Patent Application
20100236823
Printed circuit boards, or PCBs, are generally used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate. A PCB populated with electronic components is referred to as a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA). PCBs are generally rugged, inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper and faster for high-volume production. Some PCBs have trace layers inside the PCB and are called multi-layer PCBs, and may be formed by bonding together separately etched thin boards. Some multi-layer PCBs may include several layers (e.g., 4 layers, 12 layers, 24 layers, or more).
Holes are typically drilled through a PCB with tiny drill bits (e.g., made of solid tungsten carbide) in order to connect components to different layers of the PCB. The drilling may be performed by automated drilling machines, with the placement of the holes controlled by a drill tape or a computer generated drill file. The drill file describes the location and size of each hole to be drilled in the PCB. These holes are often filled with conductive annular rings to create vias, which allow the electrical and thermal connection of conductors on opposite sides of a PCB.
It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called “blind vias” when they connect an internal copper layer to an outer layer, or “buried vias” when they connect two or more internal copper layers and no outer layers. The walls of the vias, for boards with 2 or more layers, are generally plated with copper to form plated-through-holes (PTH) that electrically connect the conducting layers of the PCB.
After the printed circuit board (PCB) is completed, electronic components must be attached to the PCB to form a functional PCBA. In through-hole construction, electronic component leads are inserted in PTHs in the PCB. In surface-mount technology (SMT) construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the PCB with molten metal solder.
Through-hole technology, also spelled “thru-hole”, refers to the mounting scheme used for pin-through-hole electronic components that involves the use of pins on the components that are inserted into copper PTH drilled in printed circuit boards (PCB) and soldered to pads on the opposite side. From the second generation of computers in the 1950s until surface-mount technology became popular in the late 1980s, every component on a typical PCB was a through-hole component. While through-hole mounting provides strong electrical and mechanical bonds when compared to surface-mount technology techniques, the additional drilling required makes the boards more expensive to produce. They also limit the available routing area for signal traces on layers immediately below the top layer on multilayer boards since the holes must pass through all layers to the opposite side. To that end, through-hole mounting techniques are now usually reserved for bulkier components such as electrolytic capacitors or semiconductors in larger packages that require the additional mounting strength provided by PTH technology.
PTH electronic components may be attached to a PCB using a soldering technique referred to as wave soldering. Wave soldering is a large-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The name is derived from the use of waves of molten solder to attach metal components to the PCB. The process uses a tank to hold a quantity of molten solder, and the components are inserted into or placed on the PCB and the loaded PCB is passed across a pumped wave or fountain of solder. The solder “wets” the exposed metallic areas of the board (e.g., those not protected with solder mask, a protective coating that prevents the solder from bridging between connections), creating a reliable mechanical and electrical connection. The process is much faster and can create a higher quality product than manual soldering of components. Wave soldering is used for both through-hole printed circuit assemblies and surface mount assemblies.
While there are many types of wave solder machines, the basic components and principles of these machines are generally the same. A standard wave solder machine includes three zones: the fluxing zone, the preheating zone, and the soldering zone. An additional fourth zone, a cleaning zone, may also be used depending on the type of flux applied.
When a PCB enters the fluxing zone, a fluxer applies flux to the underside of the board. Two types of fluxers are used: a spray fluxer and a foam fluxer. For either flux application method, precise control of flux quantities is required. Too little flux will cause poor joints, while too much flux may cause cosmetic or other problems. Also, as can be appreciated, the types of flux may affect the end result.
The PCB will then enter the preheating zone. The preheating zone consists of convection heaters, which blow hot air onto the PCB to increase its temperature. Generally, preheating is necessary to activate the flux, and to remove any flux carrier solvents. Preheating is also necessary to prevent thermal shock, which may occur when a PCB is suddenly exposed to the high temperature of the molten solder wave.
The tank of molten solder has a pattern of standing waves (or, in some cases, intermittent waves) on its surface. When the PCB is moved over this tank, the solder waves contact the bottom of the board, and stick to the solder pads and component leads by surface tension. For the pins of PTH components, molten solder fills the holes around the pins by capillary action. Precise control of wave height is required to ensure solder is applied to all areas but does not splash to the top of the board or other undesired areas. This process is sometimes performed in an inert gas Nitrogen (N2) atmosphere to increase the quality of the joints.
