All,
There obviously is some confusion surrounding the calculation of land size for a
given pad stack. Hopefully I will be able to clarify some of these issues.
The calculation must start with a calculation for hole size. The hole is sized based
on whether there is a component lead in the hole or whether the hole is used as a
via. The tolerance selected for the hole size is based on these two criteria only.
There have been several good suggestions on this forum for selecting a hole size.
Paragraphs 9.2.2 and.9.2.2.3, of IPC-2222, discuss the criteria behind selecting a
hole sizes for component leads or for vias.
Once a hole size and tolerance has been selected we may proceed to calculate a
minimum land size. For outer layers, one must use the maximum finished hole size.
This includes the positive side of the tolerance. Everyone is correct when they speak
about the fabricator drilling a larger size hole to accommodate the plating. The
designer must be aware of this so that when requiring extremely small annular rings
that the over drilled hole is not larger than the imaged land. The safest route is to
use the maximum drilled size. Para. 9.3 of IPC-2222 will help you to determine drill
sizes for holes as they relate to board thickness. Again, fabricators usually drill
0.003 - 0.004 in over the nominal hole size.
For Inner layers we should only use the drilled hole size plus any etch back if
required. The reason for this difference, is in the method of measuring annular ring
between outer and inner layers. Outer layer annular rings are measured from the
inside of the plated barrel to the outside of the land. Inner layers are measured
from the outside of the plated barrel (the part that is adjacent to the laminate
material) to the outside of the inner layer land.
These numbers are then added to twice the annular ring requirement. Remember that
annular ring is measured from only one side of a land, and we need to account for
both sides of the land. See Figures 9-2 and 9-3 of IPC-2221.
Last but not least we must add a standard fabrication allowance to the equations.
This allowance takes into account variation in the fabrication process. The allowance
includes items such as positional accuracy of the drill, registration between layers,
image accuracy on each layer, etch factors, etc. This accounts for approximately
0.040in of individual tolerances. The tables in the IPC standards are based on
industry survey and a statistical approach called sum of the squares. We know that
all the tolerances will not go against us at the same time. By summing the squares we
can statistically derive a safe number for process variation.
The fabrication allowance will vary from one fabricator to another. Those fabricators
that can hold their processes tightly will have a lower standard fabrication
allowance. The tables in the IPC standards reflect the opinion of a majority of
fabricators as to what is achievable.
The table lists three levels of producibility, a clear reflection of cost. Column "A"
is fairly common and reflects the ability of the majority of fabricators in our
industry. Level "B" reduces that majority while level "C" reflects the highest cost
and lowest yield of the group. It is intended for the designer to apply level "A" as
his standard, selecting level "B" when he really must and avoiding level "C" as much
as possible.
As stated by one of our colleagues, the largest cause for deviation requests is in
the ability to meet a minimum annular requirement. This is primarily because
insufficient or no fabrication allowance has been added to the land size calculation.
We also suggest that the designer not work to minimums but add a safety margin to all
calculations. 0.001in or 0.002in annular rings for hand soldering or rework is just
not practical, and not what the standard is recommending.
The new table on standard fabrication allowances has recently come under fire since
the different board or panel sizes have been dropped. The design committee will be
meeting next week at IPC Expo and will evaluate any comments that have been received.
Regards
Gary Ferrari
Executive Director
IPC Designers Council
(860) 350-9300 Fax (860) 350-9309
[log in to unmask]
>>> "Uptain, George" <[log in to unmask]> 04/23/98 02:21PM >>>
Greetings;
Everytime I revisit hole size to pad size calculations I find a
different road to follow.
I just received my new copy of the IPC-2220 series that replaces
IPC-D-275. While reviewing it, 2222 9.1.2, I am confused by the
calculation for land size. It would seem that the maximum hole size did
not account for locational tolerance, only size. We have always
provided .022" over nominal hole size but the calculation provides only
*.014" over max. My guess is that might be .017" over nominal. Does the
fabrication allowance from 2221 Table 9-1 include the positional
tolerance? and/or size tol?
The 1st question is: How do you calculate maximum hole size? My usual
method is (nominal) + (size tol) + (1/2 DTP)
Usually that's nominal + .007"
The 2nd question is: Do you have a "normal" value to add to nominal
hole size for pad size? Ours is .022 or .026 depending on complexity.
Our usual vendor recommends .021" minimum in their design guide.
*The same calculation in IPC-D-275 shows .020" over max hole size.
5.3.2.3
Thanks;
George W Uptain, Jr.
Austin Product Center of Schlumberger Oilfield Services
(512)331-3183
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