Janice, I believe part of the problem is due to a vaguely worded surface
copper thickness requirement. For the drawing note as worded now, I would
interpret the one ounce copper foil to be a STARTING foil thickness
requirement of 0.0014" with one ounce of copper plated being equal to
0.0014". This DOES NOT mean inspect for a minimum of 0.0028" surface copper
(not even a nominal of 0.0028"), because of the variation in the copper foil
thickness as received by your board manufacturer and manufacturing
processing as you noted. For the current drawing note, I would inspect for
a total surface copper thickness of 0.0024" (0.0010" minimum foil thickness
after processing + 0.0014" copper plating on the surface). FYI, the 0.0010"
final copper foil thickness you discussed matches that in IPC-RB-276 for 1
ounce starting copper weight (internal layers).
Recommendations:
1. Be sure of what you need for your conductor thickness. If you used
Figure 3-4 of IPC-D-275 to arrive at a surface conductor thickness of 2
ounces, then be aware that these charts "include a nominal 10 percent
derating (on a current basis) for normal variations" including conductor
thickness, conductor width, etc. For this situation, lets assume you want a
nominal design value of 2 ounces of copper.
2. Clearly state your conductor thickness on the drawing. Generally, people
use ounces of copper to refer to a starting copper foil weight (thickness).
Your drawing does not clearly state a finished minimum copper thickness
(unless elsewhere on the drawing you reference IPC-RB-276. You should
translate the "ounces of copper" to be plated to a thickness in inches (or
millimeters for the rest of the world). I suggest replacing your note that says
"* one ounce copper foil and another ounce plated on the surface foil."
with one that says:
"minimum starting copper foil weight of 1 oz/ft2 plated to a total copper
thickness of 0.0024" minimum". Change the last number to 0.0025" if you
want to more closely follow the intent of the 10% derating mentioned above.
This note clearly states a finished copper thickness requirement for
inspection and allows the fabricator maximum flexibility for his processes.
3. Depending on the design and your supplier's processes, there may be a
cost premium to plate a thicker copper plating to meet your thickness
requirement. In comparison, the standard surface total conductor thickness
(foil + plated copper) for a one ounce copper foil is 0.0018" in IPC-RB-276,
para. 3.9.2.3. You are asking for at least 0.0006" additional copper
plating than the "standard".
Regards,
David Estes
PCB Commodity Manager
Texas Instruments
(214) 997-2942
------------------------------------------------------------------------------
On May 9, Janice Lund wrote:
Main question:
ON HIGH VOLUME DOUBLE SIDED POWER BOARDS. WHAT PERCENTAGE DIFFERENCE WOULD THE
PRICE INCREASE BE, OF A STANDARD MANUFACTURABLE BOARD PRICE SHOULD I SEE (of
.001" minin the hole), IF ANY, TO A TOTAL FINISHED THICKNESS OF 2 OUNCE COPPER
? (please read below to find out what my vendor says they can do without
increasng the price)
History:
My boards are power boards requiring a total of 2 ounce copper on the surface
and a minimum of .001" in the holes. These are submitted to UL for approval of
our product with the accurate requirements, because at least my prototype
house reads my print and gives me what I require.
My prints states:
* .001" minimum (not average minimum) copper in the holes
* one ounce copper foil and another ounce plated on the surface foil.
I am aware that the copper plating process plates a ratio of 1:0 in the hole
to 1:2 on the surface. Therefore, .001" minimum plate in the hole should
result in .0012" plated on top of the surface foil. I would assume that no
vendor would risk plating the absolute minimum and therefore would attempt to
plate .0012" in the hole, resulting in .0014" plate on top of the surface
foil.
I am aware that 1 ounce foil has a tolerance of +/- .0002" on the foil
thickness. I am also aware that deburring , scrub and micro-etch may remove
another .0002" from the surface of the foil. Based on this, the absolute
minimum should be -> .0010" foil + .0012" plate = a total copper thickness of
.0022" . Whereas, my copper hole requirement is .001" minimum with the maximum
controlled by the hole size and tolerance. I expected to receive boards with 2
ounce finish minimum copper thickness and copper thickness in the hole in
excess of .001" to reach the surface thickness requirement. WAS I WRONG?? (
please to not reference IPC-A-600 as my print takes precedence over that spec.)
My Production Fabrication Vendor states the best they can give us .0020" -
.0025" on the surface for our specified 2 ounce copper .0028" requirement.
Needless to say they were not giving us close to that before. (so I will
believe an improvement it when I see it).
The fact is, if we accept their minimum of .0020", aren't we now still under
the .001" minimum in the hole??
My Vendor also stated that there would be a price increase if a total of 2
ounce total was required.
Seems to me, if they took the job, they should of been able to do the work
requirements!
Lets just say I have a tolerance on the 2 ounce of .0004" minus and .001 plus
(just to have a plus) which is about 15% of .0028". Aren't I really only
asking them to produce boards that will have .0002" more that what they
should be giving me already. Should that cost me??
Thanks in advance!
|
