Preventing mechanical interference: get those tolerances just right, because a great board that can't be installed into a final product is scrap.

Success stories regarding mechanical fit and interchangeability are easy to relate to. Maybe you've sat patiently, grounded on an airliner full of passengers, waiting for a replacement part to be flown in. You learn to appreciate successful "assembleability" when the part finally shows and it slides right into the airplane.

Unfortunately, in the confines of our engineering departments, using our (nearby) fabrication and assembly facilities, we recall when an assembly has "crashed" into a mating counterpart of its next assembly. A condition known as interference has crept into the design or manufacturing process and the ugly terms "scrap" or "rework" and the tooth-grinding whirr of a Dremel tool come to mind. Fabricating a high-tolerance PCBA is not so much of a problem today, but getting the PCBA to mate successfully with its counterparts without a solid design communication process still is.

While specifying the widest PCB tolerances possible improves manufacturability, it is important to anyone who designs around a PCB that the dimensional tolerances be tight. A board designer may have good intentions when assigning a wide tolerance of +/-0.020" to the nominal 0.062" thickness of a board in order to aid fabrication. After all, it may be an eight-layer, impedance-controlled design, with no time to check what the accumulative dielectric thickness may add up to. This emotional judgment call, however, will cause any mechanical designer a headache if he's searching for an off-the-shelf card guide into which this PCB will slide. Not many card guides accept such a range of PCB thicknesses. At its least material condition (LMC) of 0.042" the PCB will chatter loosely in the card guide. At its maximum material condition (MMC) the PCB will not slide in at all. Interference!

The Mars Climate Orbiter is a different example of the havoc interference can wreak. Launched on a mission to Mars after years of planning by, yes, rocket scientists, it was hoped that this spacecraft would become the first interplanetary weather satellite. After the ship's nine-month trip through space, teams of scientists began transmitting data to navigate the satellite into a trajectory that would bring it into a safe working orbit around the planet. And like many of our own "great" PCB engineering projects, everything went fine until the project tan into something--in this case, the surface of Mars. More interference!

What happened? Post-mortem analysis concluded that the orbiter was lost due to a mix-up of imperial and metric units between collaborating teams. Additional reviews cite several other potential causes: Examining these three points will help our collaborating design and manufacturing teams prevent interference in assemblies.

Inadequate consideration of the entire mission. The PCB industry is enjoying an age of "think it/build it" manufacturing. As a board designer for over 20 years, I can remember many instances of being expected to complete a PCB layout after being handed only a schematic attached to a work order. Scheduling? Due yesterday. Size? TBD. Application? TBD. Sound familiar?

In a time-to-market engineering environment, it amazes me how some members of a design team interpret "think it/build it" as some sort of pushbutton that generates only a schematic and a work order. They take for granted--or do not fully understand--the basic design constraint requirements for successful PCB layout. As we focus on preventing mechanical interference in PCBs, it is critical to the success of the design to obtain all design criteria before beginning the layout. Failure to define overall length, width and thickness requirements, including understanding of the next assembly interface--mounting hole patterns, keepout areas and tolerance--plant the seeds of mechanical interference.

The most effective method of considering the mission of a PCB project is to call an informal planning meeting or predesign review. A meeting with representatives from key areas in the design and manufacturing process will facilitate a forum of communication between members of the design and manufacturing team. Communication is the main enemy of mechanical interference. This forum might include:

* The layout designer, who has to get it done "yesterday."

* The electronic engineer, who would never have realized the board size is only 2 x 2".

* The mechanical engineer, who would never have seen the six extra comm ports.

* The project manager, who would never have realized that the right hand didn't have a clue as to what the left hand was doing.

* The fabrication rep, who would tell you that a tolerance of +/-0.0001" is really going to cost you.

* The assembly rep, who explains that parking that BGA so close to the board edge is not a good idea. And by the way, can you add a few more fiducials?

The goal of this meeting is for the development team to walk away with a clear view of the complete design goals and constraints.

Inconsistent communications and training within the project. There are many ways to communicate mechanical information about a PCB design. A formally dimensioned fabrication drawing has been the most common way for a designer to communicate board geometry to a fabrication vendor, quality control inspector and any next assembly designers. However, inconsistent dimensioning methods and lack of training and use of popular geometric dimensioning and tolerancing techniques are often the source of mechanical problems between mating assemblies. Many designers have forsaken formal dimensioning practices and are relying on state-of-the-art CAM tools to carve the geometry of their designs at near nominal accuracy (FIGURE 1). True, a PCB can be fabricated using CAM data without a formally dimensioned fabrication drawing. But how will it be inspected? Without some kind of dimensional documentation, it is difficult to design anything that will successfully interface with that PCB. An electrically functioning PCBA is useless unless it can be installed into something.

