A NEW SURFACE FINISH FOR THE ELECTRONICS INDUSTRY Ernest Long PhD and Lenora Toscano MacDermid Waterbury, CT, USA

Transcription

1 A NEW SURFACE FINISH FOR THE ELECTRONICS INDUSTRY Ernest Long PhD and Lenora Toscano MacDermid Waterbury, CT, USA ABSTRACT The performance expectations for printed circuit board surface finishes are greater than just solderability preservation. Historically, final finishes were designed solely to protect copper from oxidation prior to the soldering of components. Now the expectations are much greater; superior solderability, contact performance, wire bondability, corrosion resistance, and all this must be achieved at low cost For the past few years the electronics industry has been investigating the effects of harsh environments on printed circuit assemblies. The general consensus is that the environments electronics experience today are particularly aggressive, being frequently exposed to high levels of contaminants such as sulfur, sulfide, SO x and NO x. One common surface finish now frequently used in the electronics industry is immersion silver. The popularity of this technology has increased over the last decade and now likely accounts for about 10-15% of final finish market. The popularity of immersion silver is driven by factors such as, excellent solderability, very low contact resistance, easy of assembly and testing, and it is a relatively low cost option. As mentioned above, electronic components are now frequently exposed to harsh, corrosive environmental and use conditions. Long term exposure to such environments can result in the onset of varying degrees of corrosion phenomena, such as creeping corrosion, peeling and coating tarnish. These phenomena can be mitigated by the adoption of best application practices and assembly processing, in conjunction with the use of specially formulated anti-corrosion coatings. A widely used alternative to the above that is frequently used for demanding, high reliability applications, is electroless nickel-immersion gold (ENIG). This technology also is not without its problems. A common example, which can arise through the use of a poorly formulated process, is the well known phenomenon of black line nickel. This issue has been of significant concern to the electronics industry for some time and has damaged the reputation of ENIG as a final finish, even limiting in some cases its wider adoption. With all of the above in mind and taking into account the very high price gold can achieve on the open market (which is currently in excess of $1100 per troy ounce) the industry is looking for an alternative final finish. Ideally, a suitable alternative would retain the positive features of the ENIG process whilst, at the same time, eliminate the black line nickel concern and, of course, be economical. This paper describes such a process, electroless nickel-immersion silver. Careful choice of the electroless nickel, and immersion silver processes results in a surface finish that has superior performance while remaining low cost. INTRODUCTION Immersion silver has served the electronics industry for 15 years as a reliable, easy to use, relatively inexpensive solderability preservative for printed circuit boards (pcb). Though every surface finish has its strengths and weaknesses, after it was introduced immersion silver very quickly set itself apart, achieving better than expected functional performance. Fabrication houses were, and continue to be impressed with its short processing time, easy chemical analysis and ease of testing and inspection. Assemblers enjoy the superior solderability performance offered, even after multiple reflows, and its compatibility with all flux chemistries. OEMs appreciate the excellent contact performance characteristics immersion silver offers, and naturally, its lower cost status. The more the market for electronic devices demands of its products the more we learn the deficiencies of every surface finish option currently available. These demands are ever increasing, be it from a performance standpoint or the ability of these devices to continue to operate under extremely harsh environmental conditions. With the above in mind, making the correct choice of final finish is critical and the decision must be made with a detailed understanding of the nature of end product usage, for example, is the product likely to be relatively disposable with a short operating life or does the end product have some critical function with a long life expectancy, perhaps decades, even when exposed to significant environmental contaminants. Understanding the expectations of the end product is essential. It is not controversial to say that today s ever increasing demands can force all final finishes offered to their performance limits. This fact drives supply houses to carry out research to develop alternative finishes which offer superior performance even when placed in stressed environments. It is fair to say that this combination of requirements is the biggest developmental driver currently in the industry. Environmental issues have impacted the electronics industry in a two fold manner. Firstly, the increasing industrialization of the developing world, particularly Asia [1], as mentioned above, has caused electronics to be exposed to increasing and very significant levels of atmospheric contamination. Secondly, and notably, is the