Advanced Materials Research
ISSN: 1662-8985, Vol. 1133, pp 391-395
doi:10.4028/www.scientific.net/AMR.1133.391
© 2016 Trans Tech Publications, Switzerland
Submitted: 2015-06-21
Revised: 2015-06-25
Accepted: 2015-09-23
Online: 2016-01-04
Deposition of Electroless Nickel Boron as Printed Circuit Board Surface
Finish
K. Hardinnawirdaa , A.M. Zetty Akhtarb, I. Siti Rabiatull Aishac*
and I. Mahadzird
Faculty of Mechanical Engineering, Universiti Malaysia Pahang, 26600, Pekan, Pahang, Malaysia
a
wirda_rose@yahoo.com, bzetty_7391@yahoo.com, crabiatull@ump.edu.my,
d
mahadzir@ump.edu.my
Keywords: Electroless nickel boron, surface morphology, surface hardness, surface roughness
Abstract. Electroless nickel boron (EN-B) had been appointed as a potential coating in several
applications in industries like aeronautics, petrochemical industry, and firearms due to its desirable
physical and mechanical properties such as high wear resistance, high hardness and provides
uniformity in coating thickness. However, in semiconductor sector, the usage of nickel boron as
coating layer is still insufficient due to lack of study in term of its potential as a coating on printed
circuit board. This study aims to investigate the coating physical and mechanical properties of
electroless nickel boron as potential printed circuit board coating layer. The study was conducted by
a subject of copper substrate to surface pre-treatment before soaking in nickel boron plating bath
solution that contained different concentration of sodium borohydrate, 0.4, 0.6, 0.8, 1.0 and 1.2 g/l.
Surface roughness was evaluated using 3D Roughness Reconstruction software while the hardness
test was conducted by using Vickers Hardness Test MMT-X7 Matsuzawa. The surface structure
was evaluated using ProgRes C3 IM7200 Optical Microscope and Field Emission Scanning
Electron Microscopy (FESEM). It was found that the surface roughness and hardness resistance
were affected by sodium borohydrate which correlated with the surface microstructure.
Introduction
In electronic packaging sector, surface finish plays an important role in determining the
reliability of the whole package. Currently, Ni-based metallization favors the electronics packaging
market since it can prevent excessive intermetallic compound (IMC) formation during packaging,
and in service [1]. While Ni-P favours among other nickel based coating, Ni-B now received much
research attention due to its ability to possess high hardness and offer superior wear resistance in the
as-deposited condition [2, 3].
According to Sankara [2], the reduction agent plays an important role in determining the overall
surface coating properties. In EN-B deposits process, the reducing agent that are commonly used
was sodium borohydrate (NaBH4) and dimethyl amine borane (DMAB). Each of reducing agent
will give different results on the coating structure where amorphous phase tends to form while using
NaBH4 and semi crystal phase opt to form when using of DMAB [4]. For reduction efficiency,
NaBH4 provided a much higher efficiency than DMAB and more cost-effectiveness in operation [5]
that makes NaBH4 usually chosen over other reducing agent. During Ni–B electroless coating,
boron enters into the film structure as a result of the oxidation reaction of borohydride (BH4).
Therefore the quantity of boron in the film is mainly controlled by the kinetics of BH4− oxidation
yielding the elemental boron deposition. The kinetics of this oxidation reaction may be affected by
BH4 concentration as well as by both the quantity of the other chemicals (like complexing agent and
stabilizer) in the bath and the coating parameters (like temperature and pH) [6].
