Process for preparation printed circuit through-holes for metallization

Disclosed herein is a process for preparing the surfaces of through-holes in printed circuit boards (especially multilayer printed circuit boards) for subsequent metallization, in which the through-hole surfaces are treated with a substantially water-immiscible organic liquid which acts upon the insulating substrate of the board, and preferably in the form of a substantially homogeneous, clear mixture comprised of the water-immiscible organic liquid, water, an alkali metal compound and a surfactant component, followed by treatment of the through-hole surfaces with an alkaline permanganate solution. The process can be used as a desmearing and/or etch-back process, a combined desmearing-conditioning process, or a conditioning process following a separate desmearing process.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of printed circuit boards and, 
more particularly, to a process for treating the surfaces of 
interconnecting through-holes in printed circuit boards to prepare the 
surfaces for subsequent metallization. 
It is now commonplace in the art to provide printed circuit boards having 
more than one plane or layer of circuitry, wherein the layers of circuitry 
are separated one from the other by an insulating substrate material, 
i.e., a dielectric. In the simplest version of such printed circuit 
boards, an insulating substrate material is provided on both sides with 
conductive circuit patterns. In other versions, which have become 
enormously popular over the past several years, a so-called multilayer 
circuit board is provided. These multilayer boards are comprised of one or 
more innerlayers of conductive circuitry in addition to layers of 
circuitry on the outer-facing surfaces of the board, with insulating 
substrate material separating each layer or innerlayer of circuitry. 
In order to provide conductive interconnection between or among the 
circuitry layers and/or innerlayers in printed circuit boards, one or more 
through-holes are formed in the board (i.e., normal to the plane of the 
board) and the wall surfaces of the through-holes are then metallized. 
Since the metallized through-holes provide the means for conductive 
interconnection between or among circuitry layers and/or innerlayers, the 
quality of the metal layer provided on the through-hole surfaces is 
extremely important. In particular, it is necessary that the metal deposit 
adhere tightly to the through-hole surfaces and that the metal deposit be 
in the form of an essentially continuous, void-free, layer. While to a 
degree these quality parameters are dependent upon the nature of the metal 
depositing solution employed and the conditions under which metallization 
is carried out, to a more significant degree these parameters are 
dependent upon the receptivity of the through-hole surfaces to acceptance 
of the metal layer. 
Generally speaking, through-hole metallization is accomplished through use 
of electroless metal depositing solutions which require that the surfaces 
to be coated with the metal be catalyzed to promote the electroless 
deposition. Numerous techniques and proposals exist in the art for 
treating or conditioning through-hole surfaces so as to enhance their 
receptivity to the deposit of catalytic species and to the 
subsequently-applied electroless metal deposit so as to arrive at an 
adherent, void-free metal coating effective to provide the desired 
conductive interconnection between or among circuitry layers on or in the 
board. 
Multilayer printed circuit boards pose unique problems in through-hole 
metallization. In the formation of through-holes (e.g., by drilling) in 
these boards, the edge portions of metal innerlayers which are exposed at 
the hole surfaces can become "smeared" with the dielectric material. Such 
smeared innerlayer surfaces at the through-hole exhibit poor receptivity 
to catalyst and deposited metal and lead to lack of adherence and/or 
incomplete coverage with respect to the metal deposit. Accordingly, it is 
known in the art to "desmear" through-hole surfaces to remove resinous 
smear from circuitry innerlayers at the through-hole surfaces, such as by 
vapor honing, secondary drilling, plasma etching or, more commonly, by the 
action of chemicals such as sulfuric acid, chromic acid or alkaline (e.g., 
sodium, potassium) permanganate. In the course of desmearing processes, 
particularly when chemical means are employed, the process also is known 
to "etch back" the dielectric material at the hole surfaces, thereby 
exposing more of the metal circuitry innerlayer surface at the through 
hole surfaces and aiding in subsequent adhesion of catalyst and deposited 
metal. 
The utilization of permanganate to desmear and/or etch-back and/or clean 
through-holes so as to condition the through-hole surfaces for subsequent 
metallization has received considerable attention in the art. British 
Patent Specification No. 1,479,556 describes a process for removing resin 
smear from printed circuit board through-holes and from holes made in wire 
assemblies by contacting the hole surfaces with an alkaline permanganate 
solution having a pH of 13-14 at a temperature between 35.degree. and 
70.degree. C., preferably also containing a wetting agent of the 
fluorocarbon variety. 
