WO1993025345A2

WO1993025345A2 - Process and device for cooling micro-drills

Info

Publication number: WO1993025345A2
Application number: PCT/DE1993/000511
Authority: WO
Inventors: Harry Züst; Walter Striedieck; Norbert Preussler; Jean-Claude Knaff
Original assignee: Atotech Deutschland Gmbh
Priority date: 1992-06-05
Filing date: 1993-06-07
Publication date: 1993-12-23
Also published as: DE4219045A1; WO1993025345A3

Abstract

The invention concerns a process and a device for cooling micro-drills (1) used in boring machines for producing micro-bores in printed circuit boards (5), as well as for removing the waste material from the close neighbourhood of the bore. A cooling fluid from a supply line (11) flows against the micro-drill (1). The cooling fluid is accelerated by a gas under pressure. The thus projected cooling fluid jet cools down and lubricates the micro-borer (1), substantially increasing its service life.

Description

Method and device for cooling micro-drills

description

The invention relates to a method and an associated device for cooling micro-drills, which are used in boring machines for the production of small holes in printed circuit boards, and for removing the drilling excavation from the narrower drill environment, in which a coolant flows against the micro-drill from a supply line.

In modern circuit board technology, the requirements for the size and quality of the circuit board holes are constantly increasing. The need for highly complex electronic circuits leads to the increased use of printed circuit boards in multilayer technology. By reducing the track widths and pads, the manufacturing tolerances with multilayers have to be narrowed more and more. The limit of the reduction is determined to a large extent by the necessary PCB holes. Through the use of SMD technology (surface mounted device), through holes are rarely required for component mounting, but the miniaturization of the components increases the complexity of the circuits. This means that more circuits per board can be accommodated with the same board size. However, this causes an increase in the number of crossing conductor tracks. A circuit breaker bypasses these intersections with so-called transfer holes and conductor tracks on other circuit board levels. Around the conductor tracks of different wiring levels. In order to be able to connect the conductor tracks of different wiring levels with each other, the transfer holes must be provided with an electrically conductive coating. In order to make optimal use of the available space on a board, the changeover bores should have the smallest possible diameter. The smaller the

Hole diameter, the smaller the diameter of the pads can be, which serve as landing sites for the transfer drilling. Small solder eyes allow a higher conductor density, which means that one or the other wiring level can be saved in a multilayer circuit board.

With several soldering eyes with small diameters lying one above the other in a multilayer in different wiring levels, high demands are placed on the accuracy of the boring machine and the geometry of the drill tip. This means that the drill must not only be positioned precisely, it must also not slip on the uppermost metal layers during the drilling process, and it must not run in the hole.

In addition to the location and shape of such bores, the quality of the borehole wall plays an important role for later plated-through holes. A faulty through-connection affects the reliability of the circuit. The quality largely depends on the structure of the board and the state of wear of the drilling tool. The majority of the multilayer boards used today consist of multilayer epoxy glass fabric layers. The poor thermal conductivity of the composite of epoxy resin and glass fabric is a major problem. The drilling tool heats up due to the poor heat dissipation through the board material. The temperature at the tip of the drilling tool reaches values that are higher than the pressing temperature during the manufacture of the blanks. As a result, the resin softens in the immediate vicinity of the tool. The glued flutes of the drilling tool prevent effective removal of the machined material. The congestion effect further increases the Borehole temperature. This means that the viscous epoxy resin that is not discharged is lubricated along the drill wall. In this way, the metallic contact areas can be insulated with epoxy resin in already drilled soldering eyes of the upper interconnect layers. It is no longer possible to make a through-connection at a later point without high hole cleaning efforts.

Furthermore, in the case of a resin-lubricated drill, the metal residues to be re-drilled are no longer cut and / or transported out of the borehole, but are displaced into the drill wall. However, this can lead to undesired contacts with conductor tracks that are in the immediate vicinity of a - possibly running - hole.

This displacement and smearing effect additionally causes a loosening of the board material, for example fraying of the glass fibers or the like in the vicinity of the borehole. Through raised ridges on the

The surface of the board can be affected by the contact to the components that will later be attached to the board. The breakouts and capillaries created in the intermediate layers also usually fill with liquid, which then evaporates suddenly during the soldering process and leads to the known tin eruptions from the holes.