resulting regulatory legislation introduced as a response to dealing with the above, such as REACH, WEEE and ROHS [2], where lead free compliance is critical but also dictated is the reduction (or elimination, where possible) in the use of harsh chemicals in surface finishing formulations, which also ultimately need to be disposed of in a safe and environmentally friendly manner. The regulatory environment within which the electronics industry needs to operate continues to become more restrictive almost by the day, with the many incarnations of the above mentioned legislation being increasingly adopted world wide. Operating in such a tough regulatory climate, will very likely ultimately mean that the use of, for instance, cyanide salts, which are the main components in immersion gold formulations used in the ENIG process will come under heavy pressure to be replaced. The use of cyanide for this application is currently not prohibited as there is no alternative available at this time. There is still tolerance, in some applications (typically, safety critical or military) even for the use of materials such as lead, however this tolerance is ever becoming less and the use, of, for example, the historically successful thick tin/lead HASL surface finishing is approaching the unthinkable. The important issue of legislative compliance, continues to need to be kept foremost in mind, when developing alternative surface finishes, however as well as being compatible with lead free solders and the associated assembly conditions, it must also be compatible with a large range of flux materials, even those less active which promote better board cleanliness, which can frequently translate to less solder flow for most finishes. Almost all assemblies require multiple heat excursions at high reflow temperatures. Boards have higher packing density so solder paste is applied in smaller volume to eliminate bridging. This will also result in exposed metal areas after assembly. In addition, and importantly, it must be understood that surface finishes now are no longer solely used as solderability preservatives, other performance requirements may include good wire bonding characteristics, good contact performance and corrosion resistance. Board construction, material thickness, along with pad size/density and above all the unique components that require planar surface finishing have propelled the growth of alternate finishes. As stated above, continued market demands force the industry to develop new and improved surface finishing. Electronics are used in a vary array of applications and are equally found in a vary array of locations worldwide, experiencing almost every conceivable environmental exposure. With this and all of the above in mind it is not difficult to understand the challenge to developing a new final finish process. In this paper a potential new process is discussed, and traditional immersion silver (Imm Ag) is compared to a combination process; that of electroless nickel plated with immersion silver (NiAg). This combination

2 process offers the benefits of both Imm. Ag and ENIG, whilst eliminating most of the negatives, such as the use of cyanide salts, as described above. For all experimentation, the test vehicles used were conventional printed circuit boards with electrolytic copper circuitry. For additional characterization of the two finishes, the silver was plated to four and eight microinches on each metal substrate. High temperature organic solderability preservatives (OSP) and electroless nickel/immersion gold (ENIG) were also used as comparative surface finishes. The process flow of electroless nickel/immersion silver (NiAg) is similar to that of ENIG (figure 1). The electroless nickel used can contain but is not limited to phosphorous containing formulations. The immersion silver bath is a modification of the current commercially well established MacDermid immersion silver process. For the purpose of this paper the electroless nickel process consistently used was a mid-phos EN plated to 180µ-in deposit thickness and an immersion silver plated to four and eight µ-in, respectively. Cleaner Microetch Tarnish and Creep Resistance The details of the MacDermid tarnish test has been published several times over the past decade [9], and so is very well established. The test was developed to evaluate immersion silver and related coatings, when exposed to a sulfur bearing environment. This test is essentially a research tool that accelerates tarnish. In the test, a sealed chamber is used. A controlled quantity of water (to induce high humidity) and hydrogen sulfide gas is introduced which creates visible tarnish on the coating under test. Specifically, the tarnish chamber used is 10 cubic feet in volume and is heated to an internal air temperature of 45 o C. The chamber contains 500mL of a sodium hydrosulfide solution, which when acidified releases hydrogen sulfide gas. The humidity level in the chamber is typically around 80%. The test is conducted on parts after plating, after one lead free reflow and after two lead free reflows. This helps to demonstrate the tarnish resistance of the silver surface between fabrication and assembly as well as resistance of the surface post-assembly when the metal will remain unprotected in its end use. Figure 3 displays the reflow profile for multiple pads sizes across the test vehicle. The