Hi Paul--
IMO, your query is not a simple response, this is because there are
additional requirements in the manufacturing ("Performance")
specifications (IPC-RB-276 or IPC-6012) that need to be included, and
are not mentioned in the IPC-A-600E
There is a "consistency" problem in the IPC-A-600E, 2.10.3 "Annular Ring
- Supported Holes" (round lands) and 2.10.5 "Annular Ring - Irregularly
Shaped Lands", basically ignore 2.10.5. (Examples in 2.10.5, Accept
Class 3, 50 micrometer ARmin — that's OK. However, Accept Class 1 & 2
is in conflict, B) 25 micrometer ARmin. (there is no documented
requirement for this requirement), which is in conflict with C) which
allows 90 degree breakout, you can't have both. So, you need to use an
expanded interpretation of 2.10.3 for all "supported" (plated-through)
hole lands.
The following embedded responses to your series of inquiries concerning
"Conductor/Land Hole Registration" are based on the full interpretation
of the technical conductor/land junction area and land/hole registration
requirements of the IPC-RB-276, IPC-6012, and IPC-A-600E. Some of these
interpretations may stimulate some additional technet discussion, but
based on current IPC standard and specification requirements, "They're
in the book(s)".
>Can anyone help with a query on land around via holes. I have a copy of
>IPC-A-600 Rev E Aug 1995 which states for plated through holes the
>minimum land between the track and pad and hole must not be less than
>0.002" as a minimum standard.
The IPC-A-600E for conductor/land junction area, and land/hole
registration does not differentiate between component and via holes.
(It is assumed "track" means conductor and "pad" is a land.)
In IPC-RB-276, 3.7.3 "Annular Ring (External)" refers to Table 6 for
"Minimum Annular Ring Requirements", and IPC-6012, 3.4.3 "Annular Ring
(External)" refers to Table 3-5 for "Minimum Annular Ring
Requirements". Then in IPC-RB-276, 3.7.3, 1st sentence, last paragraph,
and IPC-6012, 3.4.3 "Annular Ring (External)" last sentence, only
paragraph, they both have a requirement that states, "Plated-through
holes identified as vias can have up to 90 degree breakout of the
annular ring if it does not occur at the conductor and land
intersection." With minor word changes, these requirements are the
Table in the IPC-A-600E. Interpretation of these requirements per the
referenced "tables", breakout is allowed for "all" Class 1 & 2 Products
plated-through holes. There is a minimum annular ring requirement for
Class 3, and the statement addressing "plated-through holes identified
as vias" is superfluous.
The only requirement in IPC-A-600E, 2.10 "Dimensional Characteristics",
2.10.3 "Annular Ring - Supported Hole", page 46, for a 50 micrometer
minimum annular ring (ARmin) is for Class 3 products.
In the IPC-A-600E, the "Table" of requirements (from IPC-RB-276 and
IPC-6012 is essentially the same) there is no ARmin for either Class 1
or Class 2. Under Class 2 requirements the last sentence states, "The
conductor junction should never be less than 50 micrometers [0.002
inch]". (Note— the corrected metric "typo" error of 0.55 mm.) The
"should" statement should be treated as a "comment" per the IPC's use of
the words such as "must, shall, should, may, can, etc", the "should"
requirement is not a requirement, and it shall be considered as a "it
would be nice option" in accordance with our use of words.
>This means that misregistration towards the track is not acceptable
>if the pad is reduced below 0.002" even if there is no actual breakout.
>On the other hand, the hole is allowed to break out of the pad in any
>other direction by as much as half the diameter. Clearly the two are
>incompatible with each other since with registration that bad there is
>bound to be breakout towards tracks in some areas.
1) Misregistration of the hole towards the conductor in the CLJA is
acceptable because there are no ARmin requirements for Class 1 & 2
Products, "breakout" of up to 180 and 90 degrees is respectively allowed
for Class 1 and Class 2 Products of the hole to land; with an additional
requirement that "breakout" in the conductor/land junction area (CLJA)
shall not reduce the conductor width in the CLJA to less than 70% (Class
1) or 80% (Class 2) of the design conductor width minimum (DWCmin) in
the product definition data (PDD) (could be a master drawing), or the
production master nominal.
2) Because there is no ARmin for Class 1 and 2 Products, the edge of
the hole can be "tangent" to the land and the "end" of the conductor in
the CLJA (by definition, tangency is not breakout).
3) Incompatibilities - there aren't any, as in the above comment 1),
Class 1 and 2 Products allows two levels of "restricterd" breakout or
"tangency" with no ARmin requirements, and Class 3 has a 50 micrometer
ARmin requirement.
>Has there been a revision specifically for via holes only, or is there
>any other specification relating to landless via holes as this criteria
>is impossible to meet on some designs.
Per existing manufacturing specification requirements(IPC-RB-276 or
6012), there is no differentiation between component and via hole lands,
for Class 1 & 2 Products, "tangency" or limited reduction in CW in the
CLJA is acceptable.
There is a test program (through ITRI) that is evaluating "butt" or
"land-less" interlayer connections for multilayer printed boards,
unfortunately, "outer layer" interconnections are not included in the
test matrix.
Some printed board designs are "designed" for "controlled" breakout of
up to 90 degrees for Class 2 Products with very good reliability results
for specific environments. As Andy Slade (Hadco) commented, using
"filleted land patterns" (modifying the land pattern by adding a
"teardrop" or "snowman") in the CLJA will eliminate the probability of
"breakout" in the CLJA. This is what a quality design would include if
"controlled" breakout is allowed, and coupled with "reliability" are
"design" requirements.
>The implications of meeting this spec are to increase via pad size
>and/or reduce hole size which has implications for design density
>and manufacturing cost.
Not really, it depends on how "land" size is increased, use "obround",
or "elongated" hexagonal or octagonal shaped lands, by (appropriately)
orient the lands in a consistent direction on each "X" or "Y" orthogonal
conductive pattern layer, and by limiting conductors to ingress or
egress from a land only at the "ends" of the lands . If you want, I can
generate and direct e-mail to you a <*.pdf> sketch of the general design
concepts.
>Any views on this would be greatly appreciated. For production drilling
>on large panels, what minimum land do you think would be needed at the
>outset to guarantee minimum annular 0.002" on the finished panel taking
>account of drill size and etch reduction also?
You'll need to characterize all manufacturing and material tolerances.
>Best Regards
--
>Paul Gould
>[log in to unmask]
>Isle of Wight,UK
Paul, I hope this helps, if not, please feel free to contact me.
--
Ralph
Ralph Hersey & Associates
3885 Mills Way
Livermore, CA 94550-3319
PHN/FAX: 925.454.9805
e-mail: [log in to unmask]
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