By Shigeo Hashimoto, Masayuki Kiso, Yukinori Oda, Horshi Otake C.Uyemura & Co., Ltd. Central Research Laboratory, Osaka Japan
George Milad, Don Gudaczauskas
In this study, the DIG process (Direct Immersion Gold), is investigated. Direct Immersion Gold is a process in which gold is plated directly on copper as a surface finish for printed circuit board and package applications. By examining the deposition reaction of the electroless flash gold plating bath, it was confirmed that, copper
Electronic components are normally mounted on to packages and printed circuit boards using solder. Lead free solder was investigated as an alternative solder material to tin/lead based material. Although many lead free solder compositions are available, the use of Sn/Ag/Cu solder materials are widely accepted in printed circuit board and package applications for its solder joint strength and reliability. Because the peak reflow temperature for Sn/Ag/Cu solder material ranges from 240 to 260°C and is higher than that of Sn/Pb eutectic solder, there is concern that surface mounting reliability will deteriorate. In this study, for printed
It was determined that it would be difficult to achieve feature 1) and feature 2) of the above mentioned characteristics by a displacement (immersion) gold deposition reaction. Therefore, an electroless gold plating bath that mainly deposits gold by an auto-catalytic reaction was developed.
Regarding feature 3) Copper co-deposition; theoretical evaluation was conducted by investigating the oxidation potential of the reducing agent contained in the DIG bath and the deposition potential of copper. Also, by conducting Auger analysis on the deposit, it was confirmed that copper does not contaminate the gold layer. Furthermore, by measuring the amount of dissolved copper and comparing it to the amount of deposited gold it was clear that the auto-catalytic reaction percentage is >80% of the total depositing reaction, as compared to the immersion reaction.
Gold coverage obtained by the DIG bath was examined by anodic electrical current measurement. By comparing a plating layer deposited by a displacement (immersion) type gold plating bath and plating layers deposited by the Direct Immersion Gold bath in various plating times, it was demonstrated that Direct Immersion Gold exhibits superior coverage as compared to standard immersion gold. Also, gold coverage on test coupons plated under 10 minutes differed significantly from those plated in excess of 10 minutes.
Test coupons for deposit characterization were plated for 20 minutes to give a gold layer thickness of approximately 50 nm. This was shown to give the optimum gold coverage as verified by SEM evaluation.
In order to study the relationship between copper surface roughness conditions and DIG gold deposit characteristics, solder joint reliability evaluations were conducted on substrates plated by Direct Immersion Gold with the copper surface micro-roughness adjusted by various copper etching conditions. Solder
Gold wire bonding is widely used as a mounting technique
Test Procedures and Results
Confirmation of deposition reaction:
In order to determine the oxidation reaction of “DIG R”, which is the reducing agent of Direct Immersion Gold, the rest potential was measured using a gold electrode and a copper electrode. The standard electrode was Ag/AgCl, and the working electrode was copper and gold plate. Measuring temperature was 85°C.
Test method and results are shown in Fig 1
Fig 1 Rest Potential measurement of DIG plating solution
From the results of this testing method, it should be noted that during the comparison of the solution with and without “DIG-R” (comparison of solution A and B), the rest potential of the gold electrode surface and copper electrode both fluctuate to a rest noble potential. This fluctuation in potential demonstrates the possibility that “DIG-R” is oxidizing at the gold electrode and copper electrode surface. Also, this potential was near equal at the copper electrode and gold electrode surface. This type of potential fluctuation demonstrates that it is difficult for a copper corrosion current to ocuur even if copper is immersed in the plating solution.
Moreover, by conducting a plating test using a 10 L test cell and measuring the gold deposition amount and the amount of copper dissolution into the plating solution, it was determined that the main deposition reaction is an auto-catalytic reaction. The Direct Immersion Gold bath was adjusted during plating by analyzing the
Fig 2 Measurement of Cu displacement Ratio in DIG Au plating solution
From this result, it was confirmed that the dissloved copper concentration in Direct Immersion Gold solution is lower compared to the copper dissolution concentration that can be theoretically calculated assuming the plating bath deposition reaction is 100% displacement (immersion) reaction. It was also confirmed that the copper dissolution amount differs according to the exchanging cycle times of the test printed circuit boards.
