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The future of CAD libraries, Part II: Part of our story about the creation of the land pattern standard IPC-7351. Is this the future of CAD libraries? You decide.

In Part I, we discussed some of the standards that should be incorporated into a CAD land pattern library, from a land pattern naming convention and solder joint analysis through land pattern origin and land pattern courtyard. These are documented in the IPC-7351 land pattern specification.

The following are other considerations that should be implemented into a standard land pattern library.

Silkscreen Outlines

Two fundamental rules for silkscreen outlines will help make your land pattern library perfect. First, stay with a single line width throughout the entire library. I choose .008" (0.2 mm) as a good manufacturable silkscreen line width. Second, stay away from exposed copper pads. This rule is applied from the edge of the silkscreen line to the edge of the exposed copper pad (see FIGURE 5). Use .008" (0.2 mm) as a minimum, .010" (0.25 mm) as a nominal and .012" (0.3 ram) as a maximum.

[FIGURE 5 OMITTED]

Silkscreen Polarity Markings

The silkscreen polarity marking should be clearly visible to the assembly shop. A good size is a .050" (1.3 ram) solid round dot located inside the component body by Pin 1. A good practice is to place a dot outside the component near Pin 1 so that it is clearly visible after the assembly process (see FIGURE 6). This dot, known as a "post-assembly inspection dot," can be .020" (0.5 mm) in diameter.

[FIGURE 6 OMITTED]

Assembly Drawing Outlines

The assembly drawing outline (see FIGURE 7) should represent the maximum outline of the component body. Unlike the silkscreen outline which must be created in order to avoid solder pads (a fake component outline), the assembly outline is only placed on an assembly drawing that goes to the assembly shop. There is no need to fake this outline.

[FIGURE 7 OMITTED]

Assembly outlines can be created with complex drawings to illustrate the actual maximum size of the physical component features or with a simple rectangle. It makes no difference to the assembly shop that has to interpret the assembly drawing. Drawing complex shapes for the assembly outline shows off the PCB designer's artistic creativity, but once the PCB goes into production, it makes no difference because the assembly drawing is hardly ever used again.

Assembly Polarity Markings The assembly polarity marking is sometimes totally different than the silkscreen polarity marking because the silkscreen must avoid touching the solder pad. Unlike the silkscreen, the assembly drawing can illustrate robust polarity markings to ensure that the component is inserted with the correct rotation.

FIGURE 8 shows a sample assembly drawing of component polarity markings in relationship to the solder pad.

[FIGURE 8 OMITTED]

3D Model Data

Every CAD tool has a different approach to handling 3D models of component data, and some are much more elaborate than others. I use PADS PowerPCB with the IDF translator that is capable of extracting "closed polygon" data from the land pattern parts.

The PADS IDF translator requires that the component outline must be a closed polygon and it must be on a layer that does not contain any other graphic features but the component outline. In PADS, Layer_25 is used to construct a closed polygon of the maximum body size of the component outline.

The line width for the component outline should be "zero width," but PADS does not have that feature so I use a 1 micron line width.

The geometry height of the component is stored in a part-type attribute called geometry. height. The unit structure for the geometry height attribute is merely a number for mil units. It must have "mm" following the number if the units are metric and the number must be followed by the closed quotation mark (") if the value is in inches.

When you run the IDF program using PADS File/Export, the user enters a layer input field and the program looks at all the library parts for a closed polygon on that layer. If no closed polygon exists, the program will search through all the layers trying to find a closed polygon that is isolated on a layer. If you do not have a closed polygon on an isolated layer (with no other graphics), the program automatically draws a rectangular outline around the outside perimeter of the part extents and uses that shape to portray the 3D model.

The IDF program produces two files, .emn and .emp, and imports them directly into PRO-E for 3-D model illustration. SolidWorks has a translator that can be used to import the same data into their CAD tool. The CAD library of the future will have 3D model attributes (see FIGURE 9) built into every land pattern to use as a mechanical drafting aid for the reduction of errors in product packaging.

[FIGURE 9 OMITTED]

Measurement System

A good library should be based on a single unit system to simplify the land pattern construction process. Enough errors have been made while converting English units to metric and vice-versa. The measurement of choice for standard organizations worldwide is the metric measurement system. You have probably noticed that most component data sheets today use metric units. The electronics industry cannot support two measurement systems. The concept of standardization is based on the acceptance of a single measurement system.

