ELIMINATING MICROVOID RISK VIA AN OPTIMIZED SURFACE FINISH PROCESS

Transcription

1 ELIMINATING MICROVOID RISK VIA AN OPTIMIZED SURFACE FINISH PROCESS Donald Cullen, Witold Paw, John Swanson, Lenora Toscano MacDermid, Inc. Waterbury, CT USA Abstract The Pb-free transition in the electronics industry has seen immersion silver emerge as a leading circuit board finish for ROHS compliant assemblies and finished goods. The immersion silver finish has now been in high volume production for ten years and is utilized in a wide cross-section of end-use applications, both simple and technically sophisticated. The strengths of immersion silver are numerous: process simplicity, contact functionality, durability to multiple reflows, high frequency performance, and others. Recently, a phenomena arising after the soldering assembly operation, commonly referred to as microvoiding, has been linked to immersion silver processing. Studies of microvoiding have shown it to occur infrequently and somewhat unpredictably, so identification of root causes has been difficult. Reliability testing of "worst-case" microvoiding indicates it to be a problem deserving industry investigation. As an update to an earlier publication on this topic, this paper describes an investigation which identified key enablers for microvoid formation, a mechanism for their formation, and the successful implementation of an optimized silver process which minimizes microvoid formation and risk. ` Figures 1 a,b,c: Solderjoint Microvoiding by Optical Cross-Section and X-Ray Inspection Introduction Voids exist in solderjoints in many forms. The microvoids discussed in this work are characterized by three factors. First, the voids are very small, generally on the order of less than 1 microns. Next, the microvoids of concern exist at the interface of the copper substrate and the bulk solder, or more specifically, where the copper-tin intermetallic meets the bulk solder. Last, the problematic microvoids exist in large numbers, all on the same plane. These characteristics differ from other types of solderjoint voids such as bulk process voids, Kirkendall voids, and shrinkage voids. The impact of solderjoint microvoids on functional reliability is a topic of much debate. This paper will not address that topic. Intuitively, engineers believe that voiding in solderjoints reduces performance. While some work has shown that solderjoint voids may interrupt crack propagation, most engineers believe that planar microvoids, also known as champagne voids, can pose a real reliability risk when present in large proportions. In earlier investigations, a link was found between the formation of solderjoint microvoids with various PCB fabrication and assembly steps 1. A combination of extremely high silver thickness and inadequate reflow temperature was found to lead to microvoiding. Other links to microvoiding were observed empirically. The earlier work provided observations of microvoids in solderjoints and ways to reduce the voiding. The mechanism of void formation at the solder/copper interface was not fully discovered in time for the earlier publication, however. At that time, a proposed mechanism involved the interaction between flux chemistry and silver metal. Later work found this theory difficult to reproduce with some of the fluxes use in production. The industry continued to seek a more thorough mechanistic explanation of microvoid formation. As with many difficult failure modes in PCB manufacturing, the defect occurred with such infrequency that many experiments were required to collect enough data to evolve the theories of microvoid formation. The issue of contamination remains as a leading contributor to microvoid vulnerability. Early samples demonstrating microvoids were traced to fabrication process areas with process control issues. Contamination on boards included oils, developer foam, tin resist and films of sublimated soldermask volatiles. Cross-contamination from shared

