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13. PCB DESIGN, PWA LAYOUT, AND DESIGN FOR MANUFACTURE
Integrating the many available advanced ICs into one reliable, manufacturable PCB has become more difficult. The PWA designer must work with the industrial designer and the mechanical designer to find a way to fit the necessary functionality into the desired ultraportable package for products such as cell phones and personal digital assistants (PDAs). Then, too, products such as cell phones and wireless LANs depend on radiofrequency (RF) technology, ranging from a few megahertz up to a few gigahertz. In the past the RF and microwave sections of a product could be put into a specially packaged and shielded area. However, with products shrinking and with the need to have a relatively powerful computer on the same tiny PCB as the RF transmit and receive circuitry, new challenges exist. Component placement, electromagnetic interference, crosstalk, signal integrity, and shielding become extremely critical.
Traditional board technologies that evolved in the mid-1980s and into the 1990s are reaching their limits. For example, conventional etching gets linewidths down to 100 µm. Drilled holes can get down to 200 µm. It's not just that the RF section has high frequency. Personal computers contain 1-GHz processors and bus standards are at 400 MHz and moving to 800 MHz. But it's not the raw clock speed that matters when it comes to designing the PCB. The real issue is the very high edge rates on the signal pulses which have harmonic components in the 8 to 9-GHz range. For these circuits to work, trace impedance, signal transmit times, and termination impedance all need to be very tightly controlled.
Technologies requiring the use of differential pairs are becoming more common.
Board designers need to become more knowledgeable about managing these high speed issues during board design, as well as being able to use the new high speed design tools. Board designers are asked to put a radio station combined with a PC into a form factor that's becoming much smaller. Switching speeds, impedance matching, EMI, crosstalk, thermal problems, and very restricted three dimensional enclosures all combine to exacerbate the problem. They need to do this while working very closely with their extended design teams, and the ICs that they need to put on the board are in multihundred- to multithousand-pin packages that are constantly changing pin-outs. At the same time manufacturing needs must be met.
The industry is responding to these changes with new PCB technologies.
Controlling impedance requires new dielectrics that minimize signal loss at high frequencies. The very high pin and trace densities require a new class of etching technology to support the extremely fine linewidths and drilling technology to support the blind and buried microvias. The new high-density interconnect (HDI) technology generally uses laser drilling for the basic hole structures. Included in these technologies is the ability to put passive components such as printed resistors between layers. Buried capacitors and inductors will be available as well.
The HDI boards require precision automated assembly machinery.
A product design with good manufacturability will move through the production (manufacturing) environment seamlessly and thus contain an efficient labor content and have high yields, resulting in a high-quality high-reliability product. Just as important as the circuit design itself, so is the PCB design and PWA layout. The PWA serves as the interconnection medium between the various individual components placed on the PCB and between these and the rest of the system/product as well as the external world. As such, the reliability of a product or equipment is dependent on the reliability of the solder joints. Through hole components provided a robust mechanical connection to the PCB with less stress on the solder joints as compared with surface mount technology (SMT).
It is surface mount packages, however, that have been the key enablers in the drive to reduce the overall size of electronic systems and components.
It is these surface mount packages that have placed an increasing demand on solder interconnect reliability. For example, solder joints of leadless ball grid array (BGA) and chip scale package (CSP) assemblies are more susceptible to early wearout than solder joints in compliant, CTE-matched leaded assemblies.
Establishing the reliability of area array (BGA and CSP) SMT assemblies re quires robust design and assembly practices, including high-yield and high quality solder interconnects, characterization of the package and board materials, evaluation of the structural response of the entire assembly, and statistical characterization of solder joint failure distributions. Component-dependent attachment reliability trends have been established over the years based on the results of both modeling and accelerated testing programs. Figure 8 shows the impact of component types on attachment reliability and gives a relative ranking of attachment reliability for several types of surface mount packages on FR-4 PCBs. This ranking is not absolute since the reliability is application dependent and design parameters vary within a family of packages. Thus, the package styles, types, physical dimensions, construction, and materials that will be used in the physical implementation of the design must be considered during PCB design and PWA layout.
Printed circuit board design begins with the materials that constitute its construction. Problems encountered with bare PCBs by equipment and contract manufacturers alike include thin copper traces, exposed inner layer copper, insufficient copper, solder on gold tabs, outgassing and voids, flaking resist, bow and warp, poor solderability, missing solder mask, inner/outer opens, and poor hole quality. A "known good" PCB is thus required and forms the foundation for a robust and manufacturable PWA. This mandates that reliable PCB manufacturers are selected after extensive audits of their design, material supplier selection, materials analysis, and verification and manufacturing processes.
