Abstract:
Tails ( 20 ) projecting from an electrical component ( 12 ) that lies on a circuit board surface, are terminated to traces on a multi-layer circuit board ( 14 ) in a manner that minimizes the disadvantages of long through hole soldering and of surface mount techniques. A blind hole is drilled and plated to form a shallow well ( 70 ). The well is filled with a soldering composition ( 130 ). A tail ( 20 ) is projected downward into the soldering composition with the extreme tip of the tail lying above the bottom of the hole, and the soldering composition is heated to solder the tail to the hole plating.

Description:
CROSS-REFERENCE  
       [0001]     This is a continuation-in-part of U.S. patent application Ser. No. 10/461,840 that was filed Jun. 13, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Components that lie on a circuit board typically have leads or tails that must be soldered to traces on the circuit board. One technique for mounting the tails includes drilling holes through the circuit board, plating the holes and then fixing the tails in the holes. The tail can be a compliant pin which is forced into the hole in an interference fit, or can be soldered in place with the solder joined to tail portions that project from opposite ends of the hole. When a multi-layer circuit board assembly is used, the through holes extending the board assembly are long and tails to be soldered in the holes must be long. This results in considerable capacitance between the plated walls of the hole and close traces on the laminates, or layers of the assembly, and results in increased crosstalk and decreased signal strength.  
         [0003]     Another technique for soldering component tails to traces on a circuit board, is the SMT (surface mount technique) method, wherein the tails touch circuit board traces that form solder pads, and are soldered thereto. In order to achieve reliable solder joints, the tails should each touch the corresponding solder pad. This requires close tolerances and/or resilient tails. The SMT technique results in solder joints that are mechanically weak against shear forces that tend to move the tails parallel to the plane of the board upper face and against pull up forces.  
         [0004]     A technique for soldering component tails to platings or other traces on a multilayer circuit board, which avoided the disadvantages of high capacitive load for through-hole soldering, and which avoided the disadvantages of weakness and close tolerances or resilient tails for SMT, would be of value.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with one embodiment of the present invention, a solder connection is provided between a lead or tail of a component lying on a multi-layer circuit board arrangement and a plating on the arrangement, and a method for producing such solder connection, which avoids the high capacitance loading, low shear and tension strength, and the need for resilient or precision tails of the prior art. In a multi-board arrangement, a blind hole is formed that extends through only a portion of the thickness of a multi-layer circuit board. The blind hole is plated to form a solder well whose bottom is formed by the bottom of the hole. The tail of a component is inserted down into the hole and soldered in place thereat, as by a solder paste that was previously placed in the well or around the tail. The portion of the tail lying in the circuit board is short, being less than the thickness of the multi-layer board, to avoid a significant capacitive load. However, a considerable length of the tail, such as a length that is about equal to its diameter, is firmly soldered in and above the hole, so the tail resists considerable force tending to break the solder connection. The tails do not have to be formed with high precision, since a rugged connection is achieved with tails that project different distances into the solder wells.  
         [0006]     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a partial isometric view of a component lying on a multi-layer circuit board assembly, and having tails soldered to traces on an uppermost first board of the assembly.  
         [0008]      FIG. 2  is an enlarged sectional view of a portion of the component and multi-layer circuit board assembly of  FIG. 1 .  
         [0009]      FIG. 3  is a sectional view of a portion of the first board, shown after a first step of the method of the invention.  
         [0010]      FIG. 4  is a sectional view of the first board portion of  FIG. 3  and a portion of the rest of the multi-layer circuit board assembly, and with solder paste lying in a well formed in the first circuit board hole.  
         [0011]      FIG. 5  is an enlarged view of a portion of a component tail and the apparatus of  FIG. 4 , after the tail has been soldered in place.  
         [0012]      FIG. 6  is an enlarged view of a component tail fully installed in a portion of a multi-layer circuit board, in accordance with another embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]      FIG. 1  illustrates a termination system  10  of the present invention, wherein a component  12  that lies on a multi-layer circuit board assembly  14  has component pins, or leads, or tails  20  that must be soldered to traces  22  on a first layer  31  of the circuit board assembly. The particular circuit board assembly  14  includes ten layers  31 - 40  that lie in a facewise stack wherein each layer lies facewise adjacent to one or two other layers, to form a facewise stack of layers. Up and down directions are indicated by arrows U and D for the particular orientation of the component and board assembly illustrated in the figures, although it should be understood that the board assembly and component can lie in any orientation. As shown in  FIG. 3 , each layer such as  31  includes a plate  41  of insulative material. Most or all of the layers also include traces such as  60 ,  62  on one or both faces  43 ,  45  of the plate, the traces being formed of a film of conductive material.  
