Author to whom correspondence should be addressed

Authors:
Dr. Jürgen Barthelmes (Author to whom correspondence should be addressed)
Christian Wunderlich
Patrick Lam
A New Generation of Leadframe (LF) Roughening for MSL Improvement
1. Introduction
With the development of increasingly thinner and at the same time environmentally
friendly integrated circuits (ICs) the manufacturing of thermally reliable products
becomes more and more difficult. In order to achieve an equivalent reliability
standard for state-of-the-art ICs like QFNs, expressed in terms of achievable
moisture sensitivity level (MSL), new technologies have to be introduced.
A novel process has recently been added to the manufacturing sequence of
leadframes (LF), the target of which being the enhancement of the adhesion between
molding compound and the alloy substrate. The leadframe is thereby being treated
by an additive containing peroxide based micro-etch. An organo-metallic layer based
on Cu(II) is formed on top of the roughened alloy surface. During the subsequent
molding process mechanical interlocking between resin and alloy substrate is
established. Furthermore a chemical interaction increasing the thermal stability of the
IC at elevated temperatures can be observed.
2. Current Issues
As the production experience with this new type of process grows, difficulties which
had not been foreseen came to light. The employed etchants work very well on pure
Cu substrates, however when operated on Cu based alloys several issues can be
observed.
With C194, an alloy containing around 2-3 % of Iron, it is observed that after a certain
lifetime the hydrogen peroxide degrades rapidly and the process can not be
controlled any further.
With C7025, an alloy containing less than 1% of Silicon, a Si smut remains after
processing which may contaminate the Ag surface, cause wire bonding failure and
reduce the adhesion. It is imperative to remove the smut in a post treatment
operation. Current postdip technology to clean these C7025 surfaces will reduce the
surface roughness and the adhesion improvement effected by the process.
3. Electrolyte Stability with Iron Contamination
As described above it is observed that the electrolyte, an intergranular etchant,
decomposes rapidly after a certain amount of C194 is dissolved. The maximum
allowable concentration of Cu(II) in operation, based on C194, is around 5-6 g/l.
This corresponds to an Iron concentration of 120-150 ppm.
The phenomenon observed is well known in the literature, a mixture of hydrogen
peroxide and Fe 2+/3+ is called the Fenton’s reagent. Hydroxyl radicals are being
formed according to

Fe 2+ + H2O2 ----> Fe 3+ + OH - + .OH
Fe 3+ + H2O2 ----> Fe 2+ + .OOH + H+
(I)
(II)
These species are highly reactive and will rapidly attack and oxidize all organic
matter present. As the function of the etchant is based on the organic additives
included, the result of these reactions (I and II) will be detrimental for the roughening
operation.
One option to increase the lifetime could be adding a complexing agent to the
electrolyte which reduces the concentration of free Iron ions. The effect of such a
strong complexer on the stability of hydrogen peroxide with time is compared in
Figure 1 to a solution without stabilizer. As one can easily see the solution is very
stable even at high Iron concentrations.
4
3
Decomposition Rate of Peroxide
in MoldPrep LF vs. MoldPrep LF_C
No complexing agent
stoichiometrical value +10%
1/2 of stoichiometrical value
stoichiometrical value x 2
C
o
n
c 2
1
Dissolved are 1g/l Fe and 5 g/l Cu
0
0 2, 5 7, 1 12, 1 17, 2 22, 2
Effective time at 42°C
Figure 1: Decomposition of MoldPrep LF vs. MoldPrep LFC at an Iron concentration
of 1000 ppm
4. Smut Generation on C7025 Leadframes and its Removal
The smut generated on C7025 surfaces is currently removed by an acidic postdip,
which also reduces the roughness generated in the etching process. A combination
of the reengineered, stability enhanced etchant with a new alkaline postdip will grant
a smut free surface without loss of roughness.
As the appropriate roughness parameter a new concept was used which is based on
the quantification of the etched surface area as compared to an ideally flat one. The
so called relative surface area increase (RSAI) is thus most meaningful, when
adhesion properties of a certain material have to be evaluated.

Figure 2 shows the RSAI values of C194 and C7025 at different metal loading. It can
be observed that the RSAI difference between C194 and C7025 is quite small and
relates only to the different intrinsic behaviour of the alloys itself. No further attack
and RSAI reduction occurs in the new alkaline postdip.
80
60
RSAI [%]
40
20
0
New make up
R-values on
C194
5 g/l Cu R-
values on
C194
10 g/l Cu R-
values on
C194
New make up
R-values on
C7025
5 g/l Cu R-
values on
C7025
10 g/l Cu R-
values on
C7025
Figure 2: RSAI for MoldPrep LFC with improved alkaline Postdip for C194 and C7025
at different Cu(II) bath loading
As proof for the efficient removal of the Si smut from C7025 surfaces after the
alkaline postdip, Figure 3 shows the comparison between the new MoldPrep LFC
and the “conventional” MoldPrep LF process.
It can clearly be seen that no Silicon remains on the Cu alloy surface. A similar
investigation was carried out on the Ag spot or ring plated area of the leadframe.
While with the normal process sequence a Si EDX signal can be detected, no trace
of the smut can be witnessed with the new LFC sequence. This is depicted in Figure
4.
5. Button Shear Test
An adhesion comparison between the conventional MoldPrep LF and the newly
developed MoldPrep LFC process was undertaken using the button shear test
method. Table 1 compares the results obtained for different base alloys including
C194, C7025 and EFTEC 64T. As can be seen the results are generally better for the
new LFC process. All values cted were obtained for the alkaline postdip, except for
the blue value (C7025), which was measured after acidic postdip treatment, which
was the only possible option to clean C7025 surfaces until the redevelopment
reported in this paper.

100,00
2,5 0,0 3,5 0,0
% conc.
80,00
60,00
40,00
20,00
Si (k)
Cu (l)
O (k)
C (k)
0,00
LF, 0 g/l Cu LFC, 0 g/l Cu LF, 10 g/l Cu LFC, 10 g/l Cu
Parameter: 5 kV, 1000x 100sec.
Figure 3: EDX intensities expressed in concentrations for C7025 leadframes at
different Cu bath loading – MoldPrep LF versus MoldPrep LFC
MoldPrep LFC
Si Signal
MoldPrep LF
Figure 4: EDX intensities of Ag spot surfaces plated on C7025 leadframes after
treatment with different adhesion enhancement processes – MoldPrep LF versus
MoldPrep LFC

MoldPrep LFC With alkaline postdip Without postdip
C194 18,5 13,7
C7025 15,7 9,2
EFTEC 64T 16,6 11,5
MoldPrep LF
C194 17,1 13,5
C7025 16,4/9,1 10,3
EFTEC 64T 15,1 11,8
Table 1: Button shear test comparison between MoldPrep LF and MoldPrep LFC for
different base alloys; blue value for acidic postdip
5. Summary
With the new MoldPrep LFC process, a combination of a reengineered intergranular
etching solution with an innovative alkaline postdip, the two main issues with current
adhesion enhancement processes were solved.
The etchant solution is stable with respect to Iron contamination from C194 and the
Si smut from C7025 surfaces will be completely removed after the alkaline postdip,
from both Cu and Ag surfaces of the leadframe.