On Fri, 30 Jan 1998 17:39:00 CST Thad McMillan <[log in to unmask]>
writes:
> Currently we are designing a board thicker than normal with small
holes.
>
> The board is a 10 layer board, 082" thick, and uses 13.5 mil
drilled/plated holes.
>
> The board is large ~ 13 x 16" in size. It is HASL. During
component
> assembly it will see 2 IR processes and a wave. We are specifying
> the board as IPC Class 2. It will be used in high end computer
> product. Reliability is very important.
>
> In light of the above I have several questions for this
distinguished group:
>
> 1. Considering the 6 to 1 aspect ratio, using conventional FR-4
will
> the design above have reliable plated through holes?
>
This will depend entirely on the chosen fabricator's abilities to insure
good, clean hole walls (no debris), and his ability to move all his
plating
fluids through the holes, while plating (agitation, vibration, etc.),
and, as
was noted in an earlier post, the quality of his copper deposition
(ductility,
contamination). Excellent fabricators will have no problems. Lesser
ones...
>
> 2. We are considering specifying that the FR-4 be of Tg greater
>than 170 Deg C. The hope is that this will improve hole relaibility.
>The problem is that this will add significant cost to the board. Is
>it necessary to do this?
>
This, IMO, is mandatory, and as was also noted earlier, only costs about
5-6%
more than 140 Tg material (In the U.S., outside the U.S. I don't
know...).
Our rule on this is all boards over .080" thk. are Hi Tg FR4 (We spec it
as 165-180Tg)
> 3. The design is dense so that we cannot grow the hole
significantly.
> We are considering permitting hole breakout of 1 mil, so that the
> holes can be drilled larger. Is this a reliability risk?
Naturally you will tear drop, snowman or pad expand as necessary to
maintain
2 mil at trace to pad intersection. There is conflicting opinion about
this, but the ITRI
study that trumpeted " Big money lost due to excess annular ring every
year" is
rumored to be under re-examination, as perhaps having been flawed.
Breakout is certainly not as reliable as 2 mil annular ring, but only
with knowledge
of all the specifics (and perhaps it is time for you to spend the money
for some
statistically-significant sample size lots of product into ESS-type
testing.) can
your company make a truly informed decision.
As I noted earlier, reliability will certainly vary from shop to shop, as
a function
of hole quality and registration precision. Select very good shops, test
their
product thoroughly, and insist that they not "change the recipe" in any
remotely significant way, without prior notice to you, so that you may
decide
whether more product testing is warranted, before they make the change.
>
> 4. Are there other alternatives (not so expensive) to improve hole
reliability?
Make very, very clear that you do not want IPC standard hole wall copper
thickness,
(0.001" minimum average, .0008" minimum isolated) and will reject if you
see it.
You want .001" MINIMUM, and expect to see .0012", or better, average.
If you don't think that this step is worth doing, you don't know or care
where your
reliability comes from.
Also, insist on HTE (high tenstile elongation) copper foil for both
inner and outer layers.
This should be cheap, and is relatively standard (domestically in U.S.)
>
> I would appreciate any feedback.
>
> Thanks,
> [log in to unmask]
>
Bill Spalton
[log in to unmask]
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