The disadvantages mentioned, such as slipping of the drill tip during the tapping process, running of the drill in the bore and the problem of smearing and displacement, can be attributed to the high working temperature of the engaged drill part. The high working temperature promotes the quick wear of the drill. The wear shows up on the blunt of the cross and main cutting edge as well as the secondary surface chamfer. With increasing wear, the problems mentioned above increase until the micro drill breaks. US Pat. No. 4,917,547 describes, inter alia, a drilling milk supply in a circuit board drilling machine. The 'drilling milk is released in the direction of the drill via a line accommodated in the circuit board hold-down device. This wet drilling process ensures that the micro-drill is flushed around the hold-down chamber. The incoming drilling milk transports the drilling excavation out of the immediate vicinity of the drill and delivers it to a powerful suction device.

The disadvantage here is insufficient wetting of the drilling tool with drilling milk. The drilling tool, e.g. a micro drill of the type used in the present invention with a drill diameter of 0.3 mm, in order to achieve a cutting speed of 100 m / min, must be driven at a speed which is greater than 100,000 revolutions / min. The micro drill heats up to approx. 280 ° C. An annular, circumferential boundary layer flow of evaporated coolant is created around the drill casing protruding from the borehole. The steam cushion prevents effective drilling of the micro-drill with drilling milk. As a result, the service life of the micro drill and the quality of the borehole walls are only slightly higher or better with this process than with a pure dry drilling process,

From CH 596 946 a machine tool for drilling circuit boards is also known, in which an air-liquid mixture is sprayed onto the processing point, that is to say the workpiece surface, possibly with the aid of compressed air, in a suction chamber at the borehole in order to wash away chips , whereby the tool is also said to be cooled. In practice, it was found that this type of coolant supply also does not allow sufficient cooling of the micro drill. The object of the present invention is to provide a method and a device suitable for carrying it out, with which the service life of a tool for the mechanical production of, for example, micro-bores is considerably improved. The disadvantages known from the prior art for this problem are also to be avoided.

The solution to the problem is achieved by a method for cooling micro-drills, which are used in boring machines for the production of very small holes in printed circuit boards, and for removing the drilling excavation from the closer drill environment, in which the micro-drill flows with cooling liquid from a supply line ending in a nozzle becomes. According to the invention, the cooling liquid is included

Accelerated with the help of a gas under pressure inside the nozzle opening. The discharge of the coolant with pressurized gas produces a very fast flowing coolant jet at the outlet of the coolant supply. This coolant jet strikes the one surrounding the micro-drill - formed from evaporated coolant (drilling milk) -

Boundary layer flow. The energy of the coolant jet is so great that it breaks up the boundary layer flow. The coolant droplets bundled into the jet thus come into contact with the micro-drill and cool the hot drill. The drill temperature drops below that

Board press temperature. As a result, the epoxy in the borehole no longer becomes viscous. Resin and glass fabric are machined and discharged from the hole in the form of drilling dust via the chip grooves, which are no longer sticky. This drilling excavation, including the machined remains of the printed circuit board, is simultaneously removed from the drilling environment by this coolant jet or additional coolant jets.

In the method according to the invention act on the Drill surface when the boundary layer flow is torn open still the following positive physical effects: heat absorption by expansion of the gas hitting the drill and heat removal at the drill by heat of vaporization for evaporating coolant.

Lowering the drill temperature inhibits wear on the cutting edges. The extensive preservation of the cutting edge geometry guarantees the quality of the drilling over the life of the drill. The risk of slipping with a sharp chisel edge is low during the tapping process. The drill is unlikely to run as long as the main cutting edges are neither blunt nor broken. Also, the excavation can not force itself between the drill and the borehole wall as long as the minor cutting edges are not worn out. As a result, smearing of the bore wall and loosening of the individual circuit board layers is avoided.

A further advantageous embodiment of the invention is that the imaginary center line of the coolant jet tangentially touches the microbore casing above - the side facing the microbore - the circuit board surface at the point where the horizontal component of the coolant jet hits the boundary layer flow surrounding the microbore. Under this lighting condition, the

Boundary layer flow is broken up very effectively, since here the coolant jet is aimed directly at the flow surrounding it and surrounding the drill. The speed of the horizontal component of the coolant jet is much greater than the maximum peripheral speed of the boundary layer flow. As a result, the boundary layer flow breaks off in the initial area of the collision with the coolant jet, which increases the effective drill wetting area. Furthermore, the method can be optimized in that the incident angle between the imaginary center line of the coolant jet and the circuit board surface is 5 to 45 °, preferably between 15 to 35 °. With increasing angle of attack, a drill bend due to the coolant jet can be reduced in relation to a horizontal inflow, so that the coolant jet can be switched on before the drill is placed on the circuit board without fear of the drill running. By selecting a suitable angle of incidence, it can be prevented that excavated drilling, which is still between the jet nozzle and the drill, is blown into the drilling zone.

