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Contributed by Paul Stransky, CEF, Paul Stransky Associates
23 Aug 1995
Abstract
Eleven papers have been reviewed covering the time period 1986-
1991. They represent a mix of highly theoretical to those of a
more commercial nature. In general they deal with development of
models, application of these models to formulate appropriate
plating solutions, and explanations of solution components and
thier functions.
The purpose of this paper is to try to put the information in a
more practical context for platers, who do not have to formulate
plating solutions, but have to use them. To help develop an
understanding of how copper plating solutions developed for high
aspect ratio through holes (HARTH) differ from other plating
solutions and what the limitations are if used for multiple
plating tasks. For the researcher it summarizes many, but not all
of the papers on the subject. However, the references cited in
these papers represent significant works on the subject.
REVIEWS
Carano (1) discusses increasing need to plate HARTHs, the effects
of throwing power (primary and secondary current distribution),
the role of addition agents, solution agitation, other factors
such as anode / cathode placement, and ratio,as well as good
plating techniques. He concludes process windows will be
narrower, and that there has to be a synergism between
components of the plating system
(chemical,mechanical,electrical).
Amadi (2) discusses primarily plating solution chemistry
aspects of HARTH plating. He examines the effects of sulfuric
acid, copper concentration, addition agents, and current density
on the ratio of surface to hole wall plating thickness. He finds
low Copper, high acid, with sulfuric acid:Cu ratio of greater
than 20:1, and low current density necessary for maximum
uniformity.
Glantz (3) and Hazlow (4) discuss forced flooding and
ultrasonics as techniques to insure adequate solution exchange in
small holes. This work (3) is primarily about desmear. He uses a
model of laminar flow and kinematic viscosity.
Fisher, Sonnenberg and Bernards (5) write about fluid dynamics
and electrochemistry. Fundemental aspects of fluid dynamics are
discussed using a capillary tube as a model taking contact angle
between process solution and substrate into consideration as well
as surface tension. This is followed by a model for looking at
mass transfer within a hole during plating, and determining the
number of hole volume turnovers needed for a 0.1% depletion rate
of copper in the hole not to be exceeded.
Electrochemical factors such as voltage drop within a hole are
discussed and a model is developed taking Ohms law and hole
geometry into consideration. Experimental work done using a test
cell to develop a copper plating solution for HARTHs is
described. Results included optimization of metal, sulfuric acid,
and organic addition agents. Experimental results were then
confirmed with actual drilled boards. In addition general
guidelines for equipment, chemistry, and control are also given.
Some of the conclusions are:
1. Wetting is not an issue by the time you get to electroplating,
because of all the previous preparational steps.
2. Pressure requirements for forced solution flow through holes
are minimal and easily met by conventional board agitation.
3. Ohmic potenial drop is a very important factor in obtaining
uniform plating thickness throughout length of hole. It is the
limiting factor determining plating rate.
4. Sulfuric acid/Cu ratio more important than just conductivity
re; throwing power (center thickness/surface thickness).
Throwing power increases with increasing acid/metal ratio
(decreasing metal content).
5. Organic additives (carriers, brighteners, levelers) work to
increase the current density or plating rate that can be
maintained whith satisfactory throwing power.
D'Ambrisi etal (6) approach the subject of small diameter holes
(HAR) from the prospect of the entire process. From drilling
to electroless plating in this paper. The potential for excessive
heat generation and subsequent smear during drilling is
discussed.
The importance of adequate desmear, and resulting wettable wall
surface is addressed. Different desmear methods (Sulfuric acid,
chromic acid, plasma, alkaline permanganate) are covered, with
alkaine permanganate recommended.
Several steps of the electroless plating process are explained
(conditioning, acceleration, plating). Solution flow with respect
to providing appropriate mass transfer of plating solution
components (Cu, NaOH, HCHO) is discussed. The model presented is
based on laminar flow through a circular cross section and Fick's
First Law of Diffusion. The authors conclude that alkaline
desmear renders hole walls hydrophillic (wettable), increases
surface area by 6x improving adhesion, and aiding small H2 gas
bubble nucleation during electroless plating.
D'Ambrisi etal (7), in a second paper follow up with a
discussion of electrolytic and full build electroless copper
plating. Problems with ohmic resistance being the limiting factor
in uniform HARTH plating are discussed. Full Build electroless
copper is suggested as a way around the problem.conclusions
include:
1. Mass transport not problem, methods of solution agitation can
rectify this.
2. Ohmic resistance is the main problem in achieving uniform
plating thickness, with hole length being more important than
diameter.
