Direct Immersion Gold
as a Final Finish By Shigeo Hashimoto, Masayuki Kiso, Yukinori Oda, Horshi Otake C.Uyemura & Co., Ltd. Central Research Laboratory, Osaka Japan George Milad, Don Gudaczauskas
Uyemura International Corporation
Southington CT 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
does not co-deposit with gold and also that the main driving force
for deposition is an auto-catalytic reaction. In addition the effects
of the copper surface roughness and deposition time on the deposit
and solderability characteristics were examined. It was determined
that copper surface roughness affects solder spread-ability, and that solder joint characteristics were excellent when the film thickness is within the range of 30 to 80 nm. Furthermore, good wire bonding characteristics were confirmed from deposits plated by a neutral pH, auto-catalytic type heavy electroless gold plating bath, atop the
flash gold. Introduction 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
circuit board and package surface finishing applications, Direct Immersion Gold (referred to as DIG), is introduced as a viable
surface finish that is able to deposit a fine and uniform gold layer
directly on the copper surfaces. The characteristics required for
the success of the Direct Immersion Gold surface finish are
as follows:
- Minimum corrosion of the copper surface during deposition
(one of the causes of voiding after surface mounting).
- Gold plating layer has excellent coverage.
- Copper does not co-deposit with gold.
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
joint reliability was evaluated by examining solder ball spreading,
ball shear testing, and inter-metallic (IMC) formation for both
Sn/Pb eutectic solder and Sn/Ag/Cu solder. Gold wire bonding is widely used as a mounting technique
for IC chip applications. Test coupons for gold wire bonding
characterization were prepared by plating 50 nm of DIG as
above. In addition a 500 nm gold thickness test sample was also prepared by the deposition of an additional layer on top of the
50 nm of DIG. This new layer was deposited by a heavy deposition neutral pH, auto-catalytic electroless process. 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. Measuring solution:
- Solution A): DIG solution with no gold salts.
- Solution B): DIG solution with no gold salts and no reducing
agent “DIG R”
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.
If it is difficult for a copper corrosion current to generate during
plating, then the copper dissolution amount according to the
plating reaction is reduced. Also, because the oxidation reaction
of the reducing agent is nearly equal to the rest potential of copper,
it is easily seen that it is difficult for copper to deposit with the gold. 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
gold concentration amount with atomic absorption spectrometry,
and gold cyanide and “DIG-R” were replenished according to the
gold consumption amount. Copper clad laminate boards (FR-4) were used as test substrate printed circuit boards. Bath load factor was 2dm2/L, and test substrate boards were changed by two methods,
every twenty minutes and every 8 hours. The relationship between
the gold deposition amount and copper dissolution concentration
in the plating solution is shown in Fig. 2. 
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.
From these results, we assume that the main reaction for DIG
bath is an auto-catalytic reaction. This result confirm the rest
potential measurement. 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
- Deposit Characteristics
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
when a electric potential of 70mV is applied to the Ag/AgCl reference electrode. Test substrates were prepared by applying electrolytic
acid copper plating (20um) to copper clad laminate boards and
adjusting the surface area to 1 cm X 1 cm dimensions with masking tape. Measurement results of the relationship between DIG plating
time and anodic current density are shown in Fig.4. The comparison with a conventional 100% displacement (immersion) reaction
electroless gold plating bath is shown in Fig.5. Also, the
relationship between Direct Immersion Gold plating time
and gold plating layer thickness is shown in Fig.6. 
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
and gold plating layer characteristics, test cuopons with relatively
severe buff polishing after electrolytic acid copper plating were
prepared. Solder mask was applied to the test cuopons, and by
using a special copper etching agent CZ (Mec Company Ltd.)
process, test coupons of different surface roughness were made
by adjusting the CZ copper etching amounts to 0, 1, 2, and 3um.
An SPS (sodium persulfate) bath and a sulfuric acid/hydrogen
peroxide bath were utilized as a soft etching process before plating
with DIG in order to adjust surface roughness. By using test
coupons with different copper surface roughness and different soft etching conditions in the pre plating process, the influence copper surface roughness has on solder joint characteristics was confirmed.
The relation between each soft etching condition and surface
roughness is shown in Fig.8. A laser microscope (KEYENCE
Corporation VK-8550) was utilized for surface roughness
measurement. 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 Reflow Conditions 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 Dage#4000 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
in purity and had excellent gold coverage. 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
Model |
KS 4524A
Semi-auto |
|
Wire size |
25μm |
|
Capillary part # |
4047220010-320 |
|
Frequency US |
60Hz |
|
|
1st Bond |
2nd Bond |
|
Power |
130 mW |
190mW |
|
Time |
10msec |
15msec |
|
Force |
30g |
90g |
|
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. Conclusion Many 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 demonstarted.
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