Furthermore, it is advantageous if, in this process, an aqueous solution is used as the cooling liquid which contains 0.1 to 1.0% by weight of tetraethylammonium perfluoro-octane sulfonate, 0.05 to 1.0% by weight. % Polyvinyl-3-methylimi-dazolinium methosulfate and 0.4 ... 2.0% by weight para-n-nonylphenyl nonyl ethoxy polyether.

This aqueous solution shows particularly good wetting behavior. This promotes the heat given off by the drill to the coolant droplets touching it, which leads to a further reduction in the drill temperature. In addition, this drilling solution has no skin irritating properties.

The same applies to the use of aminocarboxylates or their betaines as ingredients of drilling solutions, as are known from DE-OS 41 26 513 and DE-OS 41 27 441. The proportion of aminocarboxylates and / or their betaines in a drilling solution can be 0.01 ... 10% by weight. The concentration of these proportions is preferably from 0.5 to 3% by weight. A proportion of 0.3 to 5% by weight is also conceivable.

The compounds can each be used alone or as a mixture with several compounds or as a mixture with solvents be used. Suitable solvents include water, alcohols, ketones or esters. Particularly preferred alcohols are methanol, ethanol, propanol or isopropanol.

Furthermore, suitable additives can be added to the drilling solutions in order to give the drilling solution the desired viscosity.

Compressed air should preferably be used as the gas. The drill can be supplied from the standard compressed air network via a pressure reducer. The gas speed and thus the coolant jet speed can be controlled in a simple manner with the pressure reducer. The use of other, but more expensive gases is possible if the physical properties of the gases are taken into account accordingly.

To carry out the method, a jet pump is provided, with which the cooling liquid and the gas under pressure are mixed and discharged. Such a jet pump, also called an injector pump, has a comparable design with a Bunsen burner or a water jet pump. Without moving parts, it pumps liquids or gases with an efficiency of 20 ... 25%. In the present device, the

Jet pump with the help of compressed air, the coolant to be conveyed to the drill - possibly even under pressure -. In addition to a significant increase in the speed of the coolant jet, the cooling liquid is atomized. Over other

Atomizers and / or carburettors have small dimensions and are also maintenance-free.

The entire device can be improved in that, in addition to that which emits the coolant jet,

Jet pump at least one auxiliary jet pump and / or auxiliary nozzle is provided. The auxiliary nozzles blow during the drilling process continuously or intermittently compressed air into the hold-down chamber. They are arranged almost parallel to the surface of the circuit board in order to feed the drilling excavation not captured by the coolant jet to a suction device as completely as possible. It is not your job to blow or cool the drill.

Further details of the invention result from the following description of the partially schematically illustrated embodiments. Show it:

Figure 1: Section through the lower part of a high-speed drilling machine with workpiece holder and coolant supply and a workpiece to be drilled;

Figure 2: Bottom view of Figure 1.

The section shown in Figure 1 shows the lower part of a high-speed drilling machine. A micro drill 1 is received by a drilling spindle 3 via a clamping device 2. The drilling spindle 3 is in a (not shown) bearing plate of a spindle bell

Multi-spindle automatics rotatably supported.

For the infeed and the feed of the micro drill 1, the spindle bell is in relative to the multi-spindle machine

Moved in the direction of the drilling axis.

The drilling spindle 3, the clamping device 2 and the micro drill 1 are surrounded by a tubular workpiece hold-down device 4. The workpiece hold-down device 4 presses the workpiece, e.g. a multilayer circuit board 5 via an elastic ring 8 against a drilling plate 7. Figure 1 shows the workpiece holder 4 in the working position.

