Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/03—Observing, e.g. monitoring, the workpiece
  • B23K26/034—Observing the temperature of the workpiece
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K1/00—Soldering, e.g. brazing, or unsoldering
  • B23K1/005—Soldering by means of radiant energy
  • B23K1/0056—Soldering by means of radiant energy soldering by means of beams, e.g. lasers, E.B.
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
  • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
  • H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
  • H05K3/341—Surface mounted components
  • H05K3/3421—Leaded components
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
  • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
  • H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
  • H05K3/3494—Heating methods for reflowing of solder

Definitions

• the present invention relates to a process for the brazing by laser beam of several approximately punctual metallic elements on a non-metallic substrate. It applies in particular to the manufacture of electronic circuit boards by soldering connection pins or the like (lugs, contacts, etc.) of various electronic components.
• Laser soldering has significant advantages.
• it is a contactless process in which the energy is deposited, with a high surface density, only in the zone where the solder is located, and can be controlled reliably and precisely.
• the laser method therefore has the advantage of avoiding the heating of the components.
• Degradations can indeed be encountered in conventional infra-red (IR) or vapor phase or wave soldering methods: the entire electronic card, including the sensitive components, is then subjected to temperatures of 200 ° C for several seconds or minutes.
• IR infra-red
• vapor phase or wave soldering methods the entire electronic card, including the sensitive components, is then subjected to temperatures of 200 ° C for several seconds or minutes.
• a major drawback is that this point-by-point technique is relatively slow. Furthermore, in this point-by-point technique, the amount of laser energy absorbed by each soldering point as well as the surface illuminated by the laser are sensitive parameters: if they are poorly controlled, the temperature development of the solder is poorly controlled, resulting in defects quality: local burning of the substrate, pollution by dissemination of the soldering cream. These defects occur in particular when using an Nd-YAG laser at 1.06 / Am, which is often preferred to the CO laser (at 10.6 / * ⁇ * * - m) in the point-to-point soldering, because better absorption by the metallic materials involved in the soldering of the wavelength 1.06. m.
• the object of the invention is to provide a technique allowing rapid and reliable soldering of series of connection pins or the like, with various types of laser, including CO lasers.
• the subject of the invention is a method of the aforementioned type, characterized in that a region of the substrate which is entirely located in the immediate vicinity of all of said elements is heated overall by means of the laser, to a predetermined temperature at least equal to the fu ⁇ sion temperature of the soldering products and lower than the degradation temperature of the substrate.
• the temperatures are about 220 ° C for melting of an alloy sour kind SnPb bra ⁇ and 300 ° C for temperature dé ⁇ gradation of an epoxy type substrate.
• This process achieves two conditions which are not fulfilled at the same time by the other conventional or laser soldering processes: on the one hand, a series of soldering points are produced simultaneously, in a very short time, and on the other hand, the heat affected zone is limited to the useful zone without risk of damaging the substrate or the components to be brazed.
• the heating is carried out by scanning said region with the laser beam, so that the whole of the region swept by the beam has a substantially uniform temperature.
• This scanning corresponds to a periodic movement of the impact of the laser beam on the substrate in a region entirely located in the immediate vicinity of the elements to be brazed. This movement is fast enough for the heating of the substrate to be substantially homogeneous in the scanned area, the scanning period being short compared with the thermal constants of the substrate.
• the temperature of said region is measured by means of a pyrometer which measures the intensity of a radiation characteristic of the substrate, in particular by reflex aiming of said region, this characteristic radiation corresponding to a length of wave different from that emitted by the laser.
• FIG. 1 schematically shows in perspective an apparatus according to the invention
• Fig. 2 is a plan view of an electronic circuit board produced in accordance with the invention, by means of the apparatus of FIG. 1.