3. Aspect ratios greater than 10:1 can not be plated uniformly.
Yung and Romanki (8) used a gap cell which represented a
modified version of a Hull cell equipped for solution flow, and
plexiglass drilled boards to study the acid copper PTH process.
They were concerned with both plating on the surface and hole
wall. Plating solutions with and without additives were used.
Mathematical models are developed for electrode kinetics, and
mass transfer based on electrochemical engineering principles.
Experimental results are correlated with mathematical models.
Three distinct plating regions and mechanisms are described, at
extremely low current densities (region I) charge transfer
dominates and additives can improve current distribution. At
somewhat higher current densities (region II) mixed control
prevails, both charge transfer and ohmic resistance, and
additives are still effective. In region III at higher current
densities ohmic resistance dominates and additives are no longer
effective. The authors conclude:
1. Ohmic resistance rather than mass transport is the controlling
factor in plating HARTHs at normal current densities.
2. Even with suitable agitation and flow to achieve high mass
transfer, high current densities will still give non uniform
HARTH wall thicknesses, the model will predict this.
3. Agitation must be balanced on surface and in hole.
4. When ohmic resistance dominates process, its difficult to
plate HARTHs above 10:1 without lowering current density to
20-40 mA/cm2 (19-38 ASF).
5. Possible schemes to improve copper distribution inside the
holes include increasing bath temperature and conductivity,
as well as reducing both hole diameter and length.
Hazelbeck (9) develops a nondimensional mathematical model
which includes effects of convective, diffusive, and ohmic
transport in plating through holes with additives to the bath.
The model which incorporates Laminar flow is used to examine
effects of additives and other process variables on deposit
uniformity. There are three important dimentionless parameters.
The Thiele modulus is a measure of the ratio of reaction rate
to diffusion rate. The inverse dimensionless conductivity
measures the ratio of diffusion rate to electrical migration
rate. The Peclet number provides a measure of the ratio of
convection rate to diffusion rate. Amongst his conclusions are:
1. Uniformity of plating correlated to the three dimensionless
parameters.
2. Deposition uniformity can be improved by increasing
conductivity when electrodeposition is ohmically controlled.
3. Forced convection in HARTHs is important in improving
deposition rate while maintaining uniformity.
4. Additives that decrease cathodic charge transfer coefficient
of electrodeposition significantly improve HARTH uniformity.
Hazelbeck, and Talbot (10) review development of thier two
dimensional model and other models. The two dimensional model is
used to examine the effects of plating variables on through hole
thickness uniformity in an acid copper plating bath with and
without additives. The general model is used to determine
conditions needed to achieve ohmic limited plating necessary for
HARTH uniformity. Discussion of various models of plating under
limiting conditions such as convective-diffusion, coupled
convective-diffusion / ohmic resistance and ohmic resistance are
presented. Conclusions include:
1. Uniformity of plating correlated to three dimensionless
parameters as described in earlier work above (9).
2. Increased solution conductivity and forced convection in the
hole are important in improving deposition rate while
maintaining uniformity.
3. A smaller through hole can be plated easier than a larger one
of the same aspect ratio.
4. Plating additives that reduce charge transfer can
significantly improve HARTH plating uniformity, and with flow
enhancement reasonable plating rates of 10-40 mA/cm2 (9-38ASF)
can be used. Likewise higher plating rates can be achieved for
low aspect ratio holes .
5. When using unidirectional flow solution conductivity must be
increased or charge transfer reduced by additives.
6. At very high Peclet numbers plating rate is ohmic limited with
uniform plating regardless of flow direction.
7. Uniformity of deposition can be improved by increased solution
conductivity, use of additives to reduce charge transfer,
reducing hole diameter and length while maintaining the same
aspect ratio, and lowering current density.
Hazelbeck, and Talbot (11) a model is developed for PTH when
two counter electrodes and solution flow reversal is used, and
another model for unidirectional flow with only a downstream
counter electrode. This work follows from the authors earlier
work in(9) and (10) above with the following conclusions.
1. The downstream process allows for higher current densities
than with the other configurations while still maintaining
plating uniformity.The reason being low metal ion
concentration and low ohmic losses at the down stream end of
hole with high metal ion concentration and large ohmic losses
at the upstream end .
2. Drawback to the downstream process is that it would require
holes to be plated separately from surface lines, however it
is promising for applications other than PTHs.
DISCUSSION
Some common themes come through from the various investigators:
1. Ohmic resistance within the HARTH is the major limiting factor
in obtaining uniform plating.