The workpiece hold-down device 4 includes, among other things, a multi-part coolant supply in the form of a jet pump 9. The jet pump 9 consists of a driving nozzle 10, a collecting nozzle 11 with the suction area 12 and a coolant line 13. The jet pump 9 is supplied with gas via a hose line (15), preferably with compressed air, which is under a pressure of about 3 bar. The gas flows out of the driving nozzle 10 at high speed and mixes in the collecting nozzle 11 with the cooling liquid supplied in the suction region 12 via the bore 14 from the cooling liquid line 13 with a pressure of 0.3 to 0.6. The coolant jet bundles out of the collecting nozzle 11 in order to cool and lubricate the micro-drill, which has a diameter of approximately 0.3 mm and cuts into the workpiece at 100,000 revolutions / min, in the region of the borehole. At the same time, the coolant jet flushes the drilling excavation, consisting of drilling dust of the basic board material and drilling chips of the metallic board coating (conductor tracks), on the side facing away from the jet pump 9. For reasons of simplification, it is conceivable to design the jet pump in such a way that the catching nozzle 11 and the driving nozzle 10 consist of spaced cylindrical tubes.

Opposite the jet pump 9 there is a suction pipe 18 which merges into a hose line 19. The

Hose line 19 is connected to a suction pump, not shown. The suction pipe 18 receives the drilling excavation washed away from the surroundings of the micro-drill 1.

Figure 2 shows the workpiece holder 4 with the

Coolant and suction lines. It can be seen from this illustration that the jet pump 9 is not centered on the axis of the micro-drill 1. The jet pump 9 is arranged so that its imaginary center line 16 just touches the jacket of the micro drill 1. The boundary layer flow surrounding the microbore 1 rotates towards the coolant jet and becomes from the coolant jet torn open.

For improved chip removal and cleaning, the jet pump 9 is flanked by two compressed air pipes 20, 20 ^Λ . Compressed air flows into them via lines 21, 21. Both compressed air pipes are oriented so that the projections of their center lines on the

PCB surface 6, cf. Figure 1, cut before the inlet of the suction pipe 18. The center lines of the at an angle of approximately 15 ° to the printed circuit board surface 6, cf. Figure 1, inclined compressed air tubes 20, 20 'cut the circuit board surface on both sides a few mm next to the micro-drill 1. This ensures that when these compressed air tubes are pressurized, the individual compressed air jet piles of excavation outside of

Can detect coolant jet to supply them to the intake manifold 18.

With an arrangement according to the exemplary embodiments using the specified cooling liquid, the

The service life of the micro drill can be increased by 6 - 15 times compared to a dry drilling process, while significantly increasing the quality of the drilling.

Claims

1. Method for cooling micro-drills, which in

Boring mills are used for the production of very small holes in printed circuit boards, as well as for the removal of the drilling excavation from the closer surrounding of the drill, in which the microbore is blown with cooling liquid from a supply line ending in a nozzle, the cooling liquid using a gas under pressure inside (11 ) is mixed and accelerated in front of the nozzle opening.

2. The method according to claim 1, characterized in that the imaginary center line (16) of the coolant jet formed from coolant and gas touches approximately tangentially the micro drill shell above the circuit board surface 6 at the point at which the horizontal component of the coolant jet frontally on one surrounding the micro drill 1 Boundary layer flow meets.

3. The method according to claim 1 or 2, characterized in that the incident angle between the imaginary center line (16) of the coolant jet and the circuit board surface (6) is about 5 to 45 °, preferably 15 to 35 °.

4. The method according to any one of claims 1 to 3, wherein the cooling liquid is an aqueous solution, 0.1 ... 1.0 wt .-% tetra-ethylammonium perfluoro-octane sulfonate, 0.05 ... Containing 1.0% by weight of 1 polyvinyl 3-methylimidazolinium methosulfate and 0.4 ... 2.0% by weight of para-n-nonyl-phenyl-nonyl-ethoxy-polyether.

5. The method according to any one of claims 1 to 4, wherein compressed air is used as gas.

6. Device for carrying out the method according to at least one of the preceding claims with a boring machine for small bores, a guide and a hold-down device for workpieces and a cooling device for micro-drills (1), characterized in that the cooling device for a nozzle combination (10 - 12) for the mixing and discharge of a cooling liquid and pressurized gases by means of a jet pump 9.

7. The device according to claim 6, characterized in that in addition to the, the coolant jet emitting jet pump (9) at least one auxiliary jet pump and / or auxiliary nozzle (20, 20 ') is provided for removing chips and coolant from the area of the borehole.

8. A method of cooling micro-drills comprising all of the disclosed new features and combinations of features.