U.S. Pat. No. 4,425,380 to Nuzzi, et al. also relates to the desmearing of 
through-holes in printed circuit boards and in wire assemblies, employing 
a process involving contact of the hole surfaces with an alkaline 
permaganate solution, followed by contact with compounds effective to 
remove manganese residues from the surfaces so as to avoid interference of 
these species with the subsequent electroless depositing step. 
In permanganate-based desmear/etch-back processes for through-holes formed 
in circuit boards wherein the insulating substrate material is epoxy 
(e.g., glass-reinforced epoxy resin), the art also has taught that the 
efficiency or effectiveness of the permanganate treatment generally can be 
enhanced by first treating the through-hole surfaces with a solvent for 
the epoxy to open up or soften or swell the epoxy resin structure. See, 
e.g., Kukanskis, "Improved Smear Removal Process For Multilayer Circuit 
Boards", IPC Technical Paper No. 435 (Oct., 1982); F. Tomaivolo, et al., 
"Alkaline Permanganate Treatment In Etch-Back Processes," Trans IMF, 1986, 
64, 80; U.S. Pat. No. 4,515,829 to Deckert, et al.; U.S. Pat. No. 
4,592,852 to Courduvelis, et al.; and published PCT patent application No. 
WO 85/05755 to DelGobbo, et al. In this same vein, there is described in 
the art a method for post-desmear treatment of through-holes to better 
prepare them for subsequent metallization, in which the through-hole, 
after being desmeared, is treated with a solvent for the substrate resin, 
followed by treatment with an alkaline permanganate solution. See, e.g., 
U.S. Pat. No. 4,597,988 to Kukanskis, et al. 
The epoxy solvents described in pre-permanganate treatment processes of the 
foregoing references are of the water-soluble type, a choice dictated by 
the desirability of using these solvents in aqueous solution and by the 
desirability of facilitating rinsing by insuring that the solvents will be 
miscible with rinsing water. It has been found in accordance with the 
present invention that, notwithstanding the desirability of employing 
water-soluble solvents, the effectiveness of alkaline permanganate 
treatment in cleaning and/or desmearing and/or etch back and/or 
post-desmear conditioning processes for through-holes, particularly those 
in boards where the dielectric is epoxy-based or contains epoxy, is 
generally enhanced through contact of the through-hole surfaces, prior to 
the permanganate treatment, with water-immiscible organic liquids which 
serve to penetrate and/or soften and/or swell and/or otherwise act upon 
the dielectric material. 
According to another aspect of the invention, the water-immiscible organic 
liquid is used in association with water and, optionally, other 
water-soluble compounds, in order to enhance removal of the 
water-immiscible organic liquid by subsequent water rinsing and, 
importantly, to provide a pre-permanganate treatment which will be 
optimally effective irrespective of the composition of the dielectric, for 
example, whether it be epoxy-based or a polyimide or other thermosetting 
or thermoplastic material, or the like. 
SUMMARY OF THE INVENTION 
In its broadest terms, the invention comprises a process for treating 
(e.g., cleaning and/or desmearing and/or etching back and/or conditioning 
after a previous desmear or etch-back process) the through-hole surfaces 
in printed circuit boards, particularly multi-layer circuit boards, to 
prepare them for subsequent metallization, the process steps comprising a 
first contact of the through-hole surfaces with a water-immiscible organic 
liquid which acts upon the insulating substrate material of which the 
circuit board is composed, followed by contact of the surfaces with an 
aqueous alkaline solution containing permanganate ions until the 
particular effect (e.g., desmearing or etch-back or conditioning) is 
brought about. 
Following the treatment, and optionally other treatments, the through-hole 
is metallized, typically by an electroless metal deposition process in 
which the surfaces are first catalyzed or activated, to provide an 
adherent, essentially void-free metal layer on the through-hole surfaces. 
The process of the invention, to the extent involving utilization of 
water-immiscible organic liquids prior to an alkaline permanganate 
treatment, has particular applicability to treatment of through-holes in 
circuit boards wherein the insulating substrate material is epoxy or 
epoxy-based (e.g., glass-filled or -reinforced epoxy) or epoxy-containing. 
As earlier noted, it has been found that the water-soluble solvents 
employed in the prior art in processes for treating through-holes in 
epoxy-based boards prior to a permanganate treatment, while advantageous 
because of ease of contact and rinsing, may not be as effective as 
water-immiscible organic liquids in enhancing the effectiveness of the 
subsequent permanganate treatment. 