Design for manufacture (DFM) is gaining more recognition as it becomes clear that manufacturing engineers alone cannot develop manufacturable and test able PWAs and PCAs. Design for manufacture is the practice of designing board products that can be produced in a cost-effective manner using existing manufacturing processes and equipment. It is a yield issue and thus a cost issue. It plays a critical role in printed wiring or card assemblies. However, it must be kept in mind that DFM alone cannot eliminate all PWA defects. Defects in PWAs generally fall into three categories: design related problems; incoming material related problems (PCB, adhesive, solder paste, etc.); and problems related to manufacturing processes and equipment. Each defect should be analyzed to its root cause to permit appropriate corrective action to be taken as part of the design process.
The benefits of a manufacturable design are better quality, lower labor and material costs, increased profitability, faster time to market, shorter throughput time, fewer design iterations, more successful product acceptance, and increased customer satisfaction. This happens by thoroughly understanding the manufacturing process.
Design for manufacture and design for assembly (DFA) are integral to PCB design and are the critical links between design and volume manufacturing. Utilizing DFM/A helps bridge the performance gap between the myriad functional improvements being made to packaged silicon solutions (ICs). For example, real time automated design systems gather feedback from design, test, and manufacturing and assimilate these data with the latest revisions to performance specifications and availability from component suppliers. Design then analyzes this information to enhance both the testability and manufacturability of the new product.
Design for manufacture and assembly is essential to the design of electronic products for the following reasons:
1. Products have become increasingly complex. In the last few years the sophistication of printed circuit packaging has increased dramatically.
Not only is surface mount now very fine pitch, but ball grid array and chip scale packages and flip chip technologies have become commercially viable and readily available. This plus the many high-density interconnect structures (such as microvia, microwiring, buried bump interconnection, buildup PCB, and the like) available has made the design task extremely complex.
2. Minimizing cost is imperative. The use of DFM/A has been shown in benchmarking and case studies to reduce assembly costs by 35% and PWA costs by 25%.
3. High manufacturing yield are needed. Using DFM/A has resulted in first-pass manufacturing yields increasing from 89 to 99%.
4. In the electronic product design process, 60-80% of the manufacturing costs are determined in the first stages of design when only 35% or so of the design cost has been expended.
5. A common (standard) language needs to be established that links manufacturing to design and R&D. This common language defines producibility as an intrinsic characteristic of a design. It is not an inspection milestone conducted by manufacturing. The quantitative measure of producibility directly leads to a team approach to providing a high quality cost-competitive product.
The traditional serial design approach, where the design proceeds from the logic or circuit designer to physical designer to manufacturing and finally to the test engineer for review is not appropriate because each engineer independently evaluates and selects alternatives. Worse is a situation where the manufacturing engineer sees the design only in a physical form on a PCB. This normally is the case when contract manufacturers only perform the component assembly (attachment to the PCB) process.
How should a product be designed? As mentioned previously, the design team should consist of representatives from the following functional organizations: logic design; analog design; computer-aided design (CAD) layout; manufacturing and process engineering; mechanical, thermal, component, reliability, and test engineering; purchasing; and product marketing. Alternatives are discussed to meet thermal, electrical, real estate, cost, and time-to-market requirements. This should be done in the early design phases to evaluate various design alternatives within the boundaries of the company's in-house self-created DFM document. This team should be headed by a project manager with good technical and people skills who has full team member buy-in and management support.
Manufacturing engineering plays an important role during the design phase and is tasked with accomplishing the following:
1. PC board layout maximizing the use of automation
2. Controlling the cost of raw PCB fabrication
3. Implementing design-for-test techniques
4. Creating procurement bills of materials
5. Designing and ordering SMT stencils
6. Programming the manufacturing automation equipment from design files
For the design team to design a manufacturable product, it is important to establish guidelines. Guidelines for DFM/A are essential to establishing a design baseline throughout the company. Engineering designs to a certain set of specifications or requirements, and manufacturing has a certain set of capabilities. Synchronizing requirements and capabilities sets expectations for both functions. The DFM/A guidelines form a bridge between engineering and manufacturing and become a communication vehicle. They can start out as a simple one-page list of sound practices and then evolve into a more complex and comprehensive manual, defining every component and process available. As an example, typical DFM/
A guidelines would include:
Component selection criteria
Component orientation requirements Preferred components and packages Component spacing requirements (keep-out zones) Designator and naming conventions PCB size and shape requirements Land, pad, and barrel size requirements PCB edge clearance requirements Paneling and depaneling information Trace width, spacing, and shaping requirements Solder mask and silkscreen requirements Printing and dispensing considerations Placement and reflow considerations Wave soldering and cleaning considerations Inspection and rework considerations Fiducial and tooling hole requirements Test pad size and spacing requirements Production machine edge clearance Environmental considerations
Once a DFM guideline document has been created, it must be available immediately to everyone who needs the information or else it is useless. As with any document, the guidelines must be maintained and updated so that they accurately reflect manufacturing's current capabilities. This is especially important as production automation is replaced or upgraded, new technologies and component (IC) package styles are introduced, and the manufacturing activity is out sourced.
