Patent Application
20190307029
This document generally relates to attaching a component to a circuit board using an over-filled cavity of solder.
Many electronic devices include circuit boards that have electronic components mounted thereto. These electronic components are often mounted to circuit boards with solder, which provides both physical and electrical connections to the circuit boards. The electrical connections are sometimes provided to conductive traces on a surface of the circuit board, which route electrical power and/or electrical data signals to and from such electronic components.
Conductive traces often run across a top surface of the circuit board. Still, some circuit boards also include one or more layers within the circuit board through which additional conductive traces may run. In circuit boards that have multiple layers of conductive traces, the layers may be separated by an electrically insulating material, and a conductive trace on one conductive layer may electrically connect to a conductive trace on another conductive layer with a conductive “via” that extends vertically through the circuit board to electrically connect traces of different layers. Designs that use multiple layers of conductive traces can house many different electronic components in a small footprint, with at least portions of the electrical connections being buried within the interior of the circuit board.
This document describes techniques, methods, systems, and other mechanisms for attaching a component to a circuit board using an over-filled cavity of solder.
As additional description to the embodiments described below, the present disclosure describes the following embodiments.
Embodiment 1 is a circuit-board attachment method. The method comprises providing a circuit board that defines a channel in a surface of the circuit board. The method comprises placing a stencil over the surface of the circuit board so that a slot that is in the stencil and defined by the stencil aligns with the channel in the circuit board. The method comprises applying solder to the stencil when the slot in the stencil is aligned with the channel in the circuit board to provide the solder into the slot in the stencil and the channel. The method comprises removing the stencil, leaving the solder in the channel and protruding from the channel. The method comprises placing a component in contact with the solder that is protruding from the channel. The method comprises heating the solder to attach the component to the circuit board.
Embodiment 2 is the method of embodiment 1. The circuit board is a multi-layer circuit board that includes an interior conductive layer. A conductor located within the interior conductive layer runs along a bottom of the channel and defines at least a portion of the bottom of the channel.
Embodiment 3 is the method of embodiment 2. The conductor that runs along the bottom of the channel is grounded.
Embodiment 4 is the method of embodiment 1. A length of the channel is at least ten times greater than a width of the channel.
Embodiment 5 is the method of embodiment 4. The width of the channel is greater than a width of the component at a location of the channel at which the component attaches to the circuit board.
Embodiment 6 is the method of embodiment 1. Removing the stencil leaves the solder protruding from the channel beyond the surface of the circuit board by approximately a thickness of the stencil.
Embodiment 7 is the method of embodiment 1. Heating the solder attaches the component to the circuit board such that at least a portion of the component is located within the channel below the surface of the circuit board.
Embodiment 8 is the method of embodiment 1. The component comprises an electromagnetic interference shield.
Embodiment 9 is the method of embodiment 8. The channel continuously or discontinuously surrounds a region of the circuit board. Multiple sides of the electromagnetic interference shield are located in the channel so that the electromagnetic interference shield surrounds and covers the region of the circuit board that is surrounded by the channel.
Embodiment 10 is an electronic device assembly. The device assembly comprises a circuit board that defines a channel in a surface of the circuit board. The device assembly comprises a component that is attached to the circuit board, with the component being located partially in the channel and partially out of the channel. The device assembly comprises solder that attaches the component to the circuit board, with the solder being partially in the channel and partially protruding from the channel.
Embodiment 11 is the electronic device assembly of claim 10. The circuit board is a multi-layer circuit board that includes an interior conductive layer. A conductor located within the interior conductive layer runs along a bottom of the channel and defines at least a portion of the bottom of the channel.
Embodiment 12 is the electronic device assembly of claim 11. The conductor that runs along the bottom of the channel is grounded.
Embodiment 13 is the electronic device assembly of claim 10. A length of the channel is at least ten times greater than a width of the channel.
Embodiment 14 is the electronic device assembly of claim 13. The width of the channel is greater than a width of the component at a location of the channel at which the component attaches to the circuit board.
Embodiment 15 is the electronic device assembly of claim 10. The component comprises an electromagnetic interference shield.
Embodiment 16 is the electronic device assembly of claim 15. The channel continuously or discontinuously surrounds a region of the circuit board. Multiple sides of the electromagnetic interference shield are located in the channel so that the electromagnetic interference shield surrounds and covers the region of the circuit board that is surrounded by the channel.
Embodiment 17 is the electronic device assembly of claim 16. The device assembly further comprises a computer processor that is mounted to the region of the circuit board that is surrounded by the channel and the electromagnetic interference shield.
