Last week, I spoke to a friend who works on switches; for their new project involving 25G ports, they’d opted for solder-type SFP cages. When the boards came back, they couldn’t get them to work—the eye diagrams wouldn’t open.
It took a long time to investigate before they realised the problem was residual solder. During the PCB design phase, they hadn’t allowed for back drilling; once the boards were manufactured and the cages soldered on, the through-holes were filled with solder, and it was too late to drill them—you simply can’t drill after soldering.
In fact, it’s not that soldered cages cannot be back-drilled at all; it’s simply that back-drilling must be carried out during the PCB manufacturing stage. Once soldering is complete, it is indeed impossible to rectify the issue. Furthermore, even if back-drilling is provided for during the PCB stage, the thick leads of soldered cages make it more difficult to achieve proper solder penetration during the soldering process, which leads to a lower yield rate. This is why most 25G+ designs still opt for crimp-type connectors—it is not that soldered connections are physically impossible, but rather that production yield rates are too low and the cost of rework is too high, making it uneconomical overall.
Let’s move on to repairs.
Welded cages are easy to attach, but should a problem arise, the cost of removing the welds is exorbitant—with so many thick pins, forcing them off will almost inevitably damage the PCB through-holes. Ultimately, the entire board must be scrapped.
Crimped cages can be removed and refitted using a specialised crimping tool. As components often need to be replaced during the R&D phase, this advantage is particularly evident in practical applications. When we were building prototypes previously, we had to replace the SFP cages several times due to design revisions; had they been soldered in place, those boards would have been scrapped long ago.
In terms of EMI, the soldered type is indeed slightly better.
The metallurgical bond formed by solder provides lower ground impedance than mechanical contact; in theory, therefore, soldered shielding offers better performance. A well-designed crimped connection can also meet Class B requirements, but in high-reliability applications there is a risk that the ground impedance of the mechanical contact may drift under prolonged vibration and temperature fluctuations, whereas soldered connections do not suffer from this issue.
A few points that are easily overlooked in practice:
It is essential to keep a close eye on the tolerance for crimping hole diameters; many PCB manufacturers may not be able to consistently meet the ±0.05 mm requirement. Remember to confirm this clearly with the manufacturer before dispatching the boards.
Crimping cages must be handled using hydraulic or pneumatic fixtures; under no circumstances should they be tapped by hand. Manual tapping can lead to inconsistent deformation of the ‘fish-eye’ pins, resulting in loose connections on some pins. This may subsequently cause occasional packet loss or intermittent signal issues, which are extremely difficult to troubleshoot.
Soldered cages have relatively thick pins; the stencil design for through-hole reflow soldering must be determined based on the pin diameter, pad size and stencil thickness—a one-size-fits-all approach is not acceptable. It is recommended to refer to the stencil specifications recommended in the component datasheet.
Ground vias should be densely spaced, with a pitch of no more than 5 mm, and should connect directly to the ground plane; avoid using fine traces.
As for how to choose, my personal experience is as follows:
For speeds of 25G and above, crimp-type connectors are the safer option. For 10G and below, either type is acceptable, depending on the production line’s capabilities. If components are frequently swapped during the R&D phase, crimp-type connectors are less hassle. If the production line only has wave soldering and no crimping equipment, soldered connectors are the more practical choice.
Wohu offers both crimped and soldered SFP cages, covering SFP, SFP+ and SFP28. Please check the official website for specific models.
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