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From:
Richard Mark Mazzoli <[log in to unmask]>
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Date:
Fri, 1 Oct 1999 18:27:33 EDT
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DIRECT PLATE PITFALLS

IS IT REALLY AS ROBUST AS ELECTROLESS COPPER?

RICHARD MAZZOLI


BACKGROUND STORY

Recently there was a shop that was interested in replacing their electroless
copper with one of the new alternative methods making all the headlines.
They interviewed prospective vendors, listening to the good and bad points of
each system on the market; some had long processing lines, some were
relatively unproven, and some were just poorly supported or represented in
their part of the country.

Decisions were made to test one supplier's product based on a number of
factors such as 1) it was a "tried and proven" process boasting "millions of
square feet" of high quality boards, 2) a name-brand supplier to the industry
made the process, 3) product support was considered to be the best available
and 4) the equipment and pricing was in line.  Testing commenced when the
equipment was installed.

Test data, as is often the case, was conflicting.  Basically, in order for
this process to be approved it had to stand up to 500 thermal cycles in a
liquid-to-liquid chamber.  Tests showed that if 1 mil of copper was plated on
test panels there would be a 1% to 10% failure rate.  A separation would
develop - the boards would develop circumferintial voids at the oxide
interface of the inner layers.  So, the process supplier and the board shop
mutually decided to test the panels with 2 mils of copper.  Subsequent
results were good (no separation occurred) and the board shop's largest
customer approved the process based on the latest tests.

The shop soon switched over to the new direct plate process and threw out
their old electroless copper line.  Cycle times were reduced, some unhealthy
chemistry was eliminated and everyone was happy.  Until field failures began
turning up.

The shop's largest customer began to find a significant number of failures
due to open circuits.  Analysis showed them to be wedge voids.  Things went
from bad to horrible as the shop scurried to find a solution.  They began a
series of tests designed to find the cause of the voiding and, more
importantly, how to stop it.  They began extensive quality checkpoints using
many times the normal amount of destructive testing out of copper plating in
an effort to filter out these escapes.  Tests were conducted to find out how
they could escape from electrical test.

Of course, to the customer, all of these efforts were exercises in futility.
They understood the PCB shop's problem but had problems of their own.  The
customer was quickly moving to the decision of canceling all orders and
putting the shop on a "no order" status.  They were out of patience and had
their own business to protect.  The PCB shop took 60 days to discover the
cause of the defects and construct a more robust process.  Too long in this
day and age - the customer replaced them.  All orders ceased.  The shop
replaced the direct plate process for electroless.  Too little, too late.
Several months later the shop filed for protection under bankruptcy laws.
One year later they closed their doors.

If this story sounds like fiction to you, then you'd be wrong.  It happened.
Of course, this is only the abridged version.  The lessons learned by this
shop have left an indelible mark on its personnel.  The tests that resulted
from the 60 days of failure analysis and trials have been reconstructed here
so that other PCB shops might not make the same mistakes.

THE FAILURES

Figure 1 shows an example of the failures this study evaluated.  This PCB has
been thermally cycled in a liquid-to-liquid chamber for 500 cycles.  Figure 2
shows the same sort of defect observed at only 100 cycles.  Failures were
found at 50 cycles as well.  It's important to note that these same parts
passed original electrical test then failed after cycling.  There were not
many failures per board.  In fact, out of more than 2,000 holes only 2 to 4
holes would fail.  Finding this level of defect is akin to stumbling across
the proverbial needle in a haystack.

There were many questions. What is the exact cause?  How to stop them?   How
to detect them?  Oddly enough it was only on certain board types although all
board types were being processed through direct metalization.  Why were these
parts different?

A study was conducted to determine what was different about these parts.
Taken into account was:

§ Hole size
§ Aspect ratio
§ Drill type
§ Drill supplier
§ Material and pre-preg style
§ Lamination techniques
§ Drill parameters
§ Material supplier
§ Layer count
§ Process differences between these and other parts
§ Sequential lamination techniques
§ Unique board features

Analysis showed that there was no difference in these boards than any other
product being built except that the failed parts had non-functional pads. But
other than that, there were no common features that could have accounted for
the failures being only on these particular PCB's.

THE NATURE OF THE BEAST

This direct plate process uses a micro-etch as its final chemical bath,
followed by a rinse then dry.  Panels are then ready for dry film coating.
This micro-etch bath is to remove the metalized coating from the copper
surfaces - outer layers surface and interconnections.  During failure
analysis several indications pointed to this portion of the process as being
a major cause of concern.  A good many boards were found to have slight to
moderate negative etchback.  A "wedge" or crevice was formed at the top and
bottom side (the oxide and base side) of the interconnects.  It was at these
places that failures would occur.