As the thickness of a PCB increases (e.g., above 100 mils, 150 mils, 200 mils, or more), and the mass of copper sheets increases (e.g., above 0.5 oz, 1.0 oz, 1.5 oz, 2.0 oz, or more), it may become more difficult to successfully fill the pin holes during the soldering process. One cause of the increased difficulty is that the molten solder tends to cool (“freeze”) prematurely before it has traveled from the bottom of the PCB to the top. This problem can be further exaggerated in pin holes that are used for ground and power connections. The reason for this is that a multilayered PCB may include several ground or power layers (e.g., 4 layers, 8 layers, 12 layers, or more) that include large sheets of copper. The multiple layers of copper sheets may conduct heat away from the molten solder (i.e., act as heat sinks), causing the solder to freeze prematurely, and causing the pin hole to be only partially filled with solder (e.g., 75% filled, 50% filled, or less). When the pin hole is only partially filled with solder, the mechanical and electrical integrity of the solder connection may be significantly reduced or may even be ineffective. In this regard, standards have been set to require a minimum amount of solder that fills a through hole for various components. For example, the IPC requires solder to fill at least 75% of the through hole for a signal pin, and at least 50% of the through hole for a ground or power pin.
As shown, to mechanically and electrically couple the resistor 104 to the PCB 100, solder 107 is used to connect the pin 105 to the PTH 103. Thus, the pin 105 is coupled via the solder 107 and the PTH 103 to the conductive layers 108, 114a-c, and 134. It is noted that the conductive layers 118, 124, and 130 do not contact the PTH 103 and are therefore not connected to the pin 105. In this regard, the layers 118, 124, and 130 may include signal layers, ground layers, or power layers that are connected to other components.
As shown, the solder 107 only partially fills the opening of the PTH 103. This may be due to the heat sinking effects caused by the ground or power layers 114a-c and 134 that are coupled to the PTH 103. That is, during the soldering process, molten solder 107 fills the opening of the PTH 108 from the bottom to the top via capillary action, losing heat in the process. If the molten solder 107 cools too rapidly, it may freeze prematurely, causing the opening in the PTH 103 to be only partially filled as shown. Since the PTH 103 is coupled to potentially large sheets of copper (e.g., the ground or power layers 114a-c) which have a high heat transfer coefficient, the heat of the molten solder 107 is dissipated rapidly through these electrical and heat conducting layers;
It is against this background that the “ring of power” via described herein has been invented. The present invention addresses the above problems by providing methods and systems for providing plated through-holes (PTH) in PCBs, which advantageously allow improved soldering capabilities. Such methods and systems are achieved by reducing the heat sinking effects of PTHs by providing one or more vias surrounding the PTHs to provide an electrical connection between the PTH and the bottom layer of a PCB. In this regard, the PTHs do not directly contact all of the internal ground or power layers, so the heat sinking or heat transfer effects are reduced. This feature enables molten solder to fill substantially the entire PTH before freezing. Further, one or more of the surrounding vias may be coupled to some or all of the ground or power layers without substantially increasing the heat sinking or transfer effects of the PTH. Various features and embodiments of the present invention are described in detail below.
According to a first aspect of the present invention, a multi-layer printed circuit board (PCB) is provided. The PCB includes a plurality of conductive layers with dielectric material disposed therebetween. Further, the PCB includes a through-hole that extends through the PCB, and a via, spaced apart from the through-hole, electrically coupled to one or more internal conductive layers of the plurality of conductive layers. Additionally, the through-hole is electrically coupled to the via, and the through-hole is spaced apart from at least one of the one or more internal conductive layers.