Embodiment 18 is the electronic device assembly of claim 10. The device assembly further comprises a computer processor that is mounted to the circuit board. The channel surrounds the computer processor and includes a first channel portion. The channel includes a second channel portion that is opposite and parallel to the first channel portion, the computer processor being located between the first channel portion and the second channel portion. The channel includes a third channel portion that is perpendicular to the first channel portion and perpendicular to the second channel portion. The channel includes a fourth channel portion that is perpendicular to the first channel portion, perpendicular to the second channel portion, and parallel to the third channel portion, the computer processor being located between the third channel portion and the fourth channel portion.
Embodiment 19 is the electronic device assembly of claim 18. The first channel portion connects with a first end of the third channel portion and a first end of the fourth channel portion. The second channel portion connects with a second end of the third channel portion and a second end of the fourth channel portion.
The details of one or more implementations are set forth in the accompanying drawings and the description below. Features, objects, and advantages will also be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
This document generally describes attaching a component to a circuit board using an over-filled cavity of solder. Attaching a component using an over-filled cavity of solder, rather than attaching the component to solder that is not deposited in a cavity, can allow the use of additional solder to connect a component to a circuit board without increasing a footprint of space on the circuit board required for attaching the component, or can allow a decrease in footprint required to attach a component.
Rather than using a stencil to apply solder to only a top surface of a circuit board (e.g., a printed circuit board) at which an electronic component may be attached, the circuit board can include a cavity and the stencil may be used to apply solder into the cavity. Using a stencil in such a manner may not only fill the cavity with solder, but can also leave solder extending out of the cavity above a surface of the circuit board by an amount approximately equal to a thickness of the stencil. A component may thereafter be placed over the channel that has the solder protruding therefrom, and heat may be applied to the solder to cause the component to settle at least partially into the channel. The solder may then cool, physically and electrically attaching the component to the circuit board in its settled position. The solder that is applied in such a process is often a flux-cored solder or a solder paste (e.g., a mixture of small solder balls in a flux), although this description will simply refer to such material as “solder” for simplicity of description.
In some examples, the component that is attached to the circuit board is an electromagnetic interference (EMI) shield that at least partially surrounds and covers electronic devices that may be sensitive to EMI. To protect such electronic devices from EMI (or to contain EMI produced by components within the EMI shield so that the EMI does not interfere with components located outside of the EMI shield), the periphery of the EMI shield may connect to the circuit board with solder to isolate the electronic devices that are surrounded by the EMI shield from EMI. Gaps in the connection between the periphery of the EMI shield and the circuit board may allow EMI to enter the interior of the EMI shield and may therefore reduce a level of protection provided by the EMI shield. As such, a computer processor located underneath the EMI shield may be more susceptible to interference. In some examples, a wireless circuit (e.g., to produce Bluetooth, WiFi, or NFC signals) may be located inside of the EMI shield.
As mentioned above, solder may be applied to a circuit board using a stencil. Stencils are often made of metal and define holes, slots, and other shapes of apertures at locations at which solder is to be applied to a corresponding circuit board. The apertures may be formed using laser cutting or electroforming, for example. The stencil may be placed over the circuit board, and solder applied to the circuit board, for example, with a squeegee that wipes solder across the stencil and deposits the solder into the apertures of the stencil. The stencil may then be removed, leaving solder at locations on the circuit board that correspond to the locations of the apertures of the stencil.
As components on circuit boards are manufactured with smaller overall dimensions to enable a reduction in the overall size of such circuit boards (or to allow more components to be attached to the circuit boards), it is also desirable to reduce the footprint occupied by the solder that connects such components to the circuit boards. This can be done by reducing the width of apertures in the stencil. A problem with simply reducing the width of apertures without reducing a thickness of the stencil, is that the solder may be more likely to “lift off” the circuit board when the stencil is removed. In other words, narrowing the width of stencil apertures without decreasing a thickness of the stencil may result in some solder sticking to the stencil rather than depositing onto a circuit board. For a long slot in a stencil designed to leave a continuous line of solder on a circuit board, any “lift off” may leave a discontinuous line of solder on the circuit board. An EMI shield attached using this discontinuous line of solder may therefore be attached with a gap in its connection to the circuit board, and this gap may negatively affect performance of the EMI shield.
A stencil that has less thickness may be used to limit the amount of solder “lift off,” but reducing the thickness of a stencil also reduces a height of the solder that is deposited onto a circuit board, which can negatively affect the attachment of some components to a circuit board. For instance, a heavier component or a vibrating component attached with a limited amount of solder may not form a sufficient structural connection with the circuit board. Also, if the component that is being attached to the circuit board is not perfectly planar at its location of attachment, only a portion of the intended attachment surface of the component may contact the solder if a relatively-thin layer of solder has been deposited.
