WO2016049177A1

WO2016049177A1 - Method for forming circuit boards

Info

Publication number: WO2016049177A1
Application number: PCT/US2015/051708
Authority: WO
Inventors: Richard Speer; Kenneth Grossman; David Hamby
Original assignee: Osram Sylvania Inc.
Priority date: 2014-09-23
Filing date: 2015-09-23
Publication date: 2016-03-31
Also published as: CN107206643A; DE112015004330B4; CN107206643B; DE112015004330T5

Abstract

There is herein described a method for forming a circuit board comprising: (a) obtaining a substrate of a first formable polymer material, the substrate having at least one electrical conductor on a surface; (b) laminating a coverlay of a second formable polymer material to the substrate to form a flexible circuit board wherein the at least one electrical conductor is disposed between the coverlay and the substrate, the coverlay having a plurality of holes exposing at least a portion of the at least one electrical conductor; (c) electrically connecting light emitting diodes (LEDs) to exposed portions of the at least one electrical conductor; (d) heating the circuit board while applying a force to cause the circuit board to bend and adopt a shape having arcuate cross section; (e) cooling the circuit board until the shape becomes fixed whereby a thermoformed circuit board having an arcuate cross section is formed.

Description

Method for Forming Circuit Boards

Cross Reference to Related Application

[0001] The present application is an international application, and claims the benefit of and priority to, United States Provisional Application No. 62/054,045 filed September 23, 2014, which is herein incorporated by reference in its entirety.

Background of the Invention

[0002] Standard linear fluorescent lamps are one of the most common lamp forms used to generate light. Given the large number of fluorescent fixtures installed in commercial, institutional, and industrial establishments, it is desirable to replace fluorescent lamps with other high efficiency, mercury-free lighting solutions having the same form factor so that replacement of the existing fixtures is not necessary. This has led to the development of solid-state replacement lamps which include linear arrays of light-emitting diodes (LEDs) on circuit boards disposed within hollow tubes. These new solid-state lamps require different construction methods than conventional fluorescent lamps and in particular novel techniques are required for manufacturing circuit boards with arrays of LEDs for use with the tubular lamp bodies.

Brief Description of the Drawings

[0003] Features and advantages of various embodiments of the claimed subject matter will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals designate like parts, and in which:

[0004] Figures 1 A and 1 B are perspective and cross-sectional illustrations, respectively, of a thermoformed circuit board that may made by a method of this invention.

[0005] Figures 2A and 2B are perspective and cross-sectional illustrations, respectively, of a mold that may be used in a thermoforming method according to this invention.

[0006] Figures 3A-3C illustrate a thermoforming method according to this invention using the mold shown in Figures 2A and 2B. [0007] Figure 4 is a perspective illustration of a thermoforming die for use in an inline thermoforming method according to this invention.

[0008] Figure 5 is a schematic illustration of an in-line thermoforming method according to this invention.

Detailed Description

[0009] It has been found that the rigid circuit board of conventional retrofit LED lamps may be replaced with a flexible circuit board. The ability of the flexible circuit board to bend and fit concentrically to the internal diameter of the tubular body of the lamp allows the LED arrays to be positioned further away from the front surface of the lamp permitting greater diffusion of the light emitted by the individual LEDs and thereby providing a more uniform appearance. The board's curvature also permits the LEDs to be angled with respect to each other further aiding light distribution. This invention is a novel process for thermoforming flexible circuit boards to impart the desired curvature so that they may be used in tubular LED lamp applications, and in particular, LED lamps designed as replacements for conventional linear fluorescent lamps.

[0010] Referring now to Figures 1 A and 1 B, a thermoformed circuit board 100 is shown populated with LEDs 108. The thermoformed circuit board 100 is comprised of flexible circuit board 104. Flexible circuit board 104 is comprised of a thermoplastic polymer material such as polyethylene terephtha!ate (PET) which may be thermoformed to impart a desired shape. In one embodiment, the PET is a highly reflective white PET which reflects light toward the front of the lamp. The flexible circuit board 104 has been thermoformed to yield a circuit board with an arcuate cross section that preferably conforms to the curvature of the tubular lamp with which it will be used. The flexible circuit board 104 also has electric circuitry and contacts (not shown) for providing power to the LEDs.

