Methods for forming printed circuit boards

Abstract

A method for forming a printed circuit board (100), comprising (a) Providing a substrate made of a first malleable polymer material, wherein the substrate has at least one electrical conductor on a surface; (b) Laminating a cover layer of a second malleable polymer material onto the substrate to form a flexible printed circuit board (104), wherein the at least one electrical conductor is arranged between the cover layer and the substrate, the cover layer having a plurality of holes which expose at least part of the at least one electrical conductor; (c) electrically connecting light-emitting diodes (LEDs) (108) to exposed sections of the at least one electrical conductor; and (d) Heating the flexible printed circuit board (104) by applying a force to cause the flexible printed circuit board (104) to bend and assume a shape with an arcuate cross-section; and (e) Cooling the flexible printed circuit board (104) until the shape is fixed, thereby forming a thermoformed printed circuit board (100) with an arc-shaped cross-section, wherein step (d) comprises: Arranging the flexible printed circuit board (104) in a mold (200), wherein the mold (200) has an upper half (204) and a lower half (202), wherein the upper half (204) has an elongated, convex longitudinal projection (212) having a channel (218) corresponding to a position of the LEDs (108), wherein the channel (218) has a size that prevents the LEDs (108) from touching the upper half (204) when the mold (200) is closed, wherein the lower half (202) has an elongated, concave longitudinal recess (210) for receiving the longitudinal projection (212) of the upper half (204), wherein the longitudinal projection (212) and the longitudinal recess (210) have a similar arcuate cross-section; Closing the mold (200) while the mold (200) and the flexible printed circuit board (104) are heated at a thermoforming temperature; and Holding the mold (200) in a closed position to give the printed circuit board (100) a shape with an arc-shaped cross-section, wherein the mold (200) has slots (220) in the lower half (202) for receiving longitudinal edges of the printed circuit board (100) and arranging the flexible printed circuit board (104) in the mold (200) includes inserting the printed circuit board edges into the slots (220).

Description

Cross-reference to related registration

[0001] The present application is an international application and claims the benefits and priority of US Provisional Application No. 62 / 054,045, filed on September 23, 2014, which is incorporated herein by reference in its entirety. Background of the invention

[0002] Linear standard fluorescent lamps are one of the most common lamp shapes used to generate light. Given the large number of fluorescent fixtures installed in commercial, institutional, and industrial settings, it is desirable to replace fluorescent lamps with other highly efficient, mercury-free lighting solutions of the same form factor, thus avoiding the need to replace existing fixtures. This has led to the development of solid-state replacement lamps, which contain linear arrays of light-emitting diodes (LEDs) on printed circuit boards arranged within hollow tubes. These new solid-state lamps require different design processes than conventional fluorescent lamps, and in particular, new techniques are needed for manufacturing printed circuit boards with LED arrays for use with the tubular lamp bodies.Publication US 2012 / 0281411A1 describes a method for manufacturing a lighting device with a deformable flexible printed circuit board (PCB) having an insulating film of thermoplastic resin, wherein a wiring layer and an insulating layer with a curved section are formed on the insulating film. Publication JP H04-39011A describes a printed circuit board which is temporarily transferred to a transfer film using an adhesive and affixed to a substrate film containing circuits. Summary of the invention

[0003] According to the invention, a method for forming a printed circuit board is proposed. The method comprises providing a substrate made of a first malleable polymer material, wherein the substrate has at least one electrical conductor on a surface, and laminating a cover layer made of a second malleable polymer material onto the substrate to form a flexible printed circuit board. The at least one electrical conductor is arranged between the cover layer and the substrate, the cover layer having a plurality of holes that expose at least a portion of the at least one electrical conductor. The method further comprises electrically connecting light-emitting diodes (LEDs) to exposed portions of the at least one electrical conductor and heating the flexible printed circuit board by applying a force to cause the flexible printed circuit board to bend and assume a shape with an arcuate cross-section.The method further comprises cooling the flexible printed circuit board until the mold is fixed, thereby forming a thermoformed printed circuit board with an arcuate cross-section. The method also comprises arranging the flexible printed circuit board in a mold, the mold having an upper half and a lower half, the upper half having an elongated, convex longitudinal projection that includes a channel corresponding to a position of the LEDs, the channel being of a size that prevents the LEDs from touching the upper half when the mold is closed, and the lower half having an elongated, concave longitudinal recess to receive the longitudinal projection of the upper half, the longitudinal projection and the longitudinal recess having a similar arcuate cross-section.The process also includes closing the mold while the mold and the flexible printed circuit board are heated to a thermoforming temperature, and holding the mold in a closed position to give the printed circuit board a shape with an arc-shaped cross-section, wherein the mold has slots in the lower half for receiving longitudinal edges of the printed circuit board, and arranging the flexible printed circuit board in the mold includes inserting the printed circuit board edges into the slots. Brief description of the characters

[0004] Features and advantages of different embodiments of the claimed subject matter will become apparent from the following detailed description and from the drawings, in which the same reference numerals denote the same parts and in which: The Fig. 1A and Fig. 1B each are perspective and cross-sectional views of a thermoformed printed circuit board which can be produced by a method according to the invention. The Fig. 2A and Fig. 2B each are perspective and cross-sectional views of a shape that can be used in a thermoforming process according to this invention. The Fig. 3A-3C a thermoforming process according to this invention using the in the Fig. 2A and Fig. 2B illustrates the form shown. Fig. 4 is a perspective view of a thermoforming die for use in an inline thermoforming process according to this invention. Fig. Figure 5 is a schematic representation of an inline thermoforming process according to this invention. Detailed description

[0005] It has been found that the rigid circuit board of conventional retrofit LED lamps can be replaced by a flexible circuit board. The flexible circuit board's ability to bend and conform concentrically to the inner diameter of the lamp's tubular body allows the LED arrays to be positioned further away from the front of the lamp, resulting in greater diffusion of the light emitted by the individual LEDs and thus a more uniform appearance. The curvature of the circuit board also allows the LEDs to be angled relative to each other to further enhance light distribution.This invention relates to a novel method for thermoforming flexible printed circuit boards to give them the desired curvature so that they can be used in tubular LED lamp applications, and in particular in LED lamps designed as replacements for conventional linear fluorescent lamps.

[0006] With reference to the Fig. 1A and Fig. Figure 1B shows a thermoformed printed circuit board 100, which is populated with LEDs 108. The thermoformed printed circuit board 100 comprises a flexible printed circuit board 104. The flexible printed circuit board 104 comprises a thermoplastic polymer material, such as polyethylene terephthalate (PET), which can be thermoformed to obtain a desired shape. In one embodiment, the PET is a highly reflective white PET that reflects light toward the front of the lamp. The flexible printed circuit board 104 was thermoformed to obtain a circuit board with an arc-shaped cross-section, preferably corresponding to the curvature of the tubular lamp with which it is used. The flexible printed circuit board 104 also has electrical circuits and contacts (not shown) for supplying power to the LEDs.

[0007] In a first embodiment, a flexible printed circuit board (PCB) is formed by laminating a substrate made of a malleable polymer material, which has etched copper conductors, with a second plate made of malleable polymer as a cover layer. Preferably, the cover layer is a highly reflective white PET with holes that allow LEDs to be soldered to the copper conductors. The flexible PCB is then covered with LEDs and placed in a mold with the desired cross-sectional shape. The mold further has one or more recesses that prevent the LEDs from touching the surface of the mold. The PCB is then thermoformed by heating the mold and the PCB at a temperature and for a time sufficient to give the board the desired shape, e.g., about 110°C for 20 minutes.

[0008] The Fig. 2A and Fig. Figure 2B shows a thermoforming mold 200 for use in the method according to the invention. The mold 200 is formed from two metal halves 202, 204. The lower half 202 has an elongated, concave longitudinal recess 210 with an arcuate cross-section. The cross-sectional shape of the recess 210 is designed to replicate the desired cross-sectional shape of the flexible printed circuit board. Preferably, the cross-section is a circular arc having a central angle of 120° to 150°. The elongated longitudinal recess 210 preferably has opposing slots 220 to accommodate the edges of the flexible printed circuit board for the initial phase of the thermoforming process. The upper half 204 of the mold 200 has an elongated convex longitudinal projection 212 having an arcuate cross-section that substantially corresponds to the cross-section of the recess 210.The rectangular channel 218 extends over the length of the elongated longitudinal projection 212 and has a width and depth sufficient to prevent the LEDs applied to the flexible circuit board from touching the upper half 204 of the shape 200 during the thermoforming process, which could lead to damage to the LEDs.