[FIGURE 1 OMITTED]

Designing a chassis or enclosure using edge card guides into which the PCB can be installed--without ready access to the board's thickness and length for instance--will generate a barrage of dimensional questions that will need to be answered. In order for successful interface conditions to exist, fabricators and designers must have access to the critical dimensions and tolerances relating to the PCB.

The "holy trinity" of PCB geometric features comprises the following: board edges (length, height, width), hole pattern and etched conductor patterns (FIGURE 2). The success of the board build is predicated upon the accuracy of these three features' alignment. There is one humble but key feature most commonly used to align all of these elements: a simple mounting (or tooling) hole. With its theoretical center placed in a design database using a padstack at XOY0, a hole is a feature that will surely be visible on every layer of a PCB design. In design terms a padstack is a cluster of combined data that permits software to extract graphic artwork for etching conductor pads. Padstacks also contain the data needed by drills to create holes in the PCB for mounting purposes. In the vastness of a blank design database, once a point in space has been selected and defined as XOY0, a padstack is inserted. The process of preventing next assembly interference has begun.

[FIGURE 2 OMITTED]

What is a tooling hole anyway? By definition, it is circular feature, void of material. Mechanically speaking, the benefit of a tooling hole is that it offers a most important feature, a board edge. Yes, think of a tooling hole as a mechanical board edge. In the majority of cases, this round, 360[degrees] board edge will now become the basis for all pre-fabrication dimensioning and post-fabrication inspection. With alignment between the etched pattern, the hole pattern and board outline pattern established as XOY0, dimensional information can now be added.

But what about board geometry that is seemingly "undimensionable?" In some cases, a board outline can be too complex to dimension and a designer might choose to rely on various CAM data output to define the board outline for manufacturing purposes. In this case, data may be provided along with a note: BOARD OUTLINE SHALL CONFORM TO SUPPLIED GERBER DATA WITHIN 0.010" [1.59 mm].

While this method works from a design interface standpoint, be aware that this spec pretty much excludes Quality Control from the checking process, as QC traditionally does not perform graphic CAM data comparisons with hardware.

Lack of complete end-to-end verification of navigation software. It's sad to think that a simple design review meeting to discuss units of measure could have saved the Mars Climate Orbiter, hot to mention hundreds of millions of dollars. If there is anything that we have learned from this high-profile technical failure, it is that facilitation of design review communication must take place in order to prevent scrap.

Once you've gathered the missing data revealed in the pre-design review, organized all the data into a layout, and dimensioned and documented the layout, it's time for the formal design review. Things tend to change on the fly. Have all of the changes been incorporated into the PCB layout? Has the right hand kept up with the left?

A formal design review permits design and manufacturing teams to really appreciate a clearly documented PCB package. With interface dimensions clearly documented, the PCB outline and mounting hole patterns can easily be compared and checked against mating assemblies. As carpenters say, measure twice, cut once.

Between the Internet and CAM software, a PCB design can be manufactured almost anywhere in the world within hours. Separate engineering teams can design and manufacture parts of a common assembly in record time, but the parts must interface successfully. Better still, thousands of the same parts should fit, and be interchangeable within the assembly.

Without obtaining a clear vision of PCB design constraints before starting a PCB design, without clear documentation to review before the fabrication and assembly process begins, the absence of mechanical interference upon assembly is pure luck--a term that most project managers cannot cost-justify in a project schedule. PCD&M

[FIGURE 3 OMITTED] [FIGURE 4 OMITTED] [FIGURE 5 OMITTED] [FIGURE 6 OMITTED] [FIGURE 7 OMITTED]

BIBLIOGRAPHY

IPC-2615, Printed Beard Dimensions and Tolerances, July 2000. Mars Climate Orbiter information: http://mars.jpl.nasa.gov/msp98/news/mco991110.html and seds.org/~spider/spider/Mars/ms98mco.html

KELLY DACK is a senior PCB designer at International Game Technology (igt.com). He has over 20 years of electromechanical package design experience and is a frequent speaker at the PCB Design Conferences. Dack can be reached at [e-mail].