Patent Publication Number: US-2015085504-A1

Title: Systems and Methods for Improving Service Life of Circuit Boards

Description:
RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119(e) to and incorporates herein by reference U.S. Provisional Patent Application No. 61/881,871, filed on Sep. 24, 2013, and titled “Systems and Methods For Improving Service Life of LED Boards.” 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to printed circuit boards. Specifically, the present disclosure relates to a circuit board with reduced stress on solder joints and improved service life. 
     BACKGROUND 
     Printed circuit boards (PCBs) include a layer of electrical traces which make up the desired circuit connections. The electrical traces typically include a plurality of solder pads or connection points to which respective electrical components are to be soldered, thereby electrically coupling the electrical components in the desired circuit layout. The solder pads, along with the electrical traces, are typically printed onto a base board such that the solder pads for a specific component are spaced apart and dimensioned in accordance with the spacing and dimensions of the contacts of the specific electrical component. 
     Typically, circuit boards used with surface mount light emitting diodes (LEDs) comprise an aluminum core board with a dielectric layer on which the electrical traces are printed. The LEDs are surface mounted onto the circuit board via an anode contact and a cathode contact. However, the aluminum core board has a greater coefficient of thermal expansion than does the LED package. Thus, as heat is applied to the circuit board, the distance between the anode and cathode contacts of the LED does not expand as much as the aluminum expands. Eventually, this may lead to solder cracking at the solder joints between the contacts and the circuit board, resulting in board failure. 
     SUMMARY 
     Generally, in one aspect of the present disclosure, a circuit board includes a base board and a layer of an elastic material comprising a first surface and a second surface. The layer of elastic material is adhered to the base board via the first surface. The circuit board further includes an electrical trace disposed on the second surface of the layer of elastic material. At least a portion of the layer of elastic material stretches or shrinks when the base board expands or contracts. 
     In another aspect of the present disclosure, a method of manufacturing a circuit includes obtaining an aluminum board, obtaining a layer of an elastic material, and applying a layer of adhering material to a surface of the aluminum board. The method further includes disposing the layer of the elastic material onto the layer of adhering material, and adhering the layer of the elastic material onto the aluminum board via the layer of adhering material. 
     In another aspect of the present disclosure, a method of manufacturing a printed circuit board includes obtaining a base circuit board. The base circuit board comprises an aluminum board and a layer of elastic material disposed on a surface of the aluminum board. The method further includes disposing one or more electrical traces onto the base circuit board, wherein the one or more electrical traces experience less expansion per unit surface area than the base circuit board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a cross-sectional representation of a circuit board with improved service life, in accordance with example embodiments of the present disclosure; 
         FIG. 2  is a top view of a circuit board with improved service life, in accordance with example embodiments of the present disclosure; 
         FIG. 3  is a perspective view of the circuit board of  FIG. 2  and an optics assembly, in accordance with example embodiments of the present disclosure; 
         FIG. 4  is a top view of a light module containing the circuit board and optics assembly of  FIG. 3 , in accordance with example embodiments of the present disclosure; 
         FIG. 5  is a flow diagram of a method of manufacturing a base board for a circuit board with improved service life, in accordance with example embodiments of the present disclosure; and 
         FIG. 6  is a flow diagram of a method of manufacturing a circuit board with improved service life, in accordance with example embodiments of the present disclosure. 
     
    
    
     The drawings illustrate only example embodiments of the disclosure and are therefore not to be considered limiting of its scope, as the disclosure may admit to other equally effective embodiments. The elements and features shown in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the example embodiments. In the drawings, reference numerals designate like or corresponding, but not necessarily identical, elements. 
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Example embodiments disclosed herein are directed to systems and methods for improving the service life of LED circuit boards. Specifically, the example embodiments provide the ability to relieve stress on the solder joints of LEDs and other onboard components caused by thermal expansion of the circuit board. The integrity of the solder joints is better maintained over time, thereby improving the service life of the LED circuit board. The example embodiments make reference to LEDs as an example component on a circuit board. However, the principles and techniques provided in this disclosure apply to any surface mount electrical component that is soldered to a circuit board. 
       FIG. 1  illustrates a cross-sectional view of a circuit board with improved service life, in accordance with an example embodiment of the present disclosure. FIG.  2  illustrates a top view of a printed circuit board assembly  200  using the circuit board of  FIG. 1 , in accordance with example embodiments of the present disclosure. Referring first to  FIG. 1 , in certain example embodiments, the circuit board  100  includes an aluminum board  102  and a layer of polyimide material  104  or alternative elastic material. The aluminum board  102  and the layer of polyimide  104  make up a base board  106 . In certain example embodiments, the polyimide  104  is adhered to the aluminum board  102  via a tape  108 . The tape  108  has certain appropriate qualities, such as being double-sided, thereby adhering between and to both the polyimide  104  and the aluminum board  102 . The tape  108  is also chosen to be able to withstand the high temperatures of a reflow oven such that when the circuit board  100  is subject to reflow soldering, the integrity of the tape  108  is maintained. Additionally, in certain example embodiments, the tape  108  is pressure-sensitive. In certain example embodiments, the polyimide material  104  is replaced by another elastic material. 
     Referring now to  FIGS. 1 and 2 , in certain example embodiments, the circuit board  100  further comprises an electrical or electrical trace  110  disposed on the polyimide  104  opposite the aluminum board  102 . Alternatively stated, the layer of polyimide  104  includes a first side  105   a  and a second side  105   b,  in which the first side  105   a  of the polyimide  104  is adhered to the aluminum board  102  by the tape  108  and the electrical trace  110  is laid on the second side  105   b  of the elastic material  104 . In certain example embodiments, the electrical trace  110  is fabricated from 2 oz. copper. In certain example embodiments, one or more LEDs  114  and other electrical components  202  are soldered onto one or more areas of the electrical trace  110 . Specifically, in certain example embodiments, the electrical trace  110  includes one or more solder pads for receiving and coupling to the LEDs  114  or electrical components  202 . In certain example embodiments, a solder mask  112  or dielectric is applied over the electrical trace  110 . The solder mask  112  makes the underlying electrical trace  110  more resistant to oxidation and helps prevent accidental electrical contact or shorting of the trace  110 . 
     The aluminum board  102  typically exhibits greater thermal expansion than does the electrical trace  110  and the electrical connections. However, the polyimide layer  104  has a high modulus of elasticity and acts as a buffer between the aluminum board  102  and the electrical traces  110 . Specifically, as the aluminum board  102  expands, certain portions of the polyimide layer  104  stretch accordingly. However, the portions of the polyimide layer  104  which are directly coupled to the electrical traces  110  are able to remain relatively stable. Thus, the stretching force and stress that would otherwise be felt by the electrical connections caused by disproportionally large expansion of the aluminum board  102  is largely assumed by the polyimide layer  104 . Accordingly stress on the electrical connections is reduced and the printed circuit board assembly  200  is more resilient and robust against fluctuating temperatures. As a result, the printed circuit board assembly  200  is more reliable and has an increased operational lifetime. 
       FIG. 3  illustrates the printed circuit board assembly  200  of  FIG. 2  and an optics assembly  300 . In certain example embodiments, the optics assembly  300  includes a plurality of LED optics  304  disposed on a high-density polyethylene substrate  302 , such as Tyvek®, a registered trademark of DuPont. In certain example embodiments, the substrate includes a plurality of openings  306  formed therein. In certain example embodiments, the substrate  302  includes an adhesive backing through which the substrate  302  can be applied to the printed circuit board assembly  200 . The optics  304  are disposed over the LEDs  114  and the openings  306  are disposed around the other components  202  when the optics assembly  300  is applied to the printed circuit board assembly  200 . 
       FIG. 4  illustrates an LED light module  400  in accordance with an example embodiment of the present disclosure. The light module  400  includes a housing  402  which houses the printed circuit board assembly  200  coupled to the optics assembly  300 . The housing  402  includes a plurality of openings  404  through which the optics  304  are disposed. The light module  400  further includes a plurality of wires  406  which provide power to the printed circuit board assembly  200  contained therein. The light module  400  of  FIG. 4  includes the printed circuit assembly  200  of  FIG. 2 , which includes a layer of polyimide  104  disposed between the aluminum board  102  and the electrical trace  110 . As the polyimide  104  provides a high modulus of elasticity, the effects of thermal expansion of the aluminum board  102  are substantially mitigated by the polyimide  104 . Thus, stretching forces and other stresses applied to the electrical traces  110  are decreased. Accordingly, electrical connections between the LEDs  114  and the electrical traces  110  are more secure, making for a more robust and long-lasting light module. 
       FIG. 5  illustrates a method of manufacturing  500  the base board  106  of the circuit board  100  of  FIG. 1 , in accordance with example embodiments of the present disclosure. In an example embodiment, the method  500  includes obtaining an aluminum board  102 , such as an aluminum core board (step  502 ). The method further includes disposing a layer of polyimide  104  on a surface of the aluminum board  102  (step  504 ). In certain example embodiments, a layer of an alternative elastic material is used in place of the polyimide  104 . In certain example embodiments, the method  500  includes adhering the polyimide  104  to the aluminum board  102  with a pressure sensitive, double sided, temperature resistant, transfer tape  108 , which is able to withstand the high temperatures of the re-flow oven. In certain other example embodiments, the method  500  includes securing the polyimide  104  to the aluminum board  102  using a different technique, agent, or mechanism. 
       FIG. 6  illustrates a method of manufacturing  600  the circuit board  100  of  FIG. 1 , in accordance with example embodiments of the present disclosure. In an example embodiment, the method  600  includes obtaining a base board  106  comprising a layer of polyimide  104  disposed on or adhered to an aluminum board  102  (step  602 ), such as that manufactured through the method  500  of  FIG. 5 . Alternatively, in another example embodiment, the method  600  of manufacturing the circuit board  100  includes the steps of manufacturing  500  the base board  106  as described in  FIG. 5 . The method  600  further includes laying an electrical trace  110  on the polyimide  104  of the base board  106  (step  604 ). In an example embodiments, the electrical trace  110  is created through a subtractive process over the polyimide  104 . In another example embodiment, the electrical trace  110  is created through an additive process over the polyimide  104 . In certain example processes, the method  600  includes applying a solder mask to the circuit board  100  over the electrical trace  110  (step  606 ). In certain example embodiments, the method  600  further includes disposing one or more electrical components  114  on the electrical trace  110  (step  608 ) and soldering the electrical components  114  to the electrical trace  110  (step  610 ). In an example embodiment, the electrical components  114  are soldered to the electrical trace  110  through a reflow soldering process, which may include running the board with components through a reflow oven. Alternatively, in an example embodiment, the electrical components  114  are soldered to the electrical trace  110  individually. In an example embodiment, the method  600  also includes de-panelizing the circuit board (step  612 ). 
     Although the disclosures are described with reference to example embodiments, it should be appreciated by those skilled in the art that various modifications are well within the scope of the disclosure. From the foregoing, it will be appreciated that an embodiment of the present disclosure overcomes the limitations of the prior art. Those skilled in the art will appreciate that the present disclosure is not limited to any specifically discussed application and that the embodiments described herein are illustrative and not restrictive. From the description of the example embodiments, equivalents of the elements shown therein will suggest themselves to those skilled in the art, and ways of constructing other embodiments of the present disclosure will suggest themselves to practitioners of the art. Therefore, the scope of the present disclosure is not limited herein.