Electronic assembly

An electronic assembly (10) comprises one or more electronic components (18) having solder terminations (20), and a printed circuit substrate (12) having printed circuit traces (14, 16), wherein at least one of the solder terminations of the one or more electronic components (18) and the printed circuit traces (14, 16) of the printed circuit substrate (12) has a secondary finish produced by application of an electrolessly deposited nickel film (26) containing phosphorus which is further plated with gold (28). An indium-tin-lead solder paste (22) is utilized in a soldering process to attached the one or more electronics components (18) to the printed circuit traces (14,16) on the printed circuit board (12), such that the indium-tin-lead solder (22) provides improved solder joint integrity with the secondary finish. The electronic components (18) include semiconductor devices such as ball grid arrays (1000) and flip-chip integrated circuits (1010).

FIELD OF THE INVENTION 
The present invention relates generally to electronic assemblies, and more 
particularly to electronic assemblies which utilize nickel-gold finishes 
and a solder which includes indium as a constituent. 
BACKGROUND OF THE INVENTION 
Electronic devices, such as pagers, cellular telephones, cordless 
telephones, and personal digital assistants (PDAs) just to name a few, 
have seen significant reduction in size due to the use of leadless 
components and improving reflow solder technologies. Such electronic 
devices typically utilize glass epoxy printed circuit boards upon which 
printed circuit patterns have been formed using conventional 
photolithographic processes and additive plating or etching processes. The 
printed circuit patterns are typically a one ounce copper plating, which 
after processing, is protected from oxidation using an organic solder 
preservative or tin-lead plating which has been leveled by an air-knife 
process. While both methods of protecting the copper from oxidation to 
guarantee subsequent solderability have generally proved satisfactory, the 
organic solder preservative is less costly to produce, but unfortunately 
provides little, if any post-solder protection, and the printed circuit 
board patterns of electronic assemblies often exhibit corrosion problems 
when subjected to moisture or humidity. Corrosion of the printed circuit 
pattern is especially troublesome at the connections of heat seal 
connectors such as used to interconnect liquid crystal displays to the 
copper printed circuit board runners. 
The humidity problem can be resolved by the use of an electroless 
nickel-gold plating process, but such a plating process has proven to be 
less than satisfactory for widespread use due to embrittlement of 
component solder joints as compared to conventional reflow soldering of 
components to bare copper. As a result, the use of printed circuit boards 
plated using an electroless nickel-gold plating process has been limited 
to a very small percentage of the printed circuit boards manufactured. 
Joint embrittlement has been generally attributed to phosphorus buildup at 
the solder joint and the nature of the intermetallic formed at the joint 
during the reflow solder process. Such joint degradation has been limited 
to some extent by increasing solder pad sizes and the use of relatively 
high volumes of solder at the joint, both of which are contrary for the 
production of compact electronic circuits which are capable of tolerating 
high levels of mechanical shock. 
What is needed is a solder for use in a soldering process which can 
overcome the joint embrittlement problem created in an electronic assembly 
which utilizes a nickel-gold finish. 
SUMMARY OF THE INVENTION 
In accordance with a first aspect of the present invention, an electronic 
assembly comprises one or more electronic components having solder 
terminations and a printed circuit substrate having printed circuit 
traces, wherein at least one of the solder terminations of the one or more 
electronic components and the printed circuit traces of the printed 
circuit substrate has a secondary finish produced by the application of an 
electrolessly deposited nickel film containing phosphorus which is further 
plated with gold. A solder which includes indium as a constituent is 
utilized to attached the one or more electronics components to the printed 
circuit traces on the printed circuit substrate, wherein the solder which 
includes indium as a constituent provides improved solder joint integrity 
with the secondary finish. 
In accordance with a second aspect of the present invention, an electronic 
assembly comprises a semiconductor device having solder terminations and a 
printed circuit substrate having printed circuit traces, wherein at least 
one of the solder terminations of the semiconductor device and the printed 
circuit traces of the printed circuit substrate has a secondary finish 
produced by the application of an electrolessly deposited nickel film 
containing phosphorus which is further plated with gold. A solder which 
includes indium as a constituent is utilized to attached the one or more 
electronics components to the printed circuit traces on the printed 
circuit substrate, wherein the solder which includes indium as a 
constituent provides improved solder joint integrity with the secondary 
finish. 
