Construcción y mantenimiento de los cuartos fríos

ation of a set of drawings and data that are used to manufacture and test the product. The extent of the ability of computer- based systems to generate the required documentation varies sig- nificantly. Direct output of drawings based on information taken from the design database minimizes but does not com- pletely eliminate the need for detailed checking. If design data must be transferred from a PCA design system to a mechanical drafting system to generate manufacturing drawings, than the result should be examined more closely. This is to ensure that no errors were introduced due to human or machine intervention.

PCA design systems also produce outputs in the form of data files that can be used to perform a variety of manufacturing and test functions. The primary output is data used to produce a vari- ety of phototools needed to fabricate the bare circuit board. If the phototools are produced by the design function, then they must be checked before being used for manufacturing the boards. If the correctness of the layout has been confirmed, then the data used to drive the photoplotter are also correct. It is mainly the quality of the films that the plotter produces that should be checked. Pho- toplotting is a complex process requiring a high degree of preci- sion, and there is no guarantee that artwork of acceptable quality will be produced consistently. Many opportunities for error exist, and close examination of the artwork should be performed. In addition, blemishes can occur easily that if not identified could result in fabrication of nonfunctional (scrap) boards. Typical film blemishes include pinholes, scratches, and emulsion defects.

Other system outputs used for manufacturing and test include data that can be used to drive numerical controlled (NC) fabrication equipment (drills, board profilers, etc.), assembly machines (pick-and-place, adhesive dispensers, etc.), and automated test equipment. Since these are also derived from the design data- base, the only checking usually required is to ensure that the files have been produced in the correct format and that they are derived from the correct design.

If manufacturing drawings and phototools are prepared man- ually, then a much more extensive checking effort usually is needed to verify that all data produced accurately portrays the design. For artwork, the master should be checked to ensure that all interconnections have been transposed accurately from the layout. The dimensions and placement of conductor features must be measured, especially land areas, line widths, and spac- ings. For multilayer boards, layer-to-layer alignment of pads for through-holes and vias should be checked carefully. Drawings should be checked for correctness of pictorial and dimensional data, instructional notes, and parts lists.

5.2.3

Checking Tools

The features of a PCA design produced with a CAD system are controlled by the data and design rules established in that system.

Outputs of the system, such as reports, listings, and hard-copy plots, are the primary tools used to check the design. Net and parts lists can be used to verify that the schematic input was cre- ated correctly. Drill and pad lists provide assurance that these fea- tures are placed correctly and are sized properly. Design rule verification listings ensure that there are no physical or circuit violations in the design. Check plots can be produced to show part placement, conductor routing, and drilled hole locations. The basic mechanical features of a board also can be plotted, detailing outline, tooling, and mounting-hole dimensions. Part descriptions resident in the library can be listed and plotted sep- arately to check the accuracy of the data against the latest vendor information.

The quality and accuracy of photoplotted artwork usually are checked using optical inspection and measuring equipment. This can be done manually, a labor-intensive activity that potentially introduces a significant opportunity for error due to missed defects and lack of consistency. Use of automated optical inspection (AOI) equipment, when properly pro- grammed, can locate and identify a very high percentage of art- work defects, especially for dense circuit boards. Although AOI is usually used to verify the quality of fabricated circuit layers prior to multilayer board lamination, the technology has become recognized as an excellent method for checking both master and working artwork.

Checking manually laid out PCAs involves the use of a differ- ent set of tools, especially if the artwork masters and manufac- turing drawings are also created by hand. The layout must be checked against the schematic systematically, point by point, and usually involves lining out each connection on both the schematic diagram and a copy of the layout. For multilayer designs, this is a tedious, time-consuming task. Layer-to-layer alignment of tooling and mounting holes, pad stacks, and other features must be checked. The artwork and drawings are pro- duced using the layout as the information source. These also must be checked carefully to verify that the data were tran- scribed correctly from the layout. This is especially true for manually prepared artwork, since every trace, pad, and feature must be placed by hand.

5.2.4

Checking Documentation

The results of checking activities should be recorded and the data retained for each PCA. If design-related problems arise dur- ing fabrication, assembly, test, or integration, review of checking documentation is usually a key to determining the source of the problem. In addition, checking data provides valuable informa- tion for design and checking process improvement. The kinds of discrepancies identified during checking can help establish which defect types are most common so that improvements can be focused in those areas.

If checklists are used as an aid to ensure that all required veri- fication activities were performed, these should be part of the retained data. After completion of a design, all checkplots, marked prints, listings, and other checking-related documenta- tion for a PCA should be gathered together into a file and stored for easy retrieval. Copies of meeting notes, review reports, and design analyses also should be included. Significant time can be saved by being able to quickly retrieve and review all technical data for a PCA not only for problem solving but also if design changes are required in the future or the assembly can be used for a different application.

5.3

Producibility Evaluation

A major part of the designer’s responsibility is to ensure that a PCA can be manufactured in a cost-effective way. The physical design of a PCA has a direct effect on its producibility because it is the major factor in determining which process can be used to build it. Key features of a design that most directly affect its manufacturability are

Part type selection and part placement and orientation Printed circuit board material and board construction Conductor geometry and dimensions

Overall complexity and circuit density of the PCA Type of circuitry and criticality of circuit performance

Producibility should not be an afterthought, to be considered only after a layout has been completed. It needs to have equal status with other design requirements and constraints. Unfortu- nately, absolute rules about cost-effective design are extremely difficult to establish due to the complexity of the manufactur- ing process and the broad variability of elements found in PCAs. Because different types of designs have varying cost sensitivities, easy-to-use, globally applicable cost models are hard to find.

There are, however, some basic relationships that can be used to make fundamental design decisions. These will be discussed in the following sections of this chapter. In addition, cost models have become available that can be tailored to accommodate the unique types of assemblies built by a company and take into account its unique manufacturing and assembly processes.

5.3.1

Part Type Selection, Density, and

Placement

How easily a PCA can be manufactured generally depends on the types and quantities of components used and how they are mounted to the circuit board. PCAs usually are classified for man- ufacturing as using plated through-hole (PTH) devices, surface- mount technology (SMT), or mixed technology (through-hole and surface-mount). For ease of manufacturing, a PCA is preferred to be all single-sided PTH or SMT to minimize the number of differ- ent assembly and soldering operations. If mixed technology is required, all parts should be mounted on the same side of a board. Mounting parts on both sides significantly increases manufactur- ing difficulty and should be used only when dictated by circuit density or performance requirements. Table 5.5 is a generalized list- ing of assembly types in order of increasing cost to manufacture.

Most PCA designers and engineers are familiar with the man- ufacturing process for PTH assemblies. However, the manufac- turing flow of an SMT or mixed-technology assembly is much more complex. It depends on many different design features, including the number, type, and location of individual SMT devices (and PTH components), the manufacturing and test equipment to be used, and the process experience of the manu-

facturer. It is therefore important that the manufacturing and test engineering organizations be consulted early in the design cycle to discuss and review process issues, component selection guidelines, and cost tradeoffs.

Following the manufacturing preference guideline in Table 5.5 should minimize the PCA manufacturing cost and cycle time. It should be noted, however, that a product’s total life-cycle cost is to be considered when evaluating design options. This involves weighing nonrecurring design costs against recurring manufac- turing costs, as well as considering trading component costs against their availability, reliability, maintainability, reparability, and performance.