The DFM guidelines must be verified on prototype assemblies before an item is released for high-volume production. Validation of DFM should not pose a major problem because most designs go through one or two revisions, in the beginning stages, to fine tune the electrical performance. During those revisions, manufacturing defects should be detected. Some typical examples of PCB design guidelines and the documentation required are listed in Tables 8 and 9, respectively.
TABLE 8 PCB Design Guidelines
Vias not covered with solder mask can allow hidden shorts to via pads under components.
Vias not covered with solder mask can allow clinch shorts on DIP and axial and radical components.
Specify plated mounting holes with pads, unplated holes without pads.
For TO-220 package mounting, avoid using heat sink grease. Instead use sil pads and stainless hardware.
Ensure that polarized components face in the same direction and have one axis for PTH automation to ensure proper component placement and PWA testing.
Align similar components in the same direction/orientation for ease of component placement, inspection, and soldering.
PTH hole sizes need adequate clearance for automation, typically 0.0015 in. larger than lead diameter.
Fiducial marks are required for registration and correct PCB positioning.
For multilayer PCBs a layer/rev. stack-up bar is recommended to facilitate inspection and proper automated manufacture.
Obtain land pattern guidelines from computer-aided design libraries with CAD programs, component manufacturers, and IPC-SM-782A. SMT pad geometry controls the component centering during reflow.
Provide for panelization by allowing consideration for conveyor clearances (0.125 in. minimum on primary side; 0.200 in. minimum on secondary side), board edge clearance, and drill/route breakouts.
Maximum size of panel or PCB should be selected with the capabilities of the production machine in mind as well as the potential warp and twist problems in the PCB.
PCBs should fit into a standard form factor: board shape and size, tooling hole location and size, etc.
To prevent PCB warpage and machine jams the panel width should not exceed 1.5_ the panel length.
Panels should be designed for routing with little manual intervention.
TABLE 9 PCB Design Required Documentation
CAD and gerber data Soft copy of bill of materials Gold (functional and identical) PWA Raw PCB for troubleshooting Soft and hard copies of detailed schematic diagrams Program data for programmable logic: PLDs, FPGAs, etc.
PDF data sheet for all ICs Data for custom ICs Functional test requirements
A DFM feedback process is necessary in order to effectively relay lessons learned in manufacturing to engineering. One effective technique is to have all engineering prototypes built by production personnel using the intended manufacturing processes. This is a proven method to transfer feedback on the success of building a product. Also, there are no surprises when a product is released for production because those same production processes were used throughout the design cycle. Feedback must be delivered quickly and accurately so the design team can immediately correct any problems observed by the production personnel on the prototypes.
A production readiness review that answers the questions when is an engineering product design done and when is manufacturing ready to accept a design is needed as well. This is an important step because the marketplace is competitive and customers demand high-quality, competitively priced, quick-to-market products designed for manufacturability at the onset of a new product introduction. The production readiness review measures the completeness of each deliver able from each functional group throughout the design cycle of a new product.
This is an important crossfunctional design team responsibility and enables all issues to be resolved essentially in real time rather than tossing them over the wall from one functional organization to another. For those companies that don't use crossfunctional design teams, product readiness reviews take much time and can be frustrating events.
An example of an engineering deliverable might be all drawings released to manufacturing in order to buy materials with sufficient lead time. Therefore, engineering may have x deliverables, and manufacturing may have y deliverables.
Each of these deliverables is measured at certain gates (i.e., checkpoints) through out the design cycle. The crossfunctional new product team determines when these deliverables must be completed, and the performance of the readiness is then measured. Now it becomes a very objective measure of when engineering is done and manufacturing is ready. If there are 10 different gates in a 10-month development cycle, for example, and engineering reports 50 of 75 deliverables complete while manufacturing reports 100 of 100 deliverables at Gate 10, then engineering is clearly not done and manufacturing is ready and waiting.
