Samtec has developed an innovative, yet easy-to-use alignment technique for ensuring peak connector performance of precision, high-frequency compression mount connectors in test and measurement applications. The following explains the trend we were seeing and outlines the solution we developed to the common alignment challenge that was occurring.
As data rates continue to push upward (224 Gbps PAM4) the need for precision, higher frequency compression mount test connectors, with bandwidths exceeding 90 GHz continues to increase as well. This is due, in part, to the known performance advantages of compression mounting when compared to solder-on connector solutions and the impacts of solder process variations on the PCB and connector structures.
Threaded, compression mount connectors — where the connector is compressed onto the PCB using mounting hardware — perform at higher frequencies and therefore are becoming commonplace in test applications. In addition, with the number of high-speed connections in PCBs increasing continually, compression mount connectors also provide a number of additional advantages:
- They are relatively compact, which means a number of them can be placed side by side on a PCB and take up much less real estate than traditionally needed.
- They can be placed anywhere on a PCB. This allows them to be placed close to the device the signal needs to reach.
- They can be reused.
- They can easily connect to either microstrip/CPWG or stripline traces in a PCB.
As frequency requirements increase, this inversely impacts the size of the signal pin and landing pad within an interconnect (including threaded compression mount products that we’re focused on within this article). This means, the higher the frequency then the smaller the signal pin and landing pad. Thus, making alignment much more difficult.
Precision alignment of the signal pin and landing pad is critical to peak connector performance. If misaligned, in certain cases it not only causes performance to decrease but may also cause the connection to fail completely (open circuit). But, how do you confirm precision alignment when the signal pin and landing pad are not visible when connectors are placed onto a PCB? How can an engineer easily and properly confirm alignment within their system, and therefore, have confidence the structure will perform as expected?
It is common for some to lean on mounting screws for solving misalignment. However, a factor that must be considered is that the outer diameter of the hardware screw, compared to the screw hole size itself, is not precise. Also, PCB manufacturability can be limited and tolerances can be much looser than connector assembly positional tolerances might allow. Below we see an example of mounting screws not perfectly aligned.
Even when care is taken during the assembly of the connector, as the screws are tightened to their final positions, forces on the connector can cause them to rotate. The following image provides an example (Shown: 1.35 mm, 90 GHz connector for both (a) and (b)). While the visual is magnified to clearly demonstrate misalignment, this can be extremely challenging to identify during assembly. It usually isn’t until performance degradation occurs and further examination is required that this becomes evident. Even when the misalignment becomes evident, correcting for precision alignment during board assembly can still be very difficult to achieve. Misalignment to the order shown in “(a)” will cause capacitive loading at the launch. The connector’s ground body is too close to the surface trace.
In addition, if the connector moves around, it is possible it may degrade or even destroy the landing pad on the PCB.
The below TDR chart illustrates that when measuring the same 1.35 mm connector noted above, an approximate 8 Ohm capacitive dip is introduced when the amount of misalignment shown in the image above occurs. When working with very wide bandwidths, even small misalignments can significantly impact the resulting performance. In addition, time lost on performance analysis, troubleshooting, and corrective actions can be extremely costly.
Upon examination of the same 1.35 mm connectors in the frequency domain, we see that better alignment results in improved return loss over a large frequency span:
There has been a new technique developed within the RF industry for achieving precision alignment when using threaded compression mount connectors in test and measurement applications. This new technique comes from Samtec, and could revolutionize the way these connectors are assembled onto a PCB due to ease of use. It’s straightforward and simple in execution, yet highly effective in achieving optimal connector performance.
The technique provides a visual guide; something that wasn’t possible previously since the alignment of the connector’s center pin to the landing pad is hidden as a connector is assembled onto a PCB.
The visual guide uses a combination of alignment grooves (or notches) and fiducials. Alignment grooves are milled into the foot of the compression mount connector. As PCB layout is being created, fiducials can be incorporated to match the location of these notches which makes it very easy to check the positional accuracy of the assembled connectors.
Since the notches align with fiducials on the PCB, any offset between the mounting hole location and the landing pad location is easily noticed; thereby, removing any doubt to assembly effects impacting subsequent performance measurements. Having notches aligned with the fiducial ensures the connector’s center pin is centered onto the PCB landing pad. This technique may be used for both microstrip/CPWG and stripline connectors.
Following is a recommended PCB layout, using a 1.35 mm microstrip connector, with fiducial marker:
The following shows 1.35 mm microstrip connector specifications with alignment grooves that coincide with the above recommended PCB layout:
When looking at the VSWR plot below of a perfectly aligned 1.35 mm connector that was assembled using the visual alignment process (e.g. matching the alignment grooves to fiducial markers on the PCB), we see excellent performance results; especially between 50 GHz to 80 GHz. VSWR is even lower than 1.2:1 at the connector’s cutoff frequency. Ten samples were taken for a thorough understanding of performance characteristics.
When threading the instrumentation test cables onto the compression mount connectors, anti-rotational tools may be used to hold the connectors in place. The recommended steps are:
- Secure the threaded compression mount connector to the board using the visual alignment features previously mentioned.
- Using the anti-rotational tools, thread the cable assembly connectors onto the board connectors. Use a torque wrench to turn the cable assembly connector, and anti-rotational tool to stabilize the board connector, ensuring minimal movement occurs when finalizing the assembly process.
The need for precision, high-frequency compression mount test connectors continues to increase as data rates approach 224 Gbps PAM4. However, proper alignment of the connector’s center pin to the landing pad is essential for optimal performance and for the structure to perform as expected. This new visual alignment technique, which uses a connector body with alignment grooves to easily visually align with fiducials on a PCB, is being well adopted within the industry and will likely continue to grow in popularity.
Alignment grooves discussed above are being incorporated into the next iteration of Samtec’s 1.35 mm, 1.85 mm, 2.40 mm, and 2.92 mm board-level connectors. Development is in the final stages, and connectors with alignment grooves will be fully released and available for purchase in Q2 2023. In the meantime, please feel free to reach out to Samtec’s RF Group for additional information.
Samtec RF blogs that may be of interest include:
- Wideband RF Launches: Literally Everything You Need To Know
- New 1.35 mm Precision RF Products To 90 GHz
- RF Connector Tree: From MCX To 110 GHz
- Click here to view Samtec’s complete RF product line.
(This article was written by Laura Kachnavage at Samtec)
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