Field of the Invention

The present invention relates to a method for treating, in particular for etching, of plastic substrates, in particular printed circuit boards and printed circuit foils, with a treatment solution comprising permanganate having the features of independent method claim 1 . The invention also relates to a device for an at least partial regeneration of a treatment solution comprising permanganate which is used for treatment and/or etching of plastic parts, in particular printed circuit boards and printed circuit foils, having the features of independent device claim 8.

Background of the Invention

The treating or etching of plastic substrates, such as plastic moulded parts or in particular printed circuit boards or printed circuit foils, with a solution comprising permanganate is often realised as a preparatory step for metallizing of the surfaces or of the bores of the substrate. Therefore, the substrates are brought into contact with a suitable treatment solution in a treatment tank of a dipping machine or of a tunnel machine with horizontal direction of transportation, wherein the temperature of the solution can be between 50 °C and nearly 100 °C.

EP 1 657 324 B1 discloses a method for metallizing of insulating or non- conductive substrates by etching with an etching solution, wherein the concentration, temperature and time for roughening and metallizing of the surface of the substrate is controlled. Since such a treatment solution loses its treatment properties when used for a longer period of time, it must be replenished regularly or fully or partially replaced, which in turn is costly.

A first aim of the present invention is to provide a method for treating, in particular for etching of plastic substrates, in particular printed circuit boards and printed circuit foils, using a treatment solution comprising permanganate, which enables the extended use of the treatment solution. A second aim is to provide a device for an effective and cost-saving regeneration of the treatment solution in order to achieve the aim of the method.

These aims can be achieved by the subject-matter of the independent claims. Features of advantageous preferred embodiments of the invention arise from the dependent claims. In order to achieve the first mentioned aim, the present invention proposes a method for treating, in particular for etching, plastic substrates, such as plastic moulded parts and in particular printed circuit boards and printed circuit foils with a treatment solution comprising permanganate. It is intended by this method that the carbonate concentration, based on sodium carbonate Na ₂ CO ₃ , is set to a desired value of carbonate compounds in the treatment solution in a dissolved form by the removal of carbonate compounds from the treatment solution. The dissolved carbonate compounds are converted herein to a solid form by freezing out, and subsequently removed from the treatment solution by filtration. Filtration in the context of the present invention has to be understood as a process in which solid particles are removed from a solution.

The desired value is set in the present invention to a value of less than 200 g/l, preferably the concentration of carbonate compounds in the treatment solution for the method is set to a value of 30 to 150 g/l, and more preferably the concentration of carbonate compounds in the treatment solution is set to a value of 50 to 100 g/l. Carbonate is a reaction product from the reaction of the treatment solution with the plastic substrate.

The freezing out of the carbonate compounds can be realised by fully or partially transferring of the treatment solution to a cooling tank and subsequent cooling down of the treatment solution in the cooling tank. The subsequent transfer of the treatment solution comprising cooled and frozen out carbonate from the cooling tank to a downstream filter device enables the carbonate to be removed from the solution by filtration.

To remove the carbonate compounds from the treatment solution by freezing out and subsequent filtration, the treatment solution comprises sodium permanganate. The percentage of permanganate in the treatment solution consists of more than 50% of sodium permanganate (Na ₂ MnO ₄ ).

In the present invention method the freezing out of the carbonate compounds can be conducted in particular by cooling of the treatment solution to a freezing out temperature of -12 to +12 °C, preferably from -9 to +10 °C, and more preferably from -7 to +5 °C.

The method for removing of the carbonate can be executed in such a way that the permanganate solution to be regenerated, which exhibits a treatment temperature of 50 °C to 98 °C for example, is transferred from the treatment tank to a cooling tank. The solution can hereby pass through one or two pre- coolers. In the cooling tank, the solution is cooled down to the freezing out temperature, i.e. to a temperature of 0 °C or less, so that solid carbonate crystals form in the solution as a result of the low temperature. After reaching the freezing out temperature, possibly also after a certain additional waiting time after reaching the freezing out temperature, the solution with the carbonate crystals, is either completely or, more preferably partially moved from the cooling tank to the filter device. There, the crystals are filtered off, possibly supported by a negative pressure applied to the filter. The filtrate can be moved back to the treatment tank wherein it can pass through one of the pre-coolers whereby it can be automatically heated again. The filter cake formed by carbonate crystals can be rinsed off the filter and conveyed to the effluent.

