CA2025337C

CA2025337C - Process for directly metallizing circuit boards

Info

Publication number: CA2025337C
Application number: CA002025337A
Authority: CA
Inventors: Burkhard Bressel; Heinrich Meyer; Walter Meyer; Klaus Gedrat
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 1989-09-14
Filing date: 1990-09-13
Publication date: 2001-04-17
Anticipated expiration: 2010-09-13
Also published as: KR910006516A; ES2095227T3; EP0417750B1; CN1050965A; JPH03175692A; CN1021949C; ATE145313T1; KR100216328B1; DD297669A5; CA2025337A1; EP0417750A2; EP0417750A3; DE59010565D1; JP2871828B2

Abstract

The present invention relates to a process for directly metallizing circuit boards, preferably in a horizontal throughput system, with the omission of electrolyte that requires no outside current. A multi-functional adsorption layer is produced selectively on the non-conductive parts of the circuit boards, said layer being able to adsorb an oxidation agent and/or store, this in pores or which is itself an oxidation agent, and which then is brought to reaction with such monomers as are suitable for the formation of conductive polymers, and which are metallized by electroplating.

Description

The present invention relates to a process for directly metallizing non-conducting regions of printed circuit boards, preferably utilizing a continuous horizontal process.
When through holes in printed circuit boards are metallized,it is mainly copper electrolytes that require no outside current that are used to produce thin electrically conductive coatings on the insulating surfaces, these then being brought up to the desired thickness by electroplating with copper.
In addition to a number of advantages, such as an even thickness of the coating and good adhesion both on the non-conducting base material and on the copper inner layers and the copper covering foil, this also entails a number of disadvantages:
The process is costly, for the sides of the drilled holes must first be catalytically activated by doping with noble metal after cleaning. In addition, a high level of monitoring is required in order to ensure the trouble-free operation of the currentless baths for, in most instances, they contain formaldehyde, which is hazardous to health, and the disposal of these baths is problematical because of their content of sequestering agents.
Because of these disadvantages, numerous attempts have been made to replace the metallizing of the drilled holes without the use of outside current. However, in each case, such replacement entails the production of an n-conductive intermediate layer on the insulating base material on which the copper that is required for producing a through contact can be deposited electrolytically. According to the prior art, at the present time, carbon particles and carbon layers of mixed oxides or hydroxides that contain noble metals, e.g., Sn-stabilized Pd colloids are used for such intermediate layers. In addition, there have been reports of conducting polymers by means of which electron conductivity (n-conductivity) can be produced on the non-conductive surfaces.
European application No. 0206133 dated 11 June 1986 describes conductive surface films of the polypyrrol type;
European application No. 0152632 dated 28 December 1984 describes porous conductive coatings of the chinon diimine type. German disclosure document 3321281 published December 22, 1983, describes a process for increasing the conductivity of impregnable materials, wherein the conducting system is oxydized polypyrrol.
The n-conductive intermediate layers produced according to the prior art must, however, fulfill additional demands before they can be used for producing through contacts in circuit boards:
1. There must be a stable adhesive bond between the base material (preferably glass fibre reinforced epoxide) and the electrically plated metal layer, which is preferably copper.

