PCB Dimensions: Tolerance and the IPC

Here's where we boldly confront a grim reality - nothing is perfect in the real world.

We do our best to maintain accuracy in our circuit board designs, but the manufacturing process produces imperfections.

• Our CAD systems assume a drill is perfectly centered in a round pad. It never is.

• We declare specific trace widths, and when we measure them on an actual board they are always slightly thinner or thicker than expected.

• Multiple layers are perfectly aligned on our computer screens, but the fabricators can never quite manage to duplicate it. There will always be some misregistration.

• The board design is assumed to be flat, but boards in the final product can be bowed or warped.

• We designated some traces to be impedance controlled, but our measurements differ

We could go on and on and on, but I hope you see the point. As designers, we calculate exact numbers. We design with precision. Our CAD systems show us the ideal board. In the real world, nothing is so precise. The final product will vary from the ideal in one way or another, hopefully in ways that are harmless.

How Will You Decide Which Defects Are Acceptable?

Measuring a group of supposedly identical products will show some amount of variability, so we need to define what range is acceptable for each type of measurement and at what point or limit we should reject the product as non-conforming. These ranges are called TOLERANCES.

Here’s an example: Let's assume we are given a schematic for a design that will be mounted in a metal box, and the predefined box size is larger than the area we really need for the circuitry. This type of design allows us enough room to work comfortably, placing components away from the rectangular board edge. Our goal is to make sure the board fits inside the box and the board mounting hole pattern fits the box mounting hardware. In this situation, the dimensions of the board edge aren't as important, and we may be able to allow a large tolerance.

Now let's imagine that the same circuit has to fit on a standard-size expansion card for a personal computer, and needs a plated edge connector. In this scenario, even if there is plenty of board area for the circuitry, the board edge dimensions are more critical, and the tolerance will have to be reduced to make sure the board fits properly. A smaller tolerance might still be within standard manufacturing process control, but it adds complexity, raises cost, and makes inspection more difficult.

Now let's consider a requirement to package the circuit into the confines of a cell phone enclosure. The new size constraints give us even less board area to work with, and the dimensions are more critical. The designer must clearly define these expectations for the fabricator, and this design may limit the available manufacturing partners to those that are repeatedly able to maintain these tighter tolerances.

With this example, I was hoping to illustrate that the same circuit may have differing requirements based on the end-use application, and may have different tolerance ranges. But understand that the dimensional tolerance of the board outline is just one characteristic among dozens. The board thickness may be important, or the plating thickness, hole diameters, layer registration, minimum annular ring, dielectric properties, etc. There are many parameters that contribute to a successful design, and some attention should be given to each of them as part of the process of designing a circuit board.

Now let's imagine that we have examined every way in which a circuit board parameter can vary, defined acceptable tolerances for each, and collected all of them into a specification document. When the next design comes along we may be able to use part or all of the previous specification, changing only the parameters that differ significantly. In this way, a company may have evolved a general board specification that applies to most of its products, unless otherwise specified.

This type of general specification is an effective tool for multiple designers working on similar designs or in large organizations like the military.

While proven specifications remove risk and take some of the tedious and repetitive tasks out of the design process, a few problems quickly arise. In the early years of the electronics industry, a lot of effort was put into developing specs by large companies, and there was a natural desire to keep these documents PRIVATE to maintain a competitive edge. Therefore, many of them were copyrighted and there was little open discussion or sharing of experience between organizations.

Looking at this situation from the board fabricator's point of view:

• multiple customers are providing different variations of every board parameter imaginable, making it difficult to establish consistent processes, and adding more test and inspection overhead to the entire industry.

• not every designer has a thorough understanding of the manufacturing process, and they sometimes OVER-specify tolerances without a scientific basis, adding unnecessary cost to the product.

What is really needed is a set of generic documents that can be used as a default for assigning acceptable tolerances, which can be overridden by designers when needed.

That's what an organization called the "IPC" has developed for us.

The IPC is an "Association Connecting Electronics Industries", and they develop and maintain standards and guidelines on many different aspects of the electronics industry, with periodic revision and new publications being introduced continuously to keep up with technological advancement. These are developed by committees of volunteers from every sector of the electronics industry. Some of the documents are joint publications with ANSI or JEDEC, two other worldwide standards organizations.

Here is a brief glance at some of the major players and their relationship to the electronics development process:

Source: IPC Association Connecting Electronics Industries

I'm happy to report that virtually all of the old U.S. military specifications ("Mil-Specs") have been declared obsolete in favor of the latest IPC specifications. More and more companies are adopting the IPC as a starting point, and we are all gradually reaching a consensus on how to communicate information between departments smoothly and reliably.

OK, Let's take a step back....