• a first dichroic plate 11 allowing the superposition of the two laser beams, an optical focusing system 12, and a second dichroic plate 13; an infrared pyrometer 14 to which the mirrors 3 and 4 and the blade 13 return a ray 15 characteristic of the substrate 2, emitted by the latter, and the intensity of which is representative of the temperature of this substrate.
• an organic material substrate such as a substrate epoxy
• a pyrometer which is suitable for this temperature measurement is the MODLINE type pyrometer , series. 3400, from IRCON; an electronic computer 17, such as a microprocessor, provided with software for managing and controlling the laser emission as well as the oscillating movements of the mirrors and those of the support 1, as described below , via interfaces 18 (for the laser) and 19 (for the motors 7 and 8).
• the interface 18 also serves as an interface between the pyrometer 14 and the laser 9, for modulating the power of the laser beam, as described below.
• small passive elements 20 such as two-pole miniaturized resistors 20A; larger passive elements such as resistors 21 of elongated shape comprising two projecting end connection lugs 22, or a capacitor 23 with two connection lugs 24 in juxtaposed sail ⁇ lie; complex rectangular electronic boxes with two opposite sides (boxes 25, 26, 27) or on the four sides (box 28) a series of connection pins very close to each other.
• the pins 29 practically do not project in plan view, while the pins 30 of the boxes 27 and 28 project clearly; a so-called card edge connector 31 having a plurality of connection tabs 32 projecting substantially along one edge of the substrate.
• each pin or the like ends up on a pad of a circuit track defined on the substrate.
• the pad can be pre-tinned (refusi ⁇ n technique) or coated with solder cream.
• regions of the substrate having an appropriate shape, generally rectangular, and each of which is entirely located in the immediate vicinity of a series of connection pins to be fixed by soldering. These regions are of three types:
• Regions 35 running along an edge of a component and into which a projecting pin or a series of juxtaposed projecting pins clearly penetrate (components 21-, 23, 27, 28 and 31).
• the control software of the computer 17 is adapted so that the laser beam is directed successively on each of the regions 33 to 35 and, each time, sweeps the working region by virtue of the movement of a mirror or of the two mirrors, namely a sinusoidal or sawtooth movement, in the direction of the length the region, with a frequency of 50 Hz to 1 kHz, and possibly a slow movement perpendicular to the previous one.
• the substrate emits radiation at 3.43 * m measured by the pyrometer, which "directly" sees the working region since it is arranged in reflex view.
• This pyrometer has a set temperature value at least equal to the melting temperature of the soldering products and lower than the degradation temperature of the substrate, and it emits to the laser 9, via the interface 18, a deviation signal which changes the power of the laser in the direction tending to cancel this deviation. The laser power is thus controlled at the set temperature.
• the regions 33 to 35 are defined so that the exposed substrate surface is sufficiently large relative to the metallic surface, and / or the printed circuit cir ⁇ metal tracks are covered, in their non-soldered parts, with a savings varnish absorbing the laser radiation.
• the substrate is heated in the immediate vicinity of these metal parts, and it is the substrate which heats by conduction of the brazing pads to the temperature of -fusion of the soldering products.
• the radiation at 10.6 / Am is well absorbed by the substrates used to constitute the electronic cards (plastic type Epoxy or ceramic type Al 0). This property of high absorption of the substrate at the wavelength of the lasers, which is considered to be a drawback in the conventional point-by-point technique, is thus taken advantage of in the context of the present invention.
• the CO laser can be replaced by a CO or Nd-YAG laser, the radiation of which is also strongly absorbed by the organic substrates.
• the laser used is preferably a continuously operating laser, as in the example described above. However, a pulsed laser could also be used.

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• Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Plasma & Fusion (AREA)
• Electric Connection Of Electric Components To Printed Circuits (AREA)

Applications Claiming Priority (2)

Publications (1)

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ID=9386731

Family Applications (1)

Country Status (2)

Cited By (3)

Families Citing this family (1)

Citations (2)

• 1989
  • 1989-10-24 FR FR8913937A patent/FR2653367A1/fr active Granted
• 1990
  • 1990-10-12 WO PCT/FR1990/000731 patent/WO1991006389A1/fr unknown

Patent Citations (2)

Non-Patent Citations (1)

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