[0009] Fig. 3A-3C demonstrate a batch process for manufacturing a thermoformed printed circuit board using the shape of the Fig. 2A & 2B. In an initial step, which in Fig. As shown in Figure 3A, the open mold 200 is loaded with a flat, elongated flexible printed circuit board 104 by inserting the longitudinal edges of the board into the slots 220. Once the printed circuit board 104 is in its correct position, the mold halves 202 and 204 are brought together (as indicated by arrow 230) while the mold and the printed circuit board are heated to a thermoforming temperature. As shown in Fig. As shown in Figure 3B, the upper surface 110 of the circuit board 104, on which the LEDs 108 are mounted, is first contacted by the elongated longitudinal projection 212 of the upper half 204. The channel 218 in the upper half 204 provides sufficient clearance to prevent contact with the LEDs 108. As the mold closes further, the lower surface 114 of the flexible circuit board 104 contacts the surface of the elongated longitudinal recess 210 of the lower mold half 202. When closed, the mold 200 receives the flexible circuit board 104 between the two halves 202 and 204, with the lower surface 114 and the upper surface 110 coming into full contact with the surfaces of the recess 210 and the projection 212 (except for the LED area), as shown in Figure 3B. Fig. 3C shown. The mold then remains closed for a time sufficient to give the flexible circuit board 104 the desired curvature and to obtain a thermoformed circuit board 100, as shown in the Fig. 1A and Fig. 1B illustrates this.

[0010] This type of batch forming process produces high yields and reproducible shapes. However, batch processes are inherently slow, as only one circuit board is formed at a time, even though they can be highly automated. Furthermore, the circuit boards can only be thermoformed after reflow soldering in an oven for LED mounting, followed by cooling to allow handling of the boards for further processing.

[0011] In another embodiment, the method of this invention is a continuous inline process and not a batch process and can be used in a reel-to-reel production line, which saves time and energy and improves the quality of a printed circuit board produced.In particular, the inline process may comprise: (a) obtaining a substrate of a first malleable polymer material with conductive traces; (b) laminating the substrate onto a cover layer of a second malleable polymer material to form a continuous printed circuit board (PCB) ribbon, wherein the conductive traces are arranged between the substrate and the cover layer, the cover layer having holes which allow the attachment of LEDs to the conductive traces; (c) applying a solder paste and LEDs; (d) heating the PCB ribbon in a first zone to at least partially melt the solder and electrically connect the LEDs to the conductive traces; (e) passing the heated PCB through a die in a second zone to give the PCB ribbon a shape; and (f) cooling the PCB ribbon in a third zone to retain its shape.The method may further preferably include the step of cutting the cooled printed circuit board strip to the desired length in order to produce thermoformed printed circuit boards for use in an LED lamp application.

[0012] In a preferred embodiment, the temperature in the first zone, where reflow soldering takes place, is approximately 140°C. Preferably, the solder is a low-temperature bi-tin / tin solder. Since this temperature may be above the softening point of the adhesive used to laminate the substrate's top layer, it is preferred to immediately feed the printed circuit board tape into the second zone and through the forming tool while it cools. This not only gives the tape the desired shape but also smooths out any possible delamination of the top layer caused by the higher temperature in the first zone.

[0013] Preferably, the forming step in the second zone is performed at a lower temperature than in the first zone. Particularly preferably, the forming takes place at approximately 110°C. In one embodiment, the second zone is not heated, and the forming is based on the residual heat introduced in the first zone. In particular, the printed circuit board tape is thermoformed during the cooling phase immediately after reflow soldering. This is done inline and produces the formed tape with only one heating step for both the reflow soldering and thermoforming steps, thus saving time and energy. In another embodiment, the printed circuit board tape would be thermoformed using a die located in a temperature zone downstream of the reflow section, which maintains a temperature of approximately 110°C for a time sufficient to give the tape the desired shape.During its cooling phase, the printed circuit board tape would continue to run through the die until it reaches approximately 80°C, at which point the tape would only be running in the air, outside the boundaries of the die.

[0014] Fig. Figure 4 shows a thermoforming die 400 used in an inline process according to this invention. The flexible printed circuit board 104 moves through the die 400 in the direction indicated by the arrow 420. The die 400 has a widened receiving section 408 which receives the flexible printed circuit board 104 in the form of a strip and captures the longitudinal edges 120 of the flexible printed circuit board. The receiving section 408 narrows as it approaches the forming section 410, causing the flexible printed circuit board 104 to curl and assume an arc-shaped cross-section. The forming section 410 has a gap 406 that allows the LEDs to pass through the forming section 410 of the die 400 without touching it.The forming section 410 has upper 402 and lower 404 forming surfaces separated by approximately the thickness of the flexible printed circuit board 104 (excluding the LEDs), allowing the tape to pass through while simultaneously holding the flexible printed circuit board tape in the desired shape. Specifically, the upper 402 and lower 404 surfaces have an substantially identical arcuate cross-sectional shape to impart the desired curvature to the tape. The upper surface 402 is divided into two halves separated by the gap 406. In a preferred embodiment, the upper and lower surfaces of the die are integrally formed. The thermoformed printed circuit board 100 exits the opposite end of the die 400, where it can be cut to the desired length.

