Patent Application
20220007518
Fabrication of electronic circuits, such as components assembled on a printed circuit board (PCB), typically includes soldering of components onto the PCB or other substrate. Unfortunately, defects in the fabrication process can adversely impact component operation or render an entire electronics module inoperable, for instance due to poor electrical or thermal conductivity, or insufficient mechanical connectivity. One type of defect is a void, in particular a hole or region within a solder joint in which the solder is substantially or entirely missing. The formation of voids may occur due to different reasons, such as outgassing of flux in solder paste, vias in pads, or solder paste quality impacting its rheological properties.
Certain procedures, such as reflow or adding more solder, can be employed to reduce voiding. However, these procedures may be ineffective on their own when manufacturing circuitry that includes large or heavy components, especially components with strict alignment requirements. For example, duplexers that are employed in communication circuitry may be very susceptible to voiding. It may also be difficult to detect voids beneath duplexers and other large components that may partially or completely block x-rays. Conventional manufacturing techniques can be insufficient when fabricating a communication module having duplexers or similar components. This issue can be compounded when the communication module is intended for use in extreme environments, such as the stratosphere, where very low temperatures and/or large swings in environmental conditions can accelerate system failures. These failures may be catastrophic, particularly when there is no feasible way to repair or replace the communication module when it is in the extreme environment.
Telecommunications connectivity via the Internet, cellular data networks and other systems is available in many parts of the world. However, there are many locations where such connectivity is unavailable, unreliable or subject to outages from natural disasters. Some systems may provide network access to remote locations or to locations with limited networking infrastructure via satellites or high altitude platforms (HAPs) located in the stratosphere. In the latter case, the communication equipment providing, e.g., LTE and/or 5G services, may be expected to operate for weeks, months or longer in a harsh environment without the possibility for repair should the equipment fail. Void-based failures associated with duplexers and other components thus can adversely impact the ability of the platform to provide communication connectivity, which may necessitate launching of additional HAPs in a fleet in order to address a communication coverage deficiency.
Aspects of the technology employ advanced circuit board fabrication techniques that include using alignment fixtures and solder standoffs to position dielectric filter components such as duplexers, using nitrogen gas during the reflow process, and maintaining specific temperature and “time above liquideous” (TAL) controls to ensure the temperature difference between the duplexers and other PCB elements fall within acceptable tolerance limits.
According to one aspect, a method of fabricating a printed circuit board is provided. The method comprises printing solder paste onto a plurality of contact areas of the printed circuit board; placing a set of solder preforms onto a subset of the plurality of contact areas having the printed solder paste, wherein an amount of solder paste for each of the solder preforms is selected based on a pad size of each of the subset of contact areas; placing a first set of components on the printed circuit board after printing the solder paste; placing one or more alignment fixtures on the printed circuit board; placing a second set of components on the printed circuit board, wherein the one or more alignment fixtures are placed so that the second set of components remains in place during fabrication of the printed circuit board; after the one or more alignment fixtures and the second set of components are placed, performing a reflow process to heat the solder paste and the set of solder preforms to a first temperature according to a selected temperature profile; and upon ending the reflow process and reaching a second temperature below the first temperature, removing the one or more alignment fixtures.
In one example, the set of solder preforms placed on the subset of the plurality of contact areas are arranged so that the subset of contact areas are corner pads for the second set of components. In another example, each one of the solder preforms of the set comprises a pair of standoffs placed on the printed solder paste. In this case, the amount of solder paste for each of the solder preforms may be on the order of 2 kmil3 to 15 kmil3 in volume. In one scenario, the pad size may be either a 0201 size or a 0402 size. In an example, placing the first set of components occurs in conjunction with placing the set of solder preforms.
Performing the reflow process according to the selected temperature profile may include maintaining the first temperature for a selected time above liquideous. The time at above liquideous may be between 30 and 100 seconds. Achieving the first temperature may include ramping up the temperature at a selected rate. Here, the selected rate may be no more than 1° C.-3° C. per second.
In an example, the method further comprises performing the reflow process in the presence of nitrogen gas. In another example, the selected temperature profile and selected time above liquideous are chosen so that a temperature difference between the second set of components and one or more of the first set of components is maintained within a determined tolerance limit. Here, the second set of components may comprise a pair of duplexers. In this case, the one or more of the first set of components comprises a mechanical relay disposed between the pair of duplexers.
