CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to Polish Patent Application No. P-368337, filed Jun. 2, 2004, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printed circuit board with a shielded path and a method of manufacturing a printed circuit board with a shielded path.
2. Brief Description of the Background of the Invention Including Prior Art
Various improvements have been made to shielding the paths of printed circuit boards and adjusting the impedance of their paths. This can be achieved by providing additional paths or planes placed in the proximity of the path to be shielded. The two most common examples are a “microstrip” shown in FIG. 1A and a “stripline” shown in FIG. 1B. In the “microstrip” example, a signal path 103 on a layer 101 is shielded by a path 104 connected to the ground, being placed on layer 102. This does not provide protection against interference from all sides. A better protection is provided by the “stripline”, where a signal path 115 on an inner layer 113 is shielded by paths 114 and 116, placed on layers 111 and 112. This requires at least a three-layer board.
The European patent application no. EP0800338 entitled “Devices for minimizing electromagnetic radiation in printed circuit boards and other electronic circuit carriers” presents a method for shielding of elements on the PCB by covering them with an insulating element. However, the insulating element is specific and must be suited to the shielded element. The manufacture of such a specific element, as well as its assembly on the PCB, complicates the production process and increases its cost.
An article “Circuit board guarding techniques” published in June 1990 in Research Disclosure Journal, number 31482, presents a method for shielding a path on an inner layer of a printed circuit board by means of paths placed on each side of it and on the upper and lower layers, which are connected to the ground. This method thus requires paths on the inner layer of the board, and the shield is created only by paths.
There is no known shielding solution that would allow effective shielding of paths on the outer layers, i.e. the upper and lower layers of printed circuit boards in a way that does not complicate the normal process and does not increase production costs.
SUMMARY OF THE INVENTION
PURPOSE OF THE INVENTION
It is an object of the present invention to provide effective shielding of paths on the outer layers in avoiding or at least greatly diminishing the drawbacks of prior art.
This and other objects and advantages of the present invention will become apparent from the detailed description, which follows.
BRIEF DESCRIPTION OF THE INVENTION
The idea of the invention is that in a printed circuit board with a shielded path placed on its outer layer, the shield consists of an arrangement of elements, comprising paths or planes placed along the shielded path on both sides, which are connected to the shielding arrangement, placed on the lower layer and enclosing the shielded path and connected to the ground, and a line of SMD elements placed over the shielded path, whose contacts extending outside the shielded path are connected to paths placed at both sides of the shielded path.
The shielded path can be additionally covered with a solder mask layer, and the line of SMD elements is composed of at least two SMD elements connected in series, whose contacts are interconnected. The said elements are connected by means of solder paste placed on the solder mask layer.
Distances between the SMD elements in the line can be as small as is possible using current state-of-the-art technology.
In parallel to the shielded path there can be sited on the same layer a second path to be shielded and thereby the arrangement of the elements constituting the shield can enclose both paths.
Preferably, the shielding arrangement placed on the lower layer is a conductive path or plane, and the shielding arrangement placed on the lower layer is placed on the opposite outer layer.
Preferably, symmetrically to the shielded path, there placed on the opposite layer a second path to be shielded, and the shielding arrangement placed on the lower layer is a second line of SMD elements, placed over the second path to be shielded.
The board can have three layers. There can be a path or a plane placed on its inner layer between the shielded paths, which may be in contact with the upper and lower line of SMD elements.
Preferably, the SMD elements are resistors with resistance close to zero.
