Board Dimensioning

[davepcb]

I was asked to change my tolerance for board dimensions to only 2 places after the decimal point. I usually dimension everything to 3 places after the decimal point. Is there a significant cost or setup impact by using 2 compared to 3 places after the decimal point when dimensioning the board?

Also, I’ve always used the board edge for datum “A” and one of the mounting holes for datums “B” and “C”. Then I add symbols to the fab drawing that looks like
this: (I'm not sure if the pic will show here so I'll describe it)

Circle with cross hairs on it followed by a circle with a slash in it followed by a tolerance of .005 followed by the three datum points.






My understanding was that all holes are referenced from the 3 datum points. Datum A is at the plane and datums B and C represent the X and Y coordinates for the secondary and tertiary datums. Could you clarify this for me?

Thanks,
Dave
http://D:\fab issues\DIMENSIONING.JPG

[Tom]

Dave,

Board Edge Tolerances:

Metric dimensioning should be held to two places.
Inch dimensioning should be held to three places.

Let's talk Inches. What really matters is the tolerance requirements in your fabrication drawing title block. If they state .XX = +/-.010" and .XXX = +/-.005". Now if I'm the board shop and I see this tolerance note I'm going to raise my price if you use three place dimensions because my standard tolerance is +/-.010" for my NC routing process.

You can dimension using three places but also make a note that the tolerance is +/-.010".


Datum Point:

What board manufacturers really like to see is the lower left mounting hole (or any hole in the lower left if there is no mounting hole) to be Zero, Zero. This is where the origin of your design should be. So the location of the Lower Left Mounting Hole is 0,0.

The lower left corner of the board should always be a negative coordinate. Because of NC route tolerances, the lower left corner of the board should never be 0,0 origin. Also, 0,0 origin should always be a hole and preferably a non-plated hole because it has a tighter tolerance.

All Drill Data for all PCB holes should be referenced from a hole (not a board edge).

In this manufacturing note (pertains to hole location):
All holes shall be located within 0.2mm (0.008”) Diameter of True Position. Layer to layer registration shall be within 0.125 (0.005”).

It really only refers to a single point of origin. However for your NC Routing refers to 2 places minimum and 3 places is preferred.

[davepcb]

Tom,
Thanks for the quick reply. I'm a little confused at the last two sentences. Are you describing the chart that I attached? Where did the location of holes within .008" come from? I'm not sure how to properly describe the chart that references the three datum points that I am using.

dave

[Tom]

Dave,

It's really hard to explain without pictures.

Send me your fax number.

Tom.Hausherr@WindRiver.com

Or if you have an IPC-2221 book look on pages 31 - 34.

[Tom]

I did find one picture that I had that illustrates two rigid boards joined together with a flex circuit.

[davepcb]

Tom,
Thank you. That was very helpful. It seems that I can't make one of the mounting holes be both the B and C datums for X and Y directions.

However even in the example above I can't really tell which datum represents an X or Y direction. I checked out the examples in IPC-2221 and saw the same thing. I'm going to follow the example you sent and add a maximum material condition symbol to the B datum. That's what I see in Figure 5-5B in IPC-2221. After studying the five different examples it looks like I should use the unplated tooling hole example on this particular board.

Thanks again. I'm slowly understanding this. I'll check out IPC-2615 as well.

Dave