[0011] In a first embodiment, a flexible circuit board is formed by laminating a substrate of a formable polymer material having etched copper conductors to a second sheet of formable polymer in the form of a coverlay. Preferably, the coverlay is a highly reflective white PET with holes that allow for LEDs to be soldered to the copper conductors. The flexible circuit board is then populated with LEDs and placed in a mold having the desired cross sectional shape. The mold further has one or more recesses that prevent the LEDs from contacting the mold surface. The circuit board is then thermoformed by heating for the mold and circuit board at a temperature and for a time sufficient to impart the desired shape to the circuit board, e.g., about 1 10°C for 20 minutes.

[0012] Figures 2A and 2B illustrate a thermoforming mold 200 for use in the method of this invention. The mold 200 is formed of two metal halves 202, 204. The lower half 202 has an elongated longitudinal concave depression 210 having an arcuate cross section. The cross sectional shape of depression 210 is designed to replicate the desired cross sectional shape of the flexible circuit board. Preferably, the cross section is a circular arc which subtends a central angle of 120 ° to 150 °. The elongated longitudinal depression 210 preferably has opposed slots 220 for receiving the edges of the flexible circuit board for the initial stage of the thermoforming process. The upper half 204 of mold 200 has a elongated longitudinal convex protrusion 212 that has an arcuate cross section substantially corresponding to the cross section of depression 210. Rectangular channel 218 extends the length of the elongated longitudinal protrusion 212 and has a width and depth sufficient to prevent the LEDs mounted on the flexible circuit board from contacting the upper half 204 of mold 200 during the thermoforming process which could cause damage to the LEDs.

[0013] Figures 3A-3C illustrate a batch process for making a thermoformed circuit board using the mold of Figures 2A & 2B. In an initial step shown in Figure 3A, the open mold 200 is loaded with a flat, elongated flexible circuit board 104 by inserting the longitudinal edges of the board into slots 220. Once the circuit board 104 is in its proper position, the mold halves 202, 204 are brought together (as indicated by arrow 230) while the mold and circuit board are heated to a thermoforming temperature. As shown in Figure 3B, the upper surface 1 10 of circuit board 104 on which LEDs 108 are mounted is first contacted by the elongated longitudinal protrusion 212 of upper half 204. Channel 218 in upper half 204 provides sufficient clearance to prevent contact with the LEDs 108. As the mold is further closed, the lower surface 1 14 of flexible circuit board 104 contacts the surface of elongated longitudinal depression 210 of lower mold half 202. When closed, mold 200 sandwiches the flexible circuit board 104 between the two halves 202, 204 with the lower surface 1 14 and upper surface 1 10 completely contacting (except for the LED region) with the surfaces of the depression 210 and protrusion 212, respectively, as shown in Figure 3C. The mold then remains closed for a time sufficient to impart the desired curvature to the flexible circuit board 104 and yielding a thermoformed circuit board 100 as exemplified in Figures 1 A and 1 B.

[0014] This type of batch forming process produces high yields and reproducible forming. However, batch processes are inherently slow, forming one board at a time even though they may be highly automated. Moreover, the circuit boards may only be thermoformed after solder reflow in an oven for LED attachment followed by a cool down so that the boards may be handled for further processing.

[0015] In another embodiment, the method of this invention is a continuous in-line process rather than a batch process and may be employed in a reel-to-reel manufacturing line that saves time, energy and improves quality of a formed circuit board. In particular, the in-line process may involve: (a) obtaining a substrate of a first formable polymer material having conductive traces; (b) laminating the substrate to a coverlay of a second formable polymer material to form a continuous circuit board ribbon with the conductive traces disposed between the substrate and coverlay, the coverlay having holes allowing for attachment of LEDs to the conductive traces, (c) applying a solder paste and LEDs; (d) heating the circuit board ribbon in a first zone to at least partially melt the solder and electrically connect the LEDs to the conductive traces; and (e) passing the heated circuit board through a die in a second zone to impart a shape to the circuit board ribbon; and (f) cooling the circuit board ribbon in a third zone to retain its shape. The method may further preferably include the step of cutting the cooled circuit board ribbon into desired lengths to create thermoformed circuit boards for use in an LED lamp application.

[0016] In a preferred embodiment, the temperature in the first zone where the solder is re-flowed is about 140°C. Preferably, the solder is a low temperature Bi/Sn solder. Since this temperature may be above the softening point of the adhesive used to laminate the coverlay the substrate, it is preferable to immediately pass the circuit board ribbon into the second zone and through the shaping die while it is cooling. This not only imparts the desired shape to the ribbon, but also acts to flatten out any potential delamination of the coverlay caused by the higher temperature in the first zone.

[0017] Preferably the forming step in the second zone is performed at a lower temperature than in the first zone. More preferably, the forming is done at about 1 10°C. In one embodiment, the second zone is not heated and the forming relies on the residual heat imparted in the first zone. In particular, the circuit board ribbon is thermoformed during the cooling phase directly after the solder re-flow. This would be done in-line and produce the formed ribbon with only one heating operation for both the re-flow and the thermoforming steps, saving time and energy. In another embodiment, the circuit board ribbon would be thermoformed using a die located in a temperature zone downstream of the re-flow section that maintains a temperature of about 1 10°C for a time sufficient to impart the desired shape to the ribbon. The circuit board ribbon would continue to travel through the die during its cool down phase to about 80 °C, at which point the ribbon would travel in air only, outside of the confines of the die.

[0018] Figure 4 is an illustration of a thermoforming die 400 used in an in-line process according to this invention. The flexible circuit board 104 moves through the die 400 in the direction indicated by arrow 420. The die 400 has a flared receiving section 408 which receives the flexible circuit board 104 in the form of a ribbon and engages the longitudinal edges 120 of the flexible circuit board. Receiving section 408 narrows as it approaches the forming section 410 causing the flexible circuit board 104 to begin to curl and take on an arcuate cross section. Forming section 410 has a gap 406 which allows the LEDs to pass through forming section 410 of die 400 without contacting it. The forming section 410 has upper 402 and lower 404 forming surfaces separated approximately by the thickness of flexible circuit board 104 (not including the LEDs) which allows the ribbon to pass in between while simultaneously holding the flexible circuit board ribbon in the desired shape. More particularly, the upper 402 and lower 404 surfaces have a substantially identical arcuate cross-sectional shape for imparting the desired curvature to the ribbon. The upper surface 402 is divided into two halves separated by gap 406. In a preferred embodiment, the upper and lower surfaces of the die are integrally formed. Thermoformed circuit board 100 exits the opposite end of the die 400 where it may be cut to the desired length.

[0019] Figure 5 is a schematic illustration of an in-line process according to a method of this invention. The process starts on the left and proceeds in the direction indicated by arrow 545. A first ribbon 501 of a formable polymer material is laminated to a second ribbon 502 of a formable polymer material by rollers 503 to form a flexible circuit board ribbon 504. The first ribbon 501 has etched copper conductor(s) on the surface to be mated to the second ribbon 502. The second ribbon 502 has holes 525 which correspond to the locations where LEDs will be placed further downstream in the process. The holes leave at least a portion of the etched copper conductor(s) exposed for making electric connections to the LEDs. The two ribbons may be laminated using an adhesive on one or both of the mating surfaces. The circuit board ribbon 504 continues to a first dispensing station 530 where a solder paste is applied to the desired contact locations for the LEDs and then to a second dispensing station 533 where the LEDs 508 are placed in their desired positions. The ribbon 504 with the attached LEDs 508 next enters a first thermal zone 540 wherein the solder is reflowed to make the electrical connections between the etched copper conductor(s) and the LEDs. After the solder reflow step, the ribbon 504 enters a second thermal zone 542 wherein the ribbon passes through a die 51 0 similar to that shown in Figure 4 to impart the desired curvature to the ribbon. The thermoformed ribbon 500 exists the die 510 where it further cools and is cut to the desired length by cutting station 550. Thermal zones 540 and 542 may comprise a single zone having a temperature gradient or they may be separate zones as shown in Figure 5. The temperature in 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 circuit board ribbon but low enough so that the solder solidifies upon exiting the first thermal zone 540. The ribbon preferably should remain in the die as it cools from about a 1 10°C thermoforming temperature to at least about 80 °C at which point the shape becomes fixed.