Also, by conducting Auger qualitative analysis of gold film deposited on test coupons that were plated with plating baths containing 50mg/L of copper, results showed that copper does not co-deposit within the gold (refer to Fig.3 Auger analysis).
Fig 3 Elemental analysis of DIG deposit by Auger
The standard DIG plating process utilized to evaluate the gold deposit is shown in table 1.
Table 1 Standard DIG process (Add Table)
In order to confirm the optimum plating time, the relationship between gold coverage and plating time were investigated by anodic electric current measurement. This method to measure gold coverage is performed by using 5% sodium sulfate with 0.1% tartaric acid as the electrolyte, and measuring the anodic current density
Fig 4 Effect of plating time on gold coverage
Fig 5 Comparison of gold coverage of conventional displacement bath and DIG bath
Fig 6 Gold deposition rate of Direct Immersion Gold bath
In the anodic eletrolysis conditions used in this evaluation, gold does not dissolve and only copper dissolves.
Therefore we understand it is possible to compare the gold surface coverage ratio by measuring the anodic current density. The results in Fig.4 demonstrate that the surface coverage ratio does not increase much after the first ten minutes of plating. In this study, the plating time was adjusted so that it was possible to deposit a gold plating layer of 50 nm (20 minutes) on the test substrates.
Fig 7 SEM of DIG gold deposit over time
The surface SEM photographs of variable plating time coupons are shown in Fig. 7. Because the gold plating layer thickness deposited by Direct Immersion Gold is 50 nm, copper surface roughness has a great influence on gold coverage characteristics. In order to investigate the relationship between copper surface roughness
As a result, a smoother (more even) copper surface was obtained by a sulfuric acid/hydrogen peroxide type etching bath compaired to a sodium persulfate etching bath.
Direct Immersion Gold was deposited for 20 minutes to the copper surface roughness test substrates shown in Fig. 8. Solder spreading ratio was evaluated using these test cuopons. The test results are shown in Fig.9. Also, solder spreading ratio comparison between eutectic Pb/Sn solder and lead free solder are shown in Fig.10.
Fig 8 Effect of soft etch on surface roughening
The test method for solder spread ratio measurement was as follows:
R type flux (Alpha metal Co. R5003, R Type) was applied on the test coupon and solder balls (Pb/Sn eutectic and Sn/4.0%Ag/0.5%Cu) of 0.75 mm in diameter were placed on the coupon which was then placed on a hot plate (230 oC for Pb/Sn eutectic solder, 260°C for lead free solder) for 40 seconds. At the respective temperature the solder ball would melt down and spread out on the coupon. The solder spreading ratio was then calculated as follows:
Solder spreading ratio = (Solder spreading area) / (Original solder ball volume)
Fig 9 Effect of surface roughness on solder spread ratio
Fig 10 Effect of solder ball material on solder spread ratio
The results indicate that the solder spreading ratio was larger when the copper surface was rougher. Also, lead free solder results were inferior (spread less) as compared to Pb/Sn eutectic solder. It is necessary to understand this characteristic when lead free solder is utilized.
Solder ball shear testing procedure was as follows. After the DIG finish was applied to the test substrate, Pb/Sn eutectic and Sn/Ag/Cu solder balls of 0.75 mm in diameter were soldered to 0.6 mm pads in diameter. Evaluation conditions are shown in Table 2 and test results are shown in Fig.11.