2. There must be no impairment of the adhesion of the electrically plated metal layer and already existing copper surfaces by the application of the intermediate layer.
None of the formerly known non-metallic conductive coatings, which are intended to replace copper that is deposited without current, can satisfy requirement No. 2, i.e., the n-conductive intermediate layers must be removed selectively from the metallic copper so as to leave no residue. Even very small residues of noble metal nuclei that are absorbed on the Cu coating have a marked effect on the structure of subsequently applied electrolytic metal or copper layers, at least in the boundary layer area. -The requirement for selective removal of the conductive intermediate coating represents a significant disadvantage in this process, for the selective dissolution requires the perfect control over difficult etching processes, otherwise there is a danger that the intermediate coating can be attached, damaged, or even removed from locations where it is required.
The present invention eliminates the process steps of catalytic activation of the drilled through holes in circuit boards. These holes are coated with copper electrolytes that require no outside current. Furthermore, an n-conductive polymer utilized in the process is deposited selectively only on the insulating surfaces but not on the metal.
Therefore, according to the present invention, there is provided a method of directly metallizing non-conducting regions of printed circuit boards utilizing a horizontally operating continuous system comprising:
(a) selectively producing a multi-functional adsorption layer at non-conducting regions of printed circuit boards;
(b) treating the adsorption layer with monomeric compounds suitable for the formation of conductive polymers;
(c) horizontally transferring the printed circuit board to an adjacent active bath with a conveying period of no more than seconds, whereat the monomeric compounds react in the active bath to form conducting polymers in the acidic pH range by an oxidizing effect of the adsorption layer by a complete transformation from the monomeric compounds, excess reagents and unreacted proportions are removed by acid; and (d) galvanically metallizing the conductive polymers.
According to the process of the present invention, it is possible to avoid the disadvantages of known metallizing procedures during the production of circuit boards.

A further advantage of the procedure according to the present invention is that the layer that is used for adhesion and oxidative polymerization develops during the permanganate cleaning of the drilled holes so that it is also possible to dispense with another stage in the process, which is to say the removal of manganese peroxide.
-3a-In principle, other oxide adhesive layers that can adsorb oxidation agents to a sufficient degree can also be used.
Description of Procedure 1 1. According to the procedure according to the present invention, the permanganate desmear process that is known per se is used to produce a multi-functional adsorption layer selectively on the non-metallized surfaces of circuit boards, in particular of drilled holes.
The multi-function of the adsorption layer is distinguished by the following properties:
a) Hydrophilization of the surface and increasing its wettsability by polar molecules, in particular, water.
b) oxidization agent in a subsequent step in the procedure.
c) Wettability for the monomer from which the conductive polymer is formed.
The production of the multi-functional adsorption layer can be effected in a particularly simple manner by the permanganate process, which is known per se and used for cleaning the drilled holes, but which is distinguished by the following changes according to the present invention:
1. The insoluble manganese (IV) compounds that are formed when the drilled holes are cleaned are not removed.
2. After rinsing, treatment is carried out with pyrrol or another monomer prestage of a conductive polymer (for example, another 5-ring heterocycle or a ine) that is referabl dissolved in an or anic solvent. It is also P Y g possible to use aqueous solutions of organic solvents that contain the monomer.
Another variation of the procedure according to the present invention provides for the use of gaseous monomer.
The manganese (IV) compounds of the multi-functional layer and optionally the residues of the permanganate ions that still remain in the pores act as an oxidation agent.
3. The use of horizontal technology makes it possible to move the boards to the next active bath within a few seconds (typically 5 to 10 seconds). The monomer solution that wets the manganese (IV) layer thus has no time to flow out of the drilled holes or else collect in the lower part of said drilled holes.
In addition, draining by way of the drill scoring is made more difficult since this extends horizontally when the machine throughput is horizontal.
Because of this, in the subsequent step in the process, polymers of good conductivity are obtained and these ultimately J.ead to faultless metallizing.
Were a conventional vertical plant~used, the subsequent active bath would not be reached quickly enough and the monomer solution would collect in the lower area of the drilling. This would mean that there would be insufficient monomer in the upper portion and too much in the lower portion.
Thus, in a subsequent step in the process, only poorly conductive polymer layers would be formed and these would ultimately lead to faulty metallizing.