2 rinses, organic coated conveyors, and immersion tin provided additional residue. The silver process was improved to provide adequate pre-cleaning, silver bath analysis, use of a cleaner and microetch, dedicated rinsing, and improved rework procedures. After these steps were implemented, the X-ray occurrence of microvoiding was nearly eliminated. Laboratory Methods Several methods are used in the industry for studying solderjoint microvoids. The most direct method involves physical destruction of a BGA solderjoint with techniques such as dye and pry. In the physical destructive methods, a component is assembled to the test specimen, and then physically removed, often by simple torque such as from the use of a screwdriver. The component removal can be conducted following thermal cycling tests, or other stressful environments. Alternatively, X-Ray equipment has been applied as a tool to detect and study microvoids. X-Ray equipment has reached a level of power and resolution to allow detection of microvoids. X-Ray inspection is non-destructive, but is slow and expensive and not readily used as an assembly quality control. In addition, there is not industry consensus that voids found by X-Ray can be attributed to the location within the solderjoint at the copper interface. Voids not located at the interface are not always considered to be of high importance. The most expedient method for detecting microvoids is through careful cross-sectioning. A surface under investigation is soldered, often to a BGA component or more simply, to a solder sphere. An assembled solderjoint can be optically inspected after cross-sectioning to reveal microvoids at the copper/solder interface. The crosssectioned solderjoint is more precisely studied with the use of focused ion beam. The FIB technique removes very small layers of material on the surface of the section, revealing fine detail such as voids, caves, and chimneys, as discussed later Figure : Scale Used to Rank Microvoid Severity None of the techniques listed here allows bare PCB s to be screened before assembly. Some work at various industry R&D labs has attempted to connect microvoid propensity with solderability results, thickness distribution, and even tarnish observation. Without a reliable method for predicting microvoids at the bare PCB stage, extra emphasis is placed on prevention techniques such as chemical process control. Results With cross section evaluation, it was possible to conduct the large number of experiments needed to collect sufficient data to form conclusions on microvoid formation. The scale used in Figure allowed quantification of microvoids and statistical treatment. One particular experiment produced a clear result simply by sorting the data according to the type of microetch used to prepare the copper for silver plating. Figure 3 shows the sorted data from an experiment conducted using variables including microetch type, flux type, and silver thickness. When arranged according to microvoid rating, all samples created with peroxide type microetch resulted in the highest microvoiding. This empirical result prompted further investigation into the nature of copper microetches.

3 Peroxide Microetch Modified Persulfate Microetch Su rf Figure 3: Sorted Data Demonstrating the Effect of Microetch on Microvoids Several hypotheses were proposed relating to the effects of copper surface structure on the production of solderjoint microvoids. The copper may be altered in several different processing areas. The deposition of copper itself may produce rough deposits due to current density variations. Tin stripping and soldermask preparation processing may significantly affect the copper structure prior to surface finish. The final finish process employs a cleaner and microetch, but these relatively mild solutions are not adequate to overcome extremely rough or dirty copper. Another process capable of producing a poor copper structure is silver rework. For various reasons, operators may attempt silver rework by processing the PCB repeatedly through the plating bath. This method, along with other silver stripping methods, may easily compromise the integrity of the copper and silver deposits. Once altered by any of the above processes, copper deposits affect the quality of solderjoints. Microetching of copper is a complicated system of itself. Using the methods resulting in the above experiment, the etched copper surface was studied in more detail. Figures 4 and 5 show the copper surfaces formed with peroxide sulfuric microetching system and a modified persulfate sulfuric (mixed peroxydisulfate salts) system. The type of peroxide system used in these experiments produced a structure which, when viewed under SEM and AFM, demonstrated a sharp rugged texture. The modified persulfate microetch, on the other hand, yielded a copper structure of less jagged copper interface. It has been discussed in other work that the silver coating formed with thin immersion coatings does not significantly change the surface texture of the underlying copper. So, the silver interacts with the microetched copper surface during the initial plating, and the solder interacts with a silver surface which mimics the underlying copper. At this point in the investigations, it was not known if the silver interaction with rough copper, or the solder/flux interaction with roughened silver was connected to microvoid formation. Su rf Su rf Su rf 1,, Figure 4: Structure of Peroxide Microetched Copper by SEM and AFM

