WO2005122655A2

WO2005122655A2 - Pcb including a star shaped through-hole solder pad

Info

Publication number: WO2005122655A2
Application number: PCT/IB2005/051797
Authority: WO
Inventors: Armin Schulz
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-06-09
Filing date: 2005-06-02
Publication date: 2005-12-22
Also published as: TW200614894A; WO2005122655A3

Abstract

The present invention relates to a printed circuit board (400, 500) comprising a through hole for a leaded component (412). A surface of the printed circuit board around the hole comprises a solder-pad (202, 406) having a shape with a varying radius around the center of the through hole. The solder-pad of printed circuit board (400, 500) is suitable for in a reflow soldering process. The solder pad shape is a star having at least two arms. The present invention also comprises a method that includes applying solder paste to the solder-pad (202, 406), mounting a leaded component (412) on the printed circuit board (400, 500) having a through hole with a solder pad (202, 406) having a shape with a varying radius and reflow soldering the printed circuit board (400, 500).

Description

PCB including a star shaped through-hole solder pad

FIELD OF THE INVENTION The invention relates to a printed circuit board (PCB) with through holes for mounting leaded components. More specific, the invention relates to a PCB that allows reflow soldering for leaded components. The present invention also relates to an improved method for reflow soldering that allows usage of through-hole leaded components. The present invention is particularly relevant for soldering a leaded component such as an air-coil, a pin strips etc. on a PCB in a reflow soldering process.

BACKGROUND OF THE INVENTION In the last years a usage of components on a Printed Circuit Board (PCB) has shifted more and more to Surface Mounted Devices (SMD). The number of leaded components in (especially digital) products is less and less. In case of a TV RF tuner still quite a few leaded components are used, e.g., an air-coil, a balun coil, an IEC- or F- connector, a RF shielding frame tag, and a pin-strip. Currently each component range has its own production needs, so there are a reflow process for SMD components and a wave soldering process for leaded components. A PCB that requires both SMD components and leaded components requires thus two soldering processes. There is need, e.g., for cost- and time- and factory space, PCB space-saving to use one soldering process only. The invention offers a solution that allows soldering leaded components using a reflow soldering process. The inventor also found that a leaded component needs a lot of soldering paste in a reflow soldering process. The inventors found print and solder parameters to produce a PCB with SMD components and together with leaded components that requires a minimum of (reflow) soldering paste. SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to provide an easily practiced, low cost printed circuit board suitable for a single solder process for both leaded and surface mount devices (SMD). It is another object of the invention to teach and practice a low cost method of producing a printed circuit board having leaded components that can be soldered cheaply using a reflow process. Moreover the method allows surface mount devices to be simultaneously soldered so a single soldering process is needed for both leaded and unleaded components. In one embodiment, a star shaped pad is used that provides good soldering properties for reflow soldering a component lead of a leaded component. These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: Fig. 1 shows prior art pad design; Fig. 2a shows a star-pad design in accordance with the invention; Fig. 2b to 2d show further examples of a star-pad design in accordance with the invention; Fig. 3a to 3b show even further examples of a star-pad design in accordance with the invention; Fig. 4 shows a PCB 400 (printed circuit board) with a leaded component before reflow soldering in accordance with the invention; Fig. 5 shows a PCB 500 with a leaded component after reflow soldering in accordance with the invention; and Fig.6 shows a method 600 for producing a printed circuit board in accordance with the invention. Throughout the drawings, the same reference numeral refers to the same element, or an element that performs substantially the same function. DETAILED DESCRIPTION OF THE INVENTION For SMD components there are design rules to create a stencil opening (size and shape). The height of a stencil opening determines solder paste volume. For a leaded component the inventor found a need to fill the plated through hole (that is of course minus the lead). The volume ratio between a solder paste and the solder itself is about two to one. The inventor found a minimum solder volume in order to guarantee a good solder joint quality. In a plated through hole, the inventor found that it is advantageous to have at both sides of a PCB a solder joint. For normal SMD components the inventor calculates with a fixed stencil thickness and he adjusts the stencil apertures to get the right volume for the different component sizes. A cupper pad on the PCB (on which solder paste will be put) should typically be 0,1 mm larger in all dimensions than the stencil opening to get a good print quality in mass production. The outline of a standard solder-pad for leaded components is typically defined and restricted by (e.g., company set) design rules. The introduced star-pad design is created inner value of the standard pad. The outline for the star pad itself is empirically related to the quantity of solder-paste, which is needed for reflow soldering. The quantity of melted solder-paste bases on following parameters: Diameter of drilled (via) hole Pad-design (area) Diameter of Stencil cutout Thickness of stencil Thickness of PCB An aspect to the required quantity of solder-paste is that the design must fulfill the following relation: Volume solder-paste > 2 x volume of fixation

(volume of fixation = volume of drilled hole - volume of affected pin volume solder-paste = volume stencil-opening + volume of drilled hole ) The design is a preferred one for leaded components with, as design rule violations are minimal. The ratio of the diameter stencil cut out to copper-area is 1.5, so that one third of paste volume could build up the solder meniscus. Fig. 1 shows prior art pad design. The pad design of Fig. 1 comprises a solder pad 102 that comprises a copper area (area within the one but biggest circle minus the area of the smallest circle), drill hole 104 (area within smallest inner circle) and stencil opening 106 (area within largest circle). Fig. 2a shows a star-pad design in accordance with the invention. The star-pad design of Fig. 2a comprises a star-shaped solder pad 202 that comprises a copper area (area within the star shaped design minus the area of the small circle), drill hole 104 (area within inner circle) and stencil opening 106 (area within large circle). In a typical embodiment of Fig. 2a, the star-shaped solder pad 202, typically a copper area, measures approximately 2.5 mm², drill hole 104 has a diameter of approximately 1.2 mm (Approx. area : 1.13 mm²) and the stencil area 106 has a diameter of approximately 2 mm, and a surface of approximately 3.14 mm². A lead of component has a diameter of 0.9 mm => Area : 0.63 mm², thickness of PCB: 1 mm, thickness of Stencil: 0.15 mm. With above data it follows that for a standard pad: - Volume solder-paste (1.6mm³) > 2x volume of fixation (0.5mm³) and Relation Area stencil-cutout/copper-area = 1.2 simulates an enlargement of the stencil- opening; and for a Star-shaped pad: - Volume solder-paste (1.72 mm³) > 2x volume of fixation (0.5mm³). A typical characteristic of a star-pad is that is radius varies when looking around the center of the pad (usually the center of the drill hole). Fig. 2b to 2d show further examples of a star-pad design with corresponding features as in Fig 2a. Depending on circumstances and of a process used, the pad outline may vary. Fig. 3 a to 3b show further examples of a star-pad design with corresponding features as in Fig 2a. Note that the pad outline exceeds at some points the stencil opening area. Fig. 4 shows a PCB 400 (printed circuit board) with a leaded component before reflow soldering. Fig. 4 comprises metal via-bus 402 located in a through hole, PCB carrier 404, star-shaped solder pad 406, solder paste 408, component lead 410 of component 412. Fig. 4 shows a situation after reflow paste has been applied to the PCB by a stencil process but before the actual reflow process. Fig. 5 shows a PCB 500 with a leaded component after reflow soldering. Fig. 5 comprises via metal bus 402 located in a through hole, PCB carrier 404, star-shaped pad 406, solder 508, component lead 410 of component 412. Fig. 5 shows a situation after reflow paste has been applied to the PCB by a stencil process and after a reflow process. It is also shown that solder 508 has attached to lead 410. Fig.6 shows a method 600 for producing a printed circuit board in accordance with the invention. Method 600 comprises the steps of applying solder paste 602 to a PCB that has a through hole with a solder pad (the solder pad is typically located on leaded component side, top side in Fig. 5), mounting an SMD component to a PCB 604 (top side in Fig. 5), mounting a leaded component to the PCB 606 (top side in Fig. 5), and reflow soldering 608 the PCB. The solder pad is characterized in that is has a shape with a varying radius. A person skilled in the art understands that the step of mounting the SMD components 604 and the step of mounting a leaded component 606 can he interchanged or combined. Applying method 600 would constitute only one soldering step. Is it possible to mount and (reflow) solder components on both sides of the PCB but this adds costs. Alternatively, it is possible to have a solder pad (and thus also applying solder paste) on the opposite side of the side of the PCB where the leaded component has been mounted. But this would typically constitute two soldering actions and/or component placing actions: one for the top side and one for the bottom side. This would raise the cost of producing a PCB . One of ordinary skill in the art will recognize that alternative schemes can be devised of designing a star-shaped pad. For instance, the arms of the star pad do not necessarily need to be equidistant. In addition, it is possible that no copper area (or any other solder-able material) is connecting one or more arms of the star shaped pad. Alternatively, it is also possible that more copper area (or any other solder-able material) connecting one or more arms of the star shaped pad is present (thus filling up more of the space between the arms). Moreover the star shaped pads do not necessarily need to follow a circle. They can also be shaped along an ellipse, as a matter of fact the circle or the ellipse do not even need to have a perfect (mathematical) shape. The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. Also one skilled in the art understands that the dimensions shown in the figures are not to scale. Even non- linear scaling of certain features shown will result in a figure that is in accordance with the invention.

Claims

CLAIMS:

1. A printed circuit board (400, 500) comprising a through hole for a leaded component (412) wherein a surface of the printed circuit board around the hole comprises a solder-pad (202, 406) having a shape with a varying radius around the center of the through hole.

2. The printed circuit board (400, 500) of Claim 1 wherein the solder-pad is suitable for in a reflow soldering process.

3. The printed circuit board (400, 500) of Claim 2 wherein the solder pad shape is a star having at least two arms.

4. A method of producing a printed circuit board (400, 500), the method comprising the steps of

- applying solder paste (602) to the solder-pad (202, 406); - mounting a leaded component (606) on the printed circuit board (400, 500) having a through hole with a solder pad (202, 406) having a shape with a varying radius; and

- reflow soldering (608) the printed circuit board (400, 500).

5. The method of producing a printed circuit board (400, 500) of claim 4, wherein the solder pad shape is a star having at least two arms.