substrates are becoming more popular for a variety of applications. Photo
courtesy of TDK Corp., Phoenix,
Figure 1. LTCC manufacturing methods allow for the formation
of blind vias, buried components and thru-hole vias.
Ceramic substrates have been used for many
years in the production of electronic circuits. Ceramic substrates generally
exhibit superior thermal performance when compared to organic substrates for
uses in electronic packages, although many organic packages are also used due
to cost and circuit design considerations.
Ceramic materials used in electronic manufacturing include aluminum oxide,
silicon nitride and aluminum nitride. Low-temperature co-fired ceramic (LTCC)
substrates have become especially popular in the last 10 years. LTCC substrates
are a mixture of alumina and glass, and are typically manufactured into
multilayer substrates co-fired with low-resistance metals like copper or silver
at firing temperatures of less than 1000ºC.
that is included with the ceramic lowers both the fusing point of the matrix
and also the dielectric constant, thus enhancing performance (especially at
A cross-sectional view of a LTCC
circuit is shown in Figure 1. LTCC manufacturing methods allow for the formation of blind vias, buried
components and thru-hole vias.
Final finishes may be applied to the circuit
pads for preservation of solderability or to serve as wire bonding pads. In
many instances, the final finish is used for both purposes. In this case, care
must be taken to ensure that the final finish can form a reliable solder joint
while also allowing acceptable wire bonds. Optimization of the final gold
plating thickness is crucial for optimum performance in the case of nickel-gold
In the process of forming the substrate circuitization, it is common that
electrically isolated traces are created, thereby eliminating the possibility
of electrolytic plating of final finishes unless some sort of bussing is
incorporated that is subsequently partially removed after plating.
Electroless deposition differs from
electrolytic deposition in that no external supply of electrons (such as from a
rectifier) are required. Electrolytic deposition requires the use of an
externally connected DC rectifier, which supplies electrons for metal reduction
at the cathodic surface. Electroless deposition utilizes a chemical reducing
agent that supplies electrons for metal deposition at a catalytic surface.
Examples of chemical reducing agents are formaldehyde for electroless copper
deposition and sodium hypophosphite for electroless nickel deposition.
Two common electroless final finishes for ceramic packages are
nickel-gold or nickel-palladium-gold. In certain high-frequency applications,
the nickel layer may be avoided. The initial metal layer on the ceramic
substrate and the glass materials blended with the aluminum oxide determine the
required pre-treatment prior to electroless nickel and gold (ENIG) or
electroless nickel- electroless palladium-gold (ENEPIG) deposition for LTCC
substrates. Materials in the glass portion of the LTCC substrate can sometimes
affect the metal plating process and must be cleaned from the surface prior to
Materials such as zinc, bismuth and lead, which are used to lower the fusing
temperature, may affect the plating processes either by causing background
plating (plating onto the ceramic substrate) or non-uniform nickel deposition
onto silver metallization. In some cases, these metals are partially removed
from the surface layer prior to plating. The cleaning process typically
involves the use of a glass etch and cyanide dipping.
2. LTCC plating process for a Ni-Au plate (Ag metallized).
As mentioned previously, electroless nickel is
not catalytic to the starting metal layer and, as a result, a catalyst is
normally deposited onto the starting metal layer to facilitate electroless
nickel deposition. Once the electroless nickel solution begins to deposit, the
nickel metal builds upon itself at a controlled rate. An example of
pre-treatment for a silver metallized LTCC substrate is shown in Figure 2. The
ceramic substrate and metallization layer determines the course of
In the case of silver metallization, a common conductor used with LTCC substrates, a
neutral pH palladium catalyst solution is used since it is critical to ensure
that no etching or lifting of the silver metallization occurs. Copper
metallized substrates usually require an acidic palladium catalyst prior to
electroless nickel deposition.