4 1,, Figure 5: Structure of Modified Persulfate Microetched Copper by SEM and AFM The mechanism of void formation as a function of microetched copper structure was discovered with closer inspection of solderjoints known to have high microvoid counts. In assembled PCB s showing a high concentration of microvoids, cross-section observation uncovered a new phenomenon. Small caves were found on unsoldered areas of the PCB. With more complete data collection, it was found that the caves under the silver coating were frequently, if not always associated with high microvoid occurrence. The mechanism of formation for the newly found caves was still unclear. A hypothesis was proposed linking the deep crevices produced by rough peroxide microetched copper with cave formation. In this hypothesis, the deep valleys of rough copper acted to promote extra aggressive corrosion during the silver galvanic displacement. Another hypothesis linked the modified persulfate microetch to cave prevention by its better cleaning ability. In this hypothesis, the modified persulfate acts to clean minuscule residues on the copper surface. The residues could result from earlier soldermask, tin, or handling steps prior to the silver operation. The modified persulfate is known to undercut and remove these residues more effectively than peroxide microetches. If residue is not completely removed, an aggressive silver chemical solution may selectively corrode copper at the interface between the copper and the residue coating. This effect of crevices formed in copper is known in the industry. Galvanic displacement corrosion reactions can form a localized hyper-aggression due to trapped chemistry and incomplete solution flow. Corrosive chemicals removing copper get trapped and prevent sealing of the copper surface with silver (or other immersion metals,) due to hindered solution flow in narrow local pathways. In any case, many experiments confirmed the link between the peroxide microetched copper surfaces and the formation of caves. Further, once the caves were formed, the surface was seen to have a high propensity for solderjoint microvoids upon assembly. Figure 6: Caves Detected Under Silver ace ared With Peroxide Microetch Figure 7: Caves Detected Under Silver ace ared With Modified Persulfate Microetch The mechanism of cave formation was clarified with the further discovery of small pathways leading to the caves. Because the caves are extremely small, on the order of.1.1 microns, it was difficult to locate the caves by optical, cross-sectional and X-Ray techniques. Some cross-sections revealed another even smaller level of detail.

5 Some cave cross-sections were shown to have a chimney. The chimney was a micro pore connecting the cave to the surface of the silver. The corrosion hypothesis held that as the copper was hyper-corroded in the cave area, the solution pathway to the chemical solution was maintained by the presence of this chimney. The pore would allow a one-way stream of copper ions to flow from the cave, but did not allow sufficient silver chemicals into the cave to seal the copper and quench the reaction. Samples of chimney photos are presented in Figures 8 a&b. Figures 8 a,b: Microscopic Pore Chimneys Forming a Pathway Between Caves and the Silver ace Aggressive corrosion of the copper is a clear factor leading to cave formation and, eventually, microvoids. By the above hypotheses, cave formation can be minimized and perhaps eliminated by preventing surface residue, by removal of the residues by modified persulfate microetches, and by forming a less rugged surface with the modified persulfate. These preventative measures may be augmented by modification of the silver operating parameters to reduce aggressive initiation of copper displacement in local areas. Experiments to further control the initiation rate of the silver bath did show positive results for minimizing microvoid formation. The variables studied included acid content, bath temperature, and silver ion content in the working bath. Originally, the silver working bath was controlled within relatively wide ranges, in order to allow easy use of the system at the PCB fabricator. In real use, the generous ranges for temperature, silver content, and acid control were even wider than needed to easily control the system. Prior to the discovery of microvoids, there was no reason to restrict those ranges. After demonstrating the link between microvoids and immersion silver processes run beyond the outer control ranges, it was determined that more narrow chemical operation would help enforce good silver deposition practices. By more tightly controlling these variables, the initiation rate of the silver on the localized copper areas could be well controlled. The effect of acid activity (ph) and temperature on the surface structure of immersion silver is demonstrated in Figure 9. With the surface information provided by these studies, a new set of process conditions was implemented. Better acid control was achieved through more appropriate measurement of free mineral acid, measured at low ph with a new titrimetric method. The control of acidity remains simple for the fabricator, since large amounts of chemical additions are required to modify the effective acidity to any significant degree. The updated temperature control is similarly easy to implement and control in real production. Temperature has a profound effect on the microstructure of the deposit at very high magnification. At low temperature ranges, small spherical structures are evident. This variation in texture is connected with the surface texture more vulnerable to cave formation. High surface roughness can also impact tarnishing and solderability. Maintenance of temperature in a new range of 5-54ºC is a simple matter.

