IDEFO Model Example for PWB Design.

IDEFlX Methodology: IDEFlX, a data modeling method, describes and a n a l p s the information of a complex system by offering a set of rules and procedures for creating information models. IDEFlX produces graphic diagrams that explicitly represent data semantics in terms of entities (objects), relationships, and attributes (properties). With IDEFlX, a conceptual data model is generated by a top-down analysis of data and data relationships which is suitable for supporting the full process of developing information systems. The IDEFlX model can be precisely and easily mapped into a relational database schema (in third normal form or higher) or other database schemas. An IDEFlX data model for PWB design has been constructed and a typical example is given irr Figure 10.

Figure 10. IDEFIX Model Example for PWB Design.

(3) Lop'? Design The logical design stage transforms the global conceptual schema

description of the system into a logical model according to the data model of

the DBMS which supports and manages databases. In this stage, the data model obtained from the information analysis is further refined and regrouped according to the functionalities (called normalization). The concept of multi-valued dependencies will be introduced in this stage. The resulting database schema should maintain the logical view of these basic entities and subsidiary information entities and, also, the relationships among them [37]. At the same time, a data dictionary is generated to ensure constraints and data integrity. In logical database design, the relational database model is most commonly used. However, the objed- oriented data model is gaining popularity in engineering applications due to its powerful modeling capability (data abstraction, information hiding, inheritance, dynamic binding) [38,4]. In electronic applications, many object-based systems have been developed on a research basis, but they are not suitable for using in an production environment [4,6,40]. For the TSET system, the construction of a relational database model in its fifth normal form is underway. A separate object-oriented model will be subsequently implemented using Vbase DBMS to validate these two database model paradigms and evaluate proposed information framework technology. A relation (table) example for material properties in the form of SQLIForms is shown in E i r e 11.

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Figure 11. Example for Material Properties Relation Using SQL' Forms.

(4) Physical Implementation At this stage, all the storage dependent attributes of the data and

access strategy independent of any physical storage device will be described. The entity sets and relationship sets of the logical schema are mapped into the physical schema (internal schema). The physical schema defines the internal database (the physical representation of the database) in terms of records, access paths, pointer structures, etc. This stage is achieved wing Oracle DBMS. Orade is capable of generating a report documenting the validation and evaluation of the physical schema. Inconsistencies, redundancies, or lack of data will be reported.

5. Conclusions and Future Work

Although many design tools abound in the PWB design ar&a, most them are ECAD oriented since electronic design (schematic capture, layout, routing, etc.) has traditionally been viewed as the corner stone of PWB design. As component and process technology has improved, mechanical design has becoming increasingly important. Enhanced ECAD systems and thermo-mechanical design tools must be integrated into the entire design process to improve engineering productivity and design quality. Engineering information is the most important asset in the design process. In order to better manage and facilitate the PWB design process, the information generated from various disciplines must be integrated.

In response to this concept, a computer-based PWB design system (TSET) is being developed by multidisciplinary research teams that consist of electrical design, structural mechanics, thermal science, manufacturing process, testability, and software engineering. TSET's integrated information utilizes an executive-centered management approach with a common database. The database design methodology, a systematic and rigorous approach, indudes four stages: requirements analysis, information analysis, logical design and physical implementation. Currently, the relational database model is being implemented using Oracle DBMS. In the future, the object- oriented paradigm will also be studied.

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6. References

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