In electronic industry, the used of EN-B as coating layer is still limited, probably due to the less
of study in term of the Ni-B potential as a surface coating in the electronic packaging area. Thus,
the present work focus on the deposition of EN-B using NaBH4 with the aim to investigate the
effect of NaBH4 concentration in bath solution and its effect towards EN-B characteristic properties
of Printed Circuit Board (PCB) surface finish.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.ttp.net. (ID: 132.239.1.231, University of California, San Diego UCSD, La Jolla, USA-01/02/16,06:14:47)
392
2nd Advanced Materials Conference 2014
Experimental
Copper polymer sandwich substrate (FR-4) was used as substrate materials for the deposition of
electroless Nickel-Boron (EN-B) coatings. The copper (Cu) substrates were cleaned by light grinding
and washed thoroughly with water to remove the dirt and the oxide layer. To prepare the EN–B
deposits, Cu was subjected to surface pre-treatment before soaking in an alkaline bath having nickel
chloride as the source of nickel and NaBH4 as the reducing agent also contained suitable quantities
of ethylenediamine as a complexing agent along with lead nitrate and sodium hydroxide. The NaBH4
concentration was varied with 0.4, 0.6, 0.8 1.0 and 1.2 g/l and the plating bath was maintained at 70
˚C for 1 hour. The microstructure and chemical composition were observed with a ProgRes C3
IM7200 Optical Microscope and Field Emission Scanning Electron Microscopy (FESEM). A surface
hardness test was conducted following ASTM E384 by using Vickers Hardness Test MMT-X7
Matsuzawa with 1000 g load and 15 s dwell time. The average value was taken from 10 readings to
ensure of a precise result. The surface roughness and 3D images of surface roughness was taken
using 3D Roughness Reconstruction software.
Results and Discussions
Structure and Morphology of the Coatings. Table 1 shows the surface observation of coating
with various NaBH4 content using Optical Microscope with 150x magnification and FESEM with
1000x magnification. It shows that different NaBH4 concentration gives a different coating
appearance. All samples consist of two textures which is matte grey and shine grey. Lower NaBH4
concentration (0.4 and 0.6 g/l) gave more even texture compare to sample with higher NaBH4
concentration and among all of the samples, sample that contains 0.6 M NaBH4 have the most even
and considerable appeal.
The surface topography of the coating is observed through FESEM in order to analyse the
microstructure of the deposits. In the FESEM images, the surface morphology of cauliflower like
feature plating were observed that were typically found in electroless nickel plating [7, 8]. The
morphology comparison was made and found that as the NaBH4 concentration increases, the
nodules formed also increase and become more distinct. This shows that higher concentration of
NaBH4 will lead to agglomeration of the nodules. The possible reason for this has something to do
with the amount of boron exists on the deposited substrate. The NaBH4 concentration in the
solution bath have a significant effect on the Boron content on deposition substrate and possibly
also affect the surface appearance that have a more matte grey feature [6]. This phenomena can be
explained by using mechanisms and kinetics reaction where the decomposition of NaBH4 is
catalysed by Ni2+ and H+ ions in the EN-B solution. They play a role in controlling the stability of
the solution [5] because the acceleratence of Ni2+ and H+ ions will accelerate the decomposition of
NaBH4 and lead to instability. The general reactions during electroless deposition process are
shown in Eq. 1 to Eq. 4 below.
BH¯4+ 4OH¯ = BO¯2 + 2H2O+ 2H2 + 4e¯
BH¯4 = B + 2H2 + e¯
Ni2+ + 2e¯ = Ni
2H2O + 2e¯ = 2OH¯ + H2
(1)
(2)
(3)
(4)
Eq. 1 can be considered as the main reaction that drives the nickel ion reduction (Eq. 3) due to
the release of four electrons. Eq. 2 is responsible for the presence of deposited boron in the film
structure. Eq. 3 can be assumed as being determined by Eq. 1 and the boron content of the deposited
film can be assumed as being determined by Eq. 2. Finally, direct reduction of water (Eq. 4) takes
place in the coating bath [6]. Further explanation regarding the reaction in EN-B solution were
discussed in detail by previous studies [9, 10].
Advanced Materials Research Vol. 1133
393
Table 1: Surface morphology of EN-B surface finish with different NaBH4 concentration using
(a - e) 150x and (f - j) 1000x magnification.
