Analysis of PCB Impedance Control Accuracy

Initially, the control accuracy requirement of ±10% for PCB was proposed by the application of Direc Rambus type DRAM module (RIMM) with 800MHz frequency signal in the circuit, which is to ensure that the internal circuit of the computer host and switch realizes higher speed operation. Not only computer products equipped with RIMM, but also many electronic products also require circuits on the substrate to be well matched. The characteristic impedance control accuracy of the PCB boards used by some customers is not limited to the original ±15% or ±10. %, some impedance control accuracy requirements are increased to ±8% or even ±5%, which is indeed a big challenge for PCB manufacturers. This article focuses on how to meet the customer's strict impedance control accuracy requirements, hoping to be helpful to the PCB manufacturing industry.

Analysis of Impedance Control Accuracy Generally, it is easy for the transmission line system of multi-layer board to reach 60±10%Ω, but it is a little difficult to reach 75±5%Ω, or even 50±5%Ω, and the error is 5% even for technical specifications. It is also uncommon for applications with higher requirements, but there are still some customers who put forward a requirement of ±5% for impedance control accuracy.
The following is a board produced by our company, the requirements of the board: 4-layer board, the completed board thickness is 1.0±0.10mm, the board adopts FR4, and the customer has a specified laminated structure, see the figure below

The TOP layer has single-line impedance requirements, and the reference layer is the second layer. The single-line impedance line width W1 requires 12.0MIL, and the impedance requires 50±5%Ω (50±2.5Ω). Its structure is as follows:

How to meet the customer's strict impedance control accuracy requirements? Let's talk about how our company controls it.

Simulation calculation of PCB characteristic impedance For boards with impedance control requirements, at present, a common practice in PCB factories is to design some impedance samples at appropriate positions on the side of the PCB production imposition board. These impedance samples have the same layering and thickness as the PCB. Impedance line construction. Before designing the impedance sample, some impedance calculation software will be used to simulate the impedance to predict the impedance. Among them, the CITS test system and calculation software developed by the British POLAR company have been used by many PCB manufacturers since 1991, and have simple operation and powerful functional calculation capabilities. However, no matter how powerful the system is, its computing power and field solving tools for calculating impedance depend on the use of "ideal" materials, and there will always be a certain deviation between the simulated results and the actual measured impedance results. Therefore, it is particularly important to use software with relatively high calculation accuracy for more accurate simulation prediction when the customer's impedance control accuracy is required to be ±5%. To this end, we use the latest calculation software Polar SI8000K controlled impedance fast solver developed by the British POLAR company for simulation prediction. Due to customer requirements: in order to meet the impedance of 50±5%Ω, the PCB factory can make appropriate adjustments to the stack structure. The impedance line width cannot be adjusted. For this reason, the simulation results are as follows:

According to the above simulation results, it can be seen that in order to meet the customer's 50Ω impedance requirement, the thickness of the dielectric layer from the customer's original TOP layer to the second layer needs to be adjusted from 9 mil to 7 mil. Make adjustments accordingly. Combined with the wiring density of the inner layer circuit, it is adjusted to the following laminated structure:

The production process of PCB is controlled by a parallel light exposure machine. Because non-parallel light is a point light source, the emitted light is scattered light. Therefore, these light enter the photosensitive dry film or other liquid resist film through the film negative. Exposure at various angles, the pattern developed after exposure and the pattern on the negative film will have a certain deviation, and the parallel light is irradiated to the photosensitive dry film or other liquid resist film in the vertical direction for exposure, so , the exposed wire width on the photosensitive layer will be very close to the wire width on the film negative, so that a more accurate wire width can be obtained, thereby reducing the impact of this deviation on the impedance.

Thin copper foil is selected for the outer layer of base copper. Due to the rapid development of fine circuits, thin copper foil has been extensively developed and used in an all-round way. The thickness of copper foil has changed from 1OZ in the early years to 1/2OZ. 1/3OZ and 1/4OZ, even thinner such as 1/7OZ copper foil. Because the thinner copper foil thickness is conducive to the manufacture and control of the wire width and the integrity of the wire, it is beneficial to ensure the impedance control accuracy. Since the customer's requirement for the copper thickness of the outer layer is 1OZ, we choose 1/3OZ copper foil for the outer layer when laminating the four-layer board. Thickness requirements, which not only meet the requirements of the customer to complete the copper thickness on the surface, but also facilitate the control of the uniformity of the wire width during etching.

