TECHNICALLY speaking BY GEORGE MILAD, UYEMURA INTERNATIONAL CORPORATION, SOUTHINGTON CONN

New Developments in DC Acid ACID COPPER ORGANIC ADDITIVES Copper for Vertical PlatingTanks Acid copper additives fall into three main categories: he North American printed cir- >20:1 are manufactured and plated Tcuit business continues to adjust today. This is one of the highest • Carriers to the ever-changing landscape of value added products in PCB manu- • Brighteners global market conditions. Initial facturing. • Levelers problems with supply chain manage- To meet these specification ment are being continuously requirements the board shop is Carriers increase the polarization addressed and sorted out. Most forced to seek new and advanced resistance and are current suppres- OEMs are finding it advantageous to processes in every department in the sors. The suppression is a result of manufacture their more established manufacturing process. Acid copper the carrier being adsorbed to the sur- big runners and their customary and plating comes under heavy scrutiny, face of the cathode, which, in turn, off-the-shelf (COTS) products, in the as it is the process that forms the gives rise to an increase in the effec- Far East, particularly in China. traces and the “thru hole” connectiv- tive thickness of the diffusion layer. There are certain categories of prod- ity that convey the signal from end to Suppression causes the deposit grain ucts that remain in North America. end of the final device. structure to be more uniform and These are primarily the prototype The uniformity of thickness distri- adherent. The carrier modified diffu- boards, the large backplane (>40 lay- bution on the surface and in the sion layer also improves plating dis- ers), and the boards for military use. holes is becoming ever more chal- tribution without burning the Boards with new proprietary designs lenging as board designs continue deposit. that require the respect of intellectual down the path of smaller holes and The brightener is a grain refiner— property are kept in the U.S. OEMs in finer lines as well as the need for its adsorption produces a film that the medical field continue to use U.S. impedance control. Surface unifor- will suppress crystallographic differ- manufacturing on their products that mity is sometimes plagued with the ences. Brighteners may be adsorbed are used in life-sustaining devices and occurrence of nodules, which come preferentially on particular active require the highest level of reliability from a variety of sources. Gold wire sites such as lattice kinks, growth over time. bonding applications have no toler- steps, or tops of cones, or surface The prototype boards require ance to any level of nodulation. projections; growths at these loca- elaborate front-end engineering to In addition, there is a greater tions are then blocked. Brighteners bring the part to its final configura- demand on the physical properties; give rise to fine-grained structures tion in a reasonable time frame. tensile strength and elongation that are equiaxed with no specific Prototype boards are never made in (T&E) of the plated copper, to with- direction. A fine grained structure is numbers, but they may go through stand the rigors of high temperature a bright structure. The brightener is a series of iterations before finaliza- assembly of lead-free (LF) solder as the component that has the maxi- tion. These boards cover a wide required for RoHS compliance. mum effect on the final copper phys- range of complexity with some New developments are helping ical properties, namely tensile being highly sophisticated with meet this challenge. A big part of the strength and percent elongation. high layer count, small holes, and developments are focused on: Levelers or leveling agents are buried and blind laser drilled vias. inhibitors present at low concentra- Less than 3 mil lines and spaces and • New chemical additive packages tions in the electrolyte as compared “Via Fill” were first encountered in for improved thickness to the depositing metal. In case of a the prototype shops. distribution. micro profile the diffusion layer does The backplane type boards may • Mass transfer improvements to not follow the profile contour, but is reach a thickness of 400 mils, and complement the additives maximum at the valleys and mini- although the holes are usually >20 • Nodules elimination by use of mum at the peaks. Consequently, in mils, it is the highest aspect ratio insoluble anode absence of a leveling agent, deposit- demand. Boards with aspect ration • “Via Filling”, a specific application ing ions diffuse more rapidly to the