Regardless of how much effort we put into trying to perfect a design, the actual product will have imperfections resulting from MANUFACTURING TOLERANCES. We need to know how to determine:

• What Is Preferred?

• What Is Acceptable?

• What Should Be Resolved Or Rejected?

The set of guidelines and requirements can vary for different types of circuit boards depending on the type of product being designed. You can easily imagine that what might be acceptable for a circuit board in an inexpensive toy, might NOT be acceptable for electronics in a medical product. For this reason, the designer should choose one of three Performance Classes to establish the acceptability criteria for the product he is designing. Here is how the IPC defines Class:

IPC Performance Classes

Three general classes have been established to reflect progressive increases in sophistication, functional performance requirements and testing/inspection frequency. It should be recognized that there may be an overlap of equipment categories in different classes. Users have the responsibility of specifying in the contract or purchase order what performance class is required for each product and should indicate exceptions to specific parameters where appropriate.

IPC Class 1: General Electronic Products — Includes consumer products and some computer and computer peripherals suitable for applications where cosmetic imperfections are not important and the major requirement is the function of the completed printed board.

IPC Class 2: Dedicated Service Electronic Products — Includes communications equipment, sophisticated business machines, instruments where high performance and extended life are required and for which uninterrupted service is desired but not critical. Certain cosmetic imperfections are allowed.

IPC Class 3: High Reliability or Harsh Operating Environment Electronic Products — Includes the equipment and products where continued performance or performance on demand is critical. Equipment downtime cannot be tolerated and must function when required such as in life support items or flight control systems. Printed boards in this class are suitable for applications where high levels of assurance are required and service is essential.

When sending a design out for bare board fabrication, assembly, and test, the documentation should state the Performance Class and any exceptions to specific parameters.

A good introduction to what an IPC specification is (and how the three classes of boards are used) can be seen at a glance in APPENDIX B of the document called: IPC-6012 QUALIFICATION AND PERFORMANCE SPECIFICATION FOR RIGID PRINTED BOARDS.

Appendix B shows the performance requirements for rigid boards in an abbreviated list and in alphabetical order. Here is an excerpt showing the acceptability of five different characteristics:

Source: IPC Association Connecting Electronics Industries

Pay attention to the fact that some conditions have different criteria for the three classes, but others apply the same criteria to all rigid boards regardless of class. The last column points to the associated section of the IPC-6012 document where lengthy descriptions, special conditions, and tutorial information can be found.

Many other publications have been developed for various subjects such as:

Design Guidelines, Board Storage and Handling, Manufacturer's Qualification, Material Declaration, Current-Carrying Capability, Embedded Component Technology, Data Formats, Documentation, Adhesives, Laminate Material, Fabric and Foils, Plating, Via Protection, Marking Inks, Cleanliness, BTCs, Flip-Chip, BGAs, Through-Hole, Surface Mount Land Patterns, Stencils, Rework, Optical, Test, Quality and Reliability, SPC, Thermal, Acceptability, Inspection, Dimensioning, Netlists, Hybrids, Drilling, Terms, Soldering, Marking, Shipping, Lead Free, and MORE...

There are a few IPC standards that I would consider essential for any circuit board designer. For the record, I don't receive any financial compensation for encouraging you to purchase them. Personally speaking, these documents have been invaluable to me:

IPC-2221 Generic Standard on Printed Board Design

IPC-2222 Sectional Design Standard for Rigid Organic Printed Boards

IPC-7351 Requirements for Surface Mount Design and Land Pattern Standard

IPC-6011 Generic Performance Specification for Printed Boards

IPC-6012 Qualification and Performance Specification for Rigid Boards

IPC-A-600 Acceptability of Printed Boards

IPC-A-610 Acceptability of Electronic Assemblies

If your designs use specific technologies like BGAs, HDI, BTCs, etc, make sure to look for publications on those subjects. A full list of IPC documents can be found here.

NOTE: Specifications alone will not guarantee that your boards will be what you expected them to be. It is wise to do some type of periodic verification; which could be some combination of visual inspection, measurement, destructive analysis (or other test methods), and formal statements from suppliers such as "Certification of Compliance" documents.

Summary

Standards give us a starting foundation for every aspect of product development related to circuit boards from design to final test, making the IPC truly an "Association Connecting Electronics Industries".

Published guidelines incorporate the valuable experience of engineers who came before us, their shared contributions giving us a consensus to build upon. Specifications give us a common tool to evaluate the results of our design decisions and manufacturing processes. Effective use of these tools will save resources, time, and expense.

As technology advances we learn more, materials and processes improve, standards and guidelines evolve. You are encouraged to contribute to these revisions. The IPC welcomes a wide variety of participation.

Click here to Learn More About Getting Involved with the IPC.

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