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Investigation of Immersion Silver Pcb Finishes for Portable Product Applications

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Investigation of Immersion Silver Pcb Finishes for Portable Product Applications INVESTIGATION OF IMMERSION SILVER PCB FINISHES FOR PORTABLE PRODUCT APPLICATIONS Srinivas Chada and Edwin Bradley, III APTC, Motorola, Inc 8000 W. Sunrise Blvd Plantation, FL 33322 [email protected] and [email protected] ABSTRACT Supplier 1 Supplier 2 Immersion silver is being pursued as a low-cost replacement Acid based (pH=2) Neutral (pH=7) PCB finish for both immersion gold/electroless Ni and Thickness 10 – 40 min Thickness <6 min selective gold/OSP in portable applications. Immersion Ag Drop in existing plating Needs specific Supplier finish is touted both as a cost savings, yet reliable alternative line equipment to both ENIG and OSP. Hence, this study looked at the two Vertical or horizontal Horizontal plating line only of the most mature immersion silver plating chemistries in plating line the market, as applied by several Motorola PCB suppliers and compared them to OSP and ENIG. Our study Table 1. Differences in supplier Immersion Ag plating characterized the finishes for keypad popple contact processes. resistance up to 500,000 cycles of actuation, electrochemical migration under humidity, BGA solder joint reliability, and For this study we compared PCBs finished with immersion wettability for Sn-Pb and Pb-free solder pastes. Plating Ag from two different suppliers against OSP, ENIG and process variation was evaluated by procuring samples with electroplated silver (droplet test only). SIR and typical and 2x plating thickness while, environmental elctromigration tests were conducted in accordance to IPC degradation was evaluated by aging boards in 85°C/85RH and Bellcore standards by an independent laboratory. and flowing mixed gas. Recommendations for the Furthermore, testing with exposure to extreme moisture and applicability of the conventional and silver PCB finishes voltage (droplet test) was conducted in-house to gain a will be discussed in context of these results. better understanding of dendritic growth and electromigration. BGA reliability was evaluated using the 3 INTRODUCTION point bend test while wetting and solderability was carried With the push for lower cost and lead-free electronic out using goop troop boards reflowed with several different products, the need to find alternate yet cost effective PCB solder pastes. Lastly, the finish was tested for keypad finishes to replace HASL finish has gained considerable 1,2,3,4,5 continuity applications by monitoring the changes in contact momentum . Although several other lead free finishes resistance for up to 500,000 cycles of metal popple such as organic solderability preservatives (OSP), selective actuation that mimicked the actual usage of the PCB gold, electroless nickel/immersion gold (ENIG), and surface. In order to access the process variations, samples electroless palladium, are available commercially, they are from three PCB vendors, as well as from the chemical either a more expensive alternative or have well suppliers were procured and tested. documented process related and/or reliability issues 4,6 associated with them . EXPERIMENTAL PROCEDURES SIR and electromigration tests In this paper we are focusing on Immersion silver as an Both Surface Insulation Resistance (SIR) and alternate finish for PCB surface. Two major suppliers of ElectroMigration (EM) tests were conducted at Trace immersion silver technology promote it as an economic Laboratories-East in strict accordance with IPC-TM-650 drop in replacement for OSP and ENIG without the and Bellcore GR-78 specifications, respectively. The elctromigration issues associated with electroplated Ag. samples for these tests consisted of samples with 1x and 2x Several published works have shown that silver is prone to thickness from the vendors and suppliers. A set of 5 IPC B- electromigration and dendritic growth in presence of 7,8 25 coupons were tested in the as-received condition, after 1 moisture and electric bias . However, these studies reflow at peak temperature of 225°C, with rework flux only, concentrate on thick film-fired silver. and reflow at peak temperature of 225°C with application of two Sn-Pb and one Pb-free solder pastes, respectively. SIR Even though both suppliers processes are similar in that coupons were tested for changes in resistance for up to 250 they both plate silver onto copper with a co-deposited hours, while the EM coupons were monitored for 500 hours organic, they are different in their chemistries and with periodic observation and recording of dendritic growth, processes. Table 1 shows their differences. if any. Bare Cu coupons were also run as a control. Water droplet test finish. Using a 5 mil thick stencil and 132 mil diameter This test was designed to test the tendencies for aperture, both lead-free and lead containing solder pastes electromigration of immersion Ag in presence of extreme were