Turning back to the above-described example of an EMI shield, some such shields are formed out of sheet metal that is bent at its edges to form side walls at which the EMI shield connects to the circuit board. The bottom edges of these side walls may not be perfectly co-planar, for example, because the sheet metal may not have been perfectly rectangular before the sheet metal was formed into the EMI shield, or because the forming of the sheet metal into the EMI shield may induce a curve to the bottom edge of a side wall that attaches to the circuit board. If the extent of this non-co-planarity of the point of attachment to the circuit board exceeds the height of solder deposited onto the circuit board, the EMI shield may attach to the circuit board with undesired discontinuities. Stated another way, if the bottom edge of component being attached to a circuit board is not sufficiently flat, only some of the bottom edge may settle into the solder and some of the bottom edge may remain suspended above the solder.
To address such discontinuities, a thicker stencil may be used so that more solder is deposited in the vertical direction on the circuit board. There are potential downsides to this approach, as described above. Another option is to add an additional layer of solder on top of the first layer of solder to increase the solder height in a two-step process, but adding solder in a two-step process involves additional time and may require using an additional solder machine, which can increase processing expenses. Another option is to only use EMI shields with high co-planarity tolerances, but requiring such high tolerances can increase the per-unit cost of EMI shields.
An approach that may not require such tight tolerances, and that can result in sufficient application of solder involves depositing the solder into a cavity of the circuit board and filling the cavity so that the solder extends above a surface of the circuit board, forming what this description refers to as an overfilled cavity of solder. An electronic component may then be placed on top of the solder, and heating the solder may cause the electronic component to settle into the solder. In implementations in which the portion of the electronic component that contacts the solder and that attaches to the circuit board is sized to fit within the cavity, the portion of the component that attaches to the circuit board may settle into the cavity so that at least part of the electronic component is located beneath a top surface of the circuit board.
Connecting a component to a circuit board within an over-filled cavity in this manner can allow a relatively tall column of solder to be used to connect a component to a circuit board, while constraining a width of the column of solder to a width of the cavity below the surface of the circuit board (e.g., because the solder has filled the cavity) and to the width of the stencil aperture above the surface of the circuit board (e.g., because the solder filled the stencil aperture before the aperture was removed). In some examples, the width of the stencil aperture is approximately the same as the width of the cavity, but the width of the stencil aperture can also be wider or narrower than the width of the cavity. For example, the width of the stencil aperture can be narrower than the width of the cavity, so that once the component has settled into the solder, any expansion of the solder may not extend sideways beyond a width of the channel. The following discussion explains, with reference to
In this example, cavities 110a-b terminate below a surface 120 of the circuit board 100 at interior layer 122. The cavities may be formed, for example, using a photolithographic process or a laser that removes material, and the bottom of the cavities 110a-b may be defined by a conductive trace. This conductive trace may provide a barrier that prevents the photolithographic process from etching deeper into the circuit board 100. The conductive trace can also provide an electrical interface to which one or more components may be electrically connected with solder. The depth of the cavity may be at least 0.03, 0.05 mm, 0.08, 0.1 mm, 0.12 mm, 0.15 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, or 0.7 mm. In some examples, the cavities 110a-b may terminate at a deeper interior layer, such as the second interior layer 124. In some examples, the side walls of the cavities 110a-b may be lined with a conductive material, while in some examples the side walls of the cavities 110a-b may be defined the insulating material 126a.
In this illustration, a human or machine is wiping a squeegee 140 across a surface of the stencil 130. The wiping of the squeegee 140 across the surface of the solder stencil 130 deposits solder 150 into apertures of the stencil 130.
The cross-sectional diagram that is shown in
One such electronic component is EMI shield 160, which is illustrated in
Various different types of EMI shields may be mounted to circuit boards using the techniques described herein. For example, implementation “B” shows an EMI shield 302 that connects to the circuit board 100 in such a manner that it leaves small openings (e.g., opening 310) at each of the corners of the EMI shield 302. These openings may be formed when sheet metal that forms the EMI shield 302 is bent to create the sidewalls of the EMI shield 302, producing an intended or unintended gap at the corners between the sidewalls. This gap may result in an EMI shield that does not perfectly seal its interior environment from its exterior environment, which can enable heat that is generated by electronic components mounted to the circuit board 100 within the interior of the EMI shield 302 to escape from the interior. The illustration of implementation “B” shows that solder 150 forms a continuous track around the periphery of the EMI shield, but the track may alternatively be discontinuous. For example, the solder track may comprise four unattached channels arranged in a rectangle, with the ends of the channels pointing toward the apparent corners of the rectangle but not touching.