Controlling the micro-etch bath in a direct plate process is easier said than
done.  It's not as simple as most other micro-etch baths since it relies on
impingement to work.  The spray actually has to get underneath the metalized
coating in order to "lift" it off by removing the copper underneath it.  In
that manner it remains on the non-copper areas - the areas it is intended to
metalize.  Understanding this, it's not hard to comprehend that there is
going to be some amount of copper missing from corners (knees) of
interconnects and surface copper where it meets the resin/glass.  This "gap"
is normally miniscule.  But nevertheless, it exists even under the best of
circumstances.  It is the nature of the beast.  Fundamentally it becomes a
non-conductive portion of the hole that must be plated.  The only way this
can happen is by current "jumping" the gap.  Copper must literally bridge
across this fissure and make connection to the interconnecting foil.

Figure 3 shows a drawing of this copper removal.  Further, it shows how this
condition can become amplified when coupled with any amount of nail heading
from drill.  It's not hard to imagine the metalized coating being removed by
the undercutting action of the sprayed-on micro-etch.  It stands to reason
then that when the copper interconnects have been, in effect, "widened" by
nail heading that the resulting removal of the copper will now leave an even
longer gap.  The normally miniscule fissure now becomes even harder to plate.
 The fissure now becomes a full-blown gap - a break in continuity.

Direct metalization works by way of a "moving front".  Copper travels from
each surface, through the holes until it "meets"  (Figure 4).  This is
commonly referred to as initiation.  When the moving front meets, initiation
occurs and you have successfully encapsulated the hole wall and begin to
build thickness evenly.  Until that point of initiation, copper is building
in thickness on areas that are conductive but obviously not on other areas -
like the fissure.  A wedge-like formation begins to develop which can be
miniscule to very wide, depending on how large the non-conductive fissure is.


Conversely, electroless copper has no fissure issues since it completely
encapsulates the interconnect, resin, glass and surfaces.  It begins to plate
thickness immediately when current is applied.  No moving-front is involved -
no initiation.  It's already plating on all surfaces.


NON-FUNCTIONAL PADS

Rather than debate the pros and cons of non-functional pads (NFP's) the
testing in this study included both types of designs - NFP's and Functional
Pads.  Like the moniker implies, NFP's are not functional in the sense that
they are not connected to any other pad.  They are blind pads left on by
designers for a variety of reasons but that really serve no function insofar
as electrical conductivity is concerned.

Analysis on defective parts showed that it was at the NFP that the wedge was
at its worst.  All defects were found to be at the interconnect of a NFP.
Further analysis verified that even after more than one hundred micro
sections the wedge-like defect (and subsequent separation) occurred only at
the interconnect of a NFP.  A test vehicle was devised to prove or disprove
this finding.

A test coupon was developed that included functional and non-functional pads
side by side.  The coupon was built as an 8 layer board, one hole completely
functional (linked by circuits to another pad/hole) and the next hole
completely non-functional (all interconnects "dead").  To test the possible
role of aspect ratio in the creation of the defects, hole sizes were
gradually increased across the coupon.  Tested were 5:1, 3:1, 2.5:1 and 1.6:1
aspect ratios.  Figures 5 and 6 show the coupon configuration.

Using this design the test hoped to find plating variances between functional
and non-functional interconnects and perhaps some variance due to aspect
ratio.  Since the direct plate process plated by means of the moving-front it
was decided that the initiation time delay (the time the panel entered the
plating tank and the time it actually had thru-hole contact
surface-to-surface) was important.  Going even further, it was felt that
since the fissure/wedge created extra resistance thereby making it harder to
plate, the test should include current density variables.  Therefore, the
test was outlined as described below:

1. Panels metalized with direct metalization.
2. Panels metalized with electroless copper.
3. Panels pulled from copper tank every 5 minutes and micro-sectioned
continuing for 60 minutes.
4. Panels to be plated at 10 ASF.
5. Panels to be plated at 20 ASF.
6. Panels to be plated at 30 ASF.

Panels were 18x24 and had numerous coupons merged into actual customer parts
- the same type parts that were failing in the field.  This allowed the test
vehicle to be a relatively accurate representation of the exact parts that
were failing.

The direct plate supplier was invited in and asked to perform process audits
on the metalization line.  In fact, they assisted in generating a good
portion of the data collected in the overall testing.  No significant
modifications were made to the process but it was optimized to their
recommendations.  Further, the copper baths were optimized for brightener,
copper and acid content.  Rectifiers were checked for throw and all
connections on the tanks measured for connection.