peak temperature for the profile is 245 C. 5% H2SO4 Activation 5% H2SO4 Electroless Ni Silver Pre Dip Immersion Silver Figure 1: Process Flow (rinses between steps have been omitted) HARSH ENVIRONMENTS With the increasing concern for electronics reliability in end use applications, the IPC has formed a committee specifically to consider the levels of contamination experienced in industrial and office atmospheres [3, 4]. Investigation has revealed that office environments once thought to be clean have displayed corrosion rates upwards of ISA Class G3 (figure 2) [5]. In some instances, copper foil standards have measured in excess of 2000Å of corrosion. Also, the ever increasing growth of industrialization is creating decreased air quality around the world. Some care is being taken to reduce the contamination entering office environments through better air filtering and air conditioning systems but there will always be a concern for electronics that are used in outdoor applications. The IPC committee is currently focusing on the use of mixed flowing gas (MFG) testing to replicate harsh environments and recreate associated corrosion defects, such as creep corrosion [6]. Current research centers on Battelle Class 4 MFG testing which has been altered to render it more aggressive by further increasing the H 2 S concentration present, introducing SO 2 into the test chamber and, also increasing the temperature [7,8] during the test. For the IPC test, the corrosion rate on copper will be targeted at 500 to 600nm/day of copper corrosion. For a 10 day test, this equates to 6 microns of copper corrosion and 12 microns for 20 days of exposure. The initial test phase of the committee is expected to be completed at the end of Class Temp ( C) RH (%) H2S Cl2 NO2 SO IPC Test all gas concentrations are measured in ppb Figure 2: Mixed Flowing Gas Conditions Figure 3: Lead Free Reflow Profile Corrosion rates in the tarnish chamber and in all environments are highly affected by humidity and temperature. Small increases in either can affect the corrosion rate. On average, the corrosion rate on copper for this test is 25 nm/hour. Due to fluctuations in temperature and in turn humidity, an uncoated immersion silver deposit on a copper substrate control is always used for comparison against any coating under evaluation. The control panel is also used as a gauge for test duration Below is a comparison of a traditional Imm. Ag deposit versus the proposed new coating. Visual observations of the samples show a dramatic difference in appearance between immersion silver on copper versus immersion silver deposited on electroless nickel after exposure in the tarnish chamber. Both control sets of silver on copper display heavy tarnish ranging from brown to purple in color (figure 4). The degree of corrosion product measured by Sequential Electrochemical Reduction Analysis (SERA) is over 600Å total tarnish on each test coupon [11]. It is evident that at higher silver thicknesses, reduced deposit tarnishing is seen. Observed at four microinches of silver, the deposit tarnished to purple under all test conditions. Eight microinches of silver on copper offered a greater level of resistance as displayed by tarnishing to a brown color only. Sample sets show that with increased reflow exposure, there is also an increase in tarnish resistance. This can be attributed to a light silver oxide layer being produced on reflow temperature exposure which then offers some level of tarnish protection. Due to the lack of an industry wide and universally accepted test the corrosion resistance evaluation methods presented in this paper focus on general tarnish resistance and creep corrosion, generated using a test environment developed in-house at MacDermid. The tarnish test uses H 2 S gas and small amounts of HCl gas to create tarnish on immersion silver parts [9]. To induce creep corrosion, MacDermid uses a tire factory simulated environment which includes sulfur powder as the main source of sulfur for corrosion [10]. Figure 4: (A) 4µ-in of Imm Ag on Cu (B) 8µ-in of Imm Ag on Cu

3 By comparison, the silver on nickel deposit showed only very slight differences from their as coated appearance prior to exposure in the tarnish chamber, the panels remain silver in appearance (figure 5). On close observation, there are some areas on the test panels that do display some degree of darkening, the metal appearing slightly more grey than silver like. From a cosmetic standpoint, the silver over nickel deposit appears to have a much greater degree of tarnish (and therefore corrosion) resistance. SERA analysis confirms this with a total tarnish layer of less than 200Å for each silver thickness level. elemental sulfur to the board s surface. The full test duration is 72 hours which produces 90nm of corrosion on bare copper. For all creep corrosion testing, panels are processed through mixed assembly. Samples are printed with SAC 305 solder and reflow on the A side followed by lead free wave soldering of the B side also with SAC 305 solder. MacDermid research has shown the need to mimic PCB assembly conditions [11, 12] to ensure creep corrosion generation during this test. Unsurprisingly, reflow conditioning results in the degradation or alteration of some coatings and that influence needs always to be considered. Also, the choice of both solder, and of solder flux have been