Richard, please send the photos you describe below.  We have also considered
replacing electroless with direct metallization, but I have already been
down the road where fancy expensive equipment had to be yanked in favor of
simple dip lines, and I have been fighting the change.  I greatly appreciate
your sharing this story with Technet.
Connie Huffman
Process Chemist
Avionics Production Division
Warner Robins Air Logistics Center

> -----Original Message-----
> From: Richard Mark Mazzoli [SMTP:[log in to unmask]]
> Sent: Friday, October 01, 1999 06:28
> To:   [log in to unmask]
> Subject:      [TN] Direct Plate
>
> 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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