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July 2002

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Subject:
From:
tony steinke <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 11 Jul 2002 08:23:11 -0700
Content-Type:
text/plain
Parts/Attachments:
text/plain (267 lines)
Gary,
The biggest difference that stands out from the stackup that was functional
and the the stackup that had problems
is the prepreg thickness between the two 2 oz. copper
planes. The stackup that failed had only .004 of preg. which is very
marginal considering that 2 oz. copper
is .0027 thick and you have back to back planes of this
thickness. Do you know how many sheets of preg was used to achieve the
.004?, as in 1 sheet of 2113 or 3 sheets of 106? This resin content can
become critical
when putting only .004 preg between two 2 ounce
planes.

Tony Steinke
AIT-Atlanta Inc.
----- Original Message -----
From: "gacrowell" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, July 10, 2002 11:29 AM
Subject: [TN] Stackup specification, tolerance, power plane resonance?


> I'm looking for some guidance regarding stackup specifications. Its a long
> story, I hope you'll bear with me.
>
> We had a fairly complex board fabbed and working.  Over a couple of
earlier
> versions, we had built about 60 of the boards.  The stackup for these
> versions of the board is listed below.  You can see that it has 4 internal
> impedance controlled routing layers, plus some heavy copper internal power
> planes.  The board consumes some pretty high current, and distributes
quite
> a few different voltages.
>
> Note the 3 mil spacing between planes, which is placed between power and
> ground, just because it was thought to be a good idea for the (patented,
> unmentionable) interplane "C" word.
>
> On a similar, but different board, where the stackup had also listed 3
mils
> between planes, the stackup was implemented by the fabricator with a
single
> prepreg sheet, and this apparently resulted in resin starvation in some
> areas and we got some very exciting interplane shorts.  (Other fab houses
> had implemented the same stackup with two thinner prepregs.)  We haven't
> desired to specify lamination material in the past, preferring to leave
the
> implementation to the board house.
>
> So now it comes time to do a new 'production' version (our production
> quantities are very low, never numbering more than a hundred boards or
so).
> A couple of last wires are incorporated in the layout, and just to be
safe,
> the stackup is changed to eliminate the 3 mil interplane spacings,
changing
> them to 4 mil.  This stackup is also listed below.  Other interplane
> spacings have been reduced to maintain the thickness.
>
> This new board was ordered from a contract assembler, and that assembler
> chose the board house and placed the order, using our specifications.  We
> have no issue with the board house chosen, its a perfectly reputable house
> that I knew of, and I worked with them resolving minor issues in the board
> fab on this and other boards.
>
> Now comes the problem.  On the new board, one aspect, completely unrelated
> to the minor wire changes, does not work.  Same components (even swapped
> components from a working, older version), same component placements and
> routing.  Impedance on signal layers seems to be within spec (more
checking
> to be done).  Some of the boards can be gotten to work (sometimes
> intermittently), by adding caps around the particular chip that is giving
> the problem, some gotten to work by adding bulk capacitance on the board,
> and some aren't working yet regardless of changes.
>
> The board is liberally populated with 0603 .01uF, and 0508 .1uF caps, and
> 33uF tants.
>
> The boards that work with the addition of bulk caps seems to suggest that
we
> may be shy in that area.  But change in operation with other caps seems to
> suggest that we have a power plane resonance problem.  More testing is
being
> done.
>
> The engineer on this board is understandably frustrated, and complains
that
> the stackup should not have been changed from the earlier version, and
that
> the stackup should be specified as to the exact interplane thickness.  The
> new boards are .005" thicker than the old, but within the .093 +/-.008
> tolerance.
>
> I have tried to explain that (to my knowledge) it is common to not specify
> exact laminate material, and that recipes for stackups to achieve certain
> thicknesses can change from fabricator to fabricator, Er changes from
> supplier to supplier, and a dozen other factors can change with the wind
and
> phase of the moon.  The final target as I understand it is that impedance,
> overall thickness, and copper weights are requirements that must be met
> within tolerance, but that the fabricator will adjust as required within
his
> process to hit those specs.
>
> So the questions:
>
> Is it time to specify laminate material in detail?  Where do I start?  Do
I
> get the laminate selection from each particular fabricator, or require
that
> they buy what I specify?  Will this affect cost?
>
> Should I get the exact stackup recipe from the fabricator and require this