shown to influence the overall creep reaction greatly [3, 12]. Figure 5: (A) 4µ-in of Imm Ag on EN (B) 8µ-in of Imm Ag on EN To understand the corrosion resistance on a microscopic level, samples were analyzed using Electron Dispersive X-ray Spectroscopy (EDS) before and after chamber exposure. For simplicity, the results discussed below are for samples that were not conditioned prior to the tarnish chamber exposure. EDS maps of immersion silver on copper show a uniform layer of silver. Copper can be seen in the EDS map, this is attributed to penetration of the beam through the thin silver deposit. Without areas of intense silver or copper it can be deduced that the silver layer is uniform and consistent. EDS maps of the NiAg reveal no copper on the board surface. The nickel barrier layer is too thick to allow beam penetration. Analysis of the surface finishes taken after the chamber exposure show an increase in copper observed through the silver deposit, when the silver is plated directly on copper. The EDS shows an increased concentration of copper in the deposit as a whole (figure 6). This reaction seems similar to the mechanism frequently described for creep corrosion. No copper is present in the NiAg EDS maps as coated or after the tarnish chamber conditioning. In both instances, a rich silver deposit remains (figure 7). The nickel barrier layer is preventing humidity and contamination from reacting with the underlying copper and allowing it to become mobile. Figure 6: (A) 8µ-in Imm Ag before Tarnish (B) 8µ-in Imm Ag after Tarnish Figure 7: (A) 8µ-in Imm Ag on Ni before Tarnish (B) 8µ-in Imm Ag on Ni after Tarnish Creep corrosion testing at MacDermid is also conducted in a sealed container. Parts are enclosed in the test chamber containing elemental sulfur powder and mercaptobenzithiazole in some water. The chamber is heated for the majority of the test period in an oven at 50 C. The chamber is removed every 24 hours to cool to room temperature which then forces condensation to occur onto the test vehicle surface and also transports (A) OSP (B) ENIG (C) 4µ-in Imm Ag (D) 8µ-in Imm Ag Figure 8: (E) 4µ-in Imm Ag on Ni (F) 8µ-in Imm Ag on Ni The soldermask defined pads in figure 8 above show widely varying degrees of creep corrosion depending on the type of surface finish under test. On exposure to these harsh test conditions both the immersion silver on copper samples and the high temperature OSP coating show heavy amounts of creeping corrosion. Immersion silver plated on electroless nickel shows tarnishing of the surface silver, however creep corrosion is not evident. ENIG shows no creep or tarnish. It has been well documented that the resultant creeping corrosion product is in fact copper sulfide [13, 14]. The mechanism by which the copper sulfide corrosion product forms is most likely by migration of copper through pores in the plated deposit or by migration of copper from unplated areas, such as from under the soldermask interface. Copper sulfide forms most readily when sulfur bearing contaminants in the environment, including elemental sulfur are present coupled with an elevated humidity level. The copper-sulfur salt, facilitated by the presence of moisture, can eventually migrate through the immersion silver deposit and across the printed circuit board resulting ultimately in board defects being observed. If a coating can be used to create a barrier that prevents access of contaminants to the underlying copper, then creep corrosion can be prevented from occurring. Special topcoats, applied post immersion silver, have been developed and have been demonstrated in production environments to be effective at mitigating the onset of creep corrosion. These topcoats are typically selfassembly monolayers (SAM s) and by their very thin nature the favorable, functional characteristics of the treated silver deposit is retained. What is demonstrated here is another tool to combat the creep corrosion issue. By putting down an electroless nickel under-layer beneath an immersion silver deposit, taking the lead from the well established ENIG process, creep corrosion can be effectively prevented from occurring even in very extreme environments. Some OEM s have tested and seen the level of corrosion resistance offered by electroless nickel immersion gold technology (see also figure 8 above) however, the high price of gold salts in today s market is prohibitive and prevents many from changing full production to an ENIG based surface finish. The use of immersion silver in combination with electroless nickel, now

4 made possible, offers similar performance levels to that of ENIG but at greatly reduced cost. As mentioned earlier, the current market price for gold is over 1100USD/ troy ounce, approximately, silver is almost 100 fold less at 17USD/troy ounce [15], currently, and its cost has proven historically to be much more stable than that of gold. FUNCTIONAL PERFORMANCE Solderability As one of the main functions of a final finish is to maintain solderability of the copper metal after board fabrication prior to assembly, care must be taken to demonstrate that this key performance attribute is delivered by any new final finish. For this work solderability data was generated and then analyzed for solder spread, through hole fill, and wetting balance characteristics. Through Hole solderability and solder spread were tested for