Direct application of unpackaged integrated circuit to flexible printed circuit
First Claim
1. A method for electrically connecting a plurality of flip chips, each flip chip having two or more solder bumps, to a flexible printed circuit substrate having a plurality of contact pads, the method comprising the steps of:
- providing solder paste on a plurality of active contact pads on the flexible printed circuit substrate, each active pad having a corresponding solder bump on one of the plurality of flip chips, such that the solder paste on each active contact pad is of sufficient volume to facilitate an electrical connection between the active contact pad and the corresponding solder bump;
placing the chips on the substrate to form an unsoldered chip-substrate assembly, such that the solder paste on each active contact pad is in registration with the corresponding solder bump; and
heating the chip-substrate assembly as a whole with a heating means which operates by direct infrared radiation of the assembly or by transmission of heat to the assembly from a heated working fluid, such that the solder paste on each active contact pad reflows to form an electrical connection with the corresponding solder bump.
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Accused Products
Abstract
A method is described for electrically connecting flip chips to a flexible printed circuit substrate. The method comprises (1) providing solder paste to a plurality of active contact pads located on the flexible printed circuit substrate, (2) placing the flip chips on the substrate such that solder bumps located on the flip chips are in registration with the solder paste on the active contact pads, and (3) heating the resulting assembly as a whole so that the solder paste on each active contact pad reflows to form an electrical connection with its corresponding solder bump.
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Citations
25 Claims
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1. A method for electrically connecting a plurality of flip chips, each flip chip having two or more solder bumps, to a flexible printed circuit substrate having a plurality of contact pads, the method comprising the steps of:
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providing solder paste on a plurality of active contact pads on the flexible printed circuit substrate, each active pad having a corresponding solder bump on one of the plurality of flip chips, such that the solder paste on each active contact pad is of sufficient volume to facilitate an electrical connection between the active contact pad and the corresponding solder bump; placing the chips on the substrate to form an unsoldered chip-substrate assembly, such that the solder paste on each active contact pad is in registration with the corresponding solder bump; and heating the chip-substrate assembly as a whole with a heating means which operates by direct infrared radiation of the assembly or by transmission of heat to the assembly from a heated working fluid, such that the solder paste on each active contact pad reflows to form an electrical connection with the corresponding solder bump. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24)
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25. A method for electrically connecting a plurality of flip chips to a flexible printed circuit substrate having a plurality of contact pads located on either side of the substrate, each flip chip an unpackaged silicon chip having a passivation layer on an active side of the chips for protecting active devices located on the chips from environmental contaminants, and having two or more solder bumps located on bonding pads located on the active side, each solder bump comprised of an electroplated copper pedestal attached to the active side of the flip chip and tin-lead solder having 6% tin and 94% lead and having a reflow point in the range of 316°
- C., the tin-lead solder being hemispherically-shaped and attached to the copper pedestal such that the copper pedestal is substantially between the solder and the flip chip, the flexible printed circuit substrate comprised of an adhesiveless flexible polyimide copper laminate, the method comprising the steps of;
providing solder paste on a plurality of active contact pads on the flexible printed circuit substrate by stenciling the solder paste on the active contact pads on the substrate through a 4 mil thick brass stencil having 14 mil diameter etched openings corresponding to the pads, using an automated stencilling machine having a round squeegee of 90 durometer hardness, the squeegee moving at 1.5 inches/second and applying a force of 22 psi, the solder paste comprised of solder particles in a carrier matrix of flux, the solder paste having 90% solder particles and 10% flux and having a viscosity of 900 kcps, the solder particles comprised of a 63% tin and 37% lead eutectic alloy, formed into substantially spherically-shaped objects having a diameter less than 45 microns, and having a reflow point in the range of 183°
C., each active pad having a corresponding solder bump on one of the plurality of flip chips, such that the solder paste on each active contact pad is of sufficient volume to facilitate an electrical connection between the active contact pad and the corresponding solder bump;placing the chips on the substrate to form an unsoldered chip-substrate assembly using an optical placement means, such that the solder paste on each active contact pad is in registration with the corresponding solder bump; heating the chip-substrate assembly as a whole with a heating means comprised of passing the assembly through an infrared reflow oven having a nitrogen purge option for inhibiting formation of solder oxides, at a rate of 32.4 inches/minute, and such that the temperature of the assembly rises at a rate less than 2°
C./second, the heating performed in accordance with a temperature profile comprised of;a first preheat zone for raising the temperature of the assembly from an ambient temperature to approximately 20°
-30°
C. below the reflow point of the solder paste;a second preheat zone for raising the temperature of the assembly to just below the reflow point of the solder paste; a reflow zone for raising the temperature of the assembly to approximately 40°
C. above the reflow point of the solder paste; anda cooling zone for allowing the assembly to cool to ambient temperature; such that the solder paste on each active contact pad reflows to form an electrical connection with the corresponding solder bump; cleaning the assembly using freon-based solvents and ultrasound processes; and coating the assembly with a low stress epoxy encapsulant.
- C., the tin-lead solder being hemispherically-shaped and attached to the copper pedestal such that the copper pedestal is substantially between the solder and the flip chip, the flexible printed circuit substrate comprised of an adhesiveless flexible polyimide copper laminate, the method comprising the steps of;
Specification