Table 2 Solder Ball Shear Conditions
Sn/Pb: 63/37 Senjyukinzoku, SaprkballS, 0.76mm
Sn/Ag/Cu: 95.5/4.0/0.5 Senjyukinzoku, Eco-solderball S, 0.76mm
Sn/Pb solder: 230°C, 40 sec, hot plate in air
Sn/Ag/Cu: 260°C, 40 sec, hot plate in air
Flux: Alphametal R5003 (Rtype)
Equipment and Test Conditions
Shear Speed: 4,000 μm/sec
Tool height: 50 μm
Fig 11 Effect of solder ball material on ball shear test results
In the case of Pb/Sn eutectic solder, a significant difference in ball shear test results could not be confirmed. On the other hand, Sn/Ag/Cu solder results showed that copper surface etching methods had a direct effect on voids in the solder, and therefore indicate that it is necessary for an optimum etching method to be chosen.
Fig 12 Effect of time at temperature on sheer strength
It is concluded, that the higher reflow temperature was a reason why the solder spreading ratio results for Sn/Ag/Cu solder were inferior. It may be necessary for future testing to actaully pull an IC chip after it has been mounted.
Furthermore, in order to confirm long term joint reliability after mounting, shear testing and IMC cross section observation were conducted on test cuopons that were heat treated at 150°C for 1,000 hours. Shear test results are shown in Fig.12 and IMC cross section SEM photographs are shown in Fig.13.
Fig 13 Intermetallic propagation over time at 150°C temperature
In order to confirm the wire bonding characteristics of DIG finishing, test coupons with gold thickness of 50 nm (Flash gold only) and 500 nm (Flash and heavy gold) were prepared (standard plating process is shown in Table 3).
Two types of 50 nm gold thickness test substrates were prepared, one without heat treatment and one with heat treatment at 155°C for 3 hours. Also, the 500 nm test coupons were heat treated per in-house pre-wire bonding procedure (heat treatment at 175°C for 3, 6, 10 and 16 hours), and wire bonding characteristics were evaluated. The results are shown in Fig.14 and Fig.15. The wire bonding conditions are shown in Table 4.
It is known that bonding strength will decrease or bonding would fail, if there are oxidized metals on the gold surface. Good wire bonding results were obtained from flash gold test coupons (Direct Immersion Gold finishing) that were not heat treated (Fig 14). This reconfirmed
that the Direct Immersion Gold film was high
Table 3 Standard DIG plating process with Heavy Gold
Cleaner ACL-009 50°C 5 min
Rinse Amb 1 min
Acid Dip 10%H2SO4 25°C 1min
Rinse Amb 1 min
Micro-etch 25°C 2 min
Rinse Amb 1 min
Acid Dip 10%H2SO4 25°C 1min
Rinse Amb 1min
Im Gold Flash Gold 85°C 20 min
Rinse Amb 2 min
E’less Gold Heavy Dep. 50°C 30 min
Rinse Amb 2 min
Fig 14 Wire bonding test results of Thin (50 nm) DIG gold deposit
Fig 15 Wire bonding test results of Thick (460 nm) DIG gold deposit
Fig 16 Wire bonding test results of Electroless Ni-P/thick gold after 16 hours at 175°C
Table 4 Wire Bonding Conditions
Although the wire bonding evaluation results after heat conditioning of thicker gold films (falsh and heavy dep) were inferior compared to the conventional test coupons that include a Ni-P layer (As a reference, bonding data of electroless nickel immersion gold ENIG is shown in Figs.15 and 16), it was demonstrated that it is possible to wire bond on heavy gold films which have been deposited directly on copper.
ConclusionMany processes are being proposed as a final finishes for printed circuit board and package applications. In this study, a finishing process that can directly deposit gold onto the copper surface by utilizing an electroless plating process has been presented. It was confirmed that it is possible to directly deposit gold on the copper
surface with excellent coverage and without creating defects on the copper surface, because the main gold depositing reaction is an auto-catalytic and not a displacement one. Furthermore, by combining a neutral auto-catalytic heavy gold electroless plating bath, a heavy gold layer was deposited directly on the copper
surface. The applicability of solder mounting and gold wire bonding on these gold plating layers (flash 50 nm and heavy 500 nm) directly on copper was demonstrated.