In a further example, the second temperature is room temperature. And in yet another example, the method also includes, after removing the one or more alignment fixtures, performing one or more of testing circuitry of the fabricated printed circuit board, inspecting at least some solder joints formed by the reflow process, and analyzing voids along the printed circuit board to determine whether such voids meet one or more predetermined criteria.
According to another aspect, a printed circuit board is fabricated by the method described above. Here, for instance the second set of components may comprise a pair of duplexers, and one or more of the first set of components may comprise a mechanical relay disposed between the pair of duplexers. In an example, a total size by surface area of all voids along the printed circuit board do not exceed 25%, and any individual void does not exceed 10% of the total size.
According to a further aspect, a communication system comprises one or more printed circuit boards fabricated by the method described above. In an example, the communication system may include a high altitude platform configured to operate in the stratosphere.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The technology relates to processes for reducing voids during PCB fabrication, such as for communication devices that include duplexers and other relatively large components. Reduction of voids to below a threshold level may be mission critical for systems such as communication modules deployed on HAPs intended to operate in the stratosphere for extended periods of time. Electrical, thermal and/or mechanical failures of such components due to voids may render the entire communication system of a HAP inoperable or severely degraded.
Stratospheric HAPs, such as balloon-based HAPs, may have a float altitude of between about 50,000-120,000 feet above sea level. At such heights, the density of the air is very low compared to ground level. For example, while the pressure at ground level is around 1,000 mbar, the pressure in the lower stratosphere may be on the order of 100 mbar and the pressure in the upper stratosphere may be on the order of 1 mbar. The temperature in the stratosphere generally increases with altitude. For instance, in the lower stratosphere the average temperature may be on the order of −40° C. to −100° C. or colder, while the average temperature in the upper stratosphere may be on the order of −15° C. to −5° C. or warmer. In addition, while balloons and other HAPs in the stratosphere generally fly above the clouds and most weather conditions, the HAPs can be impacted by lightning-induced transients beneath them. Such environmental conditions can cause component or system-wide failures, which can reduce or cut short the HAP's operational lifetime. This may be especially true with voids impacting electronic circuitry.
The systems and processes discussed below are configured to minimize the impact of voids on such circuitry. While these solutions are beneficial for PCB fabrication in general, including communication circuitry, they are particular helpful for communication modules operating in extreme environments such as the stratosphere.
The devices in system 100 are configured to communicate with one another. As an example, the HAPs may include communication links 104 and/or 114 in order to facilitate intra-balloon communications. By way of example, links 114 may employ radio frequency (RF) signals (e.g., millimeter wave transmissions) while links 104 employ free-space optical transmission. Alternatively, all links may be RF, optical, or a hybrid that employs both RF and optical transmission. In this way balloons 102A-F may collectively function as a mesh network for data communications. At least some of the HAPs may be configured for communications with ground-based stations 106 and 112 via respective links 108 and 110, which may be RF and/or optical links.
In one scenario, a given balloon-type HAP 102 may be configured to transmit an optical signal via an optical link 104. Here, the given balloon 102 may use one or more high-power light-emitting diodes (LEDs) to transmit an optical signal. Alternatively, some or all of the balloons 102 may include laser systems for free-space optical communications over the optical links 104. Other types of free-space communication are possible. Further, in order to receive an optical signal from another balloon via an optical link 104, the balloon may include one or more optical receivers.
The HAPs may also utilize one or more of various RF air-interface protocols for communication with ground-based stations via respective communication links. For instance, some or all of balloons 102A-F may be configured to communicate with ground-based stations 106 and 112 via RF links 108 using various protocols described in IEEE 802.11 (including any of the IEEE 802.11 revisions), cellular protocols such as GSM, CDMA, UMTS, EV-DO, WiMAX, and/or LTE, 5G and/or one or more proprietary protocols developed for long distance communication, among other possibilities. The ground-based stations may include client devices such as mobile phones, computers, etc.
In some examples, the links may not provide a desired link capacity for HAP-to-ground communications. For instance, increased capacity may be desirable to provide backhaul links from a ground-based gateway. Accordingly, an example network may also include downlink balloons, which could provide a high-capacity air-ground link between the various HAPs of the network and the ground base stations. For example, in network 100, balloon 102F may be configured as a downlink balloon that directly communicates with station 112.