The idea of the invention is also that, in the method for manufacturing a printed circuit board with a shielded path placed on its outer layer, the shield is composed of an arrangement of elements, including paths or planes placed along the shielded path at its both sides, which are connected by means of vias with the shielding arrangement, which is placed on the lower layer and encloses the shielded path and is connected to the ground, and a line of SMD elements, placed over the shielded path, whose contacts, extending outside the shielded path, are connected with the planes or paths placed along the shielded path.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings one of the possible embodiments of the present invention is shown, where:
FIG. 1A presents a “microstrip” arrangement;
FIG. 1B presents a “stripline” arrangement;
FIG. 2 presents a cross section of a single-layer printed circuit board with a shielded path;
FIG. 3 presents a top plan view of a PCB with SMD elements placed close to each other;
FIG. 4 presents a top plan view of a PCB with SMD elements placed at a certain distance to each other;
FIG. 5 presents a cross section of a double-layer PCB with a shielded path at its top outer layer;
FIG. 6 presents a cross section of a double-layer PCB with shielded paths on both outer layers block;
FIG. 7 presents a cross section of a multi-layer PCB with a shielded path at its top outer layer;
FIG. 8 presents a top plan view of a PCB with a shielded path with a width greater than a length of a single SMD element; and
FIG. 9 presents a detailed view of the cross section of the PCB shown in FIG. 8.
DESCRIPTION OF INVENTION AND PREFERRED EMBODIMENT
FIG. 2 presents a cross section of a single-layer PCB with a shielded path. Over the path 212, placed on the layer 211, there is placed an SMD element 215, which ends with in contact surfaces, called contacts 216 and 217. The contacts 216 and 217 are connected respectively to paths 213 and 214, which are connected to the ground, and are placed along the shielded path 212 at its both sides. In this embodiment, the path 212 is shielded only from one side. Such shielding is preferable especially when the bottom layer of the board is metallized. This feature makes it possible to obtain a specific path impedance.
FIG. 3 presents a top view of a PCB with a shielded path 301. On both sides of the path 301 there are placed paths and/or planes 302 and 303 connected to the ground. Over the path there is placed a line of resistors 311, 312, 313 and 314, whose contacts are connected to the paths and/or the planes 302 and 303. The contacts of the resistors 311, 312, 313 and 314 extend beyond the shielded path 301. The chain of SMD elements 311, 312, 313, 314, for example the resistors, constitutes the top part of the tunnel, which encompasses the shielded path 301. The SMD elements 311, 312, 313, 314 are placed adjacent to each other, acting additionally as the insulation for the path 301 from external influences, such as dust, and in this way preventing possible short-circuits and positively improving the durability of the design.
FIG. 4 presents an arrangement, in which there are spaces between the elements 411, 412, 413, 414, which result from the production process. Such shielding is less efficient, but it ensures a certain level of path shielding. In order to provide a fully effective shield, the distances between the SMD elements should be as small as possible (for example—the smallest distances allowed by the production process). Spacings between the SMD elements 411, 412, 413, 414 can be deliberately introduced to ensure better cooling of the path 401. This provides both shielding for the path 401 and a free flow of air. The air flows, for example, through the clearance between the elements 411, 412, inside the tunnel composed of the elements 412, 402, 403, 401 and the board surface, on which paths 401, 402 and 403 are placed, and next through the clearance between the elements 412 and 413.
FIG. 5 presents a path 512 to be shielded, which is placed on the top layer 511 of a double-layer PCB. There is an SMD element 515 placed on the path, which ends with contact surfaces 516 and 517. The contacts 516 and 517 are connected with shielding planes or paths 513 and 514, which are placed along the shielded path 512 on both sides. At the bottom surface 521 of the board, there is placed a path 522, connected to the ground, which extends along the path 512, and which constitutes the shielding arrangement. The path 522 is connected with the paths 513 and 514 by means of metallised holes, so-called vias 531 and 532. Therefore, the path 522, the vias 531 and 532, the paths 513 and 514 and the SMD element 515 surround the path 512 on all sides, so that they constitute a shield, which effectively protects the path 512 against any interference, as well as other elements against internal interference generated by the path. The SMD element might be a resistor with a resistance close to zero. For example, it can be a 1210-type SMD resistor, having the length of 3.1 mm, the width of 2.6 mm, the height of 0.6 mm, and with the width of contact surfaces of 0.5 mm. However, it is possible to use other SMD elements, for example non-zero resistors, capacitors or inductors, in order to achieve appropriate shield parameters, which allows appropriate path impedance to be achieved
The SMD elements in the line may be identical or may be made up of a line of different elements. The advantage of using the SMD elements is that no additional procedures are necessary for the production of the board. On a typical printed circuit board there are many different SMD elements. Hence the addition of further components does not pose a significant time-factor for production. Placing additional elements does not require the introduction of any special additional process which might be much more time-consuming and impose additional costs. In the presented embodiment, the indicators 513, 514, 522 are related to the paths. However, the paths may be replaced by larger planes, which are connected to the ground. The vias 531, 532 need not necessarily be placed immediately underneath the contact surfaces of the resistors, but they can be located elsewhere, the only requirement being that they connect the paths 513 and 514 with the path 522. The path 522 is presented as an example. An alternative shielding arrangement may be placed on the lower surface instead, such as for example, a resistor or another element, or a specific a conductive plane in a specific form.