20 I metalfinishing I January/February 2013 www.metalfinishing.com TECHNICALLY speaking

peaks than to the val- leys, and deposits grow more rapidly on the peaks, resulting in an exaggerated Figure 3. Via Fill DC plating in under 120 profile. With good minutes. solution agitation, the leveler will accu- minutes with 51% throw. Figure 2 mulate more rapidly shows a 15:1 aspect ratio hole plated and readily at the at 10 ASF (DC) for 120 minutes with peaks, and it will 80% throw. inhibit growth or deposition. The Mass Transfer. Mass transfer absence of leveler in becomes a key parameter that must the valleys will allow be understood and managed for high faster deposit growth aspect ratio plating. An example is and allow the valleys plating a 400 mil thick board with 20 to catch up to the Figure 2. DC Plating 6mil in 93 mil thick board; mil holes diameter a 20:1 aspect peak, thus creating 10 ASF for 120 minutes, 80% throw. ratio, a 93mil thick board with 6 mil leveling. holes, or a 125 mil thick board with 8 plating are more the result of plating mils holes, both 15:1 aspect ratios. Pulse Plating. For at low ASF for extended periods of Mass transfer is also critical for plat- the last few years the time (up to four hours) rather than ing a blind via with an aspect ratio copper plating the effect of pulse rectification. greater >1.0. industry was focused Mass transfer is influenced first on pulse plating High Throw DC Plating. A new gen- and foremost by diffusion, also and, in particular, eration of “High Throwing Power” affected by solution agitation, and periodic pulse acid copper systems have come to the part or rack agitation. Of course, a reverse as the solu- market to fill the void. These baths reduction in plating rate will always tion for all. As time are designed for today’s plating cur- improve distribution by maximiz- progressed and the rents, which are lower than the tradi- ing the role of diffusion for mass level of difficulty tional 25–30 ASF, which was com- transfer. continued to climb, mon in the days of double-sided and Diffusion refers to the movement Figure 1. DC Plating at 8 ASF the plating current simpler products. of ions through the solution in for 220 minutes; density for pulse High-throw baths are designed to response to a concentration gradi- 6 mil hole in 240 began to drop and give the desired physical properties ent. It is a consequence of random mil thick board, 51% throw. the primary advan- at current densities as low as 5 ASF molecular motion that operates to tage of plating at and as high as 20 ASF. They produce produce more uniform distribution higher current density began to dis- bright ductile deposits. throughout the solution. As soon as appear. Add to that the complexity These bath types are characterized plating begins to deplete the copper of operating a pulse rectifier with by a specific combination of organic ions in the immediate vicinity of the added definition of ASF, forward to additive package that includes a cathode (in the diffusion layer) diffu- reverse ratio, duty cycle and wave- unique leveling agent. The leveler sion drives more ions in to equalize form. In many instances, pulse also plays a key role in improving throw- the concentration. If the plating rate required an elaborate and frequent ing power particularly if it is coupled is higher than the rate of diffusion, scheme of organic regeneration to with a well designed solution move- alternate sources of “mixing” must maintain the copper thickness dis- ment system with or without air. be used to avoid burning the deposit. tribution benefits. Some of these baths can give a Most pulse plating of printed cir- throwing power >80% for a 12:1 Eductors. Eductors are used today in cuit boards today is done at very low aspect ratio drilled hole plating 1.0 many of acid copper plating tanks. current densities and for extended mil in the hole, in a plating time of They create turbulent solution flow plating times, many pulse cycles 90 minutes, in a vertical dip tank without the use of air sparging. The include a series of ever-changing mode. design and layout of the eductor pulse plating parameters. Most of Figure 1 shows a 24:1 aspect ratio sparging system is important to the advantages attributed to pulse hole plated at 8 ASF (DC) for 220 maximize the solution shearing www.metalfinishing.com January/February 2013 I metalfinishing I 21 TECHNICALLY altered throughout the life of the not require the constant regenera- speaking insoluble anode. tion of pulsed electrolytes. The use of The insoluble anode requires a DC plating allows the filling of most action at the surface for the board to continuous supply of copper ions blind vias to be completed in under be plated. If properly designed educ- brought into the system from an 120 minutes depending on the tors can preclude the need for part external source. Some of the meth- degree of difficulty or aspect ratio. agitation. The increased solution ods used to generate copper are the Via filling is based on high deposit- flow at the surface as compared to dissolution of copper oxide into the ing ion concentration coupled with a the middle of the hole could be effec- electrolyte, the electrolytic dissolu- unique additive package that is com- tive in improving the throwing tion in an external rectified cell and posed of carrier and brightener, with power, provided the chemical addi- the use of ozone to oxidize copper a low concentration of a leveling tives used are designed to respond metal in an adjunct device. agent. Most of these systems require preferentially to solution movement. The elimination of anode mainte- optimized highly uniform solution Eductors eliminate the need for com- nance, the consistency of the anode flow. The flow allows the leveler to pressed air or air blowers, and also area and the elimination of the solu- accumulate on the surface, thus provides a safer environment where ble anode as a source of nodulation inhibiting plating while the bottom acid is not constantly been blown make the use of the insoluble anode of the via continues to plate. The into the air, or the exhaust system. a viable solution for the plating chal- plating dynamics in the bottom of lenges today. the hole are very different than those Nodule Elimination Insoluble Via Fill Plating. To meet the on the board surface. Eventually, as Anode. The use of insoluble anodes demands of high density intercon- the hole fills, the difference in plat- is well established in the acid copper nect, “via filling” is quickly becoming ing dynamics even out. conveyorized equipment. It offers a a clear choice for connecting the dif- Figure 3 show DC plating of a via series of advantages over the conven- ferent layers in build-up technology completed in under two hours. tional copper slugs/balls in titanium (stacked vias and vias in pad) manu- Acid copper plating has come a baskets. The most prominent advan- facturing. They result in an overall long way since the early days of dou- tage is the absence of copper anodes, improvement in long-term reliability ble and single-sided printed wiring which need to be filmed and bagged of the PCB and the package. boards. Plating challenges will con- to contain naturally occurring Suppliers have developed new elec- tinue to increase as new product sludge from getting on the work. trolytes for plugging vias shut. Both demands (lighter smaller and more Anodes, even in their purest form, pulse plating and DC plating propri- reliable) come to market. As one are prolific sources of suspended etary “Via Filling” chemistries are leading edge milestone is conquered, matter that leads to nodules. available in the marketplace. DC a new one is set. Stay tuned.! Insoluble anodes eliminate the need plating offers a series of advantages, for dummy plating to film the as it does not require pulse rectifica- George Milad is the National Accounts anode. The need for standard anode tion and avoids the complexity of Manager for Technology at Uyemura maintenance is also eliminated. The managing a pulse wave. In addition, USA. He may be reached at gmilad@uye- anode shape and dimensions are not DC plating systems are stable and do mura.com.

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