NaBH4
Magnification
Concentration
150x
1000x
(g/l)
b
a
0.4
(0.4 NaBH4)
Shine grey
Matte grey
c
d
Shine grey
0.6
(0.6 NaBH4)
Matte grey
e
0.8
(0.8 NaBH4)
Matte grey
f
Shine grey
g
h
Shine grey
1.0
(1.0 NaBH4)
Matte grey
i
1.2
(1.2 NaBH4)
Shine grey
j
Matte grey
Surface Hardness. Hardness is one of essential properties that need to be concerned for PCB since
there are some application that needed the PCB to be drilled in order to satisfy the package
requirement. It has been observed in general that hardness of EN-B coating increases with the
increase of boron content in the coating and boron content in the coating is proportional to the
concentration of NaBH4 in the electroless solution [11]. As overall, based on results obtained on
Fig. 1, the surface hardness did increase as the NaBH4 concentration increases. However, it do not
clearly indicate the relation of NaBH4 concentration towards the surface hardness since the hardness
suddenly decrease on 1.0 NaBH4 concentration sample and the results was similar as previous
studies [6, 12] where the hardness drop on higher boron concentration before it increases again.
This effect is called as mechanical crystallization where the formation of crystalline domains in the
amorphous coating may result in interfacial deformation due to a substantial difference in specific
atomic volumes of amorphous and crystalline phases. This metastable amorphous-crystalline
structure was explained clearly by Bekish [12]. The hardness found on the 0.4 and 0.6 NaBH4
394
2nd Advanced Materials Conference 2014
Vickers Hardness (HV)
sample was almost similar and the same goes with a sample of 0.8 and 1.2 NaBH4 concentration.
The reason for this phenomena is still unclear and need further investigation. The hardness value
found from this study was also smaller compared to other research. This is because they were using
high plating bath temperature up to 95 ºC where we are only using a temperature of 70 ºC to
preserve the substrate from overheat. The difference in plating bath temperature could be the
possible reason for the difference in results. Even so, the hardness obtained by using EN-B as a
coating layer in this study was comparatively higher to the substrate (FR-4) without having any
coating layer which was 35.05 HV and in a standard range of EN deposited substrate (50-75 HV).
100
80
60
40
20
0
0.4 EN-B 0.6 EN-B 0.8 EN-B 1.0 EN-B 1.2 EN-B
NaBH4 Concentration (g/l)
Fig. 1: Graph of Vickers Hardness with different NaBH4 concentration.
Surface Roughness. Characterization of surface roughness of printed circuit board (PCB)
conductors is important since it has relation with the wettability of substrate with solder alloy that
directly influence the strength of the solder joint formed. Rode found that [13] smoother board
finishes lead to stronger and more reliable solder joints. However, Prabu [14] observed an
improvement in wetting with increasing surface roughness when Ra ranged from 0.015 µm to 1.6
µm and in agreement with findings reported by Novak [15] (Ra ranged from 0.230 to 2.194 µm).
The surface roughness also found to have an influence on surface wear resistance [16]. Fig. 2 shows
the 3D images of the surface roughness with different NaBH4 concentrations along with the
roughness value, Ra. Sample with lowest NaBH4 concentration possess the smoothest surface with
only Ra=0.4968 µm and higher NaBH4 concentration did not show much difference of surface
roughness. Even so, surface roughness of 0.6 M NaBH4 concentration had the most uniformity.
This can be related to the SEM images (Table 1(g)) that showed the cauliflower like nodule
structure uniformly distributed.
0.6 NaBH4
0.4 NaBH4
(a)
(b)
Ra=0.4968 µm
Ra =1.0442 µm
0.8 NaBH4
(c)
1.2 NaBH4
(d)
Ra =1.2418 µm
Ra =1.1386 µm
Fig. 2: 3D images of surface roughness with different NaBH4 concentration.
Summary
In this study, the surface appearance of the PCB with EN-B surface coating were depending on the
NaBH4 concentration where low concentration had more even texture and considerable appearance.
Low NaBH4 (0.4 NaBH4) concentration provided even surface texture and smooth surface, but
provide low hardenability while higher concentration shows similar hardness value. Based on the
three parameters observed, (i) surface appearance, (ii) surface hardness and (iii) surface roughness,
0.6 M NaBH4 concentration shows more potential for PCB.
Advanced Materials Research Vol. 1133
395
References
[1] G. Ghosh, Interfacial microstructure and the kinetics of interfacial reaction in diffusion couples
between Sn–Pb solder and Cu/Ni/Pd metallization, Acta Materialia. 48 (14) (2000) 3719-3738.