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4. According to the present invention, complete polymerization takes place by treating the surface that is coated with manganese (IV) compound and wetted with monomer with acid, preferably with sulfuric acid.
However, treatment with acid has additional tasks.
Excessive unreacted or only partially reacted pyrrollis effectively removed. Simultaneously, Mn(IV) compounds in acid solution act as oxidation agent and thus, together with the H+ ions of the acids, serve to bring about the complete transformation of the polymer coating to the n-conductive state.
5. Then, the end coating can be thickened as desired with an electrically plated metal, preferably copper from an .-..._ __~--_ acid Cu-electrolyte.
The technical realization requires brief and well defined transport times for Step 3 of the procedure. This can be achieved particularly well using a horizontal throughput method.
Further advantages of horizontal technology are as follows:
,~
There is no contamination of the environment by pyrrol:
This is particularly important because the monomer prestages exhibit a relatively high vapour pressure.
Description of procedure- 2 1. In the procedure according to the present invention, processes that are known per se are used to produce a multi-functional adsorption coating selectively on the non-metallic surfaces of circuit boards, in particular of drilled holes, by soaking, roughening, etc.

~~~~~uy The multi-function of the adsorption coating is distinguished by the following characteristics:
a) Hydrophilization of the surface and increasing its wettability by polar molecules, in particular water.
b) Adsorption of oxidation agent or the inclusion of such in a sufficient quantity that is required in a subsequent stage in the procedure.
c) Adsorption of the monomer from which the conductive polymer is formed.
2. The production of the multi-functional adsorption coating can be effected in a particularly simple manner by processes known per se, such as, for example, soaking in organic solvents or microporous roughening by, for example, chromosulfuric acid.
a) After rinsing, the circuit board is brought into contact with the oxidation solution.
b) The rinsing stage that follows the oxidation process is kept very short by means of horizontal technology (1-l0 seconds) so as not to leach out the oxidation agent that is adsorbed in the adsorption coating or which is in the pores, and on the other hand, however, to remove excessive oxidation agent, particularly that which is adhering to the metal surfaces.
Technical realization of the procedure requires short and well defined reaction times for Steps 1 and 2. This can be achieved in a particularly suitable manner in the horizontal throughput procedure with forced rinsing of the drill holes (surge rinsing, suction, spraying) and combinations thereof.

~~~~v~v' a) Additional advantages of horizontal-throughput technique:
No contamination of the environment by pyrrol, which is particularly important because the monomer prestages exhibit a relatively high vapour pressure.
b) The compact construction of the system modules because of short dwell periods (a cost-effective solution).
Horizontal-throughput systems for metallizing circuit boards, using known technology, are produced by the sequential arrangement of different processing modules.
The individual modules are charged with the chemicals and/or rinsing baths that are required for the sequence used in the procedure. Such systems, which are used for the conventional process, are, for example,~25 m long, are fitted with an extremely costly transportation system, and require a considerable amount of space. The length of such systems is determined by the duration of the treatment as the circuit board substrates, which have to be adapted to the metallizing process, pass through.
Thus, for example, a system length of at least 25 m results from today's state of the art that is essentially governed, at the present state of knowledge, by process chemicals which because of their sensitivity to oxygen are not only inconvenient but are also extremely costly when used in systems of this sort: this is to say nothing of their potential for environmental damage. In addition, this process requires extremely precise monitoring and control.
As has already been discussed, systems of this kind entail a space requirement of approximately 25 m, at a processing capacity of 25 m2/hr. Maintenance costs for systems of this kind are relatively high. Thus, there is a need for an economical and technically feasible reduction of the length _ g _ 2Q~~~~~
of such horizontal throughput systems and production processes.
One method that is set out in EP application 0 206 133, which is used for metallizing plastics, at first appeared to be extremely useful after careful consideration and when used in the laboratory. The transfer of the production methods to known vertical major systems proved impossible, however, since the short and process-specific processing and transport times that were associated with the above method could not be achieved by the extremely costly and difficult charging mechanisms (product carriers).
Thus, for example, processing and transport times of approximately 10 seconds duration are required for proper conduct of the procedure. In order to achieve this, the technology used in a high speed wetting system, consisting of a standing wave or surge nozzles, was proposed; this permitted the desired and necessary brief wetting in a horizontal throughput system. Such high speed systems involving horizontal throughput technology are known, for example, in a similar form, from the domain of lacquer-application technology.
When such a procedure is used, the length of a horizontal throughput system is reduced by approximately 15 m. This results in the potential, which has been desired for many years, of preceding the metallizing system by a permanganate pretreatment line, which ther_ permits linking of the process steps in the normal manner. This is particularly useful since the permanganate treatment not only serves to clean the drilled holes, but is also a necessary prestage for the subsequent metallizing process. However, the procedure is not only confined to producing conductive layers in drilled holes; it can, of course, also be used for metallizing insulating surfaces of any shape or form, i.e., for area metallizing and for generating conductor tracks. It ensures not only good adhesion of the electroplated copper not only on the organic substrate material, but also on the glass fibres.
Furthermore, other metals such as nickel, tin, tin/lead alloy, or gold, which are suitable for the technical application, can be used for the electroplate thickening. In addition to the epoxy circuit boards (FR 4) that are mainly used, other circuit boards such as multi-layer boards that consist of different fibre reinforced and non-fibre reinforced polymers can also be used, as can those with metallic inner layers. Such materials are polyimide, polyetherimide, polyethersulfone, etc.
The following examples serve to explain the present invention.
Example 1 (Through metallizing in a horizontal throughput system) 1.0 1 min. etching in persulfate sulfuric acid (80 g/1 H2S04, 120 g/1 Na-persulfate), RT, spray nozzles 1.1 Transport, approximately 10 sec.