RESULTS

The results were definite in that they clearly showed the difference in
plating thickness between functional and non-functional pads.  The
explanation for this is that functional pads are "networked" together in
various configurations.  These networks actually assist copper plating by
becoming conductive then "charging", thereby helping the moving front reach
its initiation point.  One thing was clear; wherever there was an existing
wedge there was extra resistance and the moving front of copper was forced to
somehow get across the fissure.  NFP's could have actually become anodic
(since they were isolated on both sides by the fissure), therefore offering
even more resistance to the moving front.

The tests were also definite in that they clearly showed the difference in
plating thickness between direct plate and electroless copper.  Repeated
tests showed that electroless copper yielded 25% more thickness in the same
amount of time.  That percentage climbed to as high as 75% more when looking
at holes with only non-functional interconnects.

What was not expected is that the results also clearly showed the formation
of the wedge voids.  Pulling the parts every 5 minutes for micro-sections
showed where the wedge started and how it eventually "bridged".  What was
even more interesting was to see the difference between the 10, 20 and 30 amp
current densities.  As would seem clear, the lower the current density, the
lower the plating.  Similarly, the wedge was even more pronounced at 10 ASF
than it was at 30 ASF.  However, with today's designs plating at 30 ASF is
pretty much out of the question.

Figure 7 shows the difference between direct plate and electroless copper at
20 ASF after 5 minutes in the plating solution.  Figure 8 shows a close up of
one of the interconnects from the electroless hole and direct plate hole.
The wedge is clear on the direct plate photo.  It's important to note here
that 5 minutes in a plating solution should be building copper thickness.
That's almost 10% of a normal 60 minute cycle.  It's clear that this hole is
not plating as expected.

Even at 30 ASF similar test panels showed the wedge developing.  But they
also showed something interesting.  On Figure 8 you'll see the wedge but then
you can also see that the copper has apparently bridged the gap somewhere
around the circumference of the hole, since copper is starting to plate on
the other side.  Or did it somehow "jump" across the gap?

TEST CONCLUSIONS

After repeating these tests under close scrutiny and varying the test
conditions certain conclusions were developed:

1. Non-functional pads increased resistance when using this direct plate
process.
2. Electroless copper plated better (faster and more evenly) than this direct
plate process on all coupons.
3. Even under the best of circumstances there will be some magnitude of
fissure (broadening into a gap or all-out wedge) using this particular direct
plate system.
4. This direct plate system is less robust than electroless copper when used
on designs with high amounts of non-functional pads.
5. At low current it was impossible to reliably plate NFP holes using this
direct plate process.

Does this indicate that direct plating is a poor process.  Not necessarily.
With hundreds of installations worldwide, there's plenty of data and
"millions of square feet produced" to indicate it's a viable alternative to
electroless copper.  Does the testing described in this article indicate that
direct plate has its limits?  Sure, and they prove that this direct plate
process had even more limits than what was already known.  Certainly no one
selling this process knew about its limitations on boards loaded with NFP's.
Neither the supplier nor the engineers who installed it realized it could
present such a problem.  In this shop, direct plate was certainly no drop-in
substitute for electroless copper.

BACKGROUND STORY CONCLUSION

The shop where this activity took place is no longer in business.  The
building is closed, equipment gone and the people are working in other
places.  The mistakes leading to this shop's demise are many.  They include
poor engineering practices when testing new technology, poor decision making
for allowing inaccurate test data to factor into its decision to add the
technology, poor quality control systems that allowed the failures to escape
and poor overall management for allowing all of these things to happen.  The
author is not implying that a technology change is solely responsible for
this shop's fall.  It's simplistic to think that this one problem caused the
shop to ultimately go out of business, when in fact there were many other
reasons along side it.

But the addition of this new technology should have been more thoroughly
investigated.  It had limits that even the supplier didn't know about.  Yet
it was sold as a drop-in replacement.  That's nothing new to PCB shops.
We've all installed products that don't work the way they are sold.  That
don't quite do what they are purported to do.  Experiences such as the one
outlined in this article should remind us to move cautiously when installing
new technologies.  It also reminds us that there are very few, if any, "plug
and play" replacement technologies.  And so it goes in this business, as in
life, caveat emptor (let the buyer beware).

Richard Mazzoli has been in the PCB industry for 18 years working as Process
Engineer, Manufacturing Manager and Engineering Manager.  He can be reached
at [log in to unmask] or [log in to unmask]

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