Another advantageous embodiment of the method comprises that in addition to the prescribed method, an electrolytic permanganate regeneration of the treatment solution is realised, as described for example in WO 01 90 442 A1 . The manganate, which is resulting out of the reaction of the permanganate of the treatment solution in the treatment tank with the plastic substrate, is converted back to permanganate by an anodic oxidation by bringing the treatment solution into contact with a suitable electrolytic cell. Especially by such a combination of at least one of the methods of the present invention for setting of a specific carbonate concentration in the treatment solution by electrolytic permanganate regeneration, the efficiency of the permanganate regeneration can be increased and an improved longer lifetime of the treatment solution can be achieved.

The second aim of the present invention is achieved by providing a device for an at least partial regeneration of the treatment solution comprising

permanganate by reducing of a concentration of carbonate compounds comprised in the treatment solution for the implementation of a method according to the claims. Thereby, the device comprises at least a cooling tank which is to be subjected to the treatment solution to be regenerated, which further comprises a cooling device directed to said cooling tank and which exhibits a downstream filter device for separating the carbonate compounds from the treatment solution.

The filter device can comprise at least one filter tank and one filter, e.g. a filter strainer. The filter tank can be connected to a negative pressure device in order to increase the pressure difference on the filter during filtration to achieve thereby a faster filtration.

The filter in the filter tank can be arranged movable, e.g. tiltable or rotatable, to facilitate the cleaning of the filter.

The device of the present invention according to one of the preceding embodiments of the invention is suitable for the implementation of a method according to one of the preceding method embodiments of the present invention.

Below, exemplary embodiments of the invention shall demonstrate the invention and their benefits in further details, in particular based on the enclosed figures. The size ratios of the individual elements to each other in the figures do not correspond to the current ratios since some forms have been illustrated in a simplified manner, while other forms have been shown in an enlarged ratio to other elements for providing a better visualisation.

Fig. 1 shows an embodiment of the invention wherein a device for the regeneration of the treatment solution which is used for treatment, e.g. etching, of plastic parts, such as printed circuit boards or the like, is provided.

Fig. 2 shows a schematic view of a cleaning unit of the device according to Fig. 1 .

Fig. 3 shows a schematic view of a filter unit which is part of a filter device according to Fig. 1 and Fig. 2.

Identical reference signs are used for identical elements or identical-acting elements of the invention. The embodiments shown represent solely examples how the device and the method of the present invention may look like and do not represent any conclusive limitation.

The schematic view of Fig. 1 shows an embodiment of the device 10 of the present invention for the at least partial regeneration of treatment solution 12 comprising permanganate in a treatment tank 14. The treatment tank 14 is used for the treatment, e.g. the etching, of plastic substrates. Device 10 shall reduce the concentration of carbonate compounds in the treatment solution 12. Device 10 comprises a cooling tank 16 and a downstream filter device 20 as essential elements. The treatment solution 12 to be regenerated is transferred from the treatment tank 14 to the cooling tank 16, wherein a cooling device 18 is dedicated to the cooling tank 16. The cooling device 18 gives off cold via one of the heat exchangers which has coolant flowing through it, more preferably a cooling jacket 19 around the cooling tank 16, to the treatment solution in the cooling tank 16. The drain 35 of the cooling tank 16 is connected to a filter device 20 for separating the solid carbonate compounds to be separated from the treatment solution.

A pre-cooler 22 can be installed between the treatment tank 14 and the cooling tank 16, which for example can be designed as a heat exchanger, so that cooled and regenerated treatment solution 12, i.e. the filtrate or rinsing water of the filter device 20 can be used to pre-cool the treatment solution 12 to be regenerated, which leads to the cooling tank 16.