2. Ohmic resistance is minimized by increasing plating solution
conductivity which is accomplished by increasing the ratio of
sulfuric acid to copper metal and / or increasing temperature.
3. The use of additives in plating solutions that minimize charge
transfer also is very important to hole wall uniformity and
the ratio of surface to wall thickness.
4. Plating solution flow is important, with the usuall air
sparging and panel movement being sufficient to avoid mass
transport problems.
Well what does this mean to the plater or plating engineer
actually doing the work? We usually do not formulate plating
solutions leaving that to our suppliers and at this point in time
there are many commercial plating solutions available for HARTH.
It may be helpful to understand how commercial plating solutions
available for various plating tasks differ from each other.
Table 1 contains a listing of data taken from product
literature for 4 different commercial plating solutions
(12,13,14,15) designed for different plating tasks. Obviously
there is no reference to specific additives since they are
proprietary, but for this exercise it is assumed they are
appropriate for the task.
The first represents a "standard" copper plating bath for non
electronic use (16). Baths 2 and 3 represent the "high throw"
acid copper used for many years for PTHs. Bath 4 is representative
of baths developed for HARTH plating.
From the table its easy to see the changes that have evolved to
increase conductivity, and therefore throwing power (again it is
assumed that there have been appropriate additive changes). Copper
sulfate, hence copper metal, has been reduced while sulfuric acid
has been increased. The ratio of sulfuric acid to copper metal is
1:1 for the "standard", 10:1 for the "high throw", and 38:1 for the
"HARTH" baths.
From a practical standpoint there is only one potential problem
in attempting to do multiple plating tasks form a single plating
bath. This is probably not true in a large facility where there
are dedicated lines. However in a smaller facility there may be.
If there are multiple task plating needs amongst which is HARTH,
then obviously a more recent HARTH type solution is needed. The
problem can arise if a HARTH type bath is also being used for low
aspect ratio holes or single sided boards and an attempt is made
to use higher than recommended current densities to increase
production throughput, this can not be done without burning. From
the table again it can be seen that the top end of recommended
current density of the HARTH type bath is lower than the others.
The reason for this is with low metal content the plating
mechanism becomes mass transport limited at higher current
densities, and simply put not enough copper ions can be supplied
quickly enough. However in situations like this it might be
worthwhile discussing some possible steps to increase mass
transport with your particular plating chemistry supplier. These
might include increasing solution agitation and temperature.
Increasing copper metal temporarily then dummying it down when
HARTH boards have to be run.
The other side of the coin is that a HARTH type plating
solution may be a good choice for non electronic applications
with complex geometries where throwing power is an issue.
References:
1. M. Carano,P.C.Fab,12,34,(1986)
2. S.I. Amadi,P.C.Fab,10,85,(1987)
3. E.J.Glantz,P.C.Fab,2,60,(1989)
4. R.E. Hazlow,P.C.Fab,2,78,(1989)
5. Fisher,Sonnenberg and Bernards,P.C.Fab,4,39,(1989)
6. D'Ambrisi etal,P.C.Fab,4,78,(1989)
7. D'Ambrisi etal,P.C.Fab,8,30,(1989)
8. E.K.Yung,L.T.Romankiw,J.Electrochem.Soc.,3,756,(1989)
9. D.A.Hazelbeck,J.Electrochem.Soc.,4,215C,(1989)
10. D.A.Hazelbeck,J.B.Talbot,J.Electrochem.Soc.,7,1985,(1991)
11. D.A.Hazelbeck,J.B.Talbot,J.Electrochem.Soc.,7,1998,(1991)
12. Technic TECHNI COPPER-U product data sheet
13. LeaRonal Copper Gleam PCM technical bulletin 30013
14. SelRex CUBATH M product data sheet
15. Shipley Electroposit 1000 product data sheet
16. A.Sato,R.Barauskas,Metal Finishing Guidebook,Hackensack
N.J.,1989;p216
Table 1: Comparison Of Properties From Acid Copper Plating Baths
Property Bath 1 Bath 2 Bath 3 Bath4
CuSO4 (g/L) 255 75 68 25
Cu (g/L) 57 19 17 6
H2SO4 (g/l) 60 188 173 225
H2SO4:Cu 1:1 10:1 10:1 38:1
Current Density 30-60 1-80 20-40 10-20
Range (A/ft2)
Notes:
1. Techni Copper-U
2. Copper Gleam PCM
3. CUBATH M
4. Electroposit 1000
