Bead probe technology: Difference between revisions
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{{Short description|Technique used for in-circuit testing}} |
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'''Bead Probe Technology''' (BPT) gives the [[electronics industry]] a new alternative in providing electrical access (called “node access”) to [[Printed Circuit Board]] (PCB) circuitry needed to perform In-Circuit Testing (ICT). Node access is critical for components on the PCB to be electrically connected to the [[electronic test equipment]] that is performing the In-Circuit Test. The technology combines a new test point placement method with a well-known probing technique, as described below. |
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[[File:BEAD PROBE.JPG|thumb|260px|Side and end view showing a solder bead providing access to a trace located under the solder mask]] |
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==What is Bead Probe?== |
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'''Bead probe technology''' ('''BPT''') is technique used to provide electrical access (called “nodal access”) to [[printed circuit board]] (PCB) circuitry for performing [[In-circuit test| in-circuit testing (ICT)]].<ref>{{Cite web |url=https://www.keysight.com/main/redirector.jspx?action=ref&cname=EDITORIAL&ckey=2713970&cc=US&lc=eng |title=Benefits of Keysight Bead Probe Technology |date= |website=Keysight |archive-url= |archive-date=| access-date=September 16, 2018}}</ref><ref>{{Cite web |last=Keysight |title=In-circuit Test Systems |url=https://www.keysight.com/us/en/products/in-circuit-test-systems.html |access-date=2022-05-07 |website=Keysight |language=en-US}}</ref> It makes use of small beads of [[solder]] placed onto the board's [[Printed circuit board#Patterning|trace]]s to allow measuring and controlling of the signals using a test probe. This permits test access to boards on which standard ICT test pads are not feasible due to space constraints. |
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Bead probe is a probing method used to connect electronic test equipment to the [[device under test]] (DUT). It complements traditional probing solutions that use a sharp [[test probe]] to connect the test equipment to the DUT. The BPT primarily targets high-speed and high-density printed circuit boards which generally have insufficient conductive locations (test points) on the DUT to be connected to a test probe. |
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==Description== |
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Bead Probes are made from a very small “beads” of solder that fit atop of a [[printed circuit |
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[[File:waygood.jpg|thumb|right|260px|Side view of a PCB showing a solder bead and test probe.]] |
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trace]] as shown in Figure 1. They are constructed so as to have no effect (ideally) on the width of a trace, indeed perturbing the local geometry of the trace in only the Z dimension. Beads are only a few mils in height, enough to clear the surrounding soldermask. The bead will be roughly [[elliptical]] in shape and may be 15-25 mils long. |
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Bead probe technology is a probing method used to connect electronic test equipment to the [[device under test]] (DUT) within a [[In-circuit test#Bed of nails tester|bed of nails fixture]]. The technique was first used in the 1990s<ref>Surface Mount & Mixed Technology PCB Design Guidelines by David Boswell. Page 28 {{ISBN|1-872422-01-2}}</ref> and originally given the name “Waygood Bump” after one of the main proponents, Rex Waygood. They are also commonly referred to as solder bumps.<ref>Surface Mount Technology – Principles and Practices 2nd Edition by Ray. P Prasad Page 332 {{ISBN|0-412-12921-3}}</ref> Bead probes were designed for when less than 30 [[Thou (length)|mil]] is available for test probe points on the PCB. They are used with standard ICT spring-loaded [[test probe#Pogo pins|test probe]]s to connect the test equipment to the DUT. |
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Figure 1 show a graphic representation of the bead on the trace from both end and side views. |
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==Construction== |
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[[Image:BEAD PROBE.JPG]] |
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[[File:Bead probes on PCIe lanes.jpg|thumb|right|260px|Bead probes on a [[PCIe]] lane.]] |
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Bead probes are made from a very small "beads" of solder that fit atop of the PCB traces. They are manufactured using the same techniques as other solder features. Construction requires a hole to be opened in the [[solder mask]], exposing the copper trace. This hole is sized to precisely control the amount of metal that forms the bead. [[Solder paste]] is applied to the location and [[Reflow soldering|reflowed]]. During reflow, solder flows and is drawn to the copper trace. Surface tension causes the bead to have a curved surface and rise above the solder mask, where it solidifies into a Bead Probe. The bead will be roughly [[wiktionary:obround|obround]] in shape and may be 15-25 mils long. A properly constructed bead is the same width as the trace and just enough to clear the surrounding solder mask. The bead is then accessible for testing using a probe with a flat end, which can help compensate for the tolerance build up in the test fixture and PCB. |
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==How Bead Probe created?== |
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==Advantages== |
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Bead Probes are constructed with [[solder paste]] and [[solder reflow]] technology. The solder paste bead is placed on the PCB after opening up the solder mask and exposing the copper trace. Solder paste is applied along the trace and reflowed. The resulting solder-beads are formed as a test point sitting on the PCB traces. It's than can be probed using traditional [[In_circuit_test#Bed_of_nails_tester|bed of nails]] fixtures that have been fitted with flat-headed probes. |