FIG. 6 presents a cross section of a printed circuit board with paths 612, 622 to be shielded, which are placed in parallel to each other at the outer layers 611 and 621. The shield around the paths is composed of a line of resistors 615 connected by contacts 616 and 617 with the paths 613, 614, which are connected by vias 631 and 632 with the paths 623, 624, which are connected with the line of resistors 625 with contacts 626, 627. Such shielding is effective also when the paths 612, 622 are paths transmitting a differential signal. It allows reduction of production costs, because elements to shield the paths from the lower layer are not necessary. Moreover, a decrease of production cost is obtained by miniaturizing the whole printed circuit board by placing its components on the bottom part of the printed circuit board.
FIG. 7 presents a cross section of a multilayer, for example a three-layer printed circuit board, whose path to be shielded 712 is placed on the top outer layer 711, while a shielding path or plane 722 is placed on the inner layer 721. The shield consists of a resistor 715, whose contacts 716, 717 are connected to paths 713, 714, which by means of vias 731, 732 are connected to the path 722. It allows the placement of other paths, 742 and 743, on the second outer layer 741. The paths 742, 743 can also be shielded, and their shield consists the resistor 747, whose contacts 746, 748 are connected to the paths 744, 745, which by means of vias 751, 752 are connected to the path 722. Such solution is characterized by better shielding properties than the solution depicted in FIG. 6, although production costs are raised due to the need to create the additional layer 741 of the PCB.
FIG. 8 shows a top-view of the PCB with a shielded path having a width greater than the length of a single SMD element, additionally having a variable angle of placement. The path 812 is covered with a solder mask layer made of epoxy resin. The outer layer of the shield is composed of a line of elements connected in series marked with the numbers 821 and 824, 851 and 852, 853 and 854, 855 and 856, whose contacts extending outside the solder mask layer 815 are soldered to the paths 813, 814, connected to the ground. The contacts 823, 825 of the SMD elements placed on the solder mask layer 815 are connected with each other by means of solder paste 820, put on the solder mask. Should a path 812 be required, more than two elements connected with each other in series can be emplaced. At the sides of paths 813, 814 there can also be a solder mask placed 816, 817. The solder mask can be placed on the path 812 also in the case when it is covered only with a line consisting of single SMD elements, which ensures that it has additional shielding and protection against mechanical damage.
FIG. 9 presents a cross section of the board presented in FIG. 8. There is a solder mask 815 placed on the path 812, placed on the top layer of 811 PCB, which is also placed on the sides 816, 817 of the paths 813, 814, which by means of vias 831, 832 are connected to the path 842, connected to the ground, placed on the bottom layer 841 of the PCB. The elements 821 and 824 adhere to each other by means of contacts 823, 824, which are connected together by means of solder paste 820, put on the solder mask 815. The outer contacts 822 and 826 are connected by means of solder paste 818, 819 with the paths 813 and 814. The placement of additional solder masks 815, 816, 817 additionally isolates the path 812 from external influences, leading to an increase of the durability and the resistance of the PCB.
The preferred embodiment having been thus described, it will now be evident to those skilled in the art that further variation thereto may be contemplated. Such variations are not regarded as a departure from the invention, the true scope of the invention being set forth in the claims appended hereto.
Patent Application
20050269129
