Patent Publication Number: US-2009219534-A1

Title: Imaging device for solder paste inspection

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to imaging devices, and particularly to an imaging device for inspecting solder paste deposited on a printed circuit board (PCB). 
     2. Description of Related Art 
     In the manufacture of PCBs, surface mounting components, such as resistors and capacitors, are commonly mounted on the PCBs using surface mount technology (SMT). SMT generally includes a solder pastes deposition process for depositing solder paste on conductive pads located on a PCB. 
     Manufacturing defects are common during the solder paste deposition process. If too much solder paste is deposited, one or more of the conductive pads on the PCB may make unwanted electrical connection with another nearby conductive pad on the PCB. If too little solder paste is deposited, only a poor mechanical and electrical connection might be established between one or more of the conductive pads on the PCB and the corresponding pads on surface mount components. Therefore, it is important to inspect the solder paste following the solder deposition process to determine whether the solder paste has been properly deposited. 
     One method of identifying defective solder paste uses a technology known as automated optical inspection (AOI). AOI technology generally utilizes a light source for perpendicularly projecting light beams on the solder paste, and a camera for capturing light beams reflected from the solder paste, thereby generating an image of the solder paste. Referring to  FIG. 8 , an image  10  of part of a sample PCB is illustrated. As seen, when the light beams are perpendicularly projected onto a patch of solder paste  106 , a portion of the solder paste  106  such as the portion  102  is identified as defective. 
     However, as also seen, an image of the trace  104  is also captured. This can make it difficult to identify the defective portions of the solder paste  106 . 
     Therefore, it is desired to provide an imaging device for reliably inspecting solder paste. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an imaging device in accordance with a first exemplary embodiment of the present invention, together with a sample PCB being inspected by the imaging device, wherein the imaging device comprises a camera and two light sources, and the PCB has a patch of solder paste thereon. 
         FIG. 2  is a top view of the light sources and the PCB of  FIG. 1 . 
         FIG. 3  is an enlarged view of one of the light sources and the patch of solder paste of  FIG. 1 . 
         FIG. 4  is an enlarged view of both of the light sources and the patch of solder paste of  FIG. 1 . 
         FIG. 5A  is a first image of part of the PCB, obtained by using the imaging device of  FIG. 1 . 
         FIG. 5B  is a second image of the same part of the PCB, obtained by using the imaging device of  FIG. 1 . 
         FIG. 6  is a top view of four light sources of an imaging device in accordance with a second exemplary embodiment of the present invention, together with a PCB being inspected by the imaging device. 
         FIG. 7  is a top view of three light sources of an imaging device in accordance with a third exemplary embodiment of the present invention, together with a PCB being inspected by the imaging device. 
         FIG. 8  is an image of the same part of the same sample PCB that was used in obtaining the images of  FIGS. 5A and 5B , but obtained by using a related art imaging device. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an imaging device  100  in accordance with a first exemplary embodiment of the present invention is illustrated. The imaging device  100  is configured for capturing images of at least one patch of solder paste  200  deposited on a printed circuit board (PCB)  110 , and analyzing the images to determine if the at least one patch of solder paste  200  has a defect or defects. In the illustrated embodiment, the PCB  110  is rectangular. In the following description, for simplicity, it will be assumed that there is only one patch of solder paste  200 . In addition, for convenience, the patch of solder paste  200  will generally be referred to simply as “solder paste  200 .” Furthermore, in the following description, a “defect” or “defects” may include at least one portion of the patch of solder paste  200  having too much solder, or too little solder, or solder irregularly distributed, or any combination of these defects. 
     The imaging device  100  generally includes a camera  120 , and at least two light sources  130  and  140 . The first light source  130  and the second light source  140  are capable of projecting light having different wavelengths onto the solder paste  200 . In the present embodiment, the light is visible light, and the light having different wavelengths has different colors. The camera  120  is capable of receiving reflected and scattered light from the solder paste  200 , converting the reflected and scattered light into electrical signals, and constructing an image by processing the electrical signals. 
     In the present embodiment, the first light source  130  and the second light source  140  utilize light emitting diodes (LEDs) to emit light. The first light source  130  and the second light source  140  are arranged symmetrically relative to the PCB  110 , and at a predetermined distance above the solder paste  200 , such that the light sources  130  and  140  can project the light at a predetermined angle α onto the solder paste  200 . The predetermined angle α generally satisfies the following condition (1): 
       10°≦α≦45°  (1) 
     If the predetermined angle α is less than the lower limit of 10°, a large amount of the reflected and scattered light cannot be received by the camera  120 ; thus, a clear image generally cannot be constructed. Referring also to  FIG. 3 , if the predetermined angle α is larger than the upper limit of 45°, a downward slope  201  also may be able to receive the light projected from the first light source  130 . If this happens, it may be difficult for the imaging device  100  to distinguish a defective portion from a normal portion in the image. Referring also to  FIG. 2 , the two light sources  130 ,  140  are strip-shaped, and are longer than a corresponding width of the PCB  110 . Thereby, all the solder paste  200  on the PCB  110  can be sufficiently illuminated. 
     Referring to  FIG. 3 , this shows an enlarged side view of the solder paste in association with the first light source  130 . Normally, when the solder paste is evenly distributed, an upper surface of the solder paste  200  is substantially flat. However, the upper surface of the solder paste  200  as shown in  FIG. 3  is uneven and irregular. The solder paste  200  includes downward slopes  201  facing generally away from the first light source  130 , upward slopes  203  facing generally toward the first light source  130 , and flat surfaces  205 . 
     When the first light source  130  projects light onto the solder paste  200 , the projecting light only illuminates the upward slopes  203  and the flat surfaces  205  of the solder paste  200 . That is, due to the predetermined angle of the first light source  130  relative to the solder paste  200 , the light projected from the first light source  130  is unable to illuminate the downward slopes  201 . As a result, the downward slopes  201  appear as shadows in the image taken by the camera  120 , in respect of the reflected light originating from the first light source  130 . That is, the brightness of the downward slopes  201  is less than that of the upward slopes  203  and the flat surfaces  205 . Thereby, the camera  120  can analyze the image, and determine that the downward slopes  201  correspond to defective portions of the solder paste  200 . 
     Also referring to  FIG. 4 , the first light source  130  and the second light source  140  may simultaneously project light having different wavelengths (colors) onto the solder paste  200 . The two light sources  130 ,  140  are utilized to provide color information for analyzing the images captured by the camera  120 . For example, the first light source  130  projects green light in a first direction onto the solder paste  200 . The second light source  140  projects red light in a second direction onto the solder paste  200 . 
     When the green light and the red light are simultaneously projected onto the solder paste  200 , the green light can only illuminate the upward slopes  203  facing the first light source  130  and the flat surfaces  205 , and the red light can only illuminate the downward slopes  201  facing the second light source  140  and the flat surfaces  205 . As a result, at the flat surfaces  205 , the green light and the red light are combined (mixed) together to generate yellow light according to color mixing theory in optics. 
     Therefore, the green light and the red light are reflected and scattered by the solder paste  200 , and are received by the camera  120 . The camera  120  can then construct an image to analyze the defective portions based on the color information. The downward slopes  201  are identified as defective portions, because the downward slopes  201  produce only red color information in the image. The upward slopes  203  are identified as defective portions, because the upward slopes  203  produce only green color information in the image. The flat surfaces  205  are identified as normal portions, because the flat surfaces  205  produce only yellow color information in the image. At the same time, the brightness information can also be used for identifying the defective portions. The brightness of the downward slopes  201  and the upward slopes  203  is less than that of the flat surfaces  205  in the image. 
     Referring to  FIG. 5A  and  FIG. 5B , a first image  20  and a second image  30  are obtained by utilizing the present imaging device  100  arranged to have the predetermined angle α of 15° (see above). In particular, the first image  20  is obtained by projecting green light and red light from the first light source  130  and second light source  140 , respectively, onto the solder paste  200 . Of course, the first image  20  is a monochrome rendering of the original true color image obtained. The second image  30  is obtained by emitting blue light and red light from the first light source  130  and second light source  140 , respectively, onto the solder paste  200 . Of course, the second image  30  is a monochrome rendering of the original true color image obtained. As seen in the first image  20  and the second image  30 , more defective portions are indentified than is the case with the image  10  obtained by the conventional imaging device (see above, and  FIG. 8 ), because both color information and brightness information are utilized to analyze the defective portions. Furthermore, this advantageous identification of more defective portions is even manifest in the monochrome renderings that are the first and second images  20 ,  30 . 
     As described above, unlike with a conventional imaging device for inspecting solder paste by perpendicularly projecting single-colored light, the present imaging device utilizes at least two light sources capable of projecting light with different colors at predetermined angles. The light sources are arranged at predetermined angles at two sides of the solder paste; thus, both color information and brightness information can be utilized for identifying defective portions of the solder paste. Moreover, traces on a printed circuit board typically have regular surfaces. This means all the light from the first and second light sources can illuminate and be reflected by the traces. As a result, the traces are identified as normal portions, and do not affect the results of inspecting the solder paste. 
     Referring to  FIG. 6 , this shows four light sources of an imaging device in accordance with a second exemplary embodiment of the present invention. The four light sources  150  are strip-shaped, and are arranged adjacent to four sides of the PCB  110  respectively. In particular, the four light sources  150  are arranged in two pairs. In each pair of light sources  150 , the light sources  150  are arranged symmetrically with respect to each other across the PCB  110 . The four light sources  150  may project light with at least two colors onto the solder paste  200  deposited on the PCB  110 . 
     Referring to  FIG. 7 , this shows three light sources of an imaging device in accordance with a third exemplary embodiment of the present invention. The three light sources  160 ,  170 , and  180  are strip-shaped, and are arranged generally radially symmetrically about the PCB  110 . In the illustrated embodiment, the light source  160  is arranged parallel to one side of the PCB  110 . The other two light sources  170 ,  180  are obliquely arranged with respect to the PCB  110 . The three light sources  160 ,  170 , and  180  may project light with at least two colors onto the solder paste  200  deposited on the PCB  110 . 
     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages.