Abstract: the data transmission speed of the crimped high-speed differential connector is often limited by the short pile effect caused by the unused through-hole part at the termination. This paper will provide a practical solution for how to implement reverse drilling in the application of differential high-speed connector. Ernis ermet ZD connector is a high-speed crimped connector specially designed for differential signal transmission, and each pair of signals is shielded by a special L-shaped metal sheet. Signal pin and
The data transmission speed of the crimped high-speed differential connector is often limited by the short pile effect caused by the through-hole part not used at the termination. Through reverse drilling, the transmission rate of ermet ZD can be increased from 6 Gbps to 10 Gbps. This paper will provide a practical solution for how to implement reverse drilling in the application of differential high-speed connector.
Ermet ZD? High speed interconnection solution
Ernis ermet ZD connector is a high-speed crimped connector specially designed for differential signal transmission, and each pair of signals is shielded by a special L-shaped metal sheet. The structure of the signal pins and terminals has been minimized and optimized to reduce discontinuities. A wide distance of 2.5mm is deliberately reserved between signal columns to provide isolation between signal pairs and the best combination of reverse pad / routing width. Wide enough routing not only reduces the skin effect loss, but also provides a larger reverse pad, thus further reducing the capacitance between pads. In addition, two pairs of differential signals are arranged between signal columns to increase the wiring density and reduce the required number of circuit board layers. The above advanced design concept enables ermet ZD high-speed connector to transmit up to 6 Gbps of data without any help of active components.
bottleneck
Ideally, an electronic connector should have the least impact on the interconnection system. In other words, when the line passes through the connector, it has no impact on the system, and the whole system operates as usual. However, the ideal connector does not exist in reality. In fact, each connector has a variety of effects on the system. Generally, the signal at the connector interface is the most incoherent, and the connector and the required through-hole are the structures with the most sudden changes in the waveguide. Incoherence also leads to the reflection of the sensing wave, resulting in standing wave. So the connector becomes the bottleneck of the whole system.
Integrated circuit technology has made great progress in the realization of high-speed data transmission, so how to interconnect highly integrated subsystems becomes a key factor in the design of future systems.
At present, SMT is not the best choice in the termination of backplane system, because it is difficult to ensure that the large hot backplane can be uniformly heated and welded. The vast majority of backplane connectors use crimp termination, because its interface reliability and the ability to repair a single damaged pin on a dense large backplane are proved by practice.
Short pile effect and its mechanism
However, as with all crimp connectors fitted to plated through holes in a circuit board, plated through holes can act as parasitic components or incoherence in the transmission line. In order to make up for this defect, the most critical routing must be placed on the surface closest to the bottom of the circuit board. Another solution is to drill back through-hole pipe to reduce through-hole root, so as to reduce through-hole resonance.
This resonance phenomenon is called "short pile effect" in the terminology of signal emission. When the wiring enters the top layer of the circuit board from the through-hole and then goes through the through-hole along the second layer, a section of unused through-hole is formed on the board, and the unused through-hole is like the open-ended through-hole root.
When the signal enters such through-hole and runs to the strip line layer with equivalent impedance, part of the signal will continue to be transmitted to the unused through-hole, and the energy will reach the end of the through-hole, encounter an open circuit, and then reflect to the source or convert to radiation. In the unused through-hole part, the reflection is usually more intense, because the shorter transmission path in the through-hole cannot attenuate the reflection, resulting in the reflection bouncing back and forth many times until it disappears. The resonance effect is formed and the impedance is reduced effectively.
Factors influencing short pile effect
When unused vias form a via root depends on the criticality on the transmission line and the signal rise time. The critical length is about 37.5mm for 1000 PS rise time, 3.75mm for 100 PS rise time, and 1.9mm for 50 PS rise time.
In practical application, when the transmission speed is 3.125gbps and the plate thickness is less than 5mm, the short pile effect of through-hole is not obvious. When the speed reaches 10Gbps, the effect of short pile has to be considered.
Therefore, the thickness of the back plate becomes one of the main factors, because it determines the severity of the short pile effect of plated through hole between the signal pad and the plate base. The through-hole root is like a high-frequency filter, which frequency is filtered mainly depends on the length of the through-hole root. Therefore, the thickness of the circuit board must be controlled as thin as possible.

Methods to reduce the effect of short piles
When the frequency increases, the short pile effect is also strengthened, so it is necessary to place the highest speed signal in the plate layer with the lowest short pile effect.
Reverse drilling of through hole in back plate is a common technology in back plate design to reduce short pile effect. Reverse drilling, also known as "reaming", is to drill the through-hole root or unused part of the plated through-hole length. During reverse drilling, the drilling depth of each through hole can be adjusted according to the individual length of the through hole root. However, there will always be some residual through-hole roots. Through the length of? Short through-hole, the electrical performance of the circuit board becomes thinner, so reverse drilling effectively reduces the capacitance effect of plated through-hole.
Other techniques for controlling the effect of through-hole short piles include the use of blind or buried holes. Blind holes do not penetrate the whole stack, while buried holes are embedded on the circuit board but not exposed to the top and bottom of the circuit board. These through holes are formed by a series of laminated plates in order to achieve the required through hole conversion configuration. The manufacturing cost of blind hole is more expensive than reverse drilling, while the buried hole can only be used for surface mount connector. For crimping connector, reverse drilling on plated through hole is the only feasible solution.
Practical operation guide for reverse drilling
Reverse drilling is an additional step in PCB manufacturing process, which naturally increases the cost. The cost increase usually accounts for 5-10% of the total PCB production cost. Reverse drilling is a secondary drilling operation, which removes unused through holes from the lower layer of signal routing on standard plated through holes.
There are many advantages in reverse drilling. Because part of the coating is drilled, the capacitance and through-hole root are correspondingly reduced. Although the wiring of reverse drilling is not effective without micro through hole or blind hole, it provides the same electrical performance as blind hole, but its manufacturing cost is lower. Crimp type connectors require at least 2.5 mm of plated tube length, which significantly reduces through-hole capacitance.
Manufacturing process of reverse drilling in PCB
The process of reverse drilling on Multilayer PCB is similar to the standard PCB production process. Secondary back drilling is required before the solder mask is applied and after plating and etching. Generally, CNC drilling equipment with drilling depth control is used to operate and realize process automation.
Reverse drilling diameter
An important number of reverse drilling is the diameter of secondary drilling, which must be larger than the diameter of primary drilling to ensure that all plating metal is removed. It is also important to reduce this diameter in order to avoid unnecessarily narrowing the routing channel leading to a reduction in the distance from the pin hole to the wire. Because the PCB drilling diameter of ermet ZD is 0.70mm, the recommended reverse drilling diameter is 0.80mm. The selection of this diameter takes into account the various tolerances of the reverse drilling process to ensure that all plating metal is removed and the reverse drilling diameter is kept to a minimum.
Depth of reverse drilling
The application of reverse drilling must balance production cost and electrical performance. An optimized manufacturing process can achieve a variable of 3 Sigma nominal depth ± 0.13 mm. This consists of two parts: mechanical depth and hierarchical position. The nominal depth shall be at least 0.2 mm before connecting to the last layer.
In order to reduce the production cost of the reverse drilling process, it is better to form a zone with several layers, each zone has a common reverse drilling depth. In order to simplify the production process, it is more practical to reduce the depth of reverse drilling to three zones.
Reverse drilling option: with / without pad
The pad on the plated through hole will increase the capacitance of the through hole. It is recommended to remove all unused pads on the through hole. However, it depends on the PCB manufacturers ability. Through holes also need some pads to fix the plating tube on the PCB. Therefore, the PCB manufacturer must be consulted before leaving the fixed pads.
Confirmation of reverse drilling process
In order to measure the depth of reverse drilling during drilling, a special reverse drilling sample is needed on PCB. There are different layer traces around the target layer on the reverse drilling sample. Each side of the trace is a plated through hole, and the center is a reverse hole. When the reverse drilling is deeper, more layers are broken. Because it is only possible to electrically measure whether a line is open or closed, it is necessary to stop before reaching the non cutting layer (MNC). MNC is also the layer with the highest signal speed. If it stops after MNC (MNC + 1 layer), it may damage the MNC layer but still maintain the electrical connection.
Short pile effect of long pin
The short pile effect is not only found in unused through holes, but also in pins longer than PCB thickness, especially in daughter card applications where PCB is much thinner than backplane. Therefore, ermet ZD female connector is specially designed with a short pin of 1.8mm to reduce the short pin effect. Ermet ZD male connector has two different pin lengths: 1.8mm and 3.7mm. For thinner PCB, remember to use short pin connector.
Ermet ZD reverse drilling results
In general, ermet ZD connector can be used in applications with data transmission speed up to 6 Gbps. However, with the back drilling and signal conditioning circuit, signals up to 10 Gbps can be transmitted.
As shown in the eye chart in Fig. 2, when FR4 plate and 190 mm wiring length and 0.25 mm line width are used to transmit 10 Gbps data, the eye chart opening is quite large. The device was tested without any signal conditioning circuits.
conclusion
Ermet ZD connector is specially designed to transmit high-speed differential signals. The L-shaped shield isolates each pair of differential signals. Therefore, it can not only ensure excellent signal integrity, but also withstand multiple pluggings. After the improvement of reverse drilling technology, this series of products have been proved to be able to transmit data up to 10Gbps

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