Once the metallization has been properly catalyzed, the substrates are immersed
into the electroless nickel plating solution. Electroless nickel solutions
typically operate at solution temperatures in the range of 77-90ºC and operate
at mildly acidic solution pHs. Nickel deposit thicknesses typically range from
100-300 microinches, depending on the circuit requirements. During the nickel
deposition, a portion of the reducing agent co-deposits with the nickel to form
a crystalline or semi-amorphous compound, depending on the reducing agent of
choice. The reducing agent typically is deposited in the 3-10% by weight range,
with phosphorus being a typically co-deposited material.
Precious Metal Finishes
Precious metals are deposited onto the nickel
surface to prevent the nickel from oxidizing (thus preserving a solderable
surface) and optionally to serve as a gold wire bonding surface. Either gold or
palladium plus gold are used as the final finish top coating. The nickel-gold
or nickel-palladium-gold layers exhibit excellent heat resistance for multiple
thermal cycles at assembly, especially for lead-free solders. For finishes
using gold only as the final layer (ENIG), an immersion gold layer is deposited
onto the electroless nickel.
deposit is simply a galvanic displacement reaction whereby nickel is dissolved
from the surface of the deposit and gold replaces nickel at the surface. The
nickel dissolution provides the electrons needed for gold deposition at the
surface. Two atoms of gold are normally deposited for each atom of nickel
dissolved at the surface according to the reaction:
This immersion gold reaction is normally self-limiting to approximately
0.1µm total thickness. As the available sites on the nickel surface are coated
with gold, the immersion reaction slows; typically, the rate of deposition
decreases to a very low rate after approximately 15 minutes. Following
immersion gold plating, an optional electroless gold process may be used to
increase gold thickness for wire bonding applications.
Two basic electroless gold processes are prevalent in the industry today.
The first type of electroless process deposits gold from an alkaline solution
using gold cyanide as the gold complex and either dimethyl amine borane or
sodium borohydride as the reducing agents. The second type of electroless
process deposits gold from a neutral pH solution using a gold-sulfite complex
and an organic reducing agent. Both types of electroless gold can provide a
thick, uniform, non-porous gold layer for wire bonding applications.
Another option for precious metal finishes involves the use of
electroless palladium and a flash of gold over the electroless nickel deposit
(ENEPIG). Thicknesses of the palladium are generally in the 0.05-0.1 µm range,
followed by a thin coating of gold in the 0.025-0.4 µm range.
The palladium serves to minimize possible nickel diffusion and also minimize
intermetallic growth in Pb-free assembly. ENEPIG finishes are gaining interest
due to the decreased thickness of gold required compared to traditional
electroless nickel-electroless gold (ENAG) finishes.
Figure 3. Wire bond test results after baking at 175ºC
for 16 hours (average readings).
studies have been performed comparing the functional performance of ENEPIG and
ENIG/ENAG deposits, especially with Pb-free solders. Figure 3 shows the comparative gold wire bonding pull
strengths for ENAG ENEPIG deposits after heat-conditioning the samples at 175ºC for 16
hours. It can be seen that excellent wire bond pull strengths were found
regardless of gold or palladium thickness. Thinner gold deposits with ENAG
finishes did exhibit slightly lower wire bond pull strengths due to some
diffusion of nickel to the surface during heat aging.
Figure 4. Solder ball pull test results for ENEPIG Sn/Pb
solder vs. Pb-free solder.
In addition, solder joint reliability tests
were performed on ENEPIG deposits and compared to ENIG deposits using Pb-free
SAC305 solder and eutectic Sn/Pb solder. Results indicated that ENEPIG deposits
coupled with eutectic Sn/Pb solder resulted in poorer solder joint reliability
after thermal aging compared to ENIG deposits. ENEPIG coupled with SAC305
solder yielded excellent solder joint reliability throughout the range of
thermal aging, and actually surpassed the reliability of ENIG deposits (see
LTCC substrates are becoming more popular for a variety of applications.
Electroless metal deposition processes have been developed for LTCC
metallization that yield excellent wire bond strengths and solder joint
For more information regarding electroless metal deposition, contact
Uyemura International Corp., 3990 Concours, #425, Ontario, CA 91764; (909)
466-5635; fax (909) 466-5177; e-mail firstname.lastname@example.org; or visit www.uyemura.com.