In accordance with a third aspect of the present invention, an electronic 
assembly comprises one or more electronic components having solder 
terminations and a printed circuit substrate having printed circuit 
traces, wherein at least one of the solder terminations of the one or more 
electronic components and the printed circuit traces of the printed 
circuit substrate has a secondary finish produced by the application of an 
electrolessly deposited nickel film containing phosphorus which is further 
plated with gold. A solder which includes a metal which interacts with 
phosphorus as a constituent is utilized to attached the one or more 
electronics components to the printed circuit traces on the printed 
circuit substrate, wherein the solder which includes indium as a 
constituent provides improved solder joint integrity with the secondary 
finish.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a plan view of a portion of an electronic assembly 10 utilizing a 
conventional reflow soldering process. The electronic assembly can 
represent any electronic device, such as a pager, cellular telephone, 
cordless telephone, or personal digital assistants (PDAs) which has been 
assembled using a conventional reflow soldering process. The portion of 
the electronic assembly 10 illustrates printed circuit traces 14 which 
includes solder pads 16 which have been formed on a printed circuit 
substrate 12. The printed circuit substrate can utilize any of a number of 
well known substrate materials, such as a glass epoxy laminate or a 
flexible substrate, such as a polyimide film. An electronic component 18 
having terminations 20 is attached to the solder pads 16 using a solder 
paste and a conventional reflow soldering process. The electronic 
component 18 can be a leadless capacitor, resistor, inductor or other 
leadless or leaded surface mount electrical component. Referring to FIG. 2 
which is a side view of the portion of the electronic assembly 10, during 
the reflow soldering process, a solder fillet 22 is formed which provides 
both an electrical connection and mechanical attachment of the electronic 
component 20 to the printed circuit traces 14. 
As previously described above, the printed circuit pattern can be formed 
using conventional photolithographic processes and additive plating or 
subtractive etching processes. The printed circuit traces 14 are typically 
a one ounce copper plating, which after processing are generally protected 
from oxidation using an organic solder preservative. As also described 
above, the organic solder preservative functions very well to inhibit 
oxidation of the copper printed circuit pattern, thereby insuring reliable 
solder joints are formed using a conventional reflow solder process, and a 
standard solder paste, such as an SN62 solder paste manufactured by Indium 
Corporation of Utica, N.Y., which has a composition of 62% tin, 2% Silver 
and 36% lead. Solder joint integrity to the bare copper runners has proven 
to be very good. The problem with the use of an organic solder 
preservative is that of corrosion of the copper printed circuit pattern 
due to moisture and humidity, which can lead to long term reliability 
problems within the electronic device. 
Secondary finishes have been developed which can effectively prevent 
corrosion of the copper printed circuit pattern. One such secondary finish 
is illustrated in FIG. 3 which is a cross-sectional view of a nickel-gold 
finish. Following the formation of the printed circuit traces 14, the 
printed circuit substrate 12 is immersed into a nickel plating bath. An 
auto-catalytic reaction occurs in the nickel plating bath depositing a 
nickel layer 26 on the bare copper runners 24. The nickel layer 26 also 
includes phosphorus, the relative level of phosphorus being plated out is 
controlled to a certain extent by controlling the pH of the plating batch. 
After nickel plating, a gold layer 28 is formed using an immersion gold 
plating process. In the immersion gold plating process, nickel at the 
surface is replaced by gold, and once all the surface nickel has been 
replaced by gold, the plating process stops. 
The nickel-gold secondary finish provides superior corrosion resistance as 
compared to the organic solder preservative, however, it has been found to 
produce significantly weaker solder joints when a conventional tin-lead 
solder paste is used, and compared to the solder joint obtained using the 
conventional tin-lead solder paste and a bare copper pattern. As described 
above, the joint embrittlement which results has significantly limited the 
acceptance of nickel-gold secondary finished printed circuit boards to 
those electronic device which are not subject to any substantial levels of 
mechanical shock. 