6 5ºC 47ºC 4ºC 1.8 ph Figure 9. Effects of Acid Activity and Temperature of Silver Bath on ace Structure The next parameter under revision is the silver concentration of the working bath. The silver concentration acts in coordination with temperature and acidity to affect the bath plating rate and the localized corrosion initiation. The statistical correlation of the silver content as deposit thickness is varied is presented in Figure 1. Overall, the effect of higher silver concentration added about 1.5 points to the microvoid score when compared with the new lower silver concentration. When analyzed in conjunction with excessive silver thicknesses of 1.5 microns (6 microinches,) the effect was an increase in the microvoid score of 4.5 points on the -9 scale. DESIGN-EXPERT Plot Voids X = A: Ag Conc Y = C: target Thickness C- 15. C+ 6. Actual Factors B: Acid N =.17 D: Etch Type = ME E: Reflow = std Voids Interaction Graph C: target Thickness A: Ag Conc. Figure 1: Effects of Silver Content and Resulting Silver Thickness on Microvoiding A graphical analysis of the 3-way interaction of silver concentration, bath temperature, and rate is presented in Figure 11. In this case, all samples were manufactured to achieve a target.4 microns of silver. The samples were plated at varying temperatures and silver contents, so dwell time was adjusted to produce similar thickness, represented by bars in the chart on the left y-axis. The lines, corresponding to the right y-axis, show the microvoid results. These data show that the combination of silver content reduction and increased temperature provides for the highest level of control.

7 15u-in Target Thickness C 5C 55C 6 5 Silver Thickness (microinches) Voids Actual Thickness Rate Voids Ag Conc. (g/l) Figure 11: Constant Thickness Study of Temperature and [Ag] on Microvoiding Previous Process Control Changes The experimentation conducted in earlier studies led to process modifications aimed at controlling the thickness distribution and chemical level variations. The thickness range was modified to contain the upper thickness at.45 microns. Additionally, the method for analyzing acid content of the plating bath was updated. Originally, acid was measured using methodology for detection of total acid content. In the prior technique, the effective strong acid content was masked by the additional contribution from weaker organic acids. A new technique was implemented measuring effective free acid at low ph. The mineral acid is dominant in the galvanic displacement reaction, controlling rate and deposit quality. Other weak organic acids participated in the old technique. The new analysis allowed for a huge leap forward in deposit predictability. Key Variable New Process Old Process Pretreatment Cycle Final Finish Spray Cleaner + Sterling ace Microetch Final Finish Spray Cleaner + ace Microetch or Microetch (peroxide) Silver Concentration.6-.9 g/l Silver 1.-. g/l Silver Plating Temperature 5-54 ºC ºC 6 Temperature (ºC).5 Ag Concentration (g/l) C 5-54 C 1.-. g/l.6-.9 g/l Figure 1: Summary of the Prior Processing Conditions (left bars in each graph) with New (right bars)

8 Production Verification After extensive pilot scale investigation, and scale-up work with industry partners, the new silver processing conditions were brought to the field. With positive validation of the new parameters in preventing solderjoint microvoids, the new parameters were deployed as the official operating conditions on a worldwide basis. The fabrication, assembly, and OEM sectors were notified of the changes. Many companies decided to confirm the new operating conditions within their own technical labs. In normal production, it is nearly impossible to detect microvoids due to their low level occurrence. For this reason, it was difficult to determine if the microvoid prevention benefits seen in the laboratory, created under extreme conditions, could be reproduced in production work. However, one customer, who s data, is not available for publication at the time of this printing, had been experiencing a sporadic microvoid observation by one of their customers, as observed by X-Ray inspection. With the new silver and microetch processing conditions, the microvoiding at this company was reduced from a level of about 4 to a level of less than 1 on the -9 scale as confirmed by cross-section. In addition to microvoid evaluations in production as a verification of new process conditions, numerous studies were conducted to measure solderability, contact resistance, surface texture, thickness distribution, copper crevice corrosion, and tarnish vulnerability. As predicted from the improved microetching conditions and the less aggressive silver deposition, the new conditions offered significant improvements in all categories, especially in thickness control and tarnish protection. It is recommended to implement these new operating conditions at all PCB manufacturers applying immersion silver. Conclusions and Recommendations Extending earlier studies into the recreation and prevention of solderjoint microvoiding, a great deal of scientific investigation has been undertaken by the chemical supply base and industry partners. Some of the earlier reported findings did help in microvoid prevention. However, at the time of the earlier publication, the mechanism for microvoid formation was not fully understood. More recent findings have determined that aggressively corroded copper in local areas can result in microscopic caves in the copper surface. The cave may form as a crevice corrosion at the interface of surface residues, or as an artifact of extremely rough texture from copper microetches and prior copper treatments. The localized corrosivity is demonstrated by the existence of pinhole pores or chimneys above the caves. The caves are linked to solderjoint microvoids by empirical study. The caves, filled with oxides, flux materials, or simply air, are exposed when the silver layer dissolves into the bulk solder. Material within the cave becomes a small void at the intermetallic interface. Due to the small size of the tiny voids, the void does not evolve from the solderjoint, as it buoyancy is insufficient to overcome the surface tension effects holding it to the surface. While several of the steps in this mechanism have been verified through direct observation, several others continue to be the topic of ongoing research. The reaction to the discovery of this microvoid mechanism has been a set of new process controls for the immersion silver chemistry. Once again, other voiding mechanisms will exist for other surface finishes not evaluated in this study. The new immersion silver process changes have been verified in the laboratory, with various members of the electronics supply chain, and in large scale production on real equipment. The threat of solderjoint microvoiding has been reduced to a very small possibility, and can be considered as resolved for manufacturers adhering to new, optimized immersion silver process conditions.