2.0 1 min. surge rinsing 2.1 Transport, approximately 10 sec.

3.0 2 min. soaking (advance 0.7 m/min.: module length m), 70C, surge nozzles, ultrasound assisted 1.40 3.1 Transport, approximately 10 sec.

4.0 1 min. surge rinsing 4.1 Transport, approximately 10 sec.

5.0 2 min. KMn04 treatment (advance 0.7 m/min.;

module h 1.40 m, electrolyte composition 60 g/1 KMn04, lengt 60 g/1 70C, surge nozzles NaOH, 5.1 Transport, approximately 10 sec.

6.0 1 min. surge rinsing 6.1 Transport, approximately 10 sec.

2~~~~~d 7.0 20 sec. activation (advance 0.7 m/min., module length 35 cm, electrolyte: 10% pyrrol, 30% N-methylpyrrolidon, remainder water), RT, standing wave 7.1 Transport 5-10 sec. (transport path approximately 10 cm from the activator to the next bath, advance 0.7 m/min.) 8.0 30 sec. acid treatment (advance 0.7 m/min., module length 35 cm, electrolyte 300-350 g/1 H2S04) 8.1 Transport time approximately 10 sec.

9.0 1 min. surge rinsing 9.1 Transport, approximately 20 sec.

10.0 ~ 1 min. pickling (180 g/1 H2S04), RT, standing wave 10.1 Transport, approximately 10 sec.

11.0 5 m flash copper plating (plating time 2 min., 30 sec., 10 A/dm2), RT
11.1 Drying, approximately 1 min.
Notes on the first process example:
Compared to conventional vertical tank systems, working with a horizontal throughput system entails the following advantages, which in part ensure thorough metallizing for the first time:
- Because of the improved flood rinsing of the drilled holes in the horizontal system, the processing time in procedure steps 3.0 and 11.0 are reduced by approximately one-half and the times for procedure steps 5.o are reduced 15-fold.
- Because of the improved encapsulation of the horizontal systems there is far less soiling of the work place. With conventional tank systems, there is the formation of black polypyrrol on all the acid surfaces because of the high rate of evaporation of the pyrrol, even if there is good evacuation.

