Patent Publication Number: US-2010110650-A1

Title: Soldering Strategies for Printed Circuit Board Assemblies

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     FIELD OF THE INVENTION 
     The present invention relates to soldering strategies for printed circuit board assemblies, including forming a solder joint by wicking solder through a through-hole cluster. 
     BACKGROUND OF THE INVENTION 
     Variable frequency drive controllers, also known as AC (alternating current) drives, are power conversion devices that are used to drive motors or other devices. Generally, an AC drive receives power from an AC power grid at a fixed frequency, converts this power to an intermediate DC power across an intermediate bus using a rectifier circuit, then converts the intermediate DC power to a controlled frequency, quasi-sinusoidal AC power using a switched inverter circuit. The rectifier circuit typically includes diodes, the inverter circuit typically includes semiconductor switches such as insulated-gate bipolar transistors (IGBTs), and the drive further includes capacitors and other electronic components. 
     The physical form of an AC drive can vary according to the power required by the driven device. For example, AC drives may support loads ranging from a fraction of a horsepower (HP) up to thousands of horsepower. In particular, a high horsepower AC drive, such as for example a 480 Volt 75 HP drive, is typically formed as a power structure consisting of a plurality of components connected together with multiple bus bars in order to handle the large currents required. The components of a typical power structure include a rectifying module, an output power module, a gate driver board, multiple electrolytic capacitors, snubber capacitors, and the bus bars. On the other hand, an AC drive having a lower horsepower rating can generally be formed on a printed circuit board (PCB) as a PCB assembly. The PCB supports and interconnects the various electronic components making up the AC drive and normally consists of one or more layers of laminated insulating sheets with etched copper patterns or traces providing interconnections for electrical signals between the electronic components supported thereon. 
     Different constructions exist for mounting components to a PCB. In a “through-hole” construction, the electronic components are mounted on a first (e.g. top) surface of the PCB, holes are drilled through the board to permit component leads to extend through the board, and etched copper interconnect traces (including pads) are formed on a second (e.g. bottom) surface and on other intermediate surfaces of any multiple layers. These through-holes are also known as vias. A soldering process provides a connection between each respective lead and a corresponding copper pad on the bottom surface. In a “surface mount” construction, the electronic components are mounted on the same surface as the copper interconnect pads, and the components leads are respectively connected to a corresponding copper interconnect pad. Often, through-hole and surface-mount constructions are combined in a single PCB because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both constructions is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will typically take up less space using surface-mount techniques. 
     When using a through-hole construction, soldering of the components to the PCB can be performed as part of a manual soldering process or can preferably be performed as a wave soldering process in which all components are soldered essentially simultaneously. Wave soldering is significantly more efficient and cost effective than manual soldering, and is generally the soldering method of choice for high production circuit boards. This is a continuous process in which circuit boards are passed over a wave of molten solder. The solder wicks up through the through-holes containing the component leads to form solder joints. This process lends itself to automation, as the parts are simply loaded onto a conveyor which in turn passes the parts over a wide wave of solder. Advances in PCB technology, such as double-sided boards and mixed technology assemblies, have allowed intricate and complex circuits to be implemented in a smaller footprint. 
     As noted above, high HP AC drives are typically formed as separate modules which are connected together. While attempts have been made to form high HP AC drives as PCB assemblies, the required high current capabilities of the drive dictates that the copper interconnect traces are sufficiently large, resulting in so called “heavy copper” PCBs and/or the use of multi-layer boards. These high HP AC drives also typically require discrete electronic components such as capacitors which have large lead dimensions, thereby requiring a large through-hole to match the lead dimension. Thus PCB assemblies for high HP AC drives do not lend themselves to a wave soldering process, since the through-holes are too large for the wave soldering process to adequately achieve solder hole fill. Accordingly, wave soldering of heavy copper AC drive PCBs or other high power conversion or control devices has not previously been successfully achieved. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it would be desirable to manufacture high power conversion or control devices, such as AC drives, on heavy copper printed circuit boards using a wave soldering process which achieves reliable solder joints. 
     In at least some embodiments, the present invention relates to a printed circuit board assembly including a printed circuit board having a first outer surface, a second outer surface opposite the first outer surface, and a plurality of conductive interconnect traces. The printed circuit board supports on the first outer surface at least one electronic component having a plurality of leads. The printed circuit board further includes a plurality of through-hole clusters, wherein each through-hole cluster is associated with a respective lead and includes a central hole portion and a plurality of adjacent hole portions. A plurality of solder joints are formed, wherein each solder joint is formed between a respective lead inserted in the central hole portion of a respective through-hole cluster and a corresponding one of the plurality of conductive interconnect traces. 