Design for manufacture and assembly is predicated on the use of accurate and comprehensive computer-integrated manufacturing (CIM) tools and sophisticated software. These software programs integrate all relevant data required to design, manufacture, and support a product. Data such as simulation and models; CAD and computer-aided engineering (CAE) data files; materials, processes, and characteristics; specifications and documents; standards and regulations; and engineering change orders (ECOs), revisions, parts, etc. The software programs efficiently
Communicate information both ways between design engineering and manufacturing.
Automate CAD data exchange and revision archiving.
Provide product data tracking and packaging completeness checking and support standard industry networking protocols.
Allow design for assembly by analyzing part placement, supporting multiple machine configurations, analyzing machine capacity, and providing production engineering documentation.
By having these design files available in an integrated form, PWA design and manufacturing engineers have the necessary information available in one place to develop a cost-effective design implementation, including analyzing various tradeoff scenarios such as Product definition and system partitioning (technology tradeoff )
Layout and CAD system setup
PWA fabrication design rules, yield optimization, and cost tradeoffs
SMT assembly process, packaging, component, and test tradeoffs
An example of such a tool is GenCAM (which stands for Generic Computer-Aided Manufacturing). GenCAM is an industry standard written in open ASCII format for electronic data transfer from CAD to computer-aided manufacturing ( CAM) to assembly to test in a single file. This file may contain a single board to be panelized for fabrication or subpanelized for assembly. The fixture descriptions in the single GenCAM file allow for testing the assemblies in an array or singular format, as shown in Figure 9. Some of the features and benefits of Gen CAM are listed in Table 10. A detailed description is documented in IPC-2511.
GenCAM contains 20 sections (Table 11) that convey design requirements and manufacturing details. Each section has a specific function or task, is independent of the other sections, and can be contained within a single file. The relation ship between sections is very important to the user. For example, classic information to develop land patterns is important to both the assembly and in-circuit test (ICT) functions. GenCAM files can be used to request quotations, to order details that are specifically process-related, or to describe the entire product (PWA) to be manufactured, inspected, tested, and delivered to the customer.
The use of primitives and patterns provides the information necessary to convey desired final characteristics and shows how, through naming conventions, one builds upon the next, starting with the simplest form of an idea, as shown in Figure 10. Figure 11 shows an example of various primitives. Primitives have no texture or substance. That information is added when the primitive is referenced or instanced.
TABLE 10 Benefits of the GenCAM Data Transfer Format
Improves cycle time by reducing the need to spoon-feed the sup ply chain; supply-chain management, necessary as more ser vices are outsourced; equipment re-procurement capability.
Also establishes a valuable archiving capability for fabrication and assembly tooling enhancement; and segmentation of the GenCAM file avoids the need to distribute proprietary product performance data.
Features ability to provide complete descriptions of one or more assemblies; a direct correlation with CAD library methodology. GenCAM establishes the communication link between design and manufacturing; facilitates reuse of graphical data; permits descriptions of tolerances for accept/reject criteria; brings design into close contact with DFM issues.
Provides complete description of PCB topology; opportunity to define fabrication panel, assembly subpanel, coupons, and other features; layering description for built-up and standard multilayer construction; ease of reference to industry material specifications; design rule check (DRC) or DFM review and feedback facilitation.
Also, data can be extracted to supply input to various manufacturing equipment, e.g., drill, AOI, router.
Provides complete integrated bill of materials. Identifies component substitution allowances. Accommodates several BOM configurations in a single file. Establishes flexible reuse of component package data. Supports subpanel or assembly array descriptions. Considers all electrical and mechanical component instances, including orientation, board-mount side.
Electrical bare board and ICT
Identifies one or more fixtures needed for electrical test requirements and specific location of pads or test points; describes test system power requirements, complete net list to establish component connectivity and net association, component values and tolerances. Provides reference to component behavior, timing, and test vectors.
TABLE 11 Descriptions of GenCAM Sections
Section keyword --- Purpose and content
Header Beginning of each file, includes name, company file type, number, revision, etc.
Administration Describes ordering information necessary for identifying responsibility, quantity of ordered parts, and delivery schedule.
Fixtures Panels (panelization)
Describes fixturing for bare- and assembled-board testing.
Includes description of manufacturing panels of PCBs and description of assembly arrays.
Description of boards and coupons. Includes outline of board/ coupon, cutouts, etc.
Describes engineering and formatting requirements for complete PCB and assembly descriptions.
Describes simple and complex primitive physical shapes. Includes lands, holes, and standard patterns.
Functional and nonfunctional geometries developed by the user, e.g., user macros. Includes shapes, logos, features not part of the circuit, and other user-defined figures.
Descriptions to build libraries of reuseable packs and padstack.
Provides information for handles, nuts, bolts, heat sinks, fixtures, holes, etc.
Layers Board manufacturing descriptions. Includes conductive/nonconductive layer definition of silkscreens, details of dielectric tolerances, separations, and thickness.
Padstacks CAD system data. Includes pads and drilling information through and within the board.
Packages Describes a library of component packages. Includes true pack age dimensions.
Describes logic families of components.
Component descriptions. Includes device part number.
Identifies parts. Includes reference designators where appropriate.
Power (and ground) Routes
Includes power injection types permitted.
Conductor location information. Includes location of conductors on all layers.
Test-point locations. Includes probe points, single-name test point types, etc.
Shows data changed from the design and sent to the manufacturing site.
When textured primitives are reused and named they can become part of an artwork, pattern, or padstack description. When primitives are enhanced, there are many ways in which their combinations can be reused. Primitives can also become symbols, which are a specific use of the pattern section. Figure 12 shows the use of primitives in a pattern to surface mount small-outline ICs (SOICs). In this instance, the symbol is given intelligence through pin number assignment.
Thus, the logic or schematic diagram can be compared to the net list identified in the routes section.
Within the single file, several unique assemblies can be described. (From Ref. 1.)
TABLE 12 Listing of IPC Interconnect Design Documents
Defines guidelines for selecting core constructions in terms of fiberglass fabric style and configuration for use in multilayer printed wiring board applications.
Establishes design concepts, guidelines, and procedures for reliable printed wiring assemblies. Focuses on SMT or mixed technology PWAs, specifically addressing the interconnect structure and the solder joint itself. Discusses substrates, components, attachment materials and coatings, assembly processes, and testing considerations. In addition, this document contains appendices covering Solder attachments Plated through via structures Insulation resistance Thermal considerations Environmental stresses Coefficient of thermal expansion Electrostatic discharge Solvents Testability Corrosion aerospace and high-altitude concerns
Addresses microwave circuitry, microwaves which apply to radiowaves in the frequency range of 100 MHz to 30 GHz. It also applies to operations in the region where distributed constant circuits enclosed by con ducting boundaries are used instead of conventional lumped-constant circuit elements.
Provides guidelines for design of high-speed circuitry. Topics include mechanical and electrical considerations and performance testing.
Contains industry approved guidelines for layout, design, and packaging of electronic interconnections. Provides references to pertinent specifications: commercial, military, and federal.
The design guide contains the latest information on materials, design, and fabrication of rigid single- and double-sided boards; multilayers; flexible printed wiring; printed wiring assemblies; and others.
Specifies record formats used to describe printed board products with de tail sufficient for tooling, manufacturing, and testing requirements.
These formats may be used for transmitting information between a printed board designer and a manufacturing facility. The records are also useful when the manufacturing cycle includes computer-aided processes and numerically controlled machines.
Describes an intelligent, digital format for transfer of drawings between printed wiring board designers, manufacturers, and customers. Also conveys additional requirements, guidelines, and examples necessary to provide the data structures and concepts for drawing description in digital form. Supplements ANSI/IPC-D-350.
Pertains to four basic types of drawings: schematics, master drawings, assembly drawings, and miscellaneous part drawings.
Describes the use of libraries within the processing and generation of information files. The data contained within cover both the definition and use of internal (existing within the information file) and external libraries. The libraries can be used to make generated data more compact and facilitate data exchange and archiving. The subroutines within a library can be used one or more times within any data information module and also in one or more data information modules.
Describes an intelligent digital data transform format for describing component mounting information. Supplements IPC-D-350 and is for de signers and assemblers. Data included are pin location, component orientation, etc.
Describes a standard format for digitally transmitting bare board electrical test data, including computer-aided repair. It also establishes fields, features, and physical layers and includes file comment recommendations and graphical examples.
This document is a general overview of computer-aided design and its processes, techniques, considerations, and problem areas with respect to printed circuit design. It describes the CAD process from the initial input package requirements through engineering change.