Like other HAPs in network 100, downlink balloon 102F may be operable for communication (e.g., RF or optical) with one or more other balloons via link(s) 104. Downlink balloon 102F may also be configured for free-space optical communication with ground-based station 112 via an optical link 110. Optical link 110 may therefore serve as a high-capacity link (as compared to an RF link 108) between the network 100 and the ground-based station 112. Downlink balloon 102F may additionally be operable for RF communication with ground-based stations 106. In other cases, downlink balloon 102F may only use an optical link for balloon-to-ground communications. Further, while the arrangement shown in
A downlink HAP may be equipped with a specialized, high bandwidth RF communication system for balloon-to-ground communications, instead of, or in addition to, a free-space optical communication system. The high bandwidth RF communication system may take the form of an ultra-wideband system, which may provide an RF link with substantially the same capacity as one of the optical links 104.
In a further example, some or all of HAPs 102A-F could be configured to establish a communication link with space-based satellites and/or other types of HAPs (e.g., drones, airplanes, airships, etc.) in addition to, or as an alternative to, a ground based communication link. In some embodiments, a balloon may communicate with a satellite or another high altitude platform via an optical or RF link. However, other types of communication arrangements are possible.
Each of the communication approaches noted above may employ one or more communication modules. As discussed further below, these modules may include duplexers, multiplexers and other components that may be significantly affected by voids created during the circuit board fabrication process.
The balloons of
In an example configuration, a balloon-type HAP includes an envelope and a payload, along with various other components.
The envelope 202 may take various shapes and forms. For instance, the envelope 202 may be made of materials such as polyethylene, mylar, FEP, rubber, latex or other thin film materials or composite laminates of those materials with fiber reinforcements imbedded inside or outside. Other materials or combinations thereof or laminations may also be employed to deliver required strength, gas barrier, RF and thermal properties. Furthermore, the shape and size of the envelope 202 may vary depending upon the particular implementation. Additionally, the envelope 202 may be filled with different types of gases, such as air, helium and/or hydrogen. Other types of gases, and combinations thereof, are possible as well. Shapes may include typical balloon shapes like spheres and “pumpkins”, or aerodynamic shapes that are symmetric, provide shaped lift, or are changeable in shape. Lift may come from lift gasses (e.g., helium, hydrogen), electrostatic charging of conductive surfaces, aerodynamic lift (wing shapes), air moving devices (propellers, flapping wings, electrostatic propulsion, etc.) or any hybrid combination of lifting techniques.
According to one example shown in
The one or more processors 304 can include any conventional processors, such as a commercially available CPU. Alternatively, each processor can be a dedicated component such as an ASIC, controller, or other hardware-based processor. Although
The payload 300 may also include various other types of equipment and systems to provide a number of different functions. For example, as shown the payload 300 includes one or more communication systems 308, which may transmit signals via RF and/or optical links as discussed above. The communication system(s) 308 include communication components such as one or more transmitters and receivers (or transceivers), one or more antennae, and a baseband processing subsystem (not shown). One or more duplexers (see
The payload 300 is illustrated as also including a power supply 310 to supply power to the various components of balloon. The power supply 310 could include one or more rechargeable batteries or other energy storage systems like capacitors or regenerative fuel cells. In addition, the payload 300 may include a power generation system 312 in addition to or as part of the power supply. The power generation system 312 may include solar panels, stored energy (hot air), relative wind power generation, or differential atmospheric charging (not shown), or any combination thereof, and could be used to generate power that charges and/or is distributed by the power supply 310.
The payload 300 may additionally include a positioning system 314. The positioning system 314 could include, for example, a global positioning system (GPS), an inertial navigation system, and/or a star-tracking system. The positioning system 314 may additionally or alternatively include various motion sensors (e.g., accelerometers, magnetometers, gyroscopes, and/or compasses). The positioning system 314 may additionally or alternatively include one or more video and/or still cameras, and/or various sensors for capturing environmental data. Some or all of the components and systems within payload 300 may be implemented in a radiosonde or other probe, which may be operable to measure, e.g., pressure, altitude, geographical position (latitude and longitude), temperature, relative humidity, and/or wind speed and/or wind direction, among other information.