     In at least some embodiments, the present invention relates to a method for wave soldering electronic components to a printed circuit board. The method includes providing a printed circuit board having a first outer surface, a second outer surface opposite the first outer surface, and a plurality of conductive interconnect traces, the printed circuit board supporting on the first outer surface at least one electronic component having a plurality of leads, the printed circuit board further including a plurality of through-hole clusters, wherein each through-hole cluster is associated with a single lead and includes a central hole portion surrounded by a plurality of other hole portions. The method further includes forming a plurality of solder joints by subsequently moving the printed circuit board over a wave soldering tank filled with solder, each solder joint formed between a respective lead inserted in a central hole portion of a respective through-hole cluster and a corresponding one of the plurality of conductive interconnect traces, wherein each solder joint is formed by solder which is wicked up by the through-hole cluster to extend from the second surface to the first surface. 
     Other embodiments, aspects, features, objectives and advantages of the present invention will be understood and appreciated upon a full reading of the detailed description and the claims that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are disclosed with reference to the accompanying drawings and are for illustrative purposes only. The invention is not limited in its application to the details of construction or the arrangement of the components illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in other various ways. Like reference numerals are used to indicate like components. In the drawings: 
         FIG. 1  is a top view of an exemplary AC drive printed circuit board assembly in accordance with at least some embodiments of the invention; 
         FIG. 2  is an illustration of another exemplary through-hole cluster for an electronic component such as one type of capacitor; 
         FIG. 3  is an illustration of another exemplary through-hole cluster for an electronic component such as another type of capacitor; 
         FIG. 4  is an illustration of another exemplary through-hole cluster for an electronic component such as another type of capacitor; 
         FIG. 5  is an illustration of another exemplary through-hole cluster for an electronic component such as a resistor; 
         FIG. 6  is an illustration of an exemplary flower configuration type through-hole cluster; 
         FIG. 7  is an illustration of another exemplary flower configuration type through-hole cluster; 
         FIG. 8  is an illustration showing the hole sizes for the through-hole cluster of  FIG. 7 ; 
         FIG. 9  is an illustration of solder joints formed with leads of power resistors using through-hole clusters similar to that illustrated in  FIG. 2 ; and 
         FIG. 10  is an illustration of solder joints formed with leads of power resistors using through-hole clusters similar to that illustrated in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , an exemplary AC drive printed circuit board assembly  10  is illustrated which includes a variety of electronic components supported on a first (top) surface of a printed circuit board (PCB)  12 , which can be a multi-layer circuit board. The electronic components include for example, a plurality of power resistors  14 , capacitors  16 , transformer  18 , current sensors  20 , metal oxide varistors (MOVs)  22 , a terminal connector  24 , a ribbon cable  26 , and others. The opposite surface of the PCB (not shown) includes a plurality of conductive interconnect traces, preferably of copper. These traces include a plurality of pads, which respectively connect to leads of one or more of the electronic components on the top surface. The AC drive can be a high HP drive, for example, a 480 Volt 75 HP AC drive, and the printed circuit board assembly is preferably a heavy copper multi-layer circuit board, e.g., one that includes more than approximately  2  ounces of copper per layer. 
     In at least one embodiment, the printed circuit board  12  has at least five layers, the inner layers each have at least four ounces of copper interconnect traces per layer, and the two outer layers each have at least one ounces of copper interconnect traces per layer. 
     Some of the electronic components making up the drive are through-hole components which can be soldered to the PCB preferably using a wave solder process, as more fully described below. For at least some of these through-hole components, their leads are each respectively inserted into a corresponding central hole portion of a through-hole cluster (a single lead in a single central hole portion). As shown in  FIG. 2 , an exemplary through-hole cluster  30 A includes a plurality of through-holes (also known as vias) with a central hole portion  32 A and a plurality of adjacent or surrounding hole portions  34 A (not all labeled) which aid in the soldering process. The adjacent hole portions  34 A and the central hole portion  32 A wick up solder from a wave solder tank from the bottom surface to the top surface of the PCB  12 , with the adjacent hole portions  34 A essentially providing additional surface area to induce capillary action and aid in the wicking up process. The adjacent hole portions  34 A also act as heat pipes to provide additional heat and temperature stabilization for the solder. In this manner, good solder joints can be formed and complete hole fill can be achieved. Specifically, the solder joints are formed between a respective component lead and its corresponding conductive interconnect trace (or pad) formed on the other side of the printed circuit board  12 . 