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Bead probe can be used in circuits where the pin-pitch is too fine to allow standard test pads. This is becoming more common as pin pitches continue to reduce, particularly in embedded devices. Typically bead probe widths are the width of the PCB traces with a length of about three times this. This allows a high degree of flexibility in their positioning, and can in some cases be applied retrospectively to existing layouts. Because of their small size, bead probes do not affect the signal quality of the signals transferring within the PCB trace.<ref>A New Probing Technique for High-Speed/High-Density Printed Circuit Boards by Kenneth P. Parker of Agilent Technologies</ref><ref>Applying a New In-Circuit Probing Technique for High-Speed/High Density Printed Circuit Boards to a Real-Life Product by Chris Jacobsen and Kevin Wible of Agilent Technologies</ref> This is especially useful in high speed input/output (HSIO) interconnects, where a standard test pad would interfere with the signal. |
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==Disadvantages== |
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Figure 2 to 4 show the location of typical bead probes being formed after the reflowed and probed successfully. |
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* The soldering process that forms the bead probe leaves a coating of [[Flux (metallurgy)|flux]]. Depending on the manufacturing process used, this flux can have varying levels of hardness. Flux with a waxy hardness can reduce the deformation force from the bead, preventing proper contact with the test probe during the first pass contact. This becomes less of an issue on subsequent contacts as the flux is displaced. Test probes with serrated ends of an appropriate size can also aid in measuring bead probes where flux is an issue. |
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* Bead probes require the trace being tested to be located on the [[surface]]. This makes it unsuitable for testing high-density boards with many obscured or internal traces and buried [[Via (electronics)|via]]s. |
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[[Image:Typical Bead Probe 01.JPG]] |
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==Alternatives== |
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[[Image:Typical Bead Probe 02.JPG]] |
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* [[Boundary scan]] integrates test components into the [[integrated circuit]]s (ICs) mounted on the board, giving the ability to read or drive the ICs' pins. This allows for testing of interconnects for which physical access is not an option, such as [[Ball grid array|BGA]] components or signal routes sandwiched between plane layers. A boundary scan controller uses four or more dedicated pins on the board to control test cells [[Serial communication|serially]] and receive the measured values. It has the disadvantage of needing board infrastructure to support boundary scan. |
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* Test Access Component (TAC) uses a device such as a [[Surface-mount technology|0201]] as a target for a large probe as in the solder bump examples. The advantage of this technique is that it provides two target points at each end of the package. The disadvantage of this technique is it can add process and cost to the PCB.<ref>TEST ACCESS COMPONENT FOR AUTOMATIC TESTING OF CIRCUIT ASSEMBLIES United States Patent Application 20100207651</ref> |
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==Benefit== |
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* A technique has been described<ref>VAUCHER, C., Analog/Digital Testing of Loaded Boards Without Dedicated Test Points, Proceedings of the International Test Conference, IEEE 1996, pp. 325-32.</ref> which opens up windows in the solder mask to create test points located directly on PCB tracks. This technique uses a [[conductive rubber]] tipped probe to contact the test point which could have a conductive [[HASL|Hot Air Solder Levelling (HASL)]] finish. |
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The benefit of using bead probes is in the small size of the beads. With widths of just the thickness of the PCB traces and lengths of about three times the trace width, the beads are sometimes only visible using magnifying glasses. While traditional test points require additional board real estate, the small size of the bead probes allow them to be placed anywhere on the PCB trace, providing direct access to board circuitry without the need to route dedicated test circuits. This increases design flexibility, speeds up the layout process and makes it possible to add test points without changing the layout. In addition, because of their small size, bead probes do not affect the signal quality of the signals transferring within the PCB trace, even with high speed signals that are common in today’s products. Thus the bead probe methodology is ideal for the testing of high-speed, high-density PCB assemblies. |
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==References== |
==References== |
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{{Reflist}} |
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#[http://www.home.agilent.com/agilent/editorial.jspx?action=download&cc=US&lc=eng&ckey=873640&id=873640 www.agilent.com/see/beadprobe] |
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#A New Probing Technique for High-Speed/High-Density Printed Circuit Boards by Kenneth P. Parker of Agilent Technologies |
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#Applying a New In-Circuit Probing Technique for High-Speed/High Density Printed Circuit Boards to a Real-Life Product by Chris Jacobsen and Kevin Wible of Agilent Technologies |
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#Implementation of Solder-bead Probing in High Volume Manufacturing by Madhavan (Mady) Doraiswamy and James J (JJ) Grealish of Intel Corporation |
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[[Category: |
[[Category:Printed circuit board manufacturing]] |
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[[Category:Hardware testing]] |
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Latest revision as of 08:10, 25 June 2024
Bead probe technology (BPT) is technique used to provide electrical access (called “nodal access”) to printed circuit board (PCB) circuitry for performing in-circuit testing (ICT).[1][2] It makes use of small beads of solder placed onto the board's traces to allow measuring and controlling of the signals using a test probe. This permits test access to boards on which standard ICT test pads are not feasible due to space constraints.