[0015] Fig. Figure 5 is a schematic representation of an inline process according to a method of this invention. The process begins on the left and proceeds in the direction indicated by arrow 545. A first strip 501 of a malleable polymer material is laminated onto a second strip 502 of a malleable polymer material by rollers 503 to form a flexible printed circuit board strip 504. The first strip 501 has one or more copper conductors etched onto its surface, which are connected to the second strip 502. The second strip 502 has holes 525 corresponding to the locations where LEDs are placed further downstream in the process. The holes leave at least a portion of the etched copper conductor(s) exposed to make electrical connections to the LEDs. The two strips can be laminated using an adhesive on one or both of the surfaces to be joined.The printed circuit board tape 504 continues to a first dispensing station 530, where solder paste is applied to the desired contact points for the LEDs, and then to a second dispensing station 533, where the LEDs 508 are placed in their desired positions. The tape 504 with the applied LEDs 508 then enters a first thermal zone 540, where the solder is remelted to create the electrical connections between the etched copper conductor(s) and the LEDs. After the reflow soldering step, the tape 504 enters a second thermal zone 542, where the tape passes through a die 510, similar to the one in [reference missing]. Fig. 4 shown, to give the strip the desired curvature. The thermoformed strip 500 leaves the die 510, where it continues to cool and is cut to the desired length by the cutting station 550. The thermal zones 540 and 542 can have a single zone with a temperature gradient, or they can be separate zones, as shown in Fig. Figure 5 shows that the temperature in the first thermal zone 540 is higher than the temperature in the second thermal zone 542 and must be sufficient to melt the solder in the solder paste. The temperature in the second thermal zone 542 must be sufficient to thermoform the printed circuit board tape, but low enough so that the solder solidifies upon exiting the first thermal zone 540. The tape should preferably remain in the die as it cools from a thermoforming temperature of approximately 110°C to at least approximately 80°C, at which point the shape is fixed.

[0016] While it has been shown and described what are currently considered preferred embodiments of the invention, it is obvious to the person skilled in the art that various changes and modifications can be made without deviating from the scope of the invention as defined by the attached claims.

Claims

[1] A method for forming a printed circuit board (100), comprising (a) Providing a substrate made of a first malleable polymer material, wherein the substrate has at least one electrical conductor on a surface; (b) Laminating a cover layer of a second malleable polymer material onto the substrate to form a flexible printed circuit board (104), wherein the at least one electrical conductor is arranged between the cover layer and the substrate, the cover layer having a plurality of holes which expose at least part of the at least one electrical conductor; (c) electrically connecting light-emitting diodes (LEDs) (108) to exposed sections of the at least one electrical conductor; and (d) Heating the flexible printed circuit board (104) by applying a force to cause the flexible printed circuit board (104) to bend and assume a shape with an arcuate cross-section; and (e) Cooling the flexible printed circuit board (104) until the shape is fixed, thereby forming a thermoformed printed circuit board (100) with an arc-shaped cross-section, wherein step (d) comprises: Arranging the flexible printed circuit board (104) in a mold (200), wherein the mold (200) has an upper half (204) and a lower half (202), wherein the upper half (204) has an elongated, convex longitudinal projection (212) having a channel (218) corresponding to a position of the LEDs (108), wherein the channel (218) has a size that prevents the LEDs (108) from touching the upper half (204) when the mold (200) is closed, wherein the lower half (202) has an elongated, concave longitudinal recess (210) for receiving the longitudinal projection (212) of the upper half (204), wherein the longitudinal projection (212) and the longitudinal recess (210) have a similar arcuate cross-section; Closing the mold (200) while the mold (200) and the flexible printed circuit board (104) are heated at a thermoforming temperature; and Holding the mold (200) in a closed position to give the printed circuit board (100) a shape with an arc-shaped cross-section, wherein the mold (200) has slots (220) in the lower half (202) for receiving longitudinal edges of the printed circuit board (100) and arranging the flexible printed circuit board (104) in the mold (200) includes inserting the printed circuit board edges into the slots (220). [2] The method according to claim 1, wherein the printed circuit board is heated to about 110 °C. [3] The method according to claim 1, wherein the shape (200) has a cross-section which is a circular arc. [4] The method according to claim 3, wherein the circular arc has a central angle of 120° to 150°. [5] The method according to claim 1, wherein the first and the second malleable polymer material is polyethylene terephthalate (PET). [6] The method according to claim 5, wherein the second malleable polymer material is a reflective white PET.

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