KR101289755B1 - Method for laser drilling a multilayer workpiece - Google Patents

Abstract

The present invention is a method for laser drilling holes 153, 253 in a workpiece 150, 250 having a multi-layer structure, the first laser in the partial region 390 of the cross-sectional area of the holes 153, 253 to be drilled. The first layer 152, 252 is removed by the output 111 to provide a way for a portion of the first layer 152, 252 to remain within the cross-sectional area. The second laser output removes the second layer 151, 251 within the total cross-sectional area 395 of the holes 153, 253 to be drilled, where the remaining portions of the first layer 152, 252 Layers 151 and 251 are removed and lost at the same time. Complete material removal for the first layer 152, 252 along a defined border line 370 is achieved by partially removing material along the edge of the hole. It is preferable to use the UV laser beam 111 to remove the first metal layers 152 and 252 and the IR laser beam to remove the second layers 151 and 251 of dielectric.

Description

LASER DRILLING METHOD OF MULTIPLE CONSTRUCTED PRODUCTS {METHOD FOR LASER DRILLING A MULTILAYER WORKPIECE}

The present description is directed to laser drilling a workpiece having a predetermined cross-sectional area in a workpiece having a multilayer structure, in particular a circuit board having a multilayer structure comprising a first and at least a second metal layer and a dielectric layer respectively positioned between the two metal layers. It relates to a method for doing so.

Due to the rapid development of laser technology, the processing of materials by laser beams has become more important in the last few years. In the field of electronic device manufacturing, laser machining of individual circuit boards and circuit boards as well as electronic components is an essential tool due to the trend towards miniaturization of components to make electronic assemblies as compact as possible. Has been. By doing so, holes are drilled in the multilayer substrate by laser radiation, the holes having a diameter substantially smaller than the diameter of the holes drilled by conventional mechanical drilling methods. Under conditions where the laser output of the laser beam acting on the substrate is known exactly, not only through holes but also pocket holes can be drilled. For example, a pocket hole can be drilled in a multilayer circuit board in which a plurality of metal layers are electrically non-conductively separated by a dielectric interlayer. Any metal layer may be brought into contact by subsequent pocket hole plating. In this way, the electronic circuit can be formed not only in two dimensions but also in three dimensions, and thus the integration of the electronic assembly can be reliably increased in comparison to a substrate of only one metal layer or two metal layers.

When drilling holes in multilayer circuit boards, there is a significant difference in the ablation behavior of the metal layer and the dielectric layer, which leads to the problem that an effective drilling operation cannot be performed with only one laser beam with specific laser parameters.

EP 1 169 893 B1 discloses a method of opening a plated through hole in an electrically insulating base material provided with metal layers on both sides. Here, in the hole area to be drilled, the outer metal layer, which is usually made of copper, is removed by a chemical etching process. The hole is then drilled into the dielectric base material by a CO ₂ laser emitted in the infrared spectral range.

All laser drilling methods for laser drilling multilayer substrates are known, allowing drilling of micro holes defined in electronic circuit boards while avoiding wet chemical etching processes. Since the metal layer exhibits high reflectance for infrared (IR) radiation, it is known that multilayer circuit boards can only be drilled by CO ₂ lasers under very high thermal loads. For this reason, all laser drilling on a multilayer substrate is performed by two different process steps. In one process step, the metal layer is locally removed by a laser beam in the ultraviolet (UV) spectral region. In another process step, the dielectric interlayer is removed by an IR laser beam, typically generated by a CO ₂ laser. For this reason, so-called combined laser machining devices are mainly used to drill multilayer substrates, which include two different laser light sources, for example frequency multiplied Nd: YAG lasers. UV laser light sources such as and in particular IR laser light sources such as CO ₂ lasers.

Such a compound laser processing apparatus is known from US Pat. No. 5,126,532, which includes both UV lasers and IR lasers. The two laser beams generated by the laser light source are guided to the position of the holes to be alternately drilled by the rotatably supported mirror, so that the metal layer can first be removed by the UV laser beam, and then the dielectric layer is IR laser. Can be removed by a beam.