reflowed at a peak temperature of 225°C. After reflow moisture and voltage. Electromigration of metal species the diameter of the solder sites were measured and leads to dendritic growth resulting in short circuits was compared to the original diameter to determine the monitored and compared for immersion Ag as well as for wettability of the PCB surface by the solder paste. Tests Organic solder Preservative (OSP) and Electroless Ni over were repeated with boards that were exposed to corrosive Immersion Gold (EniG) samples. IPC B-25 comb patterns mixed gas flow (mixture of Cl2, SO2, NO2 and H2S) up to 96 were connected to a 50V DC source. A droplet of deionized hours and aging at 85°C and 85%RH up to 168 hours, prior water was carefully placed bridging adjacent metal lines and paste printing and reflow. the subsequent change in potential difference was monitored across a 100kW resistor as a function of time. RESULTS AND DISCUSSION Finish appearance Keypad contact resistance test The Supplier 2 finish is much shinier and silvery in Change in contact resistance as a function of # of cycles of appearance than the Supplier 1, which has a white matte actuation were recorded using commercially available test appearance. With exposure to air reflows, the appearance of equipment manufactured by Test Systems Engineering. The each stayed practically the same. However, after exposure set up consisted of four platforms to hold the test samples to 85°C/85%RH the finish dulled a little for both supplier and pneumatically driven actuators (pistons) to push the chemistries. The flowing mixed gas reacted with the finish metal popples. Commercially available cellular phone and formed corrosion products as shown in Figure 1. keypads with immersion Ag finish from three vendors along with ENIG as reference were used. Metal popples were placed on the keypads and completed the electrical circuit when pressed by the actuators. Two wire contact resistance measurements were taken periodically to monitor the wear on the Printed Circuit Board (PCB) surface finish. Prior to testing, the keypad surfaces were subjected to various reflow and environmental conditions, including exposure to corrosive mixed gas flow (mixture of Cl2, SO2, NO2 and H2S) up to 96 hours, and aging at 85°C and 85%RH up to 96 hours, after 2 and 5 reflows, respectively. Reliability test Three-point bend test to failure was conducted by assembling Glob Top Plastic BGA (1 mm pitch, 196 I/O) on a cellular phone controller boards. Flux-only attachment results in far fewer voids in the solder joint and allows for comparisons in the effect of the finish itself on void introduction in the joint. ENIG and OSP finish boards were Figure 1. Macroscopic view of the immersion Ag surface also tested as controls. A span of 1 inch between the exposed to mixed gas flow for 96 hours showing the tarnish supports was used with a loading rate of 1mil/sec until and corrosion product. failure, to record load as a function of displacement. To test the variations in reliability resulting from reflow and aging SIR and EM tests conditions, samples were subjected to 1 or 2 reflows, From the data supplied by the SIR laboratory, it is followed by aging at 150°C for 168 hours. Also, reflow understood that all immersion Ag specimens supplied by the profiles with two different peak reflow temperatures of suppliers and vendors for both 1x and 2x thickness have 190°C and 215°C were employed to investigate the effects passed the SIR test as regulated by IPC-TM-650. All of process variation on the reliability. The aforementioned samples with the exception of a sporadic few have either 8 process variations were carried out to determine the effect of met or exceeded the reference resistance value of 10 ohms intermetallic layer thickness on the reliability of the solder even after 250 hours of testing. Figure 2 is a representative joint, which in turn is dependent on reflow temperature and plot of the resistance measurements as a function of time for time, number of reflows, aging temperature and time after both the control boards (bare Cu) as well as Immersion Ag reflows. samples. It is clear from the figure that all the samples have 8 exceeded the minimum resistance value of 10 ohms Solder wetting and spreading through the entire duration of the test. Furthermore, all the Goop Troop boards with immersion Ag finish from the samples with the exception of a very few have met the three vendors, as well as samples with OSP and ENIG were visual requirements at 10x magnification, i.e. no evidence of used to study the wetting characteristics of the PCB surface stains, dendritic growth, contamination or condensation on the comb pattern. This agrees with data reported by Intel3 and Lucent5. The ones with a growth appeared to have condensation that contributed to the growth. 1.00E+11 As Received 1 Reflow Flux only Sn-Ag-Cu Sn-Pb-Ag (3) Sn-Pb-Ag (2) Bare Cu Reference 1.00E+10 1.00E+09 Resistance (ohms) 1.00E+08 1.00E+07 0 50 100 150 200 250 300 Time (hours) Figure 2.
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