COMOSS Carried out the USB4 GEN3 240W 1-meter Certification, USB Coaxial Cable – Defect Analysis

For the current suppliers of USB Type C cable products in the world, one of the bigger issues with Type C is that the speed and stability will decrease in long-distance transmissions. In order to achieve transmission speed and stability required by the USB4 GEN3 certification, as well as the requirement for a length of 1 meter, it is necessary to introduce coaxial production process into the cable production process in order to enhance the transmission quality.
 

However, at present, USB Type C cables are primarily made of twisted pairs, resulting in high-frequency transmission characteristics that cannot meet the Association's specifications, and high-powered 240W transmission is also prone to voltage drop problems.
 

For USB4 GEN3, there are still some problems that the required certification quality cannot be achieved due to signal loss produced by the twisted pair process. Problems including cold welding and empty welding cannot be effectively avoided. Therefore, it is an imperative trend for us to introduce coaxial cable products.
 

Moreover, in addition to welding, the existing cable production processes such as cutting, stripping and splitting need to be improved in order to meet the increasingly stringent USB4 GEN3 certification requirements.
 

In light of the problems above, it is imperative for COMOSS to carry out USB4 certification and coaxial production process. Therefore, we cooperated with Allion, an impartial third-party authorized by the USB-IF Association, to complete the USB4 GEN3 240W 1-meter certification.

With the update of the USB specification to USB 3.2 and USB4, the transfer rate of the transmission cable has been increased to 10Gbps and 20Gbps, which is traditional in response to the consideration of high transmission efficiency, low loss, and low radiation impact Twisted Pair's design has been unable to meet the demand, in addition to rigorous processing methods, more manufacturers are using coaxial wire as a new generation of Type-C transmission channel. 

Figure 1: Coaxial Cable
 

The regular coaxial cable composition structure is divided into 4 layers, from the center to the outside are the inner conductor, insulation layer, shielding layer, outer quilt, as the outer quilt of the multi-coaxial cable is mostly based on copper foil myra, plus the braid layer is double shielding, which can bring better anti-interference characteristics to each coaxial cable. In addition, due to the good control of the coaxial cable to the ground (braid) distance, the performance of high-frequency transmission is better in terms of conduction characteristics (loss, differential common-mode conversion) or crosstalk. 
 

Figure 2: Coaxial Cable structure
 

In terms of twisted pair, each copper wire itself has a trace inductance. When the two copper wires are close, a charge effect (capacitance) is generated between each other, and the stability of the characteristic impedance (continuity) depends on whether the inductor and the distribution of the capacitor (L0, C0) are uniform and stable. Therefore, with reference to the following relationship, when determining the characteristic impedance, you can see that the twisted pair in addition to the core diameter (d).  And in addition to the dielectric constant (ε) of the fixed insulation layer, how to control the distance between the two wires (D) is the most important. 

 

Because of the design structure, the consistency of the control inner and outer conductors (D &d) is relatively speaking, the control of the characteristic impedance will be more stable than the twisted pair, and the continuous and smooth 

At the time of design, the type-C Cable Assembly characteristic impedance has been different from the Connector Differential 85ohm and raw Cable Differential 90ohm, so more attention needs to be paid to the impedance control of each component. Once the selected material is not matched enough or produces too many discontinuities, it may cause ILfit, IRL's Fail, and even more affect the performance of Integral Crosstalk. 

In the process of drawing the wire, the insulation layer and the cladding of the braid layer have determined the characteristic impedance of the bare wire and the quality of the channel characteristics. It is worth to be aware of in Allion's laboratory we found in some occasional insufficient cases that the benchmark value of the customer's wire material characteristic impedance has deviated from the 90 Ohm specified by USB, which will cause the signal to be transmitted in the mismatched channel, resulting in a variety of high-frequency items Fail.