Provides guidelines for the design, selection, and application of soldered surface mount connectors for all types of printed boards (rigid, flexible-rigid) and backplanes.
Provides information on design characteristics and the application of solderless surface mount connectors, including conductive adhesives, in order to aid IC package-to-board interconnection.
Contains back-plane design information from the fabrication and assembly perspective. Includes sections on design and documentation, fabrication, assembly, repair, and inspection.
Covers land patterns for all types of passives and actives: resistors, capacitors, MELFs, SOTs, SOPs, SOICs, TSOPs, SOJs, QFPs, SQFPs, LCCs, PLCCs, and DIPs. Also included are Land patterns for EIA/JEDEC registered components Land patterns for wave or reflow soldering
Sophisticated dimensioning system
Via location V-groove scoring
The goal in packaging is to transfer a signal from one device to one or more other devices through a conductor. High-speed designs are defined as designs in which the interconnecting properties affect circuit performance and require unique consideration. This guide is for printed circuit board designers, packaging engineers, printed board fabricators, and procurement personnel so that they may have a common under standing of each area.
Establishes the generic requirements for the design of organic printed boards and other forms of component mounting or interconnecting structures.
Establishes the specific requirements for the design of rigid organic printed boards and other forms of component mounting and interconnecting structures.
This standard establishes the requirements for the design of printed boards for PC card form factors. The organic materials may be homogeneous, reinforced, or used in combination with inorganic materials; the interconnections may be single, double, or multilayered.
IPC-2511 establishes the rules and protocol of describing data for electronic transfer in a neutral format. GenCAM helps users transfer design requirements and manufacturing expectations from computer-aided de sign systems to computer-aided manufacturing systems for printed board fabrication, assembly, and test.
GenCAM can handle both through-hole and surface mount components.
GenCAM accommodates through-hole components (dual in line packages, pin grid array packages, and DC/DC converters, for example) by including holes used in the CAD system in the padstack section to make connections to all layers of the PCB. For surface mount components, the relationship of vias, as shown in Figure 13, becomes an important element for design for assembly.
GenCAM handles components intermixing by combining components, padstacks, patterns, and routes information to position parts on the individual PCB and on the subpanel assembly array. Since many assembly operations use wave soldering, the general description of the component identified in the pack ages section can be transformed through (X, Y) positioning, rotation, and mirror imaging. This permits a single description of a package to be positioned in many forms to meet the requirements, shown in Figure 14.
IPC is an industry association that has taken the responsibility for generating, publishing, and maintaining extensive guidelines and standards for PCB de sign, artwork requirements, assembly and layout, qualification, and test-facilitating DFM. Table 12 provides a list of some of these documents.
Another change in PWA design and manufacturing that is driven by fast time to market is that PWAs are designed from a global input/output (I/O) perspective. This means that a given PWA is designed and the first article manufactured using embedded field programmable gate arrays (FPGAs) and programmable logic devices (PLDs) without the core logic being completed. After the PWA is manufactured, then the core logic design is begun. However, choosing to use FPGAs in the final design gives the circuit designers flexibility and upgradeability through the manufacturing process and to the field (customer), throughout the product's life. This provides a very flexible design approach that allows changes to be easily made (even extending programmability through the Internet)-all in serving the marketplace faster.
13.1 Some Practical Considerations
In the PWA manufacturing process there are numerous opportunities for potential issues to develop that could impact solderability and functionality. For illustrative purposes, three situations are presented for consideration.
Suppose that a large-physical-mass, high-current component is placed next to a smaller component that is critical for circuit timing and one of the components is not soldered properly. Which component gets soldered properly is dependent on which component the solder profile was set up for. The large part could be taking all the heat during the soldering process (due to its large mass), preventing the smaller component from receiving sufficient solder.
From a soldering perspective we don't want components placed too close together. However, from a signal integrity perspective we want the components as close together as possible to minimize the signal parasitic effects.
If we have a component with palladium leads close to a component with Alloy 42 leads, the part with palladium leads doesn't get soldered properly.
What these examples show is that careful attention must be paid by experienced engineering and manufacturing personnel to the components that are placed next to or in close proximity to each other during the PWA design (to the size of the components and the materials with which they are made). Unfortunately, these experienced manufacturing personnel are getting rarer and lessons learned have not been documented and passed on, and it takes far too long to gain that experience. The difficulties don't end there. Often, the manufacturer is separated by long geographical distances which serve to create a local on-site technical competence shortage. Suffice it to say that PWA manufacturing is in a turbulent state of flux.