Solder joint embrittlement has been traced to the concentration of 
phosphorus and its distribution within the solder joint. FIG. 4 is a 
diagram illustrating phosphorus concentration in a solder joint using the 
conventional reflow soldering process. Following conventional reflow 
soldering of an electronic component 18 to a nickel-gold secondary 
finished printed circuit substrate 12, the solder joint comprises a 
tin-lead fillet 100 which is bonded to the nickel-gold plating 104 by a 
nickel-tin intermetallic layer located within a reaction zone 102, the 
gold having been dissolved within the solder fillet 100. The formation of 
the nickel-tin intermetallic layer rejects the phosphorus from the 
reaction zone 102. As can been seen in FIG. 4, the phosphorus 
concentration 106 at the interface between the reaction zone 102 and the 
nickel-gold plating 104 is essentially that of the phosphorus 
concentration 110 of phosphorus within the nickel-gold plating 104. As can 
also be seen, the phosphorus concentration 108 immediately outside the 
reaction zone 102 increases rapidly to a level almost twice the phosphorus 
concentration 110. It has been determined that it is the high phosphorus 
concentration 108 which leads to solder joint embrittlement. 
The problem of solder joint embrittlement has been advantageously resolved 
in the electronic assembly in accordance with the present invention by the 
application of a solder paste which includes, by way of example, indium as 
a constituent, such as an indium-tin-lead solder paste which includes 
approximately 15% indium, wherein the concentration of tin and lead is 
adjusted to provide a eutectic solder. When indium is introduced into the 
tin-lead solder paste the phosphorus concentration within the nickel-gold 
layer 104 is controlled as shown in FIG. 5. When the indium-tin-lead 
solder paste is utilized in the electronic assembly in accordance with the 
present invention, the solder joint comprises a tin-lead fillet 100 which 
is bonded to the nickel-gold plating 104 within a reaction zone 102 which 
comprises a tin-nickel-indium intermetallic layer, the gold having been 
dissolved within the solder fillet 100. The formation of the 
tin-nickel-indium intermetallic layer does not reject the phosphorus from 
the reaction zone 102. As can been seen in FIG. 5, the phosphorus 
concentration 112 at the interface between the reaction zone 102 and the 
nickel-gold plating 104 is essentially that of the phosphorus 
concentration 110 of phosphorus within the nickel-gold plating 104. The 
phosphorus concentration 114 rises very little within the nickel-gold 
layer 104. As can also be seen, phosphorus is no longer excluded from the 
reaction zone 102, but rather is taken within the reaction zone 102. The 
phosphorus concentration 112 continues to fall from the phosphorus 
concentration 110 throughout the reaction zone 102, and also continues to 
fall within the tin-lead fillet 100. By spreading the phosphorus 
concentration throughout the reaction zone 102, and by controlling the 
phosphorus concentration 114 immediately outside the reaction zone 102, 
solder joint integrity has been found to approach that of conventional 
tin-lead solder pastes when utilized with a bare copper printed circuit 
substrate 12. A comparison of joint strength integrity is provided in the 
Table I below. 
TABLE I 
______________________________________ 
Pull Strength - pounds 
Sn--Pb 12% In--Sn--Pb 
25% In--Sn--Pb 
Reflow No. 
Solder Solder Solder 
______________________________________ 
1 16.13 21.36 28.70 
3 11.91 16.26 28.64 
______________________________________ 
The pull strength achieved for various combinations of printed circuit 
finish and solder paste with and without indium as a constituent is 
compared in Table I. Three solder pastes were tested, that of a 
conventional Tin-Lead solder alloy with 2% Silver content by weight (2% 
Ag--Sn--Pb); and two Indium-Tin-Lead solder alloys utilized in the 
electronic assembly in accordance with the present invention: a Tin-Lead 
solder alloy with 12% Indium content by weight (12% In--Sn--Pb), and a 
Tin-Lead solder alloy with 25% Indium content by weight (25% In--Sn--Pb). 
Part of the printed circuit boards tested were placed through the reflow 
oven once, and a part of the printed circuit boards were placed through 
the reflow oven a total of three times. As can be seen from Table I, a 
significant degradation in solder joint strength occurs when a 
conventional tin-lead solder paste is used to reflow electronic components 
to a printed circuit pattern having a nickel-gold secondary finish. As 
also can be seen from Table I, the solder joint integrity is improved 
relative to the conventional solder reflow process, and almost twice that 
of a solder joint formed using a conventional tin-lead solder paste with a 
printed circuit pattern having a nickel-gold secondary finish. It should 
also be noted that the solder joint integrity does not degrade with the 
repeated reflow solder step, due at least in part to the improved solder 
joint ductility imparted by the addition of indium, and to the scavenging 
of phosphorus in the vicinity of the reaction zone. 
