The present invention relates to a method for etching printed circuit boards.

BACKGROUND

Chemical etching of copper layers on printed circuit boards, PCBs is a well known practice within the industry when producing electronic equipment. A PCB normally comprises a copper layer fixed to a non-conductive layer, a substrate consisting of a laminate material as for example glass reinforced epoxy. In order to create a circuit with a desired shape of conductive paths, the process normally starts with applying an etching resist film comprising a mask on the copper layer. The mask represents the pattern of the conductive and the non-conductive areas on the PCB and could be produced by different pattering techniques.

The most common technique comprises photolithography in combination with chemical etching. Areas to be etched are separated from areas not to be etched by means of a light sensitive protective layer, which is exposed and developed prior to the etching. The exposure is mostly done with the aims of a transparent film which contains the intended pattern, either in negative or positive version, after which the light sensitive layer can be developed. Some other techniques use a laser to expose the light sensitive layer directly without using a transparent film to define the pattern. After the patterning of the light sensitive layer, the PCB is immersed into a chemical etch liquid (for example ferric chloride) and non protected areas are etched, leaving only the intended copper left on the laminate.

A disadvantage with chemical etching is that it creates problems when etching PCBs having a relatively thick copper layer. PCBs with a relatively thick copper layer are often required for power electronics where it is necessary to handle large electric currents. Thick copper layers reduce electrical and thermal resistance which improves energy efficiency and thermal management. With the miniaturization of the power electronics it is also necessary to have more narrow conductive and non-conductive parts on the PCB and to mount the components closer to each other. As the chemical etching of a copper layer is to a large extent uniform in direction, it means that the copper is not only etched in one direction (downwards through copper layer but also in other directions. This causes a pyramidal shape of the copper layer. This effect is illustrated by Figures 1A-1D. The pyramidal effect 160 increases with the thickness of the copper layer 220. For thin copper layers this effect is normally not a big problem, but for thicker layers the result is that it is difficult to etch enough narrow patterns that allow the use of components with smaller pitch (distance pad-pad) . This problem excludes many modern electronic components to be used in a more compact design.

It is known from prior art that laser beams can be used for drilling via holes between different copper layers in a multi layer PCB. This is for example disclosed in patent application US 7,666,320. In this patent the laser beam penetrates a first copper layer and the underlying substrate layer until it reaches a second copper layer.

However, using laser beams for etching a pattern of the copper layer (as done with chemical etching) has proved to be very difficult without destroying the substrate layer underneath the copper layer. SUMMARY

With this background it is the object of the present invention to obviate at least some of the disadvantages mentioned above. The object is achieved by a method that combines laser etching and chemical etching. The method comprises the steps of firstly applying a layer comprising a thermal absorbing material (for example brown oxide) on the copper layer of the PCB. The next step is to radiate the PCB with a laser beam. The laser beam follows a pattern to be etched and the energy of the laser beam is adapted so that the beam penetrates the etching resist layer and only down to a predefined depth of the copper layer (preferably as close to the substrate layer as possible without destroying it or causing deformations) . After the laser etching, the remaining parts of the laser etched copper layer are etched down to the substrate layer by chemical etching.

An advantage with the invention is that the pyramidal shape of the etched copper is largely reduced which in turn means that the conductive and non-conductive pattern can be more dense. This makes it possible to use components with smaller pitch and to design more compact power electronics.

The invention will now be described in more detail and with preferred embodiments and referring to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1A-1D are block diagrams illustrating the problems involved when etching relatively thick copper layers with chemical etching. Figures 2A-2C and 3A-3B are block diagrams illustrating the etching method according a first embodiment of the present invention .

Figures 4A-4C are block diagrams illustrating a second embodiment of the present invention.

Figures 5A-5C are block diagrams illustrating a third embodiment of the present invention.

Figures 6A-6D and 7A-7B are block diagrams illustrating a combination of the present invention and conventional etching.

Figure 8 is a flow chart illustrating the manufacturing steps of the present invention and different embodiments.