[2] T.S.N. Sankara Narayanan, S.K. Seshadri, Formation and characterization of borohydride
reduced electroless nickel deposits, J. Alloys and Compounds. 365 (1-2) (2004) 197-205.
[3] K. Krishnaveni, T.S.N. Sankara Narayanan, S.K. Seshadri, Electroless Ni–B coatings:
Preparation and evaluation of hardness and wear resistance, Surface and Coatings Technology. 190
(1) (2005) 115-121.
[4] Q.L. Rao, G. Bi, Q.H. Lu, H.W. Wang, X.L. Fan, Microstructure evolution of electroless Ni-B
film during its depositing process, Applied Surface Science. 240 (1-4) (2005) 28-33.
[5] Y.L. Lo, B.J. Hwang, Decomposition of sodium borohydride in an electroless nickel bath,
Industrial & Engineering Chemistry Research. 33 (1) (1994) 56-61.
[6] M. Anik, E. Körpe, E. Şen, Effect of coating bath composition on the properties of electroless
nickel–boron films, Surface and Coatings Technology. 202 (9) (2008) 1718-1727.
[7] K.L. Gilley, J.C. Nino, Y.W. Riddle, D.W. Hahn, S.S. Perry, Heat treatments modify the
tribological properties of nickel boron coatings, ACS Applied Materials & Interfaces. 4 (6) (2012)
3069-3076.
[8] C. Domínguez-Ríos, A. Hurtado-Macias, R. Torres-Sánchez, M.A. Ramos, J. GonzálezHernández, Measurement of mechanical properties of an electroless Ni–B coating using
nanoindentation, Industrial & Engineering Chemistry Research. 51 (22) (2012) 7762-7768.
[9] P. Martelli, R. Caputo, A. Remhof, P. Mauron, A. Borgschulte, A. Züttel, Stability and
decomposition of NaBH4, J. Physical Chemistry C. 114 (15) (2010) 7173-7177.
[10] E.H. Amalu, W.K. Lau, N.N. Ekere, R.S. Bhatti, S. Mallik, K.C. Otiaba, G. Takyi, A study of
SnAgCu solder paste transfer efficiency and effects of optimal reflow profile on solder deposits,
Microelectronic Engineering. 88 (7) (2011) 1610-1617.
[11] S.K. Das, P. Sahoo, Influence of process parameters on microhardness of electroless Ni-B
coatings, Advances in Mechanical Engineering. (2012) 11.
[12] Y.N. Bekish, S.K. Poznyak, L.S. Tsybulskaya, T.V. Gaevskaya, Electrodeposited Ni–B alloy
coatings: Structure, corrosion resistance and mechanical properties, Electrochimica Acta. 55 (7)
(2010) 2223-2231.
[13] C.L. Rodekohr, M.J. Bozack, G.T. Flowers, J.C. Suhling, D.A. Rodekohr, The effects of
surface finish roughness on intermetallic layer growth, intermetallic interface roughness, and solder
joint reliability. in electrical contacts (HOLM), 2010 Proceedings of the 56th IEEE Holm
Conference, 2010.
[14] K.N. Prabhu, G. Kumar, Determination of spread activation energy and assessment of wetting
behavior of solders on metallic substrates, J. Electronic Packaging. 132 (4) (2010) 041001.
[15] T. Novák, F. Steiner. The area of spread solderability test use for roughness influence
assessment in Electronics Technology (ISSE), 2010 33rd International Spring Seminar IEEE, 2010
[16] A.F. Kanta, M. Poelman, V. Vitry, F. Delaunois, Nickel–boron electrochemical properties
investigations, J. Alloys and Compounds. 505 (1) (2010) 151-156.
2nd Advanced Materials Conference 2014
10.4028/www.scientific.net/AMR.1133
Deposition of Electroless Nickel Boron as Printed Circuit Board Surface Finish
10.4028/www.scientific.net/AMR.1133.391
DOI References
[1] G. Ghosh, Interfacial microstructure and the kinetics of interfacial reaction in diffusion couples between
Sn-Pb solder and Cu/Ni/Pd metallization, Acta Materialia. 48 (14) (2000) 3719-3738.
