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. 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