> be maintained for new versions or other board houses?
>
> Should I be placing a tolerance on interplane thicknesses?  Would this
> affect cost?
>
> What should have been done to avoid this particular situation?
>
>
> Your comments and suggestions would be appreciated.
>
> Gary Crowell
> Micron Technology
>
>
> (fixed pitch font is probably required to get these to line up correctly)
> EARLY VERSION STACKUP:
>     Filename            Layer    Cu   plating  prepreg  est trace  trace
>                                  oz     oz       in.    width in.  D-Code
>     =============     ========= ====  =======  =======  =========  ======
>
>     20060r1.TSP      top paste
>
>     20060r1.TSK      top silk
>     20060r1.TSM      top mask
>
>     20060r1.BRD      board outline
>
>     20060r1.L01      top         .5     1.0                .0065
>                         prepreg                .004
>     20060r1.L02      plane       .5
>                       core                     .004
>     20060r1.L03      signal      .5                        .005
>                         prepreg                .005
>     20060r1.L04      signal      .5
>                       core                     .004
>     20060r1.L05      plane      1.0                        .005
>                         prepreg                .003
>     20060r1.L06      plane      1.0
>                       core                     .006
>     20060r1.L07      plane      1.0
>                         prepreg                .003
>     20060r1.L08      plane      1.0
>                       core                     .003
>     20060r1.L09      plane      2.0
>                         prepreg                .006
>     20060r1.L10      plane      2.0
>                       core                     .003
>     20060r1.L11      plane      1.0
>                         prepreg                .003
>     20060r1.L12      plane      1.0
>                       core                     .006
>     20060r1.L13      plane      1.0
>                         prepreg                .003
>     20060r1.L14      plane      1.0
>                         core                   .004
>     20060r1.L15      signal      .5                       .005
>                         prepreg                .005
>     20060r1.L16      signal      .5                       .005
>                       core                     .004
>     20060r1.L17      plane       .5
>                         prepreg                .004
>     20060r1.L18      bottom      .5     1.0               .0065
>
>     20060r1.BSM      bottom mask
>     20060r1.BSK      bottom silk
>
>     20060r1.BSP      bottom paste
>
>     finished thickness: 0.093" +.008"/-0.008"
>
>
>
> LATEST VERSION STACKUP:
>     Filename          Layer      Cu   plating  prepreg  est trace  trace
>                                  oz     oz       in.    width in.  D-Code
>     =============     ========= ====  =======  =======  =========  ======
>
>     20060R3.FAB      fab
>
>     20060R3.TSP      top paste
>
>     20060R3.TSK      top silk
>     20060R3.TSM      top mask
>
>     20060R3.L01      top         .5     1.0                .0065
>                         prepreg                .004
>     20060R3.L02      plane       .5
>                       core                     .004
>     20060R3.L03      signal      .5                        .005
>                         prepreg                .005
>     20060R3.L04      signal      .5
>                       core                     .004
>     20060R3.L05      plane      1.0                        .005
>                         prepreg                .004
>     20060R3.L06      plane      1.0
>                       core                     .004
>     20060R3.L07      plane      1.0
>                         prepreg                .004
>     20060R3.L08      plane      1.0
>                       core                     .004
>     20060R3.L09      plane      2.0
>                         prepreg                .004
>     20060R3.L10      plane      2.0
>                       core                     .004
>     20060R3.L11      plane      1.0
>                         prepreg                .004
>     20060R3.L12      plane      1.0
>                       core                     .004
>     20060R3.L13      plane      1.0
>                         prepreg                .004
>     20060R3.L14      plane      1.0
>                         core                   .004
>     20060R3.L15      signal      .5                       .005
>                         prepreg                .005
>     20060R3.L16      signal      .5                       .005
>                       core                     .004
>     20060R3.L17      plane       .5
>                         prepreg                .004
>     20060R3.L18      bottom      .5     1.0               .0065
>
>     20060R3.BSM      bottom mask
>     20060R3.BSK      bottom silk
>
>     20060R3.BSP      bottom paste
>
>     finished thickness: 0.093" +.008"/-0.008"
>
> --------------------------------------------------------------------------
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