2.~~~~
- Because of the short transport times (approximately sec.), the activator that is used in procedure step 7.0 cannot. collect in the drilled holes. This means that equally conductive polypyrrol coatings are formed in procedure step 8.0 and these contribute to faultless metallizing of the drillings in procedure step 11Ø Collection of the activator solution that is applied in procedure step 7.0 is made more difficult by the drilling scoring that extends horizontally.
In conventional tank systems, because of the longer transport times (approximately 30 sec.), the activator flows into the lower area of the socket of the drilling jig. For this reason, sufficiently conductive polypyrrol coatings are only formed in the lower part of the drilled holes.
The process that has been described would lead to only inadequate metallizing of the drillings in conventional tank systems and could not be used for the production of circuit boards.
In this instance, collection of the activator solution is not made more difficult by the now vertical drilling scoring.
Example 2 A 35 x 18 cm board of copper-coated k'R-4 epoxy base material was cut to size and treated as follows in order to produce a circuit board with metallized walls in the holes:
1. Production of the drilled holes 2. 30 sec. cleaning in an aqueous solution of sodium peroxide disulfate (120 g/1) and H2S04 (70 g/1) 3. 2 min., 30 sec. soaking in a commercial soaking agent solution (70°C), followed by 1 min. of rinsing in water 5. 2 min., 30 sec. treatment with aqueous alkaline permanganate solution containing KMn04 60 g/1 NaOH 60 g/1 (70°C) 6. Brief rinsing in water 7. 10 sec. treatment in aqueous monomer solution containing 30%-wt N-methylpyrrolidon 10%-wt pyrrol and the remainder water 8. 10 sec. treatment in aqueous sulfuric acid solution containing 300 g/1 H2S04 9. 1 min. rinsing in water 10. 1 min. pickling 11. 6 min., 30 sec, electro copper plating in a conventional acid bath, current density 4 A/dm2 12. 1 min. rinsing in water 13. Drying, mask printing 14. Electro copper plating of the areas not covered by the mask in a conventional bath for a period that is sufficient to achieve the required thickness of copper coating 15. Electro plating of a lead-tin coating 16. Removal of the masking coating, etching of the exposed copper (and, next, if desired, removal of the lead-tin coating) The resulting circuit board was faultless and the walls of the holes were copper plated at a very high quality and at extremely good adhesion.
Example 3 As in Example 2, however, with the additional procedure step of drying of the plate after treatment with the permanganate. The dried plate displayed extremely good storage properties.
After storing and prior to further processing in the monomer solution, the plate was rinsed briefly. The results corresponded to those achieved in Example 2.
Example 4 A copper coated base material was cut to the desired size and treated as in Example 2, Steps 1 to 9, or as in Example 3. Then the plate was dried and processed further as follows.
a) Application of a masking coating corresponding to the negative of the track pattern.
b) Etching in H2S04 sodium-peroxide disulfate solution, rinsing in water, pickling (5 vol-% H2S04 in water).
c) Introduction into an acid bath for electro copper plating for a time that was sufficient for the copper layer to build up to the desired nominal thickness of the conductor tracks.
d) Further processing as in Example 2, Steps 15 and 16.
The circuit boards produced in this manner satisfied all quality control requirements.
Example 5 A board of copper coated base material was cut to the desired size and then treated as in Example 2 or 3, Steps 1 to 10. Then the board was placed in an acid electroplating copper bath for a period that was sufficient to obtain the desired end thickness of the coating (20-40 m) of the coppering on the walls of the holes. Further processing took place in the known manner and provided circuit boards of excellent quality.
Example 6 A circuit board measuring 38 x 38 cm of FR-4 base material which, instead of a copper coating, had a layer of adhesive coupling agent on both sides was processed as follows:
1. Production of the drilled holes 2. As in Example 2, Steps 5 to 9 3. Drying 4. Application of a masking layer that corresponds to the negative of the desired track pattern 5. Immersion in an acid copper electroplating bath for a time sufficient to build up the conductor tracks including the coppering on the walls of the holes to the required nominal thickness 6. Removal of the masking layer 7. Processing for 30 sec. in an aqueous solution of KMn04 60 g/1 NaOH 40 g/1 (70°C) 8. Immersion in an oxalic acid solution for a time sufficient to free the surface between the conductor tracks of all residues from the procedure, either completely or to a very large extent Example 7 A board of base material coated with adhesive was processed as in Example 2 or 3, Steps 1 to 14. The masking layer was then removed and the exposed thin copper layer was etched. Subsequent treatment was as in Example 6, Steps 7 and 8. The results corresponded to those obtained in the previous example.
Example 8 Examples 5 and 6 were repeated with comparable results although, however, this proceded from uncoated base material without a layer of adhesive (a resinous laminate surface).
Example 9 A board of FR-4 base material, 38 x 18 cm, which was produced by pressing prepregs and copper coated inner layer laminates with the objective of having no copper coating on the outer sides of the laminate, and which was not provided with a layer.of coupling agent, the outer surfaces of the laminate not necessarily being highly resinous compared to the proportion of glass fibre, was processed as follows:
1. Production of the drilled holes 2. 10 min. of soaking in an aqueous solution consisting of 45%-vol. dimethylformamide 20%-vol. diethylene glycol dimethyl ether (30°C) 3. 1 min. rinsing in water (50°C) 4. 10 min. of processing in an aqueous solution of chromium trioxide (300 g/1 (70°C)) 5. 1 min. rinsing in water (50°C) 6. 1 min. rinsing in water 7. Reduction of adhering Cr(VI) ion residues in a solution of NaHS03 8. 1 min. of rinsing in water 9. As in Example 3, Steps 5 to 14 ~~2~ ~~'a The masking layer was then removed and the thin copper layer that this exposed was etched off. Subsequent processing was carried out as in Example 6, Steps 7 and 8.
The results corresponded to the results obtained in the previous examples.
Example 10 Example 2 was repeated, except that the procedure after Step 10 was interrupted and the dried board of base material that was covered with an electrically conductive polymer layer was stored for two weeks under normal environmental conditions prior to further processing. For purposes of further processing the board was once again immersed in the sulfuric acid solution used in Step 9, and then rinsed and then processed as described heretofore.
Tests revealed that the procedure according to the present invention also provided excellent results for the production of multi-level circuit boards.