Description
[edit]
Bead probe technology is a probing method used to connect electronic test equipment to the device under test (DUT) within a bed of nails fixture. The technique was first used in the 1990s[3] and originally given the name “Waygood Bump” after one of the main proponents, Rex Waygood. They are also commonly referred to as solder bumps.[4] Bead probes were designed for when less than 30 mil is available for test probe points on the PCB. They are used with standard ICT spring-loaded test probes to connect the test equipment to the DUT.
Construction
[edit]
Bead probes are made from a very small "beads" of solder that fit atop of the PCB traces. They are manufactured using the same techniques as other solder features. Construction requires a hole to be opened in the solder mask, exposing the copper trace. This hole is sized to precisely control the amount of metal that forms the bead. Solder paste is applied to the location and reflowed. During reflow, solder flows and is drawn to the copper trace. Surface tension causes the bead to have a curved surface and rise above the solder mask, where it solidifies into a Bead Probe. The bead will be roughly obround in shape and may be 15-25 mils long. A properly constructed bead is the same width as the trace and just enough to clear the surrounding solder mask. The bead is then accessible for testing using a probe with a flat end, which can help compensate for the tolerance build up in the test fixture and PCB.
Advantages
[edit]Bead probe can be used in circuits where the pin-pitch is too fine to allow standard test pads. This is becoming more common as pin pitches continue to reduce, particularly in embedded devices. Typically bead probe widths are the width of the PCB traces with a length of about three times this. This allows a high degree of flexibility in their positioning, and can in some cases be applied retrospectively to existing layouts. Because of their small size, bead probes do not affect the signal quality of the signals transferring within the PCB trace.[5][6] This is especially useful in high speed input/output (HSIO) interconnects, where a standard test pad would interfere with the signal.
Disadvantages
[edit]- The soldering process that forms the bead probe leaves a coating of flux. Depending on the manufacturing process used, this flux can have varying levels of hardness. Flux with a waxy hardness can reduce the deformation force from the bead, preventing proper contact with the test probe during the first pass contact. This becomes less of an issue on subsequent contacts as the flux is displaced. Test probes with serrated ends of an appropriate size can also aid in measuring bead probes where flux is an issue.
- Bead probes require the trace being tested to be located on the surface. This makes it unsuitable for testing high-density boards with many obscured or internal traces and buried vias.
Alternatives
[edit]- Boundary scan integrates test components into the integrated circuits (ICs) mounted on the board, giving the ability to read or drive the ICs' pins. This allows for testing of interconnects for which physical access is not an option, such as BGA components or signal routes sandwiched between plane layers. A boundary scan controller uses four or more dedicated pins on the board to control test cells serially and receive the measured values. It has the disadvantage of needing board infrastructure to support boundary scan.
- Test Access Component (TAC) uses a device such as a 0201 as a target for a large probe as in the solder bump examples. The advantage of this technique is that it provides two target points at each end of the package. The disadvantage of this technique is it can add process and cost to the PCB.[7]
- A technique has been described[8] which opens up windows in the solder mask to create test points located directly on PCB tracks. This technique uses a conductive rubber tipped probe to contact the test point which could have a conductive Hot Air Solder Levelling (HASL) finish.
References
[edit]- ^ "Benefits of Keysight Bead Probe Technology". Keysight. Retrieved September 16, 2018.
- ^ Keysight. "In-circuit Test Systems". Keysight. Retrieved 2022-05-07.
- ^ Surface Mount & Mixed Technology PCB Design Guidelines by David Boswell. Page 28 ISBN 1-872422-01-2
- ^ Surface Mount Technology – Principles and Practices 2nd Edition by Ray. P Prasad Page 332 ISBN 0-412-12921-3
- ^ A New Probing Technique for High-Speed/High-Density Printed Circuit Boards by Kenneth P. Parker of Agilent Technologies
- ^ Applying a New In-Circuit Probing Technique for High-Speed/High Density Printed Circuit Boards to a Real-Life Product by Chris Jacobsen and Kevin Wible of Agilent Technologies
- ^ TEST ACCESS COMPONENT FOR AUTOMATIC TESTING OF CIRCUIT ASSEMBLIES United States Patent Application 20100207651
- ^ VAUCHER, C., Analog/Digital Testing of Loaded Boards Without Dedicated Test Points, Proceedings of the International Test Conference, IEEE 1996, pp. 325-32.