The filter device 20 can be coupled to a negative pressure unit 24 which facilitates the separation of solid parts from the treatment solution. It has been shown that a significant higher amount of carbonate crystals can be filtered out if the filtration process takes place in a fast manner. As a result of the negative pressure unit 24, there is a considerably higher pressure difference on the filter for the filtration which enables a significant faster filtration. The filter cake comprising the filtered off carbonate crystals can be rinsed using a flushing device 28 to remove adhesive permanganate. The resulting rinsing water solution can be transferred to the treatment tank 14. The filter cake is rinsed off the filter by the flushing device 28 while the resulting solution or sludge can be conveyed via the drain pipe 26, e.g. to a drainage. Filtrate which accrues in the filter device 20, so regenerated treatment solution, is preferably transferred back to the treatment tank 14. Therefore, the regenerated treatment solution flow through the pre-cooler 22 wherein it is automatically heated up again, wherein the treatment solution, which is transferred from the treatment tank 14 to the cooling tank, is cooled down.

The cooling tank 16 is provided preferably with a, for example motorised, agitation device 32 which for instance has rotating stirrer blades with vertical bars, which are typically moved closely along the inside wall of the tank to detach the carbonate seed crystals which form on the inside wall of the tank from said wall, and to distribute them in the solution. The stirrer blades are preferably extended in a funnel-shaped floor 34 of the cooling tank 16 (not shown here) and extend into it in order to be able to capture more or less all solid parts of the treatment solution 12 which have been deposited on the walls. The distance between the bars and the wall can for example be less than 20 mm, in particular less than 5 mm, preferably about 1 mm. The agitation device can extend up to into the drain 35 of the cooling tank to be able to also remove carbonate crystals from that wall. The regeneration of the treatment solution 12 serves the prolongation of their period of use, while the usual lifetime of the treatment solution 12 is generally relatively short. This depends strongly from the formation of carbonate compounds in the solution which can have a negative effect on the etching behaviour. For this reason, the regeneration device 10 of the present invention is used to reduce the carbonate concentrations to a value which is preferably less than 200 g/l, in particular to values between 30 and 150 g/l, wherein concentration values between 50 and 100 g/l (measured as gram of sodium carbonate per litre of treatment solution) are more preferred to obtain optimal results. To set the desired concentration range of the treatment solution 12, carbonate, which is formed by the etching process, is removed from the solution 12 using the device 10. This is done by freezing out the carbonate from the treatment solution 12 in the cooling tank 16, whereby it is converted from a dissolved state to a non-dissolved form. For economic reasons, as little permanganate as possible should be converted to a non-dissolved form. By using sodium permanganate a suitable freezing out temperature can be selected, wherein the carbonate can be frozen out without freezing out too high amounts of permanganate.

The filter device 20 is used for fast separation of the frozen out carbonate with a possible dissolution of the frozen out carbonate.

The temperature in the cooling tank 16 is to be set in such a manner that the carbonate crystals are forming in the treatment solution or at the inner wall. To freeze out the carbonate, a solution temperature of -12 to +12 °C is set, in particular from -9 to +10 °C, and preferably from -7 to +5 °C. The temperature of the cooling liquid is to be lower, respectively.

The quantity of treatment solution transferred to the cooling tank 16 is to provide in such a way that the temperatures required to freeze out the carbonate, are not exceeded in order to prevent a back dissolution of carbonate seed crystals which have already formed, which would otherwise considerably reduce the formation rate of frozen out carbonate. The transfer of parts of the treatment solution to the cooling tank 16 and the transfer of treatment solution comprising cooled and frozen out carbonate from the cooling tank 16 to the downstream filter device 20 can be done in such a way that the temperature of the treatment solution in the cooling tank does not exceed the freezing out temperature. By preference, there is also a feeding in and feeding out of partial volumes of the treatment solution in the cooling tank 16, e.g. less than 10 I, based on the operating volume of the cooling tank 16, e.g. more than 50 I. The corresponding partial volumes can be set and conveyed by the supply pump 21 and transfer pump 37. The return pump 23 returns the decarbonated regenerated treatment solution and the rinsing solution from the rinsing of the filter cake back to the treatment tank 14, wherein these solutions can flow through the pre-cooler 22.