DETAILED DESCRIPTION

Figures 1A-1D illustrate the problem of using traditional chemical etching on a PCB 200 with a copper layer 220 that is relatively thick compared to the desired width of the conductive paths 150 and the spaces 140 between the conductive paths 150. On the copper layer 220 an etching resist film 110 is applied (Figure 1A) . When applying an etching liquid E on the unprotected parts 140 of the copper layer 220 as illustrated by Figure IB, the liquid E etches not only in the vertical direction but also in other directions of the copper layer 220. When the etching liquid E has reached the substrate layer 230 the shape of the etched space becomes pyramidal (160 in Figure 1C) . Figure ID illustrates the PCB 200 after that the etching resist film 110 has been removed.

The pyramidal effect increases with the thickness of the copper layer 220. For example in a typical design rule for PCBs with thicker copper layers, the minimum conductor width is 18mil (-0.4 mm) for a 120Z (-0.4 mm) thick copper layer. This leaves no space left for using electronic components with a lead pitch less than 0.65 mm if one wants a soldering surface of 0.25 mm. This limitation excludes many of the modern electronic components to be assembled on a thick copper layer.

Figures 2A to 6D illustrate different embodiments of the present invention. Figures 2A-2C and 3A-3B illustrate a first embodiment and Figure 8 illustrates a corresponding flow chart starting at (A) . Figure 2A illustrates a PCB 200 with a copper layer 220 fixed to a substrate layer 230. The copper layer 220 is covered (step 840 in Figure 8) by a layer 210 of a thermal absorbing material. The thermal absorbing material could for example be of tin or brown oxide. It could also be a conventional photo-resist film that has inherent thermal absorbing properties. The purpose of the thermal absorbing layer 210 is to improve the absorption of laser energy thus creating a stable process window for a following laser work. The PCB 200 is in Figure 2B radiated (step 850 in Figure 8) by a laser beam L where the beam L follows the pattern to be etched. The laser beam L is controlled by a laser cutter arrangement that is programmed with executable CAD software in order to provide the desired pattern. The energy level for the laser beam L is also programmed so that when the beam L follows the pattern to be etched it penetrates the thermal absorbing layer 210 and down to a certain depth 221,222 of the copper layer 220. It is preferred that the thickness of the remaining parts 221b, 222b of the copper layer 220 is less than the thickness of the penetrated parts 221,222 or that the penetrated parts 221,222 are as close to the substrate layer 230 as possible, but without destroying or causing deformations to said layer 230. The next step (step 860 in Figure 8) is to perform chemical etching by using a chemical etching liquid E in order to remove the remaining non penetrated parts 221b, 222b of the laser etched copper layer 220 down to the substrate layer 230. This is shown in Figure 2C. This is normally done by immersing the PCB 200 into a bath containing the chemical etching liquid E.

As the remaining parts 221b, 222b are relatively close to the substrate layer 230, the pyramidal effect is significantly reduced as illustrated in Figure 3A.

Optionally the thermal absorbing layer 210 is removed as in Figure 3B and in step 870 in Figure 8. The removal can be done either by mechanically cleaning (brushing) or by chemical cleaning.

The PCB 200 illustrated in Figures 2A-2C and 3A-3B has only one copper layer. The inventive concept is however also applicable to two-layer PCBs and multilayer PCBs and a combination of these. A two-layer PCB is here defined as a PCB having two copper layers on opposite sides of the substrate layer and a multilayer PCB is here a PCB having multiple copper layers and substrate layers laminated to each other. For a two-layer PCB the method of the present invention is illustrated in Figures 4A-4C. Figure 4A illustrates a two-layer PCB 300 having a first copper layer 220 and an opposite second copper layer 240. Applied on each copper layer 220,240 is a thermal absorbing layer 210,250. The first copper layer 220 has already been partly penetrated in step 850 by the laser beam L as described above. The second copper layer 240 is partly penetrated by the laser beam L in the same way as the first layer 220. It is also possible that both copper layers 220,240 are laser etched simultaneously with two opposite laser beams.

After both copper layers 220,240 have been laser etched, the PCB 300 is immersed in a bath containing the chemical etching liquid E and the remaining parts of the laser etched copper layers 220,240 are chemically etched down to the substrate layer 230 in step 860 (Figures 4B and 4C) .