         [0014]      FIG. 2  shows that the first or uppermost layer  31  has walls forming a plated hole  50 . The layer hole  52  within the plating is preferably formed, as by drilling with a drill or boring tool or laser, in the dielectric or insulative substrate  53  that forms most of the layer. After drilling, a plating is applied that includes a hole plating portion  54  and top and bottom face traces parts  60 ,  62  that lie on upper and lower faces  64 ,  66  of the substrate of the first layer. As shown in  FIG. 1 , the upper face trace parts  60  merge with the rest of each trace  22  that extends from a hole to other circuitry on the circuit board.  
         [0015]     Each hole  50  of  FIG. 2  forms a solder well  70  whose well lower wall  72  is formed by an upper face  74  of the second layer  32 . Thus, the well has bottom and side walls, so it can contain solderable material during a soldering operation when the solder is flowable. A lower tail end portion  84  of each component tail projects into one of the plated holes  50  and is soldered by the solder  80  lying in the well and displaced and attracted by capillary action to lie above the well. In the soldering operation, the solder forms a fillet  86  that adheres to the top face plating part  60  and to portions  90  of the tail.  
         [0016]     The solder connection  100  formed between the tail end portion  84  that projects into and moderately above the solder well  70  and the plating on the walls of the hole and on the upper surface of the first layer, provides a rugged joint of minimal capacitance. The small capacitance is due to the short length B of the tail that lies within the multi-layer circuit board assembly  14 , the length B preferably being no greater than the thickness D of the first layer  31 . The very small thicknesses of the face traces  60 ,  62  can be considered part of the thickness D of the board. The resistance of the tails to upward U pullout is much greater than for a surface mount solder connection, and almost as great as that for a very long tail that extends through the entire thickness of the multi-board circuit board assembly  14 . The resistance to damage when the component tends to move laterally L relative to the circuit board assembly is much higher than for an SMT connection and almost as high as for a tail that extends completely through the circuit board assembly.  
         [0017]     The component  12  shown in  FIG. 1  has standoffs  110 ,  112  that leave a space  114  ( FIG. 2 ) above the first layer upper face  64 . This leaves room for the solder fillets  86  and leaves room for washout fluid to clean the solder connections and possibly to allow observance of the solder connections for defects.  
         [0018]     The length of each tail  20  should be about the same, but with large tolerances allowed. In  FIG. 2 , the tails  20 A,  20 B,  20 C are of different lengths. The tail  20 A extends a minimal distance into the hole  50 , of slightly more than half the height of the hole, and is about the shortest tail. The tail  20 B extends along most of the depth of the hole. The tail  20 C extends almost to the bottom of the hole, and is about the longest tail within tolerances. All of the tails lengths  20 A- 20 C provide a rugged connection.  
         [0019]     In a system that applicant has designed, the insulative plate of each layer has a thickness of 1.25 millimeters, and each complete layer has a thickness D of about 1.30 mm. Each hole  50  has a diameter A of 1.25 mm, which is about the same as the thickness of the layer. Each tail has a width C slightly smaller than the hole diameter, to allow solder to extend all along the depth of the hole between the tail and the plated walls of the hole, with the tail width C being about one millimeter. The tolerances in tail lengths are ±0.25 mm. The faces of the layers carry signal traces and/or ground planes. The layers are held together by epoxy which is shown at  120 .  
         [0020]      FIG. 3  shows a first step in the construction of the termination system. A hole  52  is formed in the first layer  31  and a plating is applied at  54 ,  60  and  62  to cover the walls of the hole and form trace parts of the top and bottom faces of the layer. Some of the traces such as  60  extend, as shown at  22  in  FIG. 1 , along the layer to connect the tail  20  to circuit components (which may be connectors) on the board. After the plating process, the layer  31  of  FIG. 4  is assembled in a stack of layers  14 , that may be held together with a bonding material such as epoxy  120 . Then, a quantity  130  of solder paste is placed in the solder well  70  that is formed between the plated walls  54  of the hole and the upper face  72  of the second layer  32  which forms a well bottom  73 . Of course, the second layer is not perforated under the well (unless the well extends through the second layer). The solder paste  30  can include multiple tiny balls of solder in a solder flux, or tiny donuts or tubes of solder paste initially placed around the tails, etc. Although applicant shows a solder well  70  extending through one layer it is also possible to form a well  132  that extends through two or more layers, though it does not extend through most of the layers. A solder well can extend through a plurality of layers in order to reach traces on a layer that lies deep under the top of the board assembly.  
         [0021]     In  FIG. 5 , a component tail  20  has been pressed down into the solder paste, and heat has been applied to melt the solder paste and form the solder joint  100  that includes solder in the hole and the solder fillet  86 .  
         [0022]      FIG. 6  illustrates another system in which a multi-layer circuit board assembly  150  has formed blind holes  152  with bottom ends  154 , wherein at least some of the bottom ends have extreme lower ends  156  that lie between the top and bottom surfaces  160 ,  162  of a layer  173  of the board assembly  150 . In this system, the board assembly  150  is assembled by bonding together layers  171 - 175 . The formed holes such as  152  are formed by drilling or are otherwise formed to extend through one or more layers. Then, each formed hole  152  is plated with a plating  180  to form a plated hole  182 . Solder  183  is placed in the plated holes or on the component tail  184 , the component tail is inserted into the plated hole, and the solder is melted to form a solder joint  186 .  
         [0023]     In  FIG. 6  the particular component tail  184  is electrically connected through the solder and plating to a circuit board trace  190  on the lower face of the second layer  172 . Usually, there are a plurality of traces on a surface of one layer. The forming of a hole through more than one layer provides a longer hole than a hole extending through only the first layer  171 , although it results in slightly greater capacitance if the hole does not extend through most of the layers. The conical lower end  154  of each blind formed hole  152  is a result of drilling. A milling tool could be used to bore holes with flat bottoms. The “bottom”  154  of the hole is where the hole wall extends by many (at least 7) degrees from the sides  192  of the hole as seen in a sectional view taken on the hole axis  194 . In any case, by not requiring the bottom of the holes to lie precisely at the top of a layer of the multi-layer assembly, drilling or even boring can be accomplished while requiring only moderate precision.  
         [0024]      FIG. 7  illustrates another system in which a multi-layer circuit board assembly  200  has through holes  202 . Upper portions  204  of the through holes are plated with a plating  206  that is electrically connected to a trace  210  at the bottom of a layer  212 . Lower portions  214  of the holes are not plated. The upper hole portions and the plating  206  form plated holes  216  that extend between points  217 ,  218 . A component tail  219  and solder with flux are placed in the hole and heated to form a solder joint  220 . The solder will not wick down below the bottom  218  of the plating of the plated hole.  
         [0025]     To construct the system of  FIG. 7 , the upper hole portions  204  are drilled from the top of the board assembly and the upper hole portions are plated. The lower hole portions  214  are then drilled up from the bottom of the stack. The lower hole portions  214  serve no purpose other than an attempt to avoid the patent.  
         [0026]     Although terms such as “top”, “bottom”, etc. have been used to describe the invention as it is illustrated, the invention can be used in any orientation with respect to the Earth.  
         [0027]     Thus, the invention provides a termination system that includes the combination of a component and a multi-layer circuit board assembly. The invention provides much of the high strength previously supplied by component tails that extend completely through the circuit board assembly, while avoiding the high capacitance, and provides many of the advantages of SMT (surface mount technology) soldering of low capacitance while providing further advantages of high strength. A plated hole extends only partially through the circuit board assembly, and may extend through only one layer (which may be the uppermost layer during manufacture). The plated hole forms a well with a well bottom formed by the upper face of the next layer in the stack or by a wall bored (e.g. drilling) partially through a layer. A solderable composition such as a solder paste is placed in the well (possibly with the tail) and a component tail is placed in the well and soldered in place. The invention allows the use of uninterrupted routing traces on layers below the top one or two layers, and uses a minimum of solder for easy melting, in addition to the low capacitive coupling and other advantages described above.  
         [0028]     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.