Another aim of the present invention addresses the desire to provide the 
user with a single treatment composition, for use preparatory to 
permanganate treatment, which is equally (i.e., highly) effective--in 
enhancing permanganate effectiveness and efficiency--irrespective of the 
composition of the material used as the insulating substrate material in 
the circuit board and, specifically, irrespective of whether the board is 
composed of epoxy-based or polyimide material. 
As noted, and according to the invention, the water-immiscible organic 
liquids are those of choice for the commonly-employed epoxy-based printed 
circuit boards. However, for the processing of printed circuit boards 
substantially based upon non-epoxy materials, e.g., polyimide, these same 
water-immiscible organic liquids, alone, are not as effective in acting 
upon polyimide-based boards so as to enhance subsequent permanganate 
treatment. Indeed, the most effective such pretreatments found for use 
with polyimide-based boards are those containing alkali metal compounds 
(e.g., caustic). Since these compounds are water-soluble, their use, in 
turn, requires that aqueous systems and water-soluble organic solvents be 
employed in arriving at a suitable pretreatment composition for 
polyimide-based printed circuit boards. 
On the one hand, then, the pre-permanganate processing of typical epoxy 
boards has been found to be most advantageously carried out utilizing 
water-immiscible organic liquids, while the pre-permanganate processing of 
many other board types (e.g., polyimide-based) has been found to be most 
advantageously carried out utilizing systems containing water-soluble 
alkali metal compounds and, hence, requiring water and water-soluble 
organic solvents. 
In order to compatabilize these inconsistent requirements, the present 
invention makes use of an admixture comprised of water-immiscible organic 
liquid, water and an alkali metal compound, in the form of a substantially 
homogeneous, substantially clear mixture as achievable utilizing any of a 
wide variety of suitable materials (which may be commonly referred to as 
surfactants, wetting agents, coupling agents or emulsifiers). 
By proceeding in this manner, the invention provides a pre-permanganate 
treatment composition which, by reason of the presence therein of 
water-immiscible organic liquid, is highly effective for use with epoxy 
boards and, by reason of the presence therein of the aqueous component, in 
which the alkali-metal compound is dissolved, is highly effective for use 
with boards other than those based substantially on epoxy materials, e.g., 
polyimide-based boards. In addition, by reason of the aqueous component, 
the composition can effectively be rinsed from the through-hole surfaces 
using conventional water rinses. 
In the present invention, then, a process is provided for treating a 
printed circuit board through-hole to prepare it for subsequent 
metallization. This process has applicability to through-holes formed in 
double-sided printed circuit boards and in multi-layer printed circuit 
boards, although it is in the latter case where the invention demonstrates 
its most significant applicability. The process can be employed, for 
example, to desmear and/or etch back a multilayer board through-hole 
surface, following which the through-hole is metallized, or can be 
employed as a post-desmear and/or post-etchback treatment to further 
prepare a multilayer board through-hole surface for metallization. In all 
such cases, it is found that the process of the invention provides 
through-hole surfaces which possess a micro-roughened topography which is 
ideally suited for accepting an adherent, essentially void-free metal 
deposit.

DETAILED DESCRIPTION OF THE INVENTION 
In a typical manufacturing sequence for printed circuit boards involving 
the process of the present invention, an insulating substrate material, 
which may be metal- (e.g., copper foil) clad on one or both sides and 
which may contain one or more metal innerlayers separated by insulating 
substrate material, is employed as the starting material. The substrate 
material may be any suitable non-conductive, insulating material, such as 
thermosetting or thermoplastic resins, inorganic materials such as mica, 
and the like, and may be filled or reinforced with various inorganic 
materials such as glass. Based upon the current state of technology, the 
most widely used insulating substrate materials will be either epoxy-based 
(e.g., glass-filled epoxy resins of the difunctional or tetrafunctional 
epoxy type or cresol or phenol novolaks, bis-maleimide triazine, and the 
like) or polyimide-based compositions. 
Through-holes are punched or drilled in the starting board and, according 
to one embodiment of the invention, the through-hole surfaces thus exposed 
are then treated to prepare them for subsequent metallization so as to 
provide conductive interconnection between or among the layers and/or 
innerlayers of metal circuitry provided on or within the board surfaces. 