Since the output power of the laser light source emitted in the ultraviolet spectral region is usually insufficient to remove the metal layer in the entire cross section of the hole by one or more laser pulses, the removal of the metal layer is often performed by so-called trepanning. Is performed. Because of this, the laser beam on the metal layer is focused on a diameter substantially smaller than the diameter of the hole to be drilled. The laser beam is then guided into a circle along the edge of the hole to be drilled by a deflection unit comprising two mirrors supported to be movable so that the metal layer is removed along the circle line. In general, a cover plate made of a metal layer will pop out by itself after at least a number of completed circles have been rounded. Alternatively, the stepping process may also be carried out at a different radius, or the laser beam may be helically guided within the cross sectional area of the hole to be drilled. The problem with all laser drilling on multilayer electronic circuit boards is that the laser power of a typical UV laser light source is clearly lower than that of a CO ₂ laser light source. Thus, the process step of removing the metal layer is certainly slower than the subsequent process step of drilling the dielectric layer. As such, the speed at which the metal layer is drilled determines the speed of the overall drilling process and also determines the process yield, ie the maximum number of holes per hour drillable.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for laser drilling a hole in a workpiece having a multi-layer structure, which provides a high speed drilling of a metal layer and thereby a high speed drilling as a whole.

This object is achieved by a method comprising the features of claim 1, which is an independent claim. According to the invention, the first layer is removed by the first laser power only within a portion of the cross-sectional area of the hole to be drilled, so that a part of the first layer remains in the hole area. The second layer is removed by a second laser power in the entire cross-sectional area of the hole to be drilled, with the remaining part of the first layer being removed at the same time as the second layer is removed.

The present invention is based on the recognition that even if the first layer is not sufficiently removed, the remaining portion of the first layer is automatically removed when the second layer is removed. Since only part of the first layer is removed by the first laser power, the first step is certainly faster than removing the entire area of the first layer in the entire hole area. As such, the method of the invention for laser drilling can certainly achieve higher drilling speeds, especially if the complete removal of the first layer takes longer than the removal of the second layer because of the different removal properties between the two layers of material. The present invention is suitable not only for drilling pocket holes and blind holes, but also for drilling through holes, and thus can be widely used in the field of laser drilling of circuit boards having a multilayer structure. It should be noted that for the drilling of through holes, the bottom metal layer must be removed by conventional drilling methods since there is no further dielectric layer underneath when viewed from the processing laser beam.

According to claim 2, the material of the first layer is further removed along the edge of the cross-sectional area by the first laser output. In particular, it is possible to form passages or grooves in the first layer along the edge of the hole to be drilled, which preferably extends in the area of the second layer or the first layer just adjacent the surface. Due to this structure with a perforation effect, high quality holes can be produced since the edges of the holes clearly defined by the second laser power are guaranteed when the first layer is completely removed.

According to claim 3, the first laser output comprises a laser beam in the visible or ultraviolet spectral region. Because UV radiation is relatively weakly reflected on the metal layer compared to infrared laser radiation, UV radiation is particularly suitable for removing metal materials. In this case, the thermal stress of the workpiece is lower than that of IR radiation, because each atomic bond and metal bond between individual atoms and molecules of each metal layer can be broken directly by UV radiation. Is clearly reduced.

According to claim 4, the first laser power is generated by a soak plate-state laser, in particular a frequency multiplied laser. The active laser medium can be Nd: YAG, ND: YVO ₄ or ND: YLF, for example, which can generate laser radiation with a fundamental wavelength of 1064 nm. The solid state laser is preferably pumped using a semiconductor diode. Since this can be arranged around the active laser medium, the corresponding solid state laser does not require an external pump light source and can be implemented in a compact structure. To generate UV laser radiation the laser light source additionally comprises an optical nonlinear medium for frequency multiplication. Such optical nonlinear media, commonly known in laser technology, can be disposed on the inside as well as on the outside of the resonator. Lasers of the type described above having a fundamental wavelength of 1064 nm can obtain frequency multiplying radiation having wavelengths of 532 nm, 355 nm, and 266 nm. This frequency multiplication takes into account the case where the fundamental wavelength is divided by 2, 3 or 4 as an example. Especially in modern and very powerful laser systems, frequency multiplications of coefficients 5 and 6 and above can also be considered. Frequency multiplication is advantageous in that laser radiation in the visible or ultraviolet spectral region can be generated in a simple manner and is particularly suitable for removing metal layers such as copper.