       FIGS. 3-7  show other exemplary through-hole clusters  30 B- 30 F, each through-hole cluster including a corresponding central hole portion  32 B- 32 F and a corresponding plurality of other adjacent hole portions  34 B- 34 F. Each central hole portion  32 A- 32 F is typically substantially centered with respect to its corresponding interconnect trace or pad  36  formed on the second surface, which pads can take a variety of shapes. The through-hole clusters  30 A- 30 F can be divided into different types, as shown in  FIGS. 2-7 . For example, as shown in  FIGS. 2-5 , a first through-hole cluster type is where the central hole portion  32 A- 32 D is separate from its corresponding plurality of adjacent hole portions  34 A- 34 D, with PCB material in between. Alternatively, as shown in  FIGS. 6-7 , another through-hole cluster type is where the central hole portion  32 E- 32 F and the corresponding surrounding hole portions  34 E- 34 F overlap to form a flower configuration, e.g., a configuration with scalloped edges. Varying amounts of overlap are contemplated as  FIGS. 6 and 7  illustrate. In both types of through-hole clusters, the surface area of the through-hole cluster forming the solder joint is increased over the surface area of a central hole alone. 
     The size of a central hole portion is generally determined by the size of a corresponding electronic lead which needs to be inserted in that central hole portion, and its shape can vary. Further, the size, shape and number of the adjacent hole portions can vary. Preferably, the diameter of one or more holes in the plurality of adjacent hole portions  34 A- 34 F is in the range of approximately 0.020 to 0.035 inches (i.e. 20 to 35 mils), and these hole portions need not be the same size as the central hole portion or each other. Generally, the larger the central hole portion, the greater number of surrounding hole portions required to achieve appropriate hole fill and thus a reliable solder joint. 
     With respect to preferred sizes,  FIG. 2  illustrates the through-hole cluster  30 A with the central hole portion  32 A having a size of 0.078 inches, and with the eight equally spaced hole portions  34 A surrounding the central hole portion (also called a ring of vias) each having a size of 0.024 inches.  FIG. 3  illustrates the through-hole cluster  30 B with the central hole portion  32 B having a size of 0.040 inches, and with the four equally spaced surrounding hole portions  34 B each having a size of 0.024 inches.  FIG. 4  illustrates the through-hole cluster  30 C with a central hole portion  32 C having a size of 0.051 inches, and with the four equally spaced surrounding hole portions  34 C each having a size of 0.024 inches. The through-hole clusters configured as shown in  FIGS. 2 ,  3 , and  4  can be used to attach leads of various capacitors of the PCB assembly  10 . Clearly other sizes of central hole portions and surrounding hole portions are contemplated, and the surrounding hole portions need not be equally spaced.  FIG. 5  illustrates the through-hole cluster  30 D which is meant for a lead of a power resistor, with a central hole portion  32 D having a size of 0.057 inches, and with the three adjacent hole portions  34 D each having a size of 0.024 inches. As shown, a distance between a central hole portion and the adjacent hole portions is preferably approximately 0.010 inches to 0.015 inches (with a tolerance of plus or minus 0.004 inches), and a distance between one of the adjacent hole portions and an outer pad edge  36  is preferably approximately 0.015 inches (with a tolerance of plus or minus 0.004 inches). 
       FIG. 8  is an illustration showing the through-hole cluster  30 F of  FIG. 7 , and specifying the hole sizes. In particular, through-hole cluster  30 F includes a central hole portion  32 F having a size of 0.035 inches (35 mils), and four adjacent hole portions  34 F each having a size of 0.014 inches (14 mils). Note that these hole sizes are the same for the through-hole cluster  30 E shown in  FIG. 6 , with a different amount of overlap between the central hole portion and the others. In practice, the outer surrounding hole portions  34 F are preferably formed, such as by drilling, prior to forming the overlapping central hole portion  32 F. 
     The hole portions forming the through-hole cluster in the PCB  12  can be constructed by drilling the PCB  12  using tiny drill bits which can be made of solid tungsten carbide for example. Such drilling can be performed by automated drilling machines with controlled placement capabilities, to control the size and location of the desired holes. If very small holes are required, drilling with mechanical bits can be costly because of high rates of wear and breakage. In this case, the hole portions of the through-hole cluster can be formed by lasers. The holes formed are typically cylindrical in shape, although this not need be the case. Oval or elongated and other shaped holes and hole portions are also contemplated. The walls of some of the holes, for boards with two or more layers, can be plated with copper to form plated-through holes that electrically connect the conducting layers of the PCB  12 . 
     The solder joints of the through-hole components supported on PCB  12  are preferably formed using a selective wave solder system and process in which the PCB assembly  10  (PCB  12  and supported electronic components) is moved, using a conveyor system at a predetermined speed, to different locations or stages where different processing activities occur. As a general overview, in the wave solder process, flux is applied, one or more heating stages occur, and then solder is applied via a “wave” to the bottom surface of the printed circuit board. Appropriate heating is required to allow the flux to remain within a desired temperature range such that the flux remains active (does not become exhausted) for the appropriate amount of time. Further, appropriate heating allows the electronic components to be heated in a manner that prevents excessive heat stress, yet allows the solder to be applied at an appropriate temperature. Parameters relating to the various process stages are preferably selected by a teaching process, which determines an appropriate conveyor speed and appropriate heating steps though evaluation of time vs. temperature graphs of the various layers of a PCB assembly  10  during one or more trial runs. 
     In an exemplary wave solder process for PCB assembly  10 , using a commercially available wave solder system, an appropriate conveyor speed is determined to be approximately 82 centimeters per minute. At a first process stage, a flux is applied to both sides of the circuit board assembly  10  prior to the heating stages. 
     In a first heating stage, a first bottom side preheater is set to 130 degrees C. In a second heating stage, a second bottom side preheater is set to 150 degrees C. and a top side preheater is set at a 40% on duty cycle. In a third heating stage, a third bottom side preheater is set to 200 degrees C. and a top side preheater is set to a 60% on duty cycle. A solder which is preferably lead-free is then applied from a solder wave tank, and the solder is wicked up into the through-hole clusters to form solder joints to secure the supported electronic components. A preferred solder tank temperature is 270 degrees C. The wave solder system is preferably operated under the control of a control system, which controls the conveyor speed and heating stages. During automated production, the conveyor system will move to each of the stored process stages and the pre-programmed action will be conducted. 
     Using the above processing parameters, and the through-hole clusters such as illustrated and described above for at least some of the electronic components, acceptable solder joints are formed for the AC drive. During the soldering stage, solder is wicked up through the through-hole clusters from the bottom surface of PCB assembly  10  to the top surface to form solder joints  40 A,  40 B such are shown in  FIGS. 9 and 10 , which illustrate both non-overlapping and overlapping types of through-hole clusters. 
     In particular,  FIG. 9  illustrates on the left the solder joint  40 A for a lead  42 A of a power resistor using a through-hole cluster similar to that illustrated in  FIG. 2 , wherein surrounding holes each have a diameter of 0.020 inches and lead  42 A is inserted in the central hole. Similarly, illustrated on the right is the solder joint  40 B formed in a through-hole cluster wherein the surrounding holes each have a diameter of 0.030 inches and a lead  42 B is inserted in the central hole. Further,  FIG. 10  is an illustration of solder joints formed in respective flower type through-hole clusters having varying degrees of overlap of the hole portions. Using varying forms and sizes of through-holes such as shown in  FIGS. 9 and 10  allow the solder joints to be examined and an optimal design determined for one or more through-hole clusters. As shown, complete hole fill can be achieved resulting in reliable solder joints. 
     In this manner, the AC drive can be formed as a printed circuit board assembly using an automated wave soldering process or other soldering process wherein the solder is wicked through a through-hole cluster to form a solder joint. The printed circuit board assembly provides a reduction in size and weight for the AC drive as compared to the AC drive being constructed as a power structure having multiple components. Clearly, the same through-hole constructions can be beneficial in the assembly of other devices, specifically other high power devices, such as other power conversion devices (e.g. DC drives) and power control devices. The PCB assembly construction provides savings, improves process time, and at least in the case of a wave soldering process, improves manufacturability by allowing a wider process window for the wave soldering process. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.