During the process of certification, we sometimes encounter customers involve Paddle Card designers in Debug. Both the manufacturer and the Layout designer believe they work on the project according to the requirements on the design drawings. Then, why does the finished product Fail?
 

The reason for causing the problem is that after receiving the case, the Layout side designs Trace and and Pad according to the impedance set by the specification. The wire manufacturer obtains the Paddle Card after the design was completed and solders & glues the bare wire. Then the problem suddenly comes. The original characteristic impedance falls down because of the capacitance effect after processing. This then causes an increase in the impedance discontinuity (as shown in the following figure). The gap happens because the manufacturer and Layout designer do not discuss the subsequent processing and the corresponding impact when the contract was issued. The efficient way to do this is to estimate the difference in the characteristic impedance after processing, and first improve the design of the characteristic impedance when layouting, so that the processed waveform falls into the strike zone.
 

Figure 3: Paddle Card is different before and after soldering

 

If the negative effects caused by the thread extraction process are removed , when analyzing the unqualified samples of the certification case, Allen found that the vast majority of Fail will occur in the process of processing, and the common bad causes are roughly divided into 3 types:

1.    Poor soldering

Common soldering defects include empty (floating) soldering, loose wire core, core fracture, excessive soldering alignment error, tin wire solder balls, or tin oil residues, etc., which may cause insufficient characteristics. For example, when the core is loose, the shape of the surface area and the tin eating situation are not easy to control, the stability of the characteristic impedance is insufficient, and the soldering displacement may cause the patch between the signals to be too small, which in turn affects crosstalk. In addition to increasing the possibility of crosstalk, the generation of tin wire and solder balls will cause the parasitic capacitance effect generated by Stub, which should be avoided when soldering. 

Figure 4: Examples of poor soldering
 

Wire damage is divided into core damage or external damage. In mass production, we usually choose Hot Bar Soldering Machine when soldering. In some cases, we use high-frequency soldering that the force of stripping must be paid attention to. We must also pay attention to the heating time and clamping may cause external deformation or holes in hot bar displacement stroke, temperature adjustment.
 

Before mass production, the length of each layer of the bare wire should be stripped. The characteristic impedance of a bare wire is determined by the material and thickness of each solid layer, and the characteristics and difficulty of processing in the later mass production should be taken into account when removing each layer. Once the stripping length definition is completed, whether it is the pretreatment of the bare wire or the cable assembled with the connector as a finished cable, it should be complied with together. If the stripping wire definition is 1.5mm for each level, it should be as consistent as possible when managing or soldering, and do not change the outer quilt and shield length due to processing, because these changes even if the difference of 0.5mm may directly affect the final high-frequency result. 
 

 

Figure 5: Divergence of impedance discontinuities caused by stripping lengths

Excepting the Full-Featured board-side connector of Type C can be USB4 Gen3 certified, all certifications of USB 3.2 Connectors and Plug are excluding high-frequency testing. Therefore, choosing the right connector for product production will be an important part for manufacturers in the development stage. 
 

In the product development stage, the first question that manufacturers face is, how to choose the right material and parts for assembly? The composition of the finished product line is roughly divided into connectors, Paddle card, Raw Cable. Taking USB Type C as an example, the association has specifically listed the reference loss (dB/m) of Coaxial/Twisted in the specification , the manufacturer can use such example data to check the supplier's bare wire. 

Table 1: Loss Example for Twisted Pair Cable (Source: USB Type-C Spec R2.1).
 

Table 2: Loss Example for Coaxial Cable (Source: USB Type-C Spec R2.1).

 

In the assembly, in addition to paying attention to the design of Paddle Card Impedance, it is also necessary to define the type of glue covered in the processing area, to consider the characteristics of the glue and if in he mass production it can meet the processing convenience, moisture content, curing time, etc. Most products use UV glue (UV Resin). All these variables for the characteristic impedance need to be considered in the PCB Layout at the beginning of the product design. 

Figure 6: Effects of the characteristic impedance when the glue is selected