9 Microetch preceding plating Silver Immersion Plating Subsequent reflow/assembly High Risk Process Cycle: Ag + Cu ++ Ag + Cu ++ Ag + Cu ++ Ag + Cu ++ Ag + Cu++ ++ Unfavorable peroxide etch creates micro-topography and sites susceptible to subsequent cave formation Caves formed when high plating rate electrolyte forms silver roof over microtopography and high risk sites. Cave volume magnified as roof forms During reflow cycle, caves are exposed via solder melt, silver dissolution, and copper consumption. Voids formed do not escape Optimized Process Cycle: Ag + Cu++ Ag + Cu++ Ag + Cu++ Preferred etch creates rolling topography and less high risk sites for subsequent cave formation Minimal cave formation: Lower rate plating electrolyte permits controlled plating and copper displacement over rolling copper surface No significant void formation due to minimal cave presence entering reflow operation References. 1. D.Cullen, Characterization, Reproduction, and Resolution of Solderjoint Microvoiding, IPC APEX/EXPO February 5.. A.Primavera, R.Sturm, S.Prasad, K.Srihari, Factors That Affect Void Formation in BGA Assembly, Proceedings of Institute for Interconnecting and Packaging Electronic Circuits, October IPC Association Connecting Electronic Industries, IPC-795, Design and Assembly Process Implementation for BGA s, August. 4. R.Coyle, G.Wenger, D.Hodges, A.Mawer, D.Cullen, P.Solan, The Effect of Variations in Nickel/Gold ace Finish on the Assembly Quality and Attachment Reliability of a Plastic Ball Grid Array, IEEE Transactions on Component and Packaging Technologies, Dec D.Banks, T.Burnette, Y.Cho, W.deMarco, A.Mawer, The Effects of Solder Joint Voiding on Plastic Ball Grid Array Reliability, ace Mount International 1996, Sept C.Chiu, K.Zeng, R.Stierman, D.Edwards, K.Ano, Effect of Thermal Aging on Board Level Drop Reliability for Pb-free BGA Packages, IEEE ECTC S.Eckel, N.Kini, D.Le, R.Jay, Impact of PCB ace Pad Finish and Contamination on BGA Solder Joint Voiding, SMTA International, Sept D.Bernard, K.Bryant; Does PCB Pad Finish Affect Voiding Levels in Lead-Free Assemblies? SMTA International, Chicago, September 4 9. W.Jones, Universal Instruments SMT Laboratory: So What's in Cu-Pads, and How Do We See It? Universal Webinar, September D.Henderson; Kirkendall Voiding and Other Embrittlement of Cu-Pads. Universal Webinar, September T.Woodrow; High Vibration Testing of Solder Joints, Universal Webinar, September D.Angelone; Soldermask Contamination of Copper Foil, MacDermid R&D Internal Report, November P.Roubaud, J.Gleason, C.Reynolds, K.Lyjak, M.Kelly, J.Bath, Development of Baseline Lead-free Rework and Assembly Processes for Large Printed Circuit Assemblies. 14. IPC Association Connecting Electronic Industries, IPC-4553, Specification for Immersion Silver Plating for Printed Circuit Boards (final draft,) October M.Wickham; Voiding, Occurrence and Reliability Issues with Lead-Free, NPL Originally published in the Proceedings of the International Conference on Lead-free Soldering, Toronto, Ontario, Canada, May 6.