According to claim 5, the second laser output is generated in the infrared (IR) spectral region. Corresponding laser radiation is preferably generated by the CO ₂ laser because the high available laser power quickly removes the dielectric second layer.

The use of pulsed laser radiation according to claim 6 is advantageous in that because of the cooling time occurring between the laser pulses, the removal of material exerts a relatively low stress on the workpiece to be drilled.

According to claim 7, the partial region of the first layer is removed by trapping with a laser beam guided along the edge of the partial region. This creates a structure along the entire edge of the partial region and extends to a depth up to the second layer. In this way, a disc lying on the second layer is produced and usually sticks out of itself from the hole area. If the manufactured disc remains unexpectedly in the second layer, the manufactured circular disc can be guided around the edge of the subregion or directed to the inner region of the disc, with one or more cycles of circulation. Is removed very reliably from the hole area by the high energy input.

According to claim 8, the second layer is removed by trapping or punching. When trapping, it is first ensured that the second laser power is directed directly to the second layer in the area of the hole exposed into the cross-sectional area of the hole. As such, due to the direct energy input into the second layer, it is ensured that the remaining portion of the first layer is removed before the second laser output is directed to the cross-sectional area portion, which is the portion not yet removed by the first laser output. .

During so-called punching, the laser beam is directed to the same point of the workpiece to be drilled as one or more continuous pulses. By using suitable optics, the spot size of the second laser output can be adjusted to different hole diameters.

According to claim 9, the partial region is located between the center of the hole and its edge. First, by directing the first laser beam towards the hole center, the laser beam can be quickly approached the hole edge. Since a deflection unit, which generally exhibits some mechanical inertia constantly guiding the laser beam, is required to be operated with relatively short movements, two processing steps, namely, the formation of passageways along the edge of the hole and The first layer removal step in the partial region can be performed effectively.

Preferably, the partial region is a circular region having a diameter about half the size of the total hole diameter. As such, the traverse path of the first laser output around the subregion is only about half the length of the traverse path around the entire hole, while the area to be removed of the first layer is one quarter of the total hole cross-sectional area. Corresponds to As a result, the energy input per zone to be removed becomes so much larger that a reliable removal for the cut-out cover plate is already ensured when the first laser power is once rounded around the partial region. In addition, the overall drilling process can be performed quickly due to the short travel path of the first laser power with respect to the first layer as a whole.

In addition, the advantages and features of the present invention can be seen from the detailed description accompanying the preferred embodiment described below.

1 shows a laser processing apparatus for drilling holes.

2 shows a cross-sectional view of a drilled pocket hole.

3 shows the pivotal movement of the first laser output according to a preferred embodiment of the present invention.

It can be seen that the reference numerals for the corresponding parts in the figure differ only in their first digit.

The laser processing apparatus 100 of FIG. 1 includes a laser light source 110 that emits a laser beam 111 in the ultraviolet spectral region. The laser light source 110 is a diode-pumped solid state laser, in particular an Nd: YLF laser optically pumped by a semiconductor diode disposed around an active laser medium. In a known manner, UV laser radiation 111 is generated by frequency multiplication by an optical nonlinear crystal.

The laser beam 111 is irradiated to the deflectometer 130 formed in a conventional manner including a galvo mirror. The laser beam deflected by the deflectometer 130 is guided as a processing laser beam 141 onto the substrate 150 to be processed via an imaging optical system 140 such as, for example, an F-theta optical system.

The substrate 150 includes a dielectric layer 151 whose upper and lower sides are covered with a metal layer 152, respectively. The metal layer is formed in a manner not shown to form a circuit path. Micro holes 153 are drilled and their walls can be plated in a known manner to create an electrical connection between the two metal layers 152. Each time the fabricated laser beam 141 is centered by a jump movement 155 onto the drilling position 154 for the manufacture of the micro holes 153 and then the spot size F adjusted by the imaging optics 140. The micro holes are each produced by moving in circular motion within the region of the drilling position 154 with In the following description, the manufacture of the micro holes 153 will be described with reference to FIG. 3.

