PCB Design Perfection Starts in the CAD Library - Part 1

PCB Design Perfection Starts in the CAD Library

Guidelines and Recommendations - Tom Hausherr Mentor Graphics Corp.

 

This is the first article of a series that is intended to introduce readers to creating a high quality CAD library. The CAD library is the starting point that affects every process from engineering and PCB layout through PCB manufacturing and assembly. There are dozens of things to consider when creating a CAD library that are often overlooked or not even considered, which will directly affect the quality of the part placement, via fanout, trace routing, post processing, fabrication, and assembly processes. This article, part one of a series, introduces aspects that should be considered when creating chip CAD library parts and the impact that each feature of the CAD library has in the PCB process. Subsequent articles will delve more deeply into these concepts.

 

A high quality CAD library can make a huge difference in the quality of your PCB design layout and your overall productivity levels throughout the PCB design process. Also, you need to have the necessary tools that can quickly build new CAD library parts that are consistent quality that have identical features in every library part. You also need an organized CAD library management system that allows you to quickly locate the correct land pattern for each component in your PCB layout.

To begin, all of the components and their related land patterns can be found in the free LP Viewer that you can download here - http://www.mentor.com/go/lpwizard

 

Creating a CAD Library

Creating a CAD library requires answers to these questions: 

  • Why do we snap the lands (pads) to a 0.1 mm grid?
  • Why is the assembly outline different than the silkscreen outline?
  • Why do we snap the via fanout to a 1 mm grid?
  • Why is it important to use metric units when creating your CAD library?
  • What is the IPC-7351B standard and why is it important?

I will never forget my friend Wolfgang’s description of a PCB design error: “A small error on an ugly PCB layout is a big deal, but the same exact error on a nice looking PCB layout is minor issue”.

 

Every good PCB designer has the heart of an artist and the mind of a mathematician and each PCB design is a different piece of art with mechanical precision. A good PCB designer takes pride in their workmanship as they strive to make each new PCB layout more perfect than the last. After 35 years of laying out over 2,000 PCB designs I can say without any reservation that PCB design perfection starts in the CAD library.

 

Measurement Units

One of the secrets of today is that 90% of all component manufacturers are providing their component package dimensions and pin pitches in metric units. Texas Instruments only provides metric units for all 982 of their component packages. TI is following the metric mandate by all world standards organizations and 99% of all world governments. So the CAD library should also be built using metric units. Using metric units for PCB design layout is the future, so you should transition to metric as soon as possible and quit wasting your time building an Imperial unit working environment. The longer you wait to transition the harder it becomes. If you’re a PCB design artist in search of perfection, this paper will clearly illustrate why metric units for PCB layout is vastly superior.

 

The IPC-7351B Standard

Let’s briefly look at the IPC-7351B Standard. We will reference this standard throughout the series. IPC-7351B standard uses a 3-Tier CAD library system:

  1. Most land protrusion (Density Level A) – for military and medical applications
  2. Nominal land protrusion (Density Level B) – for controlled environment desktop
  3. Least land protrusion (Density Level C) – for cell phones and hand held devices

 

The IPC-7351B standard focuses on these key items:

  1. Land size and spacing calculations
  2. Placement courtyard boundary
  3. Land pattern name
  4. Zero component orientation
  5. Padstack name

 

Three types of CAD library parts are defined: Through-hole (PTH), Surface Mount (SMD), and a combination of the two technologies. SMD and PTH CAD libraries are distinctively different but the same basic rules apply to both technologies – “snap” and “round-off” CAD library land (pad) shapes are defined in 0.05 mm increments. We will look at why this standard is so important and how it relates to our CAD library shortly in the series.

 

In addition to the items in the IPC-7351B standard, other considerations are important and are basic guidelines and drafting recommendations for your CAD library. These are summarized here and will be fully explored as we continue.

  1. Pad Spacing with DRC Checking and Pad Trimming when necessary
  2. Pad Size and space round-off
  3. Four Outlines
    1. Silkscreen
    2. Assembly
    3. Placement Courtyard – adjustable sizing for IPC 3-Tier environment levels
    4. 3D Model
  4. Polarity Markings for both silkscreen and assembly
  5. Two Reference Designators with center/center justification (Ref Des Origin), right reading orthogonal, located at the land pattern origin with Height 1.5 mm and line width 10% of Height

 

Coming Up

Additional brief topical articles will appear in future InnoFour newsletters. You can also read more detail in my blog, which can be found at: http://blogs.mentor.com/tom-hausherr/

 

Written by Tom Hausherr CID+

EDA Library Product Manager

Mentor Graphics Corporation