Claims

1. A method of directly metallizing non-conducting regions of printed circuit boards utilizing a horizontally operating continuous system comprising:
(a) selectively producing a multi-functional adsorption layer at said non-conducting regions of printed circuit boards;
(b) treating said adsorption layer with monomeric compounds suitable for the formation of conductive polymers;
(c) horizontally transferring said printed circuit board to an adjacent active bath with a conveying period of no more than 10 seconds, whereat said monomeric compounds react in said active bath to form conducting polymers in the acidic pH range by an oxidizing effect of the adsorption layer by a complete transformation from the monomeric compounds, excess reagents and unreacted proportions are removed by acid; and (d) galvanically metallizing said conductive polymers.

2. The method according to claim 1, said multi-functional adsorption layer serving as an oxidizing agent.

3. The method according to claim 2, wherein said oxidizing agent is adsorbed at said non-conductive regions.~

4. The method according to claim 2, wherein said oxidizing agent is stored in pores in said non-conductive regions.

5. The method according to claim 2, wherein permanganate is used as said oxidizing agent to produce said adsorption layer, in that soluble manganese oxides and manganese hydroxides formed at said non-conductive regions of the printed circuit board remain at these locations until said conductive polymers are formed and serve as the oxidizing agents in the reaction with said monomeric compounds.

6. The method according to claim 1, wherein said non-conductive regions of the printed circuit board are prepared by roughening these regions to form a micro-porous surface to serve as a storage medium for an oxidizing agent, in that oxidizing capacity of said oxidizing agent stored in said adsorption layer is maintained in a subsequent rinsing cycle which lasts between 1 and 10 seconds, so that conductive polymers during a subsequent reaction of said monomeric compounds with oxidizing agent stored in said adsorption layer are selectively formed only at said non-conductive regions of said printed circuit board.

7. The method according to claim 6, wherein said oxidizing agent is permanganate.

8. The method according to claim 6, wherein said oxidizing agent is chromate/dichromate.

9. The method according to claims 1, 2, 3, 4, 5, 6, 7 or 8 wherein said printed circuit boards are treated in a continuous system which may be used for cleaning bore holes in printed circuit boards with permanganate and in that cleaning of said bore holes and direct metallization of the printed circuit boards are effected in a modular unit having a variable advancing speed.