In this treatment process, the through-hole surfaces are contacted with a 
water-immiscible organic liquid or with a composition, containing a 
water-immiscible organic liquid (e.g., the previously-noted homogeneous 
clear mixtures), which has the overall effect of acting upon the 
insulating substrate material in a way which enhances the effectiveness of 
the subsequent permanganate treatment. Thereafter, typically after rinsing 
the through-hole surfaces with water, the through-hole surfaces are 
contacted with an alkaline solution containing permanganate ions (e.g., as 
made using potassium or sodium permanganate). By virtue of the 
pre-treatment with water-immiscible organic liquid and the conditions of 
permanganate treatment, the permanganate solution effects a 
micro-roughening of the insulating substrate material exposed at the hole 
surfaces so as to alter the surface topography in a way which increases 
receptivity of the surfaces to, e.g., adsorption of the catalytic species 
used in electroless metal depositing and adherence of the subsequent 
electroless metal deposit. 
The process as above-described can constitute the sole preparation of the 
through-hole surfaces or can form a part of a multi-treatment hole 
preparation process. For example, the water-immiscible organic 
liquid/permanganate process cycle can be relied upon to effect any 
required desmearing and etchback of the through-hole surfaces and, 
following rinsing and preferably treatment to remove and/or neutralize any 
residual manganese species, then proceed to the metallization process. 
Alternatively, however, any required desmearing and/or etch-back can be 
preliminary accomplished by any of the means known in the art (e.g., 
secondary drilling, vapor honing, plasma treatment, contact with acids 
such as chromic acid or sulfuric acid, contact with alkaline permanganate 
solutions), following which the water-immiscible organic liquid 
contact/permanganate contact cycle of the present invention is then 
employed to further prepare the through-hole surfaces for metallization. 
See, e.g., U.S. Pat. No. 4,597,988 to Kukanskis for a discussion of this 
post-desmear processing sequence. Indeed, the process of the present 
invention may be employed in a cycle wherein the through-hole is first 
treated in accordance with the water-immiscible organic 
liquid/permanganate process combination of the invention (e.g., to effect 
desmearing and etch-back) and then further treated with the same 
water-immiscible organic liquid/permanganate contacting steps to effect 
yet further preparation of the through-hole surfaces. 
With respect to the subsequent metallization of the through-holes after 
treatment in accordance with the invention, any number of techniques can 
be employed. However, the typical process is one of electroless metal 
deposition in which the surfaces to be plated are first activated by 
adsorption thereon of species (or precursors thereof) catalytic to the 
metal depositing process. The most commonly-employed catalytic materials 
for this process are tin-palladium catalysts, used in a one-step 
activation process involving true or colloidal solutions of tin and 
palladium chloride. See, e.g., U.S. Pat. No. 3,011,920 and U.S. Pat. No. 
3,532,518. This activation process is generally followed by an 
acceleration step which functions either to remove excess tin deposits or 
alter the valence of the tin compounds or other mechanism to stabilize the 
catalyst on the substrate surface and ensure its exposure in the 
subsequent electroless metallization. Where an acceleration step is 
employed, particularly preferred is the use of an oxidizing accelerator as 
described in U.S. Pat. No. 4,608,275 to Kukanskis, et al. One-step 
catalysts also are known which do not require acceleration, such as the 
organic acid-containing compositions described in Canadian Patent No. 
1,199,754 of Rhodenizer. Following activation, an electroless metal (e.g., 
copper, nickel) depositing solution is brought into contact with the 
activated surfaces. These solutions may be of the formaldehyde-reduced 
variety or may be based upon other types of reducing agents, such as 
hypophosphite. See, e.g., U.S. Pat. Nos. 4,209,331 and 4,279,948. 
As previously noted, prior to the electroless metallization, it is 
generally preferred and desirable to insure that residual manganese 
compounds on the through-hole surfaces, resulting from the permanganate 
treatment, be removed and/or neutralized. While water rinsing may be 
capable of effecting this removal, it generally is preferred to utilize 
neutralizing or reducing agents such as hydroxylamine or salts thereof. 
Among the features of the present invention is the use of a 
water-immiscible organic liquid, preparatory to the alkaline permanganate 
treatment of the through-holes, for those insulating substrates which can 
effectively and efficiently be acted upon with water-immiscible organic 
liquids. Notable in this regard are the epoxy-based printed circuit board 
materials which, for reasons already noted, have in the past been softened 
or swelled preparatory to a permanganate treatment using water-soluble 
solvents (e.g., ketones, ethers, ether alcohols and the like) or aqueous 
solutions containing these solvents. In the present invention, the organic 
liquids utilized for epoxy-based boards and other like insulating 
substrate materials are of the substantially water-immiscible type, and 
preferably are selected from organic liquids represented by the structural 
formula 
##STR1## 
where n is an integer from 1 to 4 and preferably 2 or 3. Particularly 
preferred are the water-immiscible ethylene and propylene glycol phenyl 
ethers, such as those available from Dow Chemical Company, Midland, Mich., 
under trade name Dowanol EPh and Dowanol PPh. 
The functionality of the water-immiscible organic liquid (or composition 
containing it) in the present invention is to act upon the insulating 
substrate material in a way which enhances the effectiveness of the 
subsequent permanganate treatment, i.e., with respect to the 
permanganate's cleaning or desmearing capability and/or its capability in 
etching back through hole surfaces and/or its capability in generally 
effecting a micro-roughening and/or alteration of topography of 
through-hole surfaces either as part of a cleaning or desmearing or 
etch-back process or, alternatively, as a post-desmearing or 
post-etch-back treatment. Generally speaking, the water-immiscible organic 
liquid or composition containing it penetrates and/or softens and/or 
swells the insulating substrate material as the means by which subsequent 
permanganate functionality is enhanced. However, the specific method by 
which the water-immiscible organic liquid operates on the insulating 
substrate material to enhance the effectiveness of the subsequent 
permanganate treatment is not entirely understood, may be other than 
penetration or softening or swelling, and may differ for particular 
water-immiscible organic liquids and particular insulating substrate 
materials. For this reason, the functionality of the water-immiscible 
organic liquid is referred to herein as acting upon the insulating 
substrate material so as to enhance the subsequent effectiveness of a 
permanganate treatment of through-holes formed in boards composed of that 
substrate material. As noted, the effectiveness of the permanganate 
treatment generally has reference to its micro-roughening or 
topography-altering capability vis-a-vis the insulating substrate (in 
terms of increased degree of roughening or etching and/or improved 
topography), but also can have reference to cleaning or desmearing or 
etch-back capability. As such, the choice of particular water-immiscible 
organic liquid may, at least in part, take into account the purpose for 
which the subsequent permanganate treatment is employed. Also generally 
speaking, the water-immiscible organic liquid will be a solvent for the 
particular insulating substrate material; however, since this is not 
always the case, the present application makes use of the terminology 
water-immiscible organic liquid to define the liquid organics used to 
contact the insulating substrate, such terminology embracing liquid 
organics which may or may not literally be solvents for the substrate. 
Also, water-immiscible as used herein is intended to embrace liquid 
organic materials which are at least substantially immiscible in water. 
In the most preferred embodiment of the invention, the water-immiscible 
organic liquid is admixed with water and at least one water-soluble alkali 
metal compound, typically an alkali metal hydroxide, carbonate, silicate, 
phosphate or the like, and most preferably an alkali metal hydroxide such 
as caustic, and this admixture then processed to provide a substantially 
homogeneous, substantially clear mixture through the use of materials such 
as surfactants, wetting agents, coupling agents, or emulsifiers of the 
type and in the amount required to result in the desired mixture 
characteristics regarding clarity and homogeneity. As noted, operation of 
the process with compositions of this type is advantageous in terms of 
facilitating contact and rinsing owing to the presence of the aqueous 
component. However, its most important advantage is in providing a 
pre-permanganate process which will have across-the-board excellent 
functionality irrespective of the composition of the insulating substrate 
material of the circuit boards. Thus, compositions of the type described 
provide the water-immiscible organic liquid found to be effective in 
treatment of epoxy-based and other like substrate materials, while at the 
same time providing the alkali metal compound found to enhance 
effectiveness in other substrate materials such as those which are 
polyimide-based. 
Typical compositions of the type described preferably will contain the 
alkali metal compound in as great an amount as can be employed while still 
retaining the substantial homogeneity and substantial clarity required to 
insure uniformity and uniform effectiveness in the composition as a 
pre-permanganate treatment. Typically, the amount of alkali metal compound 
employed will be in the range of from about 0.1 to about 15% by weight, 
preferably from about 2 to 8% by weight, and most preferably from about 3 
to 7% by weight. Typical compositions will contain on the order of 30 to 
75% water by weight, preferably about 40 to 60% by weight, and 
water-immiscible organic liquid in an amount of from about 5 to 30% by 
weight, preferably from about 8 to 20% by weight, and most preferably from 
about 8 to 12% by weight. 