10.1016/s1359-6454(00)00165-8
[2] T.S.N. Sankara Narayanan, S.K. Seshadri, Formation and characterization of borohydride reduced
electroless nickel deposits, J. Alloys and Compounds. 365 (1-2) (2004) 197-205.
10.1016/s0925-8388(03)00680-7
[3] K. Krishnaveni, T.S.N. Sankara Narayanan, S.K. Seshadri, Electroless Ni-B coatings: Preparation and
evaluation of hardness and wear resistance, Surface and Coatings Technology. 190 (1) (2005) 115-121.
10.1016/j.surfcoat.2004.01.038
[4] Q.L. Rao, G. Bi, Q.H. Lu, H.W. Wang, X.L. Fan, Microstructure evolution of electroless Ni-B film during
its depositing process, Applied Surface Science. 240 (1-4) (2005) 28-33.
10.1016/j.apsusc.2004.07.059
[5] Y.L. Lo, B.J. Hwang, Decomposition of sodium borohydride in an electroless nickel bath, Industrial &
Engineering Chemistry Research. 33 (1) (1994) 56-61.
10.1021/ie00025a008
[6] M. Anik, E. Körpe, E. Şen, Effect of coating bath composition on the properties of electroless nickelboron films, Surface and Coatings Technology. 202 (9) (2008) 1718-1727.
10.1016/j.surfcoat.2007.07.031
[7] K.L. Gilley, J.C. Nino, Y.W. Riddle, D.W. Hahn, S.S. Perry, Heat treatments modify the tribological
properties of nickel boron coatings, ACS Applied Materials & Interfaces. 4 (6) (2012) 3069-3076.
10.1021/am3004297
[8] C. Domínguez-Ríos, A. Hurtado-Macias, R. Torres-Sánchez, M.A. Ramos, J. GonzálezHernández,
Measurement of mechanical properties of an electroless Ni-B coating using nanoindentation, Industrial &
Engineering Chemistry Research. 51 (22) (2012).
10.1021/ie201760g
[9] P. Martelli, R. Caputo, A. Remhof, P. Mauron, A. Borgschulte, A. Züttel, Stability and decomposition of
NaBH4, J. Physical Chemistry C. 114 (15) (2010) 7173-7177.
10.1021/jp909341z
[10] E.H. Amalu, W.K. Lau, N.N. Ekere, R.S. Bhatti, S. Mallik, K.C. Otiaba, G. Takyi, A study of SnAgCu
solder paste transfer efficiency and effects of optimal reflow profile on solder deposits, Microelectronic
Engineering. 88 (7) (2011) 1610-1617.
10.1016/j.mee.2011.02.104
[11] S.K. Das, P. Sahoo, Influence of process parameters on microhardness of electroless Ni-B coatings,
Advances in Mechanical Engineering. (2012) 11.
10.1155/2012/703168
[12] Y.N. Bekish, S.K. Poznyak, L.S. Tsybulskaya, T.V. Gaevskaya, Electrodeposited Ni-B alloy coatings:
Structure, corrosion resistance and mechanical properties, Electrochimica Acta. 55 (7) (2010) 2223-2231.
10.1016/j.electacta.2009.11.069
[13] C.L. Rodekohr, M.J. Bozack, G.T. Flowers, J.C. Suhling, D.A. Rodekohr, The effects of surface finish
roughness on intermetallic layer growth, intermetallic interface roughness, and solder joint reliability. in
electrical contacts (HOLM), 2010 Proceedings of the 56th IEEE Holm Conference, (2010).
10.1109/holm.2010.5619460
[14] K.N. Prabhu, G. Kumar, Determination of spread activation energy and assessment of wetting behavior
of solders on metallic substrates, J. Electronic Packaging. 132 (4) (2010) 041001.
10.1115/1.4002899
[15] T. Novák, F. Steiner. The area of spread solderability test use for roughness influence assessment in
Electronics Technology (ISSE), 2010 33rd International Spring Seminar IEEE, (2010).
10.1109/isse.2010.5547277
[16] A.F. Kanta, M. Poelman, V. Vitry, F. Delaunois, Nickel-boron electrochemical properties investigations,
J. Alloys and Compounds. 505 (1) (2010) 151-156.
10.1016/j.jallcom.2010.05.168