Payload 300 may include a navigation system 316 separate from, or partially or fully incorporated into control system 302. The navigation system 316 may implement station-keeping functions for the HAP to maintain position within and/or move to a position in accordance with a desired communication coverage or other service requirement. In particular, the navigation system 316 may use wind data (e.g., from onboard and/or remote sensors) to determine altitudinal and/or lateral positional adjustments that result in the wind carrying the balloon in a desired direction and/or to a desired location. Lateral positional adjustments may also be handled directly by a lateral positioning system that is separate from the payload. Alternatively, the altitudinal and/or lateral adjustments may be computed by a central control location and transmitted by a ground based, air based, or satellite based system and communicated to the HAP. In other embodiments, specific HAPs may be configured to compute altitudinal and/or lateral adjustments for other HAPs and transmit the adjustment commands to those other HAPs.
An environmental sensor system 318 is also shown, which may encompass some or all of the probes and other sensors mentioned above. In addition, the environmental sensor system 318 includes other sensors configured to detect information associated with lightning and other environmental conditions.
As seen by arrows 706, spaces for mechanical relays (not shown) are disposed between pairs of the duplexers 702. Another fabrication challenge is that the mechanical relays may be thermally sensitive. Even though a certain temperature profile may be desired in order to prevent void formation beneath the duplexers, this profile may be too hot for the mechanical relays, which can result in damage to the relays. In addition, manual addition of solder balls along edges of a duplexer (e.g., solder ball 508 in
According to one set of criteria, e.g., IPC-A-610, the total size (by surface area) of all voids should not exceed 25%, while any individual void should not exceed 10% of the total size. In order to satisfy such criteria, one process according to aspects of the technology, the peak solder reflow temperature at the duplexers is limited to no more than 232° C., while the maximum temperature difference (delta) between the duplexers and the mechanical relays is 21° C.
The time above liquideous (TAL) should be carefully controlled as well. In one scenario, the TAL is between 30-100 seconds and the peak temperature range is on the order of 235° C.-245° C.
Additional solder is added to the pads prior to placement of the duplexers. For 0201 size pads (measuring 0.024×0.012 in. or 0.6×0.3 mm), each preform of additional solder may be approximately 2 kmil3 in volume, while for 0402 size pads (measuring 0.04×0.02 in. or 1.0×0.5 mm) each preform of additional solder may be approximately 15 kmil3 in volume.
In one alternative, a single standoff having an 0402 size could be used in place of the pair of 0201 standoffs. This may be done depending on the size of the components of interest (e.g., duplexers or similar devices).
At block 1206, a first set of components is placed on the PCB. These components may include, e.g., inductors, capacitors, resistors, diodes, transistors and other relatively SMT devices, mechanical relays and the like. This placement may occur in conjunction with (e.g., at the same time, or just before or just after) the placement of the solder preforms, since the solder preforms are intended for use with other, selected components. At block 1208, one or more alignment fixtures are placed on the PCB. The alignment fixtures are positioned so that the one or more selected components are held in place during the reflow process. Depending on the type and/or size of the selected components, alignment fixtures may not be employed. Then, at block 1210, the one or more selected components, such as duplexers, are placed on the PCB so that the alignment fixtures maintain the positions of these components.
A reflow process is performed at block 1212 once the different components and alignment fixtures are on the PCB. The reflow process includes control of the TAL, as noted above. For instance, the ramp up rate may be no more than 1° C.-3° C./sec, and the TAL may be between 30 and 100 seconds. These factors may be impacted by the size of the PCB, the characteristics of the selected components, and other issues. Once the reflow process is finished and a determined temperature has been reached (e.g., room temperature, such as about 20° C.-22° C., or more or less), the alignment fixture(s) are removed at block 1214. At this point, the circuitry of the PCB may be tested, the solder joints may be inspected, and voids may be analyzed to determine whether they meet any predetermined criteria (e.g., IPC-A-610). By way of example, the process may result in the total size (by surface area) of all voids along the PCB not exceeding 25%, and/or any individual void not exceeding 10% of the total size.
The foregoing examples are not mutually exclusive, but may be implemented in various combinations to achieve unique advantages. As these and other variations and combinations of the features discussed above can be utilized without departing from the subject matter defined by the claims, the foregoing description of the embodiments should be taken by way of illustration rather than by way of limitation of the subject matter defined by the claims. In addition, the provision of the examples described herein, as well as clauses phrased as “such as,” “including” and the like, should not be interpreted as limiting the subject matter of the claims to the specific examples; rather, the examples are intended to illustrate only one of many possible embodiments. Further, the same reference numbers in different drawings can identify the same or similar elements.