An embodiment of how to etch copper layers for a multilayer PCB is illustrated by Figures 5A-5C. Figure 5A corresponds to Figure 3A, i.e. the result achieved after the sequential laser and chemical etching of the copper layer 220 according to the present invention. In order to produce a multilayer PCB a plurality of copper layers and substrate layers are stacked and laminated together. But before adding a second substrate layer 270, an additional brown oxide layer 260 is applied to the PCB 200 as illustrated in Figure 5B and in step 880 in Figure 8. The purpose of the additional brown oxide layer 260 is to increase the adhesion between the copper layer 220 and the substrate layer 270.

After adding the second substrate layer 270 in step 890, a second copper layer 280 is added in step 895 as illustrated in Figure 5C. After this, the process of the sequential laser and chemical etching of the second copper layer 280 is repeated (A) and performed in the same way as already described and illustrated by steps 840,850,860 in Figure 8 and by Figures 2A-2C.

The sequential laser and chemical etching process can also be combined with an additional and conventional chemical etching process of an additional pattern. This is for example applicable when some parts of the PCB 200 do not have the same requirements with regard to narrow patterns as the laser etched parts. The additional chemical etching process is performed in the same way as illustrated in Figure 1A-1D but precedes the sequential laser and chemical etching process and starts at (C) in Figure 8. That is, an etching resist film 110 with a certain pattern is applied on the copper layer 220 as illustrated in Figure 1A and in step 810 in Figure 8. The conventional chemical etching is performed in step 820 as earlier described and illustrated by Figure IB. In step 830 the etching resist film 110 is removed as illustrated in Figure ID. Figure 6A illustrates a PCB 200 after the process steps 810,820,830. In Figure 6B the layer 210 of thermal absorbing material is applied as in process step 840. The PCB 200 is in Figure 6C radiated (step 850 in Figure 8) by a laser beam L where the beam L follows the pattern to be etched. Again, the energy level for the laser beam L is programmed so that when the beam L follows the pattern to be etched it penetrates the thermal absorbing layer 210 and a part 223,224,225 of the copper layer 220. After the laser etching the PCB 200 is in step 860 immersed into a bath of chemical etching liquid E so that the remaining non penetrated parts 223b, 224b, 225b of the laser etched copper layer 220 dov/n to the substrate layer 230 are removed. This is shown in Figure 6D. The resulting pattern is illustrated in Figure 7A and again, as an option, the layer 210 of thermal absorbing material is removed in step 870 and as illustrated in Figure 7B.

The inventive concept can in fact be included as a part of a multitude of different manufacturing processes of the PCB 200,300. The different processes are structured as follows:

1. Inner layer process For the process of manufacturing the inner layers of a multilayer PCB, different embodiments are possible.

1.1 Laser only defined copper:

In this process, the copper layers are etched using the sequential laser-chemical etching process but without any additional conventional etching process. The thermal absorbing material can be either brown oxide or a photo-resist film.

1.1.1 Thermal absorbing brown oxide

In this embodiment a brown oxide is applied as the layer 210 of a thermal absorbing material in step 840 and as illustrated in Figure 2A. The brown oxide layer 210 and a part 221,222 of the copper layer 220 is penetrated by a laser beam L in step 850 and as illustrated by Figure 2B. The chemical etching E of the remaining parts 221b, 222b of the laser etched copper 220 is performed in step 860 and illustrated by Figure 2C. When the PCB is etched as illustrated in Figure 5A, a new layer of brown oxide 260 is applied in step 880 and as illustrated by Figure 5B.

After adding a second substrate layer 270 in step 890, a second copper layer 280 is added in step 895 as illustrated in Figure 5C. After this, the process of the sequential laser and chemical etching of the second copper layer 280 (steps 840,850,860) can repeated as illustrated by option (A) in Figure 8 or using the combined process (also including steps 810,820,830) in option (C) . An embodiment of option (C) is also described in paragraph 1.2.1 below.

