TITLE OF INVENTION

BENDABLE CIRCUIT STRUCTURE FOR LED MOUNTING AND INTERCONNECTION

FIELD OF THE INVENTION

This invention is directed to a flexible substrate structure used for LED (light emitting diode) mounting and Interconnection.

TECHNICAL BACKGROUND OF THE INVENTION

Insulated metal substrates are being used for the mounting and interconnection of light emitting diodes (LEDs) in applications such as display back lighting, automotive lighting and general commercial and consumer lighting uses. The substrates currently being used are generally rigid and are not able to be deformed and bent without degradation or damage to their mechanical, electrical and thermal properties. It is desirable to be able to form or bend such substrate materials to make angles, curves or bends, while maintaining mechanical and thermal integrity and without causing degradation of the substrate or of its electrical properties.

Multi-functional electronic substrate materials of various kinds are known in the art. Some of the layers of these substrates can be made of various polymer materials such as polyimide (hereinafter referred to as "Pl"). In general, these are rigid structures with little flexibility, unless the structure is grooved.

US 2007/0076381 A1 , for example, describes a flexible heat spreader circuit board with a heat sink. The flexibility of its circuit board results from a pattern of grooves that make an upper surface and a lower surface flexible when the substrate is bent. Aluminum substrates are known for use in visible LED Circuits.

Canadian Electronics Magazine e-magazine, for example as shown in an article titled "Aluminum Substrates Make light work of visible LED Circuits" by Tom Morris, dated September 18, 2007. Economical solderable polymer thick film conductors can be screen printed directly onto such

substrates, without degradation. The article mentions that aluminum alloy substrates can be extruded, die cast and made in special shapes. The thermally conductive aluminum alloy material enables design engineers to mount high power LED components directly to it. However, no mention is made of any flexibility of such alloy substrates.

In an article titled "Poor fixtures threaten to jeopardize the illumination potential of LEDs" by Richard Stevenson, Compound Semiconductor, June 2007, the author points out that the low efficiency of many fixtures in the industry results from lack of skills relating to working on the design aspects of these fixtures, by the workforce of many companies, who work with the electrical systems used in powering LEDs. These techniques are needed for thermal management and optical design.

As applications expand requiring more complex mechanical configurations and circuitry becomes thinner and more complicated, it is desirable for the substrates to be shapeable without any resulting degradation of the substrate surface characteristics or without interfering with their thermal or electrical properties. It is also desirable for chosen substrates to be cost effective, be easy to handle and be able to be used in fixtures allowing the best use of the light from an LED. The laminate described in the present patent application is useful in such fixtures as it can be shaped for improved optical properties without any degradation of its electrical properties and with its thermal properties maintained. The ability to bend a laminated structure without degrading the electrical circuitry allows the configuring of LED's in a "3D" format. Such structures are not limited to a planar format. Having this bendable ability provides the device designer with more options to tailor and optimize the device design. Overcoming the limitations in LED mounting and LED structure and fixtures imposed by rigid substrate materials and interconnection materials allows the designer to customize and optimize the mechanical features found in Solid State lighting components.

The present flexible structure is expected to increase overall optical electrical efficiency from 30% to 40% of the range currently seen in LED structures and to optimize the configuration manually by allowing the

bending at the most efficient configuration to concentrate and disperse LED light.

SUMMARY OF THE INVENTION

The present invention concerns a laminate comprising, on a substrate the following layers:

(a) a copper or aluminum metal layer;

(b) a polyimide or adhesive layer, the metal layer being adjacent to the polyimide or adhesive layer

(c) copper foil layer and (d) liquid or film solder mask layer.

The laminate may be a thin polymer based copper clad filled polyimide (Pl), for example a CooLam™ thermal laminate such as LC (Alumina Filled Pl) and LX (Alumina Filled Pl), used in conjunction with an aluminum alloy composition substrate. The laminate may also be an anodized aluminum CooLam™thermal laminate, such as LU (Anodized Aluminum and with adhesive). Such laminates must be designed to be used in a composition of the proper thickness for LED applications. Use of such laminates creates a versatile and bendable structure for ease of use in LED packaging.

As pointed out above, a suitable family of materials useful in this invention is the CooLam™ thermal laminates. Examples are embodiments such as LC ( Alumina Filled Pl), LU (Anodized Aluminum with adhesive) and LX (Alumina Filled Pl). A CooLam™ structure consists of a metal, optionally an adhesive (only in the LU (Anodized Aluminum with adhesive) embodiment), polyimide (Pl), copper foil and a solder mask. See drawings Figure 3A and 3B (key). LU products may also be alumina filled.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 shows a typical laminate substrate, with bend locations 6, 7 and 8.

Figure 2 shows the detail of test lines 6. Two traces that were measured are labeled 9 and 10.

Figure 3(A) shows the stacking arrangements of CooLam™ Laminates. Figure 3(B) serves as a "key" to the drawing in Figure 3(A). In Figure 3(B) the 14 ounce to 4 ounces Cu, in metric units, is "14.79 milliliters to 118.29 milliliters".

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figures 3(A) and 3(B), the term "laminate" herein is referred to as the metal (1 ) and the polyimide (3), and adhesive (2) where adhesive is present, plus the copper foil (4). The term "substrate" refers to the metal itself (1 ). The finished product with patterned copper foil and solder mask (5) is called a "Metal Core Printed Circuit Board" or an "Insulated Metal Substrate" (IMS).

In one embodiment, the copper is at 18 urn, the dielectric is at 6um and aluminum is at 100 urn. In another embodiment the copper is at 35 urn, dielectric is at 12 urn and the aluminum is at a thickness selected from the following group of thicknesses: 200 urn and 247 urn.

The composite films of the present invention are generally derived from a multi-layer structure having one or more polyimide composite layers. These polyimide composites and layers can have thermal conductivity, electrical conductivity, electrical resistivity, electrical capacitance and other desirable properties. They can also be layered in various ways to create other sought after properties. An example of such a layering plan for a coating on a substrate is described in US Patent 7,100,814, which is incorporated herein by reference. Some of the layered structures known in the art lack flexibility which results in cracking and breakage upon their use. The laminate thickness is from about 125 microns to about 3 millimeters. Allowing substrate flexibility results in cost reduction, the need for fewer pieces, need for less handling and greater ease of installation, along with improved appearance and the ability to create custom shapes and more efficient configurations to concentrate or distribute light, offering improved illumination and appearance characteristics.

EXAMPLES

Tests to determinate interconnection integrity of the present laminate.

Tests to determine interconnection integrity of CooLam™ laminate under bending stress were performed on substrates fabricated with 17micron thick CooLam™ thermal laminate and 2 mm thick Aluminum Alloy 5005 using a LCD Backlight Unit circuit design. The purpose of this testing was to determine the feasibility of offering this type of construction for applications in LED lighting.

The overall sample size was 17mm x 423mm (5/8" x 17") and consists of 6 identical segments is shown in Figure 1.

Three locations on each of six pieces were tested as shown in Figure 1. The resistance of two 11.8 mil 1.4 mil thick traces, shown as 9 and 10 in Figure 2, in each location was measured before and after bending. The bend point centerline on each segment was positioned on the centerline of a 1 1/8" diameter mandrel and bent by hand to an approximate 90 degree bend. The laminate/conductor was on the outside of the bend putting the material in tension. The actual bend profile for each location tested was traced or compared to prior sample traces and entered in a notebook with the test data. The following Table 1 tabulates the resistance test results for the configurations. Visual examination at the bend point evidenced no cracking or crazing of the solder mask. Resistance measurements were performed using an Electro Scientific Industries 1700 series Micro-Ohm meter system using the 20 milliohm range (resolution of 1 micro Ohm). The average change in resistance of all the circuit lines was 1.55 milliohms with a maximum of 3 milliohms and a minimum of 0.8 milliohms. This testing indicates that the CooLam™ material used in a metal-backed structure has the potential to be bent to 90 degrees or greater angles with reliable interconnections.

Table 1