According to a second aspect of the present invention, a method of manufacturing a PCB is provided. The method includes providing a multi-layer PCB that includes a plurality of conductive layers with dielectric material disposed therebetween. The method also includes generating a through-hole that extends through the PCB, and generating a via, spaced apart from the through-hole, that is electrically coupled to one or more internal conductive layers of the plurality of conductive layers. Further, the through-hole is electrically coupled to the one or more vias, and the through-hole is spaced apart from at least one of the one or more internal conductive layers.
According to a third aspect of the present invention, a method for mechanically coupling an electrical component to a multi-layer PCB is provided. The method includes providing a multi-layer PCB that includes a plurality of conductive layers with dielectric material disposed therebetween, a through-hole that extends through the PCB, and a via, spaced apart from the through-hole, electrically coupled to one or more internal conductive layers of the plurality of conductive layers. The through-hole of the PCB is electrically coupled to the via of the PCB, and the through-hole is spaced apart from at least one of the one or more internal conductive layers. The method further includes providing an electrical component that includes a pin, and positioning the pin of the electrical component into the through-hole of the PCB. Additionally, the method includes soldering the pin to the through-hole to form a secure mechanical and electrical connection, wherein the solder substantially fills the through-hole.
Various features and refinements to the above-noted embodiments are also provided. For example, the multi-layer PCB may include a conductive surface land that couples the through-hole to one or more vias. As another example, the through-hole and/or the one or more vias may be plated with copper, or another conductive material. Further, the one or more internal layers may include ground layers, power layers, or signal layers.
In addition to the above, the size and number of the through-hole and the vias may be varied. For example the number of vias may be one, two, four, eight, or more. Further, the diameter of the vias may be substantially the same as the diameter of the through-holes, or different. As an example, in one embodiment, the diameter of the through-hole is about 60 to 100 mils, wherein the diameter of the one or more vias is about. 10-25 mils. Additionally, the through-hole may be spaced apart from one internal conductive layer that is coupled to a via, or more than one internal conductive layer.
In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following descriptions.
a-3c illustrate various embodiments of an exemplary “ring of power” via of the present invention.
a-6c illustrate various electronic components that include pins that may be coupled to a PCB using a “ring of power” via of the present invention.
Embodiments of the present invention are directed to methods and systems for providing PTHs in PCBs, which advantageously allow improved soldering capabilities. Such methods and systems are achieved by reducing the heat sinking effects of PTHs by providing one or more vias surrounding the PTHs to provide an electrical connection between the PTH and the bottom layer of a PCB. In this regard, the PTHs may not directly contact internal ground or power layers, or may contact fewer ground or power layers, so the heat sinking effects are reduced. This feature enables molten solder to fill substantially the entire PTH before freezing. Various features and embodiments of the present invention are described in detail below with reference to
a-3c illustrate top views of various embodiments of exemplary “ring of power” vias (or PTH assemblies) 300, 320, and 350 of the present invention.
b illustrates another “ring of power” via 320 which includes a PTH 322 which is surrounded by four vias 324. The vias 324 and the PTH 322 are coupled together through a conductive layer 326. Similarly, another “ring of power” via 350 is shown in
It should be appreciated that the embodiments shown in
In the example shown in
By not directly coupling the PTH 403 to the internal ground layers, the PTH 403 has relatively low heat transfer characteristics. This feature permits molten solder 410 to “wick” up through the PTH 403 to substantially fill the entire PTH 403 during the soldering process. As a result, the electrical and mechanical connections between the pin 416 and the associated PCB are relative secure.
It should be appreciated that although the embodiment shown in
a-6c illustrate various electronic components that include pins that may be coupled to a PCB using PTH technology. More specifically,
In the example shown in
By not directly coupling the PTH 703 to the internal ground layer 726c, the PTH 703 has relatively lower heat transfer characteristics. This feature permits molten solder 710 to “wick” up through the PTH 703 to substantially fill the entire PTH 703 during the soldering process. As a result, the electrical and mechanical connections between the pin 716 and the associated PCB are relative secure.
It should be appreciated that although the embodiment shown in
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character. For example, certain embodiments described hereinabove may be combinable with other described embodiments and/or arranged in other ways (e.g., process elements may be performed in other sequences). Accordingly, it should be understood that only the preferred: embodiment and variants thereof have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.