1.1.2 Photo-resist film

In this embodiment a photo-resist/etching-resist film is applied as the thermal absorbing layer 210 in step 840. The photo-resist/etching resist film 210 and a part 221,222 of the copper layer 220 is penetrated by the laser beam L in step 850 and the remaining parts 221b, 222b of the laser etched copper 220 are chemically etched E as in step 860. After the etching, the pho o-resist/etching resist film 210 is stripped off in step 870. After the photo-resist/etching resist film 210 is removed in step 870, a layer of brown oxide 260 is applied in step 880 and the process continues by adding an additional substrate layer 270 in step 890 and an additional copper layer 280 in step 895.

1.2 Combination with conventional chemical etching 1.2.1 Thermal absorbing tin layer

In this embodiment the sequential process (steps 840,850,860) of laser and chemical etching is preceded by an additional conventional chemical etching process (steps 810,820,830) illustrated by Figures 1A-1D.

A photo-resist/etching resist film 110 is applied to the copper layer 220 in step 810. The photo-resist/etching resist film 110 is exposed and developed following conventional methods resulting in a certain pattern 140,150 to be etched. In step 820 the copper layer 220 is chemically etched down to the substrate layer 230. The photoresist/etching resist film 110 is stripped from the copper layer in step 830. In step 840 a layer of tin 210 is applied. The sequential process (steps 840,850,860) of laser and chemical etching is now performed with the tin layer 210 as the thermal absorbing material. When the etching is ready and after the tin layer 210 has been removed in step 870, the process continues by adding a brown oxide layer 260 in step 880, an additional substrate layer 270 in step 890 and an additional copper layer 280 in step 895.

2 One or two-layer PCB outer process

With or without having performed the inner layer process as described in section 1 above in advance, the outer process can be performed according to at least any of the following embodiments . 2.1 Laser only defined copper:

As in section 1.1, the copper layer is etched using the sequential laser-chemical etching process but without any additional conventional etching process. The thermal absorbing material can be either brown oxide or a photo-resist film.

2.1.1 Thermal absorbing brown oxide

This embodiment involves the steps of initially preparing the PCB 200. The embodiment includes the steps of: - drilling a plated-thru-hole (PTH) needed to make signal connections within the PCB 200;

- a desmearing process;

- an electroless copper process, resulting in a thin (1-5 My) copper layer on the PCB 200; - separating areas to be etched from areas not to be etched by means of a light sensitive photoresist/etching resist film, which is exposed and developed prior to the etching;

- electroplating, a process whereby more copper is applied until a predefined thickness of the copper layer is reached;

- applying a layer of tin (tin plating) ;

- stripping the photo-resist/etching resist film;

- applying a layer 210 of brown oxide (step 840) . After these steps, the sequential laser-chemical etching process as described above is performed by penetrating the brown oxide layer 210 and a part 221,222 of the copper layer 220 by the laser beam L in step 850 and chemically etching the remaining parts 221b, 222b of the copper layer 220 in step 860. Finally the tin layer is stripped off.

2.1.2 Photo-resist film This embodiment follows similar steps as in section 2.1.1, but without the step of applying the brown oxide as the photo-resist/etching resist film is acting as the thermal absorbing layer 210. The photo-resist/etching resist film is removed in a step preceding the step of stripping off the tin layer. The embodiment includes the steps of:

- PTH drilling;

- desmearing;

- electroless copper process;

- applying light sensitive photo-resist/etching resist film, which is exposed and developed prior to the etching (step 840) ;

- electroplating;

- tin plating;

- laser etch (step 850) ;

- chemical etch (step 860);

- photo-resist/etching resist film strip (step 870);

- tin strip. 2.2 Combination with conventional chemical etching 2.2.1 Thermal absorbing tir. layer

Again, in this embodiment the sequential process (steps 840,850,860) of laser and chemical etching of the copper layer 220 is preceded by an additional conventional chemical etching process. The steps are:

- PTH drilling;

- desmearing;

- electroless copper process; - applying light sensitive photo-resist/etching resist film, which is exposed and developed prior to the etching;

- electroplating;

- tin plating; - photo-resist/etching resist film strip;

- chemical etch {step 820) ;

- tin plating (step 840) ;

- laser etch (step 850);

- chemical etch (step 860) ;

- tin strip (step 870)