The schematic block diagram of Fig. 2 illustrates, in comparison to Fig. 1 , additional constructive details of the cleaning unit 36 which essentially comprises the filter device 20. The filter device 20 can essentially comprises a filter tank 42 and a filter, such as a filter strainer 56.

To obtain a high yield of filtered carbonate, it has been proven to be

advantageous to execute the filtration process in a time period as short as possible, measured from the time the cooled down treatment solution is added, by preference a partial quantity of the treatment solution in the cooling tank 16, to the filter device 20 until the end of the formation of the filter cake. The filtration process is therefore completed in less than 10 minutes, preferably in less than 200 seconds. Alternatively, the filter device 20 can also be cooled down, e.g. using a cooling jacket. Longer filtration times are then also possible.

The cooling tank 16, transfer line 38 between the cooling tank 16 and the filter device 20, as well as their tank 42, can be thermally insulated.

An improved filtering effect and separating effect can be achieved by using a pressure difference. Via the vacuum pipe 46, the tank 42 can be charged with negative pressure from the negative pressure unit 24.

After filtration, the filter cake can be rinsed with water, preferably with cooled water of less than 10 °C, which is introduced from the rinsing water line 40 through the nozzles 52 (see Fig. 3). Any adhesive permanganate solution can hereby be washed off and fed back to the treatment tank 14. The rinsing water can thus be sucked through the filter 56 with negative pressure or vacuum. After the filtration process of the filter cake, which consists of the carbonate crystals, the filter 56 is cleaned with water, preferably uncooled or heated water, through the nozzles 52 and the carbonated solution or sludge is removed via the drain pipe 26.

The schematic view of Fig. 3 illustrates a possible embodiment of the present invention of the filter device 20. Around the top area in the filter tank 42, the filter tank 42 comprises nozzles 52 at the height of the filter or filter strainer 56. They are used to rinse the filter cake or to clean the filter from the filter cake by water from the rinsing water line 40. The filtrate is removed via the return pipe 44.

The filter 56 can be arranged tiltable, e.g. rotatable around a horizontal axis. The filter 56 is in the operational modus in a horizontal position and can be moved to an inclined or vertical position for cleaning purposes. It can also be tilted or rotated by 180°. In order to properly clean the filter strainer 56, the nozzles 52 are at a height above or below the level of the horizontal arranged strainer 56. During cleaning, the filter strainer 56 is tilted or rotated and the top and the bottom sides are sprayed with rinsing water.

The ventilation 30 and the negative pressure pipe 46 of the negative pressure unit 24 meet in an upper area, but below the filter, in the tank 42. The negative pressure unit 24 can essentially consist of a negative pressure pump 59, a negative pressure tank 58 and a cut-off valve 57.

As Fig. 1 shows, the device 10 for setting a certain carbonate concentration in the treatment solution can also be combined with an electrolytic regeneration device 1 1 for the oxidation of manganate, which is produced during the etching reaction, in permanganate. Both regeneration devices can then be connected to the treatment tank 14 via the supply and return pipes 25, 44, 64, 65. Reference list:

10 Device

1 1 Electrolytic regeneration device

12 Treatment solution

14 Treatment tank

16 Cooling tank

18 Cooling device

19 Cooling jacket

20 Filter device

21 Feed pump

22 Pre-cooler

23 Return pump

24 Negative pressure unit

25 Supply pipe

26 Drain pipe, disposal

28 Flushing device

30 Ventilation

32 Agitation device

34 Floor, funnel-shaped floor

35 Drain

36 Cleaning unit

37 Transfer pump

38 Transfer line

40 Rinsing water line

42 Tank, filter tank

44 Return pipe

46 Negative pressure pipe

52 Nozzles

56 Filter strainer

57 Cut-off valve

58 Negative pressure tank

59 Negative pressure pump

64 Return pipe

65 Supply pipe