With respect to the surfactant or wetting agent or coupling agent or 
emulsifier in the composition (for ease of reference, referred to herein 
as the surfactant component, which may be one or a combination of such 
materials), its choice, and the amount in which it is employed in the 
composition, are simply those which will result in a substantially 
homogeneous, substantially clear mixture of all the components under 
conditions of use (generally elevated temperature) and, most preferably, 
under ambient conditions as well. This in turn is substantially dependent 
upon the relative ratios of water and water-immiscible organic liquid and 
the amount of alkali metal compound present in the composition. The 
surfactant component can be selected from a wide variety of known 
compounds having ability (e.g., based upon their hydrophile-lipophile 
balance or HLB), alone or in combination with other surfactants, to 
solubilize or otherwise couple water and water-immiscible organic phases 
into a homogeneous, clear mixture. Typically, the surfactant component 
will be present in an amount of from about 2 to about 50% by weight, 
preferably from about 7 to about 25% by weight, and most preferably from 
about 10 to about 20% by weight of the overall composition. The preferred 
surfactants for use in the invention are disulfonated alkyl diphenyl 
oxides, available from Dow Chemical Company, Midland, Michigan, under the 
trade name Dowfax. The foregoing ranges for the surfactant component are 
intended to be based upon the surfactant-active portion thereof, 
recognizing that many commercial surfactants are obtainable only in 
admixture with water or with other ingredients. For this and other 
reasons, it should be understood that the criteria regarding the amount of 
the surfactant component is simply that amount (in whatever form 
available) which will result in a substantially clear, substantially 
homogeneous mixture of the particular ingredients, i.e., water, particular 
water-immiscible organic liquid, particular alkali metal compound, given 
the particular amounts in which each is present, and that the above-stated 
ranges for the surfactant component are intended only as general 
guidelines. Also, to the extent any surfactant component is employed 
which, as purchased, contains compounds in addition to the active 
surfactant material, care obviously must be taken that such additional 
compounds not be inconsistent in any way with the intended functionality 
of the homogeneous, clear mixture according to the invention herein in 
which the surfactant component is used. Still further, to the extent the 
surfactant component might itself, as purchased, contain ingredients 
(e.g., alkali metal compounds) which are intentionally present in the 
homogeneous, clear mixture used according to the invention herein, the 
amount of any such compounds in the surfactant component obviously is 
taken into account in formulation of the homogeneous, clear mixture 
according to the invention. 
Typically the pH of compositions of the type described will range from 
about 8 to about 14 depending largely upon the amount and type of alkali 
metal compound present, the preferred pH being in the range of from about 
8 to about 13. In less preferred embodiments of the invention, advantages 
are realized utilizing compositions without alkali metal compounds, i.e., 
admixtures of water and water-immiscible organic material rendered 
substantially homogeneous and clear through use of a surfactant component. 
In such cases, the pH of the composition generally will be close to 
neutral. 
The conditions under which contact of the through-hole surfaces with the 
water-immiscible organic liquid or composition will be conducted may vary 
widely depending upon the particular water-immiscible organic liquid or 
components of the composition and the particular insulating substrate 
material of the board. Generally, the treatment temperature for the 
water-immiscible organic liquid or composition will be in the range of 
from about room temperature to about 170.degree. F., preferably from about 
130.degree. to 150.degree. F., and the time of contact (e.g., residence 
time of the board in a tank of the organic liquid or composition) will be 
on the order of anywhere from, e.g., 1 to 20 minutes, and more typically 
about 2 to 10 minutes. 
Following a water rinse, the through-hole surfaces are then contacted with 
the aqueous alkaline permanganate solution. This solution is generally 
comprised of water, a water-soluble salt of permanganate and sufficient 
alkali to obtain a solution pH in the alkaline range, and preferably a pH 
of at least about 13.0. Typically, the solution will contain the 
water-soluble salt of permanganate in a concentration of from about 1 g/l 
to saturation, and most preferably from about 40 to 250 g/l. Treatment 
with the permanganate solution generally will be at an elevated 
temperature (e.g., from about 130.degree. F. to about 180.degree. F., and 
most preferably from about 160.degree. to 170.degree. F.), and for a time 
(e.g., from about 3 to 20 minutes) sufficient to effect the conditioning 
(e.g., desmearing and/or etch-back and/or further alteration of surface 
topography) needed to result in improved catalyst adsorption and improved 
adhesion of an essentially void-free electroless metal deposit on the 
through-hole surfaces. 