[0020] While there have been shown and described what are at present considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

Claims

Claims What is claimed is:

1 . A method for forming a circuit board comprising:

(a) obtaining a substrate of a first formable polymer material, the substrate having at least one electrical conductor on a surface;

(b) laminating a coverlay of a second formable polymer material to the substrate to form a flexible circuit board wherein the at least one electrical conductor is disposed between the coverlay and the substrate, the coverlay having a plurality of holes exposing at least a portion of the at least one electrical conductor;

(c) electrically connecting light emitting diodes (LEDs) to exposed portions of the at least one electrical conductor; and

(d) heating the circuit board while applying a force to cause the circuit board to bend and adopt a shape having arcuate cross section; and

(e) cooling the circuit board until the shape becomes fixed whereby a thermoformed circuit board having an arcuate cross section is formed.

2. The method of claim 1 wherein step (d) comprises:

placing the circuit board in a mold, the mold having an upper half and a lower half, the upper half having an elongated longitudinal convex protrusion having a channel corresponding to a location of the LEDs, the channel having a size that prevents the LEDs from contacting the upper half when the mold is closed, the lower half having an elongated longitudinal concave depression for receiving the

longitudinal protrusion of the upper half, the longitudinal protrusion and longitudinal depression having a similar arcuate cross section;

closing the mold while heating the mold and circuit board at a thermoforming temperature; and

holding the mold in a closed position to impart a shape having an arcuate cross section to the circuit board.

3. The method of claim 2 wherein the mold has slots in the lower half for receiving longitudinal edges of the circuit board and placing the circuit board in the mold comprises inserting the circuit board edges into the slots.

4. The method of claim 1 wherein the circuit board is heated to about 1 10°C.

5. The method of claim 1 wherein the shape has a cross section that is a circular arc.

6. The method of claim 5 wherein the circular arc subtends a central angle of 120° to 150°.

7. The method of claim 1 wherein the first and second formable polymer material is polyethylene terephthaiate (PET).

8. The method of claim 7 wherein the second formable polymer material is a reflective white PET.

9. The method of claim 1 wherein the substrate and coverlay comprise ribbons of the first and second formable polymer materials;

step (b) yields a circuit board ribbon;

step (c) comprises applying a solder-containing paste and LEDs to the circuit board ribbon and heating the circuit board ribbon in a first zone to at least partially melt the solder to electrically connect the LEDs to the at least one electrical conductor; and

step (d) comprises passing the heated circuit board ribbon through a die in a second zone to impart the shape to the circuit board ribbon.

10. The method of claim 9 wherein the method further includes the step of cutting the cooled circuit board ribbon into desired lengths.

1 1 . The method of claim 9 wherein the die has a forming section comprising upper and lower forming surfaces having a similar arcuate cross section, the upper surface having a gap that allows the LEDs to pass through the die without contacting it.

12. The method of claim 9 wherein circuit board ribbon is at least partially cooled while in the forming section so that the shape is substantially fixed when the ribbon exits the die.

13. The method of claim 9 wherein the first zone is heated to a solder reflow temperature and the second zone is heated to a thermoforming temperature.

14. The method of claim 13 wherein the solder reflow temperature is about 140 °C and the thermoforming temperature is about 1 10°C.

15. The method of claim 9 wherein the second zone is not heated.

16. The method of claim 1 1 wherein the die has a flared receiving section that receives both edges of the circuit board ribbon.

17. The method of claim 9 wherein the first and second formable polymer material is polyethylene terephthaiate (PET).

18. The method of claim 17 wherein the second formable polymer materia! is a reflective white PET.

19. The method of claim 9 wherein the shape has a cross section that is a circular arc.

20. The method of claim 19 wherein the circular arc subtends a central angle of 120° to 150°.