A little background: The United States is now the only industrialized country in the world that does not use the metric system as its predominant system of measurement. Congress, recognizing the necessity for the U.S. to conform with international standards for trade, included new encouragement for US industrial metrication in the Omnibus Trade and Competitiveness Act of 1988. This legislation amended the Metric Conversion Act of 1975 and designates the metric system as the preferred system of weights and measures for US trade and commerce. The legislation states that the Federal Government has a responsibility to assist industry, especially small business, as it voluntarily converts to the metric system of measurement.

The current effort toward national metrication is based on the conclusion that industrial and commercial productivity, mathematics and science education, and the competitiveness of American products and services in world markets, will be enhanced by completing the change to the metric system of units. Failure to complete the change will increasingly handicap the nation's industry and economy.

Companies sometimes ask if they must convert to the metric system of measurement. The simple answer is no--the law does not require conversion and the government cannot force businesses to convert. As a matter of fact, competitors (especially overseas competitors) might even prefer that American companies not convert to metric. Some workers may be relieved to hear they do not need to learn a new system, and companies may wish to postpone transition expenses (although the competitive reality is that postponement will be very temporary and subsequent costs may be higher than anticipated).

The best answer is yes. Make a conscious and strategic decision and convert. Companies that delay conversion will miss out on some of the future economic benefits that will ultimately surpass any short-term transition costs. Companies should convert if they make or sell any product or service that they or anyone else might want to sell in foreign markets, if they want to be assured of being able to sell to the US government in the future, and if they want to be able to enjoy a long-term return on their investment. In short, companies should actively plan and manage their transition, and not wait for circumstances that will force them to do so. By then, it may be too late for some firms to gain back years of competitive advantage and some may even have difficulty surviving.

Clearly, US companies that do not provide products or services in metric specifications will risk being increasingly noncompetitive in world markets. Japan has identified the lack of metric usage by American companies as a strategic impediment to their access to the Japanese home market. In addition, consolidation of the European market product standards will make sales of non-metric products increasingly difficult and uncertain. Most US companies understand that using metric units is essential to future economic success. Their hesitation to convert may be due to uncertainty as to when and how to convert.

Through their actions, federal agencies are demonstrating an increasing determination to use the metric system of units in business-related activities. The results are not yet very visible to the public, which is not a direct target of current federal transition activities. The target is industry, which is becoming increasingly aware of the government's progress. Industry acceptance of metric transition is due partly to the realization that producing to metric specifications and surviving in tomorrow's economic environment are synonymous. Industry also understands that government agencies are committed to working cooperatively with industry.

All the world standard groups involved in the electronics industry (IPC, IEC, NIST, JEDEC, EIA and JEITA) have made the transition to the metric measurement system. They formed an alliance to stop using English units and publish data in metric units only, and you can bet the CAD library of the future will be in metric units.

As this article is written, IPC, in conjunction with IEC and the world electronics industry standards groups, is in the process of establishing the IPC-7351 standard for CAD land pattern specifications. The IPC-7351 specification introduces the following standards for CAD libraries:

1. A strict land pattern naming convention that will help in the standardization of electronic schematic symbols for engineering.

2. A zero component rotation so that all CAD land patterns are built with the same rotation for the purpose of assembly machine automation.

3. A three-tier specification that supports various levels of product complexity. The three-tier CAD library system supports the following:

i. Least environment land pattern for miniature devices in which the land pattern has the least amount of solder pattern to achieve the highest component packing density.

ii. Nominal environment land pattern for normal everyday consumer devices. The solder pattern is average size.

iii. Most environment land pattern for high-shock, high-vibration or life support systems. The solder pattern is robust and can be easily reworked.

4. The placement courtyard has been redefined to accommodate the three-tier specification.

5. The silkscreen and polarity marking sizes, copper-to-ink clearances and locations.

6. Land pattern origins to aid pick and-place machine automation.

7. Mathematical algorithms to determine pad sizes and spacing have been defined for the three-tier environment. These algorithms account for fabrication and assembly tolerances and component tolerances to calculate a precise land pattern.

8. 3D modeling for mechanical verification using maximum component outlines and maximum component height.

REFERENCES

Free IPC-7351 libraries in different CAD formats, as well as technical papers and documents related to this article, can be downloaded from www.PCBLibraries.com/IPC-7351.

TOM HAUSHERR is CEO and director of technology for PCB Libraries Inc. He has been involved with PCB design for over 30 years. Hausherr can be reached at tom@pcblibraries.com.
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Title Annotation:CAD Libraries
Author:Hausherr, Tom
Publication:Printed Circuit Design & Manufacture
Date:Mar 1, 2005
Words:1824
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