There are two heating methods: electric heating and steam heating, and our company uses a multi-layer vacuum press produced by Italian CEDAL company using ADARA technology. The copper foil surrounds the prepreg and the inner layer of the laminate layer by layer, and the copper foil is energized in the laminator to achieve the effect of heating, temperature distribution, and the temperature distribution of the entire laminate can reach 177±2°C , Due to the fast heating and uniform temperature distribution, the resin fluidity is relatively uniform during the lamination process, the flatness of the laminated board thickness can reach ±0.025mm, and the thickness of the interlayer dielectric layer is relatively uniform.

Production by whole-board electroplating In order to obtain wires of relatively uniform thickness and width to ensure that the impedance is within the specified tolerance range, the PCB is directly produced by full-board electroplating after perforation, in which the current density is appropriately reduced. After hole formation, it directly enters the whole plate electroplating. Under certain plating solution conditions, the entire plate making surface of the whole plate receives a uniform current density, so the copper thickness in the entire plate surface and in the hole is relatively uniform. It is beneficial to control the uniformity of copper thickness and wire width on the surface (because uneven copper thickness will bring disadvantages to etching uniformity), which is beneficial to control the characteristic impedance of the PCB and reduce its fluctuation.

Of course, in order to meet the customer's impedance control requirements of 50±5%Ω (50±2.5Ω), the etching circuit, silk screen green oil, etc. should also be controlled to ensure the uniformity of the wire width and the thickness of the green oil layer on the wire surface.

Impedance Measurements of PCBs Impedance measurements are typically done using Time Domain Reflectometry (TDR), which has become an established technique for measuring characteristic impedance on printed circuit boards. Impedance measurement is also very important for the characteristic impedance that requires an accuracy of ±5% for measuring impedance. It is necessary to ensure the correctness of the measurement, otherwise the board with qualified impedance will be mistakenly measured as unqualified.

The traceable impedance standard is used for calibration before measurement. Because TDR for impedance measurement is a high-precision RF measurement tool, during the measurement process, TDR measurement requires the same environment as the front-end and back-end DC conditions of the trace. Most impedance COUPONs are unterminated, so it is best to use a reference air line calibrated to a traceable standard. The impedance measurement error can be reduced by calibrating the TDR with a high precision load resistance.

Do not put your hand on the impedance COUPON during measurement. When you put your hand or finger on the impedance COUPON, the impedance structure on its surface changes, resulting in a drop in the measured impedance. Place your hand or finger on the impedance COUPON.

During the test, a fixed test fixture is used to fix the impedance COUPON to test the impedance. Generally, the impedance COUPON is often tested by placing the impedance COUPON directly on the workbench, which will affect the measurement results, because the workbench has its own insulation constant, and the impedance COUPON If it is in direct contact with the work surface, the obtained impedance test results will be low. Of course, it is acceptable for the case where the impedance control accuracy is not very strict, but it should be used when testing the characteristic impedance with an accuracy of ±5% similar to the measurement impedance. Use a fixed test fixture to test the impedance COUPON fixedly.

Check for RF cable and probe wear during measurement
The service life of RF cables and probes is limited, and users will wear them during use. Once the RF cables and probes are damaged, it will affect the impedance measurement results. Therefore, check the RF cables and probes for wear during measurement to ensure that the measurement is guaranteed. correctness.

Of course, in order to ensure the accuracy of the measurement, the mobile phone near the test area is required to be turned off during the measurement process, and the TDR impedance test probe is required to be in good contact with the impedance test COUPON during the measurement.

Results and discussion The following are the impedance test results of the board using the TDR test system. From the results, it can be seen that the impedance tested by the board is between 47.5∽52.5Ω, which fully meets the customer's 50±5%Ω (50±2.5Ω Ω) impedance requirements. Therefore, it can be seen that for the customer's impedance control accuracy requirements of ±8% