Abstract:
There is disclosed herein an electronic circuit assembly, such as a printed circuit board, having solder resist windows with one or more enlarged solder resist pullback zones, thereby facilitating solder paste overprinting.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to solderable electronic circuit assemblies, and more particularly to electronic circuit assemblies having solder resist windows which facilitate solder paste overprinting. 
     2. Disclosure Information 
     Traditional electronic circuit assemblies (such as printed circuit boards) generally includes a substrate  20  with conductive circuit traces  22  and mounting pads  24  thereon, as shown in FIG.  1 . Once the traces and pads have been placed on the substrate, a thin, solder-resistant layer of material known as “solder resist”  28  is typically applied over the surface of the substrate. 
     The solder resist layer  28  typically covers substantially all of the circuit traces, but not the mounting pads; this is accomplished by forming windows  30  in the solder resist such that each window  30  generally conforms in shape with and is situated concentrically about a respective mounting pad  24 . The edges  35  of each window  30  generally conform with and run generally parallel to adjacent, corresponding edges  25  of the mounting pad  24 , as shown in FIG.  1 . Typically, all the window edges  35  are spaced apart from their respective pad edges  25  by the same distance P; this distance P is commonly referred to as the “pullback”. For screenprinted solder resist, the pullback P is typically about 10-20 mils, whereas for liquid photoimageable solder resist (LPISM) the pullback P is typically 0-5 mils. 
     While it is preferred to keep the solder resist window at least as large as the respective pads at a minimum, it is common industry practice—and, in fact, a tenet of industry design standards (e.g., ANSI/IPC-D-275)—to keep the pullback P as small as possible. One reason for this is to conserve real estate on the substrate; making the solder resist openings/pullback larger rather than smaller takes up valuable board space. It is also common practice and teaching to use the same phototool to produce the masks/stencils needed for laying down the mounting pads, the solder resist layer/windows, and the solder paste depositions. For example, a given phototool may be (1) non-magnified to produce the mounting pad mask, (2) magnified to produce the solder resist layer/window mask, and (3) de-magnified to produce the solder paste stencil. This avoids having to independently design three separate masks/stencils, and making them from the same phototool provides better registration, and better avoids tool-to-tool discrepancies, than if three separate phototools were used. 
     Once the solder resist is applied to the substrate surface, the mounting pads and any other circuit structures exposed through the windows may be tinned, followed by solder paste being applied thereto, typically by screen-printing or deposition. When solder paste is applied to mounting pads, the common practice is to print or deposit paste-to-pad in a 1:1 or less ratio, as illustrated by the smaller-than-mounting pad deposition  40  illustrated in FIG.  1 . For example, for a typical 90×140-mil rectangular mounting pad, the solder deposited atop such a pad will usually be a similarly shaped deposition measuring 90×140 mils or less (e.g., 60×100 mils) that is centered with respect to the pad. After the paste has been printed/deposited, and the substrate populated with electronic components, the populated assembly is then subjected to reflow processing. 
     Applicants have discovered that it may be advantageous in some applications to go against standard industry practice and to intentionally overprint the mounting pads. For example, for a 90×140-mil pad, a 120×180-mil solder paste deposition may be desired. Such overprinting may be useful, for instance, in the creation of specially shaped solder joints during reflow. Applicants believe it is preferable to position the overprinted deposition such that substantially all of the deposition lies on the pad  24  and on one or more outboard edges  26  of the pad. (As used herein, the “outboard” edges  26 / 36  of the pad  24  and window  30  are those which are located generally outside the component footprint  50 , while the “inboard” edges  27 / 37  are those which are generally within the footprint  50 . For example, in FIG. 1, each rectangular mounting pad  24  and window  30  has one inboard edge  27 / 37  and three outboard edges  26 / 36 .) 
     However, if one were to follow standard industry practice in providing a 0- to 20-mil pullback between the pad and window edges, there would not be enough room between the pad edges  26  and the window edges  36  in which to overprint the paste without printing onto the adjacent outboard solder resist layer  28 , as illustrated by reference numeral  42  in FIG.  1 . Overprinting onto the solder resist layer  28  is undesirable because there would then be an increased risk of forming migratory solder balls and/or solder bridges between adjacent pads during reflow. 
     Therefore, it would be advantageous to provide a way of accommodating the desire to overprint which overcomes the aforementioned drawback. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of prior art approaches by providing an electronic circuit assembly offering improved overprintability. One configuration of the assembly comprises: (a) a dielectric substrate having a plurality of circuit traces and at least one mounting pad disposed thereon, wherein each mounting pad is arranged in matched relation with a respective termination of a surface-mount electronic component; and (b) a solder resist layer generally covering the substrate and having at least one window therethrough, wherein each window has at least one inboard window edge generally within a projected footprint of the electronic component and at least one outboard window edge generally outside the footprint, wherein each window generally conforms in shape with and is arranged about a respective one of the mounting pads. Each inboard window edge is spaced a first predetermined pullback distance P 1  away from a respective adjacent inboard mounting pad edge, and each outboard window edge is spaced a second predetermined pullback distance P 2  away from a respective adjacent outboard mounting pad edge, such that P 2 &gt;P 1  for at least one of the at least one outboard window edge. 
     It is an object and advantage that the solder resist windows of the present invention provide one or more enlarged pullback zones outboard from the component footprint into which solder paste may be deposited, thereby facilitating solder paste overprinting. 
     These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of an electronic circuit assembly after solder paste deposition and before component population, according to the prior art. 
     FIGS.  2 - 5  are various plan views of an electronic circuit assembly after solder paste deposition and before component population, according to the present invention. 
     FIGS.  6 - 8  are plan views of an electronic circuit assembly showing various mounting pad/window configurations according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIGS.  2 - 8  show various configurations of an electronic circuit assembly having improved overprintability according to the present invention. One configuration of the assembly comprises: (a) a dielectric substrate  20  having a plurality of circuit traces  22  and at least one mounting pad  24  disposed thereon, wherein each mounting pad is arranged in matched relation with a respective termination of a surface-mount electronic component; and (b) a solder resist layer  28  generally covering the substrate and having at least one window  30  therethrough, wherein each window has at least one inboard window edge  37  generally within a projected footprint  50  of the electronic component and at least one outboard window edge  36  generally outside the footprint  50 , wherein each window  30  generally conforms in shape with and is arranged about a respective one of the mounting pads  24 . Each inboard window edge  37  is spaced a first predetermined pullback distance P 1  away from a respective adjacent inboard mounting pad edge  27 , and each outboard window edge  36  is spaced a second predetermined pullback distance P 2  away from a respective adjacent outboard mounting pad edge  26 , such that P 2 &gt;P 1  for at least one of the at least one outboard window edge  36 . 
     To assist the reader in understanding the present invention, all reference numbers used herein are summarized in the table below, along with the elements they represent: 
       20 =Dielectric substrate (e.g., FR-4) 
       22 =Circuit trace 
       24  Mounting pad 
       25 =Edges of mounting pad 
       26 =Outboard edge(s) of mounting pad 
       27 =Inboard edge(s) of mounting pad 
       28 =Solder resist layer 
       30 =Window in solder resist 
       35 =Edges of window 
       36 =Outboard edge(s) of window 
       36   2 =Second lateral outboard window edge 
       36   3 =Third lateral outboard window edge 
       36   4 =Fourth outboard window edge 
       37 =Inboard edge(s) of window 
       40 =Solder paste deposition (typical/underprinted) 
       42 =Solder paste deposition (overprinted onto resist) 
       44 =Solder paste deposition (overprinted within overprint zone) 
       50 =Footprint of electronic component 
       60 =Phantom line indicating where prior art window edge would be located 
       70 =Overprint zone(s) provided by additional pullback of outboard window edge(s) 
     P=Pullback of solder resist edge from pad edge 
     P 1 =First predetermined distance between inboard edges of pad and window 
     P 2 =Second predetermined distance between outboard edges of pad and window 
     The dielectric substrate  20  may be a conventional flat, planar FR-4 glass/epoxy laminate; a semi-rigid to rigid plastic part (e.g., molded out of ABS or polypropylene); a flexible polyester, polyimide, or polyetherimide film; and so forth. The circuit traces  22  are typically copper, and the mounting pads  24  are typically rectangular or round copper pads which are formed on the substrate  20  by well-known plating and etching processes. The electronic component is preferably a surface mount component (SMC) with two or more terminations thereon, such as a bi-terminated leadless chip component (LCC) such as a 2512 resistor; however, in some cases (e.g., wirebonded power transistors), the component may have only a single termination that is placed on a mounting pad, so only one mounting pad  24  is required for the present invention. 
     The solder resist  28  is a generally non-solder-wettable layer attached to the top surface of the substrate  20 . The resist  28  typically has at least two windows  30  therein (but may have only one), wherein each window generally conforms in shape with, and is arranged about a respective one of, the at least one mounting pad  24 . For example, if a given mounting pad is rectangular, the corresponding window about this pad will be similarly rectangular (and usually, but not necessarily, larger in length and width than the pad). 
     As illustrated in FIGS.  2 - 4 , each window  30  has at least one inboard window edge  37  located generally within a projected footprint  50  of the component. The footprint  50  generally conforms to the underside surface of the component as projected onto the substrate top surface. Each window  30  also includes at least one outboard window edge  36  located generally outside the footprint  50 . Each inboard window edge  37  is generally disposed along and proximate to a corresponding inboard mounting pad edge  27  with a first predetermined distance or gap P 1  therebetween, while each outboard window edge  36  is generally disposed along and proximate to a corresponding outboard mounting pad edge  26  with a second predetermined distance or gap P 2  therebetween. The gap P 1  between the inboard mounting pad and window edges  27 / 37  is typically kept the same as in the prior art; namely, generally between 0 and 20 mils. However, the gap P 2  between one or more respective outboard mounting pad and window edges  26 / 36  is generally at least 10 mils larger than P 1 . 
     In most actual applications, the mounting pads  24  are rectangular in shape, as illustrated in FIGS.  2 - 5 . In such a case, the at least one inboard window edge  37  comprises a single first inboard window edge, while the at least one outboard window edge comprises: (a) second and third lateral window edges  36   2 / 36   3  each contiguous with and generally orthogonal to the first inboard window edge, and (b) a single fourth window edge  36   4  generally parallel with the first inboard window edge and contiguous at each end thereof with a respective one of the second and third lateral window edges  36   2 / 36   3 . Alternatively, the pads  24  may be round, or may have a combination of straight and arcuate edges, as illustrated in FIG.  6 . The at least one inboard window/pad edge may comprise a single straight or arcuate edge, or multiple straight/arcuate edges. Likewise, the one or more outboard window/pad edge may comprise, for example, a single arcuate outboard edge, two straight edges forming a “V” shape, three straight edges forming three sides of a rectangle, and so forth. The configurations shown in the drawings are merely representative of the various configurations possible, and are not intended to encompass all the possible configurations within the scope of the present invention. 
     Three different embodiments are illustrated in FIGS.  2 - 4 . In FIG. 2, P 2 &gt;P 1  for only the fourth outboard window edge  36   4 , while gap P 2  is generally equal to gap P 1  with respect to the second and third lateral edges  36   2 / 36   3 . In FIG. 3, the reverse is true; P 2 &gt;P 1  for the second and third lateral edges  36   2 / 36   3 , while P 2 ≈P 1  for the fourth edge  36   4 . And in FIG. 4, P 2 &gt;P 1  for all three of the outboard window edges  36   2 / 36   3 / 36   4 . 
     Note that phantom lines  60  indicate where the solder resist window edge(s)  36  would be placed according to conventional, prior art approaches; this helps to show how the window  30  has been selectively enlarged in each case to provide one or more “overprint zones”  70  to accommodate a desired overprinting strategy. In FIGS.  2 - 4 , one of the two mounting pads  24  shown includes an overprinted solder paste deposition  44  thereon. In each case, the deposition  44  extends beyond one or more outboard edges  26  of the mounting pad  24 ; however, unlike prior art overprinting strategies which extend paste onto the solder resist  28 , the selectively enlarged outboard window edges of the present invention provide solder-resist-free overprint zones  70  into which the paste may ex tend. This helps prevent solder balls from migrating across the solder resist; furthermore, the zones  70  tend to keep any solder balls contained therein, since the higher, surrounding window edge  36  acts as a wall or barrier which the solder ball would have to climb over to escape. 
     It should be noted that although FIGS.  2 - 4  show pad/window configurations for a component having only two terminations, components having other than two terminations may be accommodated by the present invention. For example, FIG. 5 illustrates a pad/window configuration for a component having three terminations in a triangular arrangement. Additionally, FIGS.  7 - 8  illustrate cases wherein the component has only a single termination to be bonded to a substrate mounting pad  24 . For instance, such a component might be a power transistor whose bottom surface is to be soldered to a single mounting pad, and whose two top-surface aluminum bond pads are to be wirebonded to an adjacent connector leadframe (not shown). In such a case, there is no “inboard” mounting pad/window edge, only one or more “outboard” edges. (For round pads  24 , there would be a single arcuate outboard edge, while for the rectangular pads  24  shown there would be four straight outboard edges.) For these applications, the window may be selectively widened to provide one or more overprint zones  70  about the pad. In FIG. 7, two overprint zones  70  are provided, while in FIG. 8, an overprint zone  70  is provided about the entire perimeter of the pad. (Or, FIG. 8 may be viewed as having four contiguous zones  70 , one adjacent to each of the four pad edges.) In such single-pad applications, the main difference between the prior art and the approaches illustrated here is that the gap or width P 2  of each overprint zone  70  is dramatically larger than the 0- to 20-mil pullback recommended by conventional industry design standards and practice. 
     Various other modifications to the present invention will, no doubt, occur to those skilled in the art to which the present invention pertains. For example, although reference is made herein to “solder”, “solder paste”, and “reflow” processing, the present invention applies equally to forming joints with conductive adhesive rather than solder paste, and optionally using an alternative heating or curing process (e.g., ultraviolet/infrared/laser radiation, exposure to hot air or other gases, etc.) other than conventional reflow oven processing. Also, it should be understood that “overprinting” is not limited to the use of conventional screenprinting, but may also include dispensing, transfer printing, compression printing, solder preform placement, or other methods for placing bonding material (i.e., solder paste, conductive adhesive, etc.) in distinct, predetermined depositions atop the PCB mounting pads. Additionally, while the drawings show each window edge  36 / 37  being generally parallel to or conforming in contour with its respective pad edge  26 / 27 , it is also possible that the respective edges may deviate in shape from one another, if desired. Other modifications not specifically mentioned herein are also possible and within the scope of the present invention. It is the following claims, including all equivalents, which define the scope of the present invention.