Typically, different layers will be processed by different laser processing devices. A laser processing apparatus having a UV laser light source is provided for drilling a metal layer, and a laser processing apparatus having an IR laser light source is provided for drilling a dielectric layer. Instead of using two laser machining devices, a so-called combined laser machining device comprising two different laser light sources can be used.

2 shows a cross-sectional view of the micro holes 253 drilled into the multilayer substrate 250. The micro holes 253 are pocket holes, and only the upper metal layer 252 and the dielectric intermediate layer 251 are removed. The two metal layers 252 may be connected to be electrically conductive by the plating process of the subsequent pocket hole 253.

3 shows the movement of the processing laser beam 141 guided onto the upper metal layer 252 by the UV laser beam 111. To better understand the present invention, the drilling operations previously used to remove the upper metal layer 252 will first be described.

According to this, the laser beam is first moved over the circle 360, starting from the center M of the microhole 253 to be drilled to a point A of the edge of the microhole 253 to be drilled, with a very low UV laser power. The laser light source 110 is controlled while the laser beam travels along the semicircle 360, thereby emitting a non-pulsed, so-called cw laser beam (continuous wave laser beam). After reaching point A, the output of the UV laser beam 111 is correspondingly increased by activating the laser light source 110 and is typically set to a repetition frequency of 20 kHz. The laser beam is then guided to the outer raceway 370 along the edge of the micro hole 253 to be drilled. Depending on the given conditions (material and thickness of the upper metal layer, laser power, laser wavelength, etc.), the laser beam is guided along the outer trajectory 370 in one cycle, or in many successive cycles. This movement along the outer raceway 370 is performed several times until the circular cover plate formed on the upper metal layer 252 is removed. The laser beam then moves back along the second semicircle 380 to the center laser M at a low laser power in cw mode.

Typically, the removal of the dielectric layer 251 to be performed will be performed in a different laser processing apparatus, preferably by punching, ie by repeated collisions of the IR laser beam on the cross section corresponding to the cross section of the entire micro hole 253. . When the drilling operation is completed, the deflector 130 is directed to the center of the next micro hole to be drilled by the jump movement.

According to a preferred embodiment of the invention, the pulsed UV laser beam is guided at full power from the center of the microhole 153 to be drilled along the first semicircle 360 to point A. The laser beam will then be guided back to point A along the outer orbit 370 over the complete circle. As such, each groove or trench structure will be formed in the top metal layer 252, which will then facilitate removal of the top metal layer 252 within the full diameter of the micro holes 253. The groove structure, preferably produced by one circular movement, or by several circular movements along the outer raceway 370, preferably causes removal of the material down to the dielectric layer 251. Desired material removal on the outer orbit 370 may be achieved by adjusting the speed of travel and / or laser power. In either case, it should be ensured that the metal layer 252 remaining in the hole region will be completely removed after the underlying dielectric layer 251 is removed.

The UV laser beam is then guided back along the second semicircle 380 to the center M at the full power of the laser. If material removal has already been performed along the two semicircles 360 and 380 down to the dielectric layer 251, the cut cover plate fabricated on the upper metal layer 252 having a diameter corresponding to the distance between point A and M is It will pop out on its own. If the laser energy irradiated for both semicircles was not sufficient to remove the metal layer in the subregions 390 defined by the two semicircles 360 and 380, then the UV laser beam would be exposed when the subregions 390 were fully exposed. You will be guided once more along the inside circle. Optionally, the UV laser beam may be directed to another circle with a smaller radius to ensure reliable removal of the upper metal layer in the partial region 390.

After the partial region 30 is exposed, the substrate 150 will be transferred to another laser processing apparatus that includes a CO ₂ laser light source. The resulting IR laser beam will be directed to be centered with the center M on the substrate 250 with a spot size F corresponding to the diameter of the micro holes 153 to be drilled. As such, particularly because large energy transfer will be performed to dielectric layer 251 in partial region 390, metal layer 252 remaining within the hole diameter will also be removed automatically. The groove structure previously fabricated by the movement of the UV laser beam along the outer raceway 370 is now formed from the dielectric layer 251 by the upper metal layer along the clear waste edge extending along the outer raceway 370. 252) causes the removal to occur. After the top metal layer 252 is completely removed within the cross section 395 of the entire hole, the material of the dielectric layer 251 will be removed in a known manner by the IR laser beam.