The invention is further described with reference to the following 
examples. 
EXAMPLES 1-18 
A series of experiments were conducted utilizing a number of different 
multilayer printed circuit board materials and a number of different 
compositions preparatory to permanganate treatment. 
The boards treated were identical in all respects except for the material 
of the insulating substrate. Thus the first series of boards (Sample 1) 
were epoxy-based boards available under the tradename N.V.F.; the second 
series of boards (Sample 2) also were epoxy-based, available under trade 
name Polyclad Tetrafunctional; and the third series of boards (Sample 3) 
were polyimide-based, available under tradename Nelco Polyimide. 
Six (6) different compositions were employed as pre-permanganate 
treatments. Sample A was a 50% (by weight) solution of caustic in water. 
Sample B was 100% by weight N-methylpyrrolidone. Sample C was 50% by 
weight N-methylpyrrolidone in water. Sample D was 100% by weight 
water-immiscible ethylene glycol phenyl ether (Dowanol EPh). Sample E was 
50% by weight Dowanol EPh dispersed in water. Sample F was a substantially 
homogeneous, substantially clear mixture containing (by weight) 45% water, 
17% Dowanol EPh, 5% caustic, and 33% surfactants (50:50 mixture of Triton 
BG-10 and Dowfax 2A1, the latter being in the form of a 45% weight mixture 
of active ingredient with water, and the former being 70% active, with the 
remaining 30% being 15% caustic and 15% water). 
The cycle to which each of the through-hole-containing boards was subjected 
was as follows. Five (5) minutes contact with the composition at 
150.degree. F.; two (2) minute water rinse at ambient temperature; fifteen 
(15) minute contact with potassium permanganate solution (50 g/l.; 
pH=13.0) at 165.degree. F; two three (3) minute rinses with water at 
ambient temperature; and five (5) minute contact at 110.degree. F. with a 
neutralizing solution of hydroxylamine and hydrochloric acid. 
For each sample, an etch rate was determined based upon measurement of the 
weight loss resulting from the processing. In addition, inspection of 
surface topography of the through-hole was made by means of scanning 
electron microscopic photography, and, for each series of board samples, 
the topographic results were ranked as best (5) to worst (1). The results 
are set forth in Table I. 
TABLE I 
______________________________________ 
Etch-Rate 
Example Description (% wt. loss) 
Topography 
______________________________________ 
1 1A .136 1 
2 1B .210 4 
3 1C .197 2 
4 1D .227 4 
5 1E .180 1 
6 1F .210 4 
7 2A .151 1 
8 2B .197 5 
9 2C .188 3 
10 2D .224 5 
11 2E .116 2 
12 2F .201 5 
13 3A .231 4 
14 3B .154 1 
15 3C .139 1 
16 3D .183 2 
17 3E .122 1 
18 3F .213 4 
______________________________________ 
From these results, it can be seen that the water-immiscible Dowanol EPh is 
highly effective as a pre-permanganate treatment for both epoxy and 
polyimide multilayer boards; that a composition containing caustic is 
highly effective for polyimide boards but of very low effectiveness for 
epoxy boards; and that single compositions can be formulated to provide 
uniformly excellent effectiveness for both epoxy and polyimide boards. 
In terms of the preferred aspects of the invention, a water-immiscible 
organic liquid is employed as a pretreatment for through holes preparatory 
to treatment of the through holes with alkaline permanganate in processes 
for cleaning, and/or desmearing and/or etching back and/or conditioning 
the through hole surfaces for subsequent metallization. The 
water-immiscible organic liquid may be used alone, but preferably is used 
in combination with water along with a surfactant which brings about a 
substantially homogeneous, substantially clear mixture. Most preferably, 
the water-immiscible organic liquid is used in association with water and 
water-soluble alkali metal compounds, again with effective amounts of a 
surfactant component to provide a substantially homogeneous, substantially 
clear mixture among all the components. Although not preferred as such, in 
any of the foregoing embodiments there may additionally be present 
water-soluble organic liquids. 
Having thus described the invention, it is to be noted, and will be readily 
understood by those in the art, that various other modifications or 
alternatives are possible without departing from the scope or spirit of 
the invention, as defined by the appended claims.