unconditioned samples and samples that had been exposed to multiple lead free reflows. Through hole solderability was conducted on two DIMM patterns with hole diameters of 25 and 30mil. All through-hole solderability observed was 100% for immersion silver on copper, immersion silver on electroless nickel and immersion gold on electroless nickel. There was some degradation of the hole-fill on OSP after multiple reflows but all samples passed according to IPC J-Std 003. The conventional finishes performed as expected for solder spread. The immersion Ag/EN process, at both silver deposit thicknesses tested were very similar in spread to immersion Ag on copper. There was no degradation of spread on the Imm Ag on copper, Imm. Ag/EN, or ENIG after heat treatments. OSP is historically known to have little to no solder spread after heat treatments, and such was the case during this testing as well. ENIG displayed the greatest solder spread resulting in full solder wetting even on pads that were only printed to 20% of the total pad area. To go into greater detail on these tests is beyond the scope of this paper. Instead closer attention has been allotted for the solderability analysis by wetting balance. Wetting Balance Solderability tested by wetting balance was conducted in accordance with IPC J-std 003. To introduce a more challenging aspect to this test, lead free SAC 305 solder globules were used with Kester 959-T No clean solder flux. The choice to veer from the exact specification was generated in part due to a preference to use of a flux more typically employed in routine, everyday assembly. For all surface finishes tested, solderability proved to be very good as coated and also after one and two reflow excursion. This test included high temp OSP, ENIG, Imm Ag at 4 and 8 microinches on copper substrate, as well as coating under test, i.e., Imm Ag/EN at 4 and 8 microinches silver thicknesses. The robustness and the degree to which any surface finish is likely to degrade can be evaluated, being particularly evident after 24 hour exposure to 85 C/85% RH. This type of conditioning resulted in discoloration to the thin immersion silver deposit on copper (figure 9) and resulted in decreased solderability for many of the other surface finishes included in this test (figure 10). Figure 9: (A) 4µ-in Imm Ag on Cu (B) 8µ-in Imm Ag on Cu. Both After 24-hour 85 C/85% RH Exposure The wetting balance graph (figure 10) displays passing IPC criteria for all surface finishes except the organic solderability preservative. The graph is an average of 12 pads soldered per surface finish after 24 hours of 85 C/85% RH exposure. When comparing Imm Ag on copper and Imm Ag/EN all display extremely fast time to buoyance (T b ) with times well below the 1-second IPC specification. Comparison of the wetting forces gives more insight to the differences between these finishes. Imm Ag at 8 microinches on copper and the two test surface finishes, Imm Ag/EN at 4 and 8 microinches all display uniform wetting with maximum forces around 3mN. The immersion silver on copper at 4 microinches of silver shows degradation of solderability after the 24 hour 85 C/85% RH exposure. This series of data shows a large deviation in wetting force. Of the 12 pads tested, the force ranges from 1.25 to 3mN. Increasing the silver thickness on copper or using the electroless nickel barrier layer produce a more consistently solderable surface which is a reflectance of corrosion resistance under heat and humidity exposure. Figure 10: Wetting Balance Data After 24-hour 85 C/85% RH Exposure Galvanic Corrosion One of the biggest process issues associated with immersion silver on copper is sporadic occurrence of solder mask interface attack (SMIA). Though the triggers for SMIA are well known and documented, instances still occur when board designs, board construction and material changes occur. It is not until the panels reach the immersion silver line and have been plated that the defect becomes evident. Having an electroless nickel barrier layer should eliminate this defect from occurring. Figure 11: Zygo Image of SMIA area on Imm Ag trace to pad The mechanism of SMIA is activated when an improper soldermask foot is created. This interface is small enough to trap chemistry without allowing solution exchange to occur, thereby preventing the replenishment of silver ions necessary for the plating process to take place. The area is then left with an acidic solution in place which facilitates the corrosion of copper at that location, whilst the silver plating reaction continues at other locations on the board where silver ions have access. For the purposes of this paper test coupons were created with a negative soldermask foot, silver plating time was extended to further exaggerate the defect and, subsequently the soldermask was removed to better understand the resultant attack. Figure 11 above shows a copper trace on the right side that was under soldermask and the wider pad on the left that has been plated in silver. A zygo representation top down on the left side shows variation in copper height at the interface between the two metals. A closer look at the cross sectional view on the right reveals areas where the copper has been etched away. When the same part is processed through the electroless nickel/immersion silver process, the copper etching is eliminated (figure 12).

5 Measurement and Control Systems: Airborne Contaminants, Instrument Society of America, [6] Task Group 3-11 g [7] ASTM International, Standard Guide for Mixed Flowing Gas (MFG) Tests for Electrical Contacts, ASTM B , June 10th, Figure 12: Zygo Image of SMIA area on Imm Ag trace to pad The right image shows no variation is trace or pad height as the entire analyzed area is displayed in orange. The cross sectional view on the left shows the slight increase in pad height due to the plated nickel. Significantly, there is no reduction of the copper trace. Black Line Nickel Attack When discussing electroless nickel/immersion gold (ENIG), as mentioned earlier, an industry concern remains the possibility for black line nickel occurrence. Chemical suppliers have made adjustments to the cleaning and, in particular, their electroless nickel bath formulations to considerably reduce the propensity of the defect. Soldermask applications have also been adjusted and have been found to help. Pcb fabricators are well educated on the subject and know proper maintenance of the electroless nickel/immersion gold process is also critical for black line nickel mitigation. [8] Chen, Xu., Creep Corrosion of PWB Final Finishes: Its Cause and Prevention, IPC/APEX April [9] Toscano, L., Cullen, D., The Study, Measurement, and Prevention of Tarnish on Immersion Silver Board Finishes, IPC July 03. [10] Toscano, L., Long, E., Swanson, J., Creep Corrosion on PCB Surfaces: Improvements of Predictive Test Methods and Developments Regarding Prevention Techniques, SMTA International Orlando, Oct [11] Bratin, P., Pavlov, M., PC Fab, 1999 May p [12] Toscano, L., Long, E., Creeping Corrosion of PWB Surfaces in Harsh Sulfur containing Environments, SMTA International Orlando, Oct [13] Schuller, R., Creep Corrosion on Lead Free Printed Ciruit Boards in High Sulfur Environments SMTA International Orlando, Oct [14] Sandia National Laboratories, The Effects of Varying Humidity on Copper Sulfide Film Formation. Sand Report, Feb [15] [16] Abbott, W., The Development and Performance Characteristics of Mixed Flowing Gas Environment, IEEE Trans. Components, Hybrids, manufacturing Technology, Vol. CHMT-11:1, 1988, p Figure 13: Cross Sectional SEM of Electroless nickel/immersion silver Interface, Free of Black Line Nickel Attack Through the use of a specifically formulated electroless nickel process, employed in combination with a Sterling immersion silver bath that has been modified to ensure both the desired deposit thickness and required deposit cosmetics are achieved, the tendency for such a defect to occur is dramatically reduced. When processed using best practice conditions the defect is essentially eliminated. Future Work Additional work is now being conducted to further optimize the Imm. Ag- EN process flow and the new surface finish performance characteristics. Future reports on this exciting new surface finish will include wire bonding data and additional solderability data in the form of ball shear testing. CONCLUSIONS A combination of electroless nickel/immersion silver offers a surface finish with the beneficial characteristics of both immersion silver and ENIG but at a greatly reduced cost when compared to ENIG. This process also delivers superior functional performance particularly when applied as a final finish on components destined for aggressive end use applications REFERENCES [1] Improving Air Quality in Asian Developing Countries, Chinese NRI Activities, Phase 1 Final Report, Asian Regional Research Programme on Environmental Technology (ARRPET), May [2] [3] Helen Holder, HP presentation Metal Finishes Data Acquisition Task Group (IPC 3-11g) committee conference calls. [4] Benchmarking Urban Air Quality Management and Practice in Major and Mega Cities of Asia, APMA, [5] ISA-S , Environmental Conditions for Process