Implementation “C” shows an example of an EMI shield 304 that includes a gap 312 in its sidewall (and in the solder track) through which conductive traces 320 route to connect electronic components within the EMI shield 304 to other electronic components attached at the circuit board. The presence of the gap 312 in the sidewall allows the conductive traces 320 to cross the boundary defined by the periphery of the EMI shield 304 on a surface layer of the circuit board 100 rather than such conductive traces 320 having to cross the boundary defined by the periphery of the EMI shield 304 in an interior layer of the circuit board 100.
Implementation “D” shows an example of an EMI shield 306 that includes holes (e.g., hole 330) in a horizontal surface/cover of the EMI shield 306. Such holes may enable heat to escape the interior of the EMI shield 306 and can allow outgas to escape during a solder heating reflow process.
Implementation “E” shows an example of an EMI shield 308 that has more than five sides, therefore having more than four sidewalls. In this example, EMI shield 308 includes five walls. EMI shields may have various other shapes, for example, they may be circular, have three walls, or have six walls. Common to these various examples of EMI shields is that all have sidewalls that surround any one or more electronic components attached to the circuit board 100 within an interior of the respective EMI shield, to protect such components from EMI.
Other components may be attached to a circuit board using the techniques herein. The components need not attach with lengthy walls, as illustrated with the various EMI shields shown in
At box 410, a circuit board that defines a channel is provided. For example, a person or machine may place circuit board 100 (see, e.g.,
A conductive trace located within the inner layer may run along the bottom of the channel (box 414). This conductive trace may be grounded, and may provide the electrical connection to which an electronic component placed within the channel will connect. In some examples, no other conductive trace is located at the bottom of the channel. The conductive trace may define a bottom of the channel. A width of the channel may be greater than a width of a component that will be placed into the channel, at a location at which the component will attach to the circuit board (box 416). As such, the channel may form a trench within which the component will mount to the circuit board.
At box 420, a stencil is placed over a surface of the circuit board 100. The stencil may define multiple apertures into which solder will be deposited. An aperture may correspond to a shape of the channel, and placing the stencil over the surface of the circuit board 100 may include aligning the stencil so that the aperture that corresponds to the shape of the channel is aligned with the channel so that solder deposited into the aperture fills an entirety of the channel.
At box 430, solder is applied to the stencil. The solder may be applied when the apertures in the stencil are aligned with corresponding locations on the circuit board 100 at which solder is to be deposited (with some locations being on a surface of the circuit board, and at least one other location being a cavity formed in the surface of the circuit board). Applying the solder to the stencil can include wiping solder across the stencil with a squeegee, or by performing other processes by which solder may be inserted into apertures in the stencil.
At box 440, the stencil is removed, leaving solder protruding from the channel. For example, after the solder has been inserted into the stencil aperture that has a shape that corresponds to the channel, the stencil is removed to leave solder not only in the channel, but also protruding from the channel at approximately a thickness of the stencil (box 442). The protruding solder extends beyond a surface of the circuit board an amount that is approximately equal to the thickness of the stencil because, absent the stencil, the solder may have been wiped from the surface of the circuit board by a squeegee running thereacross.
At box 450, a component is placed is contact with the solder. In some examples, the component comprises an EMI shield (box 452). In other examples, the component comprises a vibrating device. The component may be placed in contact with the solder using a locating pin. The solder may also or alternatively have sufficient adhesion to hold the component in place until the solder is heated (as described below).
At box 460, the solder is heated. Heating the solder to attach components is sometimes called reflowing. Heating the solder may cause the electronic component to settle into the solder such that, upon the solder cooling, the solder may form a physical and electrical connection between the component and the circuit board 100. The settling of the component into the solder may leave at least a portion of the component located within the channel and therefore below the surface of the circuit board (box 623), as illustrated in
At box 464, the channel continuously or discontinuously surrounds a portion of the circuit board, and multiple sides of the shield are located in the channel, such that the shield at least partially surrounds the portion of the circuit board. For instance, as shown in
In this disclosure, references to the vertical direction, up, down, or the ‘z’ direction, a dimension normal to a front surface of the circuit board. References to a footprint on a circuit board, the ‘x’ and ‘y’ dimensions, and side-to-side, represent the dimensions that define the front surface of the circuit board.
Although a few implementations have been described in detail above, other modifications are possible. Moreover, other mechanisms for performing the systems and methods described in this document may be used. In addition, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other implementations are within the scope of the following claims.