Since material removal of the upper metal layer 252 by the UV laser beam 111 is performed only in the partial region 390 which is reduced compared to the cross section 395 of the entire hole, the first process step, i.e., by the UV laser beam The material removal step is obviously faster than the conventional method in which the first process step includes complete removal of the metal layer 252 in the entire hole area. Since material removal of the dielectric layer 251 by the IR laser beam can be performed significantly faster than the first process step (the available laser power in the IR spectral region is clearly higher than the power in the UV spectral region). The overall drilling process is obviously faster than the conventional drilling method.

As pointed out, the two process steps can also be carried out with a single so-called complex laser drilling machine comprising a UV laser light source and an IR laser light source.

Naturally, it will be appreciated that the present invention can also be used to drill workpieces, not only two but basically a plurality of metal layers, two adjacent metal layers each separated by an electrically insulating dielectric layer. . As such, a pocket hole of a predetermined depth in such a multilayer workpiece can be manufactured by removing each metal layer by the method described with reference to FIG. 3.

In summary, it can be expressed as:

The present invention is a method for laser drilling holes 153, 253 in a workpiece 150, 250 having a multi-layer structure, the first laser in the partial region 390 of the cross-sectional area of the holes 153, 253 to be drilled. The removal of the first layers 152, 252 by the output 111 provides a way for some of the first layers 152, 252 to remain within the cross-sectional area. The second laser output removes the second layer 151, 251 within the total cross-sectional area 395 of the holes 153, 253 to be drilled, where the remaining portions of the first layer 152, 252 Layers 151 and 251 are removed at the same time. Complete material removal for the first layer 152, 252 along the defined boundary line 370 is achieved by partially removing material along the edge of the hole. It is preferable to use the UV laser beam 111 to remove the first metal layers 152 and 252 and the IR laser beam to remove the second layers 151 and 251 of dielectric.

Claims (9)

In the laser drilling method of the workpiece of the multilayer structure which performs laser drilling of the hole 153, 253 which has a predetermined cross section to the to-be-processed object of a multilayer structure, The workpiece includes a circuit board 150 having a multilayer structure having two first layers 152 and 252, which are metal layers, and second layers 151 and 251, which are dielectric layers disposed between the first layers 152 and 252, respectively. As a laser drilling method for a workpiece having a multilayer configuration of 250), Using the first laser output 111, the first layer 152, 252 is completely removed in the partial region 390 of the cross section of the holes 153, 253 to be formed, and also the peripheral edge of the cross section. Along 370 to remove the material of the first layers 152 and 252 to form grooves, Using the second laser power, the spot size is equal to the diameter of the holes 153 and 253 to be formed, and the second layers 151 and 251 are cut in the whole of the cross section 395 of the hole, Simultaneously with the removal of the second layer 151, 251, the remaining portions of the first layer 152, 252 except for the partial region 390 are removed together. Way. The method of claim 1, And said first laser output comprises a laser beam (111) in the visible spectral region or near ultraviolet spectral region. The method of claim 2, Wherein said first laser output is generated by a solid state laser (110), by one or more of frequency multiplying and diode pumped solid state lasers. 4. The method according to any one of claims 1 to 3, And said second laser output has a laser beam in an infrared spectral region. 4. The method according to any one of claims 1 to 3, At least one of the first laser output and the second laser output comprises pulsed laser radiation. 4. The method according to any one of claims 1 to 3, A method of laser drilling of a workpiece in a multi-layer configuration, wherein the subregions (390) of the first layer (152, 252) are removed by trepanning. 4. The method according to any one of claims 1 to 3, A method for laser drilling a workpiece in a multilayer configuration, wherein the second layer (151, 251) is removed by punching. 4. The method according to any one of claims 1 to 3, And the partial region (390) is present between the center point (M) of the hole and the circumferential edge (A) of the hole. delete

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