Patent Publication Number: US-11647592-B2

Title: Exposure system, circuit board, and method for making circuit board

Description:
FIELD 
     The subject matter herein generally relates to circuit board manufacture, specifically an exposure system, a circuit board, and a method for making the circuit board. 
     BACKGROUND 
     Generally, a circuit board includes a circuit layer and a solder resist layer covering the circuit layer. The solder resist layer may be a patterned dry film ink. However, when the dry film ink is to be exposed and cured, the bottom of the ink may not be completely cured, resulting in serious undercut. In addition, the residual liquid in the undercut may become dangerous due to subsequent processes, causing the circuit board to explode. 
     Therefore, there is room for improvement in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present disclosure will now be described, by way of embodiment, with reference to the attached figures. 
         FIG.  1    is a schematic diagram of a working principle of an exposure system according to an embodiment of the present disclosure. 
         FIG.  2    is a schematic flow chart of a method for making a circuit board applying the exposure system in  FIG.  1    according to an embodiment of the present disclosure. 
         FIG.  3    is a cross-sectional view of a circuit substrate in block S 1 , created in the method of  FIG.  2   . 
         FIG.  4    is a cross-sectional view showing dry film ink covering a surface of the circuit layer away from the base layer in  FIG.  3   . 
         FIG.  5    is a cross-sectional view showing a solder mask layer obtained by patterning the dry film ink in  FIG.  4   . 
         FIG.  6    is a cross-sectional view showing a slot of the solder mask layer in  FIG.  5   . 
         FIG.  7    is a top view of the circuit board in  FIG.  5   . 
         FIG.  8    is a cross-sectional view showing a backlight board obtained by connecting a light-emitting element to the solder pad of  FIG.  7   . 
         FIG.  9    is a cross-sectional view of a display device according to the present disclosure applying the backlight board in  FIG.  8   . 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the exemplary embodiments described herein may be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the exemplary embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features of the present disclosure. 
     The term “comprising” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references can mean “at least one”. 
       FIG.  1    shows a working principle of an exposure system according to an embodiment. As shown in diagram (a) of  FIG.  1   , the exposure system  10  includes a plurality of light sources for emitting light of different wavelengths within the range of 365 nm to 440 nm. The light sources of the exposure system  10  can output light of at least four different wavelengths, three of such wavelengths being substantially of a single wavelength, and the fourth light source emitting a spectrum of light, within the range of 425 nm to 440 nm. 
     The exposure system  10  can use light of at least four different wavelengths in the range of 365 nm to 440 nm to expose and cure a dry film ink  13 . Short-wavelength light (e.g., light with a wavelength in a range of 365 nm to 425 nm) is good for curing surface of the dry film ink  13 , and long-wavelength light (e.g., light with a wavelength in a range of 425 nm to 440 nm) is effective for curing the interior and bottommost part of the dry film ink  13 . Therefore, the light output by the light sources is in the range of 365 nm to 440 nm, which can penetrate a surface portion of the dry film ink  13  and act on the inner and deep portions of the dry film ink  13 . Thus, the bottom portion of the ink  13  is not susceptible to being washed away by developer fluid during a subsequent development process, which avoids undercutting caused by incomplete curing of the bottom portion of the dry film ink  13 . Furthermore, when the dry film ink  13  is patterned and used as a solder resist, the solidity of being in non-liquid form avoids erosion of the sides, shrinkage of a gap is less, and risk of circuit board explosion caused by residual liquid in gaps during subsequent processes is reduced. 
     Some conventional exposure systems use mixed light sources, their wavelengths of light emitted by the mixed light sources are in a range of 320 nm to 400 nm. Since absorption rate of dry film ink to light (especially UV light band) is affected by the wavelength of the light source, and more energy is absorbed near the surface of the ink, while the deeper the ink (also called the bottom portion of the ink) gets less energy. This becomes an acute problem as the thickness or depth of ink increases. This situation varies with the wavelength range of the light sources. The wavelength range of the light sources of the traditional exposure system is 320 nm to 400 nm, and this can only reach the ink depth of about 30 μm at most. When the ink thickness is greater than 30 μm, 320 nm to 400 nm light cannot reach the bottom portion of the ink at all, making the curing of the bottom layer incomplete. During exposure, the surface portion of the ink is cured more fully than the bottom portion. In the development step, when washing with a developer, the weakly cured bottom portion has a weaker bond with the substrate  11 , and side erosion occurs, so that a width of the bottom portion is undercut and is less than a width of the surface portion. A cross-section of the patterned dry film ink along the thickness direction of the patterned ink is generally of an inverted trapezoid shape. Therefore, the shrinkage of the patterned dry film ink is relatively large (greater than 45 μm), which tends to hide the liquid in the subsequent process and affect quality of the circuit board with the patterned dry film ink as a solder mask layer. 
     In some embodiments, the mixed light sources of the exposure system  10  include one or a combination of a light source  12  for emitting light of a single wavelength of 365 nm, a light source  14  for emitting light of a single wavelength of 385 nm, and a light source  16  for emitting light of a single wavelength of 405 nm. The light sources  12 ,  14 ,  16 , and  18  may all be LEDs. In other embodiments, the light sources  12 ,  14 ,  16 , and  18  may all be laser light sources. 
     As shown in diagram (a) of  FIG.  1   , the mixed light sources of the exposure system  10  include four light sources, which are light sources  12 ,  14 ,  16 , and  18 . Short-wavelength light (e.g., 365 nm) can reach the surface portion of the dry film ink  13 , and as the wavelength of the light source increases, the depth to which irradiate the dry film ink  13  is increased. The light source  14  with a wavelength of 385 nm and the light source  16  with a wavelength of 405 nm can reach the middle portion of the dry film ink  13 . The light source  16  of a wavelength of 405 nm can reach the ink depth of about 30 μm, and the light source  18  of a wavelength in the range of 425 nm to 440 nm can reach the deep portion (more than 30 μm) of the dry film ink  13 . Therefore, in the exposure system  10 , the wavelengths of the output light are in the range of 365 nm to 440 nm, which can penetrate not only the surface portion of the dry film ink  13  but further act on the inner and deeper portions (more than 30 μm) of the dry film ink  13 . 
     As shown in diagram (b) of  FIG.  1   , after the dry film ink  13  is exposed and developed, since light with a wavelength in the range of 425 nm to 440 nm can reach the deep portion of the dry film ink  13 , the bottom of the ink is not easily washed away by the developer, which avoids the problem of undercutting caused by incomplete curing of the bottom portion of the film ink  13 . A cross-section of the side wall of the patterned dry film ink  13  in the thickness direction is a convex arc in shape, like a bullet. In other words, after being patterned, the dry film ink  13  has a “U”-shaped side wall that is inclined at 90°. As shown in diagram (b) of  FIG.  1   , a width at the widest position of the patterned dry film ink  13  is W 1 , and a width at the narrowest position of the patterned dry film ink  13  is W 2 . An undercutting, in terms of distance, is defined as half of a width difference between the widest position and the narrowest position of patterned dry film ink  13 . That is, the undercut distance is (W 1 −W 2 )/2. Since the dry film type ink  13  has less shrinkage after being patterned (i.e., W 2  increases), the undercut distance decreases. Therefore, compared with the inverted trapezoidal cross-section, the patterned dry film ink  13  has less shrinkage after being patterned. Therefore, when the patterned dry film ink  13  is used as a solder mask layer of the circuit board, the risk of malformations of the circuit board and other dangers of a hidden liquid in the gaps during the subsequent process is reduced. 
     In addition, the exposure system  10  may further include a controller (not shown) and optical elements such as a mirror (not shown). The controller controls time sequence of the light emitted by the mixed light sources. In one embodiment, during the exposure process, the controller controls the mixed light sources to irradiate the dry film ink  13  in the order of wavelengths from longest to shortest. For example, the controller controls the mixed light sources to first use the light source  18  with a wavelength in the range of 425 nm to 440 nm to irradiate the dry film ink  13 , so that the light with the wavelength in the range of 425 nm to 440 nm penetrates the surface portion of the dry film ink  13  and acts on the deepest, the deeper, and the inner portions of the dry film ink  13  (more than 30 μm). After the dry film ink  13  absorbs the energy of light with a wavelength of 425 nm to 440 nm, its curing strength on the substrate  11  is effectively higher. That is, the deep inside of the dry film ink  13  is cured by light with a wavelength in the range of 425 nm to 440 nm, so that the dry film ink  13  will not easily be detached from the substrate  11  during the subsequent development process, which is beneficial to improve the gloss of the dry film ink  13  after being patterned. Then, the control system controls the mixed light sources to irradiate the dry film ink  13  with the light source  16  with a wavelength of 405 nm, then the light source  14  with a wavelength of 385 nm and the light source  12  with a wavelength of 365 nm in sequence to act on the surface portion and the inner middle portion of the dry film ink  13 , to further solidify the surface and inner middle portions of the dry film ink  13 . 
     In addition, the controller can further control an energy ratio of the light emitted by each light source in the at least four different wavelengths of light in the combined light source, so that the dry film ink  13  achieves a better curing effect. 
       FIG.  2    shows a flowchart of a method for a circuit board according to an embodiment. The example method is provided by way of example, as there are a variety of ways to carry out the method. The method described below can be carried out using the configurations illustrated in  FIGS.  3  through  7   , for example, and various elements of these figures are referenced in explaining the example method. Each block shown in  FIG.  2    represents one or more processes, methods, or subroutines carried out in the example method. Furthermore, the illustrated order of blocks is by example only, and the order of the blocks can be changed. Additional blocks can be added, or fewer blocks can be utilized, without departing from this disclosure. The example method can begin at block S 1 . 
     Block S 1 : a circuit substrate is provided, wherein the circuit substrate includes an insulating base layer and a circuit layer on a surface of the base layer. 
     Block S 2 : a surface of the circuit layer away from the base layer is covered with dry film ink. 
     Block S 3 : the dry film ink is patterned to form a solder mask layer. 
     The method for making the circuit board will be described in detail below with reference to  FIGS.  3  through  7   . 
     Block S 1 : a circuit substrate is provided, wherein the circuit substrate includes an insulating base layer and a circuit layer on a surface of the base layer. 
     As shown in  FIG.  3   , the circuit substrate  20  includes four circuit layers, one dielectric layer, and two layers of build-up materials. In order from top to bottom, the circuit substrate  20  includes a first circuit layer  21 , a first build-up material  26 , a second circuit layer  22 , a dielectric layer  25 , a third circuit layer  23 , a second build-up material  27 , and a fourth circuit layer  24 . The insulating base layer in block S 1  is the first build-up material  26 , and the circuit layer on the surface of the base layer in block S 1  is the first circuit layer  21 . The first circuit layer  21 , the second circuit layer  22 , the third circuit layer  23  and the fourth circuit layer  24  are electrically connected by conductive holes  28  penetrating the first build-up material  26 , the dielectric layer  25 , and the second build-up material  27 . In other embodiments, the circuit substrate  20  is not limited to include four circuit layers, it may include one circuit layer, or two, three, and more than four circuit layers. 
     Block S 2 : a surface of the circuit layer away from the base layer is covered with dry film ink. 
     As shown in  FIG.  4   , the dry film ink  13  covers the surface of the first circuit layer  21 , infills a through hole  29  penetrating the first build-up material  26 , the dielectric layer  25 , and the second build-up material  27  and covers the surface of the fourth circuit layer  24 . That is, both the first circuit layer  21  and the fourth circuit layer  24  are covered with the dry film ink  13 . In other embodiments, forming the dry film ink covering the surface of the fourth circuit layer  24  and forming the dry film ink covering the surface of the first circuit layer  21  are two separate processes. 
     In one embodiment, a thickness of the portion where the dry film ink  13  covers the first circuit layer  21  is in a range of 30 μm to 60 μm. 
     Block S 3 : the dry film ink is patterned to form a solder mask layer. 
     Patterning the dry film ink  13  to form the solder mask layer  30  includes exposure, development, and post-curing. In the exposure step, at least four different wavelengths of light with a wavelength range of 365 nm to 440 nm irradiates the dry film ink  13  in the order of wavelength from long to short, and the light of at least four different wavelengths includes light of a wavelength in a range of 425 nm to 440 nm. Thus, the deep portion, the inner middle portion, and the surface portion of the dry film ink  13  can all be irradiated. In some embodiments, the light of at least four different wavelengths includes light with a single wavelength of 365 nm, light with a single wavelength of 385 nm, and light with a single wavelength of 405 nm, and light with a wavelength of 425 nm to 440 nm, light with a single wavelength of 365 nm. In the light of at least four different wavelengths, energy ratios of light of a wavelength in a range of 425 nm to 440 nm, light of a single wavelength of 365 nm, light of a single wavelength of 385 nm, and light of a single wavelength of 405 nm are defined as a, b, c, d, respectively; where 0&lt;a≤36%, 0&lt;b≤25%, 0&lt;c≤15%, and 0&lt;d≤24%. Thus, by adjusting the energy ratios of the light emitted by the mixed light sources with the light of different wavelengths, the dry film ink  13  achieves a good curing effect. 
     In addition, since the deep portion, inner middle portion, and surface portion of the dry film ink  13  can be irradiated during the exposure step of the method for making the circuit board, it has a better curing effect. Therefore, after exposure, the development step can be directly performed without pre-baking. Thus, the pre-baking step after the exposure process is eliminated, the process is simplified, and the production efficiency is improved. 
     As shown in  FIG.  5   , after development and post-curing, a portion of the dry film ink  13  covering the first circuit layer  21  is patterned to form a slot  31  exposing the first circuit layer  21 . The exposed portion of the first circuit layer  21  forms a solder pad  211 . The solder mask layer  30  includes a side wall  32  at the slot  31 . 
     As shown in  FIG.  6   , along a thickness direction of the solder mask layer  30 , a cross section of the side wall  32  is a convex arc in-shape facing the slot  31 . Compared with the traditional inverted trapezoidal cross-section, the dry film ink  13  has less shrinkage after being patterned. Therefore, when the patterned dry film ink  13  is used as a solder mask layer of the circuit board, risk of the circuit board explosion due to the residual liquid in gaps during subsequent processes is reduced. 
     In some embodiments, after the dry film ink  13  is patterned, the undercut distance at the slot  31  is less than 45 μm. Therefore, compared with the traditional inverted trapezoidal cross-section, the dry film ink  13  has less shrinkage after being patterned, which can effectively reduce the length of gap when the patterned dry film ink  13  is used as the solder mask layer  30  of the circuit board, risk of the circuit board explosion due to the residual liquid in gaps during subsequent processes is reduced. 
       FIG.  7    shows a top view of the circuit board shown in  FIG.  5   . The slots  31  opened in the solder mask layer  30  are arranged in a matrix including rows and columns. Each slot  31  exposes two solder pads  211 . The two solder pads  211  are spaced apart from each other for connection with an external circuit (such as an LED chip). 
     An embodiment of the present disclosure provides a circuit board  110  shown in  FIGS.  5  and  7   . The circuit board  110  includes an insulating base layer, a circuit layer formed on the surface of the base layer, and a solder mask layer  30 . The solder mask layer  30  covers the surface of the circuit layer away from the base layer, the solder mask layer  30  has slots  31  exposing the circuit layer, and each exposed portion of the circuit layer forms a solder pad  211 . The solder mask layer  30  is a patterned dry film type ink  13 , the solder mask layer  30  includes a side wall  32  at the slot  31 , and in the thickness direction of the solder mask layer  30 , the cross section of the side wall  32  is a convex arc in-shape facing the slot  31 . The thickness of the solder mask layer  30  is 30 μm to 60 μm. The undercut distance of the patterned dry film ink  13  at the slot  31  is less than 45 μm. In other embodiments, the circuit board  110  is not limited to having four circuit layers. The cross section of the side wall  32  of the solder mask layer  30  of the circuit board  110  is a convex arc in-shape facing the slot  31 . That is, the solder mask layer  30  has a side wall like a bullet structure, or a “U”-shaped side wall with a similar slope of 90°. Therefore, the problem of undercutting is avoided, so that the shrinkage of the solder mask layer  30  is less, which can effectively reduce the risk of circuit board explosion caused by the circuit board  110  due to the liquid hidden in gaps in the subsequent manufacturing process. 
     As shown in  FIG.  8   , an embodiment of the present disclosure provides a backlight module  100 . The backlight module  100  includes at least one light-emitting element  120  and the circuit board  110 . The light-emitting element  120  is on the solder pads  211  and electrically connected to the solder pads  211 . In some embodiments, the light-emitting element  120  is a mini-LED with a size range of approximately 50 μm to 200 μm. 
     In some embodiments, the distance between two adjacent mini-LEDs is in a range from 0.1 mm to 1.0 mm. That is, the distance between two adjacent exposed portions of the circuit layer is in a range from 0.1 mm to 1.0 mm. Thus, the number of mini-LEDs increases, and the arrangement of mini-LEDs is dense, so that when the backlight module  100  is used in a display device, the display device has a high contrast, a good color gamut, and a fine area light control effect to achieve a high dynamic range image. 
     As shown in  FIG.  9   , an embodiment of the present disclosure provides a display device  1000 . The display device  1000  includes a display panel  200  and the backlight module  100 . The display panel  200  and the backlight module  100  is stacked. The display panel  200  is a liquid crystal display panel, which includes a color filter substrate  230 , a liquid crystal layer  220 , and a thin film transistor array substrate  210  that are sequentially stacked. The backlight plate  100  serves as a direct type of backlight source and is on a side of the display panel  200  away from the display surface to provide backlight for the display panel  200 . In addition, the backlight module  100  further includes optical films such as brightness enhancement films, diffusion sheets, light guide plates, and reflection sheets that are laminated. 
     In some embodiments, the display device  1000  may be an outdoor display screen, a TV, a notebook computer, a tablet computer, a car touch screen, etc. Since the display device  1000  includes the backlight module  100 , it also has the advantage of good reliability. 
     It is to be understood, even though information and advantages of the present exemplary embodiments have been set forth in the foregoing description, together with details of the structures and functions of the present exemplary embodiments, the disclosure is illustrative only. Changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the present exemplary embodiments to the full extent indicated by the plain meaning of the terms in which the appended claims are expressed.