FIG. 6 is a depth profile illustrating the metal distribution in a 
fractured solder joint using the conventional soldering process. The depth 
profile 600 illustrates the metal distribution within a solder joint which 
has been reflow soldered using a conventional tin-lead solder and a 
nickel-gold secondary finished printed circuit board. The depth profile 
600 is measured using a Scanning Auger Microprobe manufactured by Physical 
Electronics of Eden Prairie, Minnesota from the point of break in the 
solder joint into the nickel-gold plating layer. As illustrated, the break 
occurred within the intermetallic layer in reaction zone 102. Curve 602 
represents the atomic concentration of nickel within the joint, curve 604 
represents the atomic concentration of phosphorus within the joint, and 
curve 606 represents the atomic concentration of tin within the joint. As 
described above, curve 604 shows the high phosphorus concentration which 
leads to joint embrittlement. 
FIG. 7 is a depth profile illustrating the metal distribution in a solder 
joint using an indium-tin-lead solder paste in the electronic assembly in 
accordance with the present invention. As illustrated, the break also 
occurred within the intermetallic layer in reaction zone 102. Curve 602 
represents the atomic concentration of nickel within the joint, curve 604 
represents the atomic concentration of phosphorus within the joint, curve 
606 represents the atomic concentration of tin within the joint, and curve 
608 represents the atomic concentration of indium within the joint. As 
described above, curve 604 does not show the high phosphorus concentration 
which leads to joint embrittlelment, but rather shows that the phosphorus 
is absorbed into the reaction layer at a very low concentration, as 
compared to the conventional tin-lead solder joint. 
FIG. 8 is a depth profile illustrating the metal distribution in an intact 
solder joint using an indium-tin-lead solder paste in the electronic 
assembly in accordance with the present invention. As described above, 
three identifiable regions are shown, the solder fillet 100., the reaction 
zone 102, and the nickel-gold plating 104. Within the reflowed solder 
fillet 100, curve 602 represents the atomic concentration of nickel, and 
curve 604 represents the atomic concentration of phosphorus within the 
joint, and it is noted that only small atomic concentrations of nickel and 
phosphorus enter the solder fillet 100 during the reflow soldering 
process. Curve 606 represents the atomic concentration of tin, and curve 
608 represents the atomic concentration of indium within the solder 
fillet. (the atomic concentration of lead was purposefully omitted from 
FIG. 7). Within the reaction zone 102, the atomic concentration of nickel 
increases significantly, while the atomic concentration of indium 
decreases significantly. The atomic concentration of phosphorus remains 
relatively constant. With the inclusion of nickel, a 
nickel-tin-indium-phosphorus intermetallic is formed within the reaction 
zone 102. 
Within the nickel-gold plating 104, the atomic concentration of phosphorus 
and nickel increases to essentially the concentrations of phosphorus and 
nickel within the nickel-gold plating 104. There is no doubling of the 
phosphorus concentration as observed in a conventional tin-lead solder. 
The atomic concentration of tin decreases rapidly towards zero within the 
nickel-gold plating 104, as does the atomic concentration of indium. 
In conclusion, the introduction of indium into a tin-lead solder paste used 
to reflow components to a nickel-gold finish enables the assimilation of 
phosphorus into the reaction zone, thereby effectively preventing the 
build-up of phosphorus which leads to joint embrittlement as described 
above. It will be appreciated that there are other metals, such as zinc, 
cadmium, and germanium, which would function in a manner similar to 
indium. Zinc, cadmium, and germanium dissolve completely in a tin-lead 
solder system and do not form solid components which would melt at a 
temperature higher than the melting point of eutectic tin-lead solder, as 
is the case with indium. In addition, zinc, cadmium, and germanium 
interact with the phosphorus to form compounds which are assimilated into 
the reaction zone, which would prevent the build-up of phosphorus which 
leads to joint embrittlement as well. 
It has been further demonstrated from Table I above that indium 
concentrations as high as 25% will improve solder joint integrity when an 
electronic component is reflow soldered to a nickel-gold plated printed 
circuit substrate. An indium concentration as low as 5% can also provide 
improved solder joint integrity as compared to a conventional tin-lead 
solder paste when an electronic component is reflow soldered to a 
nickel-gold plated printed circuit substrate. When indium is introduced 
into a solder, such as a tin-lead solder paste, an added advantage is 
obtained, that of reduction of the melting point of the solder paste. A 
conventional tin-lead solder paste melts at 183.degree. C., whereas, as 
shown in FIG. 9, which is a graph showing the melting temperature of a 
solder paste as a function of the percentage of indium incorporated, an 
indium-tin-lead solder paste having a 15% indium concentration melts at 
161.degree. C. It will be appreciated that for any concentration of indium 
added to the tin-lead solder paste, the concentration of tin and lead is 
adjusted to produce a eutectic solder. Not only is the solder joint 
integrity improved significantly when a nickel-gold secondary coated 
printed circuit substrate is used, but also components which cannot 
tolerate the high reflow solder temperatures encountered when using a 
conventional tin-lead solder paste, can now be successfully reflow 
soldered at the lower reflow solder temperature. required. At indium 
concentrations much higher than 25%-30%, the cost of the solder starts to 
become prohibitive except for very special applications, and the ductility 
of the solder becomes too high for many applications. It will be 
appreciated that the use of a solder paste which includes indium as a 
constituent in an electronic assembly in accordance with the present 
invention would provide improved joint integrity when the terminations of 
the electronic components have a secondary nickel-gold finish when used 
with a printed circuit substrate having printed circuit traces which have 
a nickel-gold secondary finish, or when the printed circuit traces 
utilizes only an organic solder preservative. While the electronic 
assembly in accordance with the present invention is described primarily 
in terms of a reflow solder process, it will be appreciated that the 
electronic assembly in accordance with the present invention would also 
provide improved solder joint integrity for leaded components using either 
a wave soldering process or a hand soldering process utilizing an 
indium-tin-lead solder as well. 
FIG. 10 is a bottom view and FIG. 11 is a cross-sectional view 11--11 of a 
ball grid array (BGA) 1000 semiconductor device. The BGA array 1000 
comprises a substrate 1002 upon which solder paste is reflowed onto 
contact pads to form solder bumps 1004. The solder bumps provide 
electrical connection to an integrated circuit die (not shown) which has 
been assembled to the opposite side of the substrate 1002. The integrated 
circuit die is typically covered with a die cap or conformal coating 1006 
which protects the integrated circuit. In accordance with the present 
invention, the solder bumps 1004 of the BGA array 1000 can be processed 
with a secondary nickel-gold finish, and as such would benefit from the 
use of a solder paste which includes indium as a constituent in an 
electronic assembly in accordance with the present invention. 
FIG. 12 is a top view and FIG. 11 is a cross-sectional view 13--13 of a 
flip-chip integrated circuit 1010 semiconductor device. The flip-chip 
integrated circuit 1010 comprises a semiconductor die 1012 having an 
active area 1016 and bonding pads upon which solder paste is reflowed to 
form solder bumps 1014. The solder bumps provide electrical connection to 
the flip-chip integrated circuit 1010. In accordance with the present 
invention, the solder bumps 1014 of the flip-chip integrated circuit 1010 
can be processed with a secondary nickel-gold finish, which when soldered 
to a printed circuit substrate would benefit from the use of a solder 
paste which includes indium as a constituent in an electronic assembly in 
accordance with the present invention. Flip-chip integrated circuits are 
often reflow soldered to a flexible substrate, such as a polyimide film, 
which has been processed with a secondary nickel-gold finish The solder 
joint integrity would benefit from the use of a solder paste which 
includes indium as a constituent in an electronic assembly in accordance 
with the present invention 
The electronic assembly in accordance with the present invention utilizes a 
solder paste which includes indium as a constituent to solder electronic 
components and semiconductor devices to nickel-gold plated printed circuit 
substrates. Electronic assemblies which utilize nickel-gold plating 
include such electronic devices as a pager, cellular telephone, cordless 
telephone, or personal digital assistants (PDAs), and have improved solder 
joint integrity when a solder paste which includes indium as a constituent 
is used as compared to that derived with the use of standard tin-lead 
solders. The solder paste which includes indium as a constituent also 
allows lower reflow solder temperatures, thereby enabling a greater number 
of electronic components to be reflowed as compared to a conventional 
reflow solder process. The electronic assembly in accordance with the 
present invention can utilize printed circuit substrates which have a 
secondary nickel-gold finish applied or electronic components which have a 
secondary nickel-gold finish applied. The electronic assembly in 
accordance with the present invention can also utilize semiconductor 
devices which have a secondary nickel-gold finish applied. When utilized 
with semiconductor devices, the semiconductor devices can be packaged, 
such as BGA arrays or SLIC (single in-line integrated circuits), DIP (dual 
in-line integrated circuits) or other packaged integrated circuits, or 
unpackaged, such as flip-chip integrated circuits.