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February 2009

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Subject:
From:
Kim Sparrow <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Kim Sparrow <[log in to unmask]>
Date:
Mon, 2 Feb 2009 01:22:09 -0600
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I've been working with some old PTH padstacks lately, and thought I'd see how 
they "stack up" against the IPC-7251 recommended patterns. Really at first I 
was looking at the spreadsheet generated by the LP Calculator, both from PCB 
Matrix. Then to see where the LP Calculator came up with its numbers, I've 
been consulting the draft IPC-7251.

I think I've found an inconsistency in hole and land diameter relation that in 
some circumstances causes insufficient annular rings, by IPC-2221/2222 
standards.

In section 3.1.3, paragraph 1 it says, "Each dimension given is considered part 
of the equation which takes the maximum hole size, twice the annular ring 
requirement, and an appropriate manufacturing allowance."  That's the minimum 
land size = a + 2b +c from IPC-2221 sec. 9.1.1. So far so good.

So we move to the series of tables 3-1 through 3-9, where the PTH lands are 
defined based on the maximum lead size. We see the numbers from IPC-2222 
Table 9-3 (as cited by Jeffrey) for defining the _minimum_ hole diameter.

In the next row, the values in "Int. & Ext. Annular ring Excess (added to hole 
dia.)" correspond to IPC-2221 Table 9-1 minimum standard fab allowance + 
double the Table 9-2 minimum external layer annular ring, which should be 
added to the _maximum_ hole diameter. But as far as I can tell, this value is 
instead intended to be added to the minimum hole diameter from the previous 
row. The LP Calculator adds this value to the minimum 

What seems to be missing is some accounting for IPC-2222 Table 9-5, 
"Minimum Plated-Through Hole Diameter Tolerance Range". Without it, it's as if 
drill sizes have zero tolerance, which we know isn't the case!

I've been searching validation (or otherwise) on the Internet but to no avail. 
I'm not sure if I'm on to something or if I'm missing one key point, or if I've just 
totally lost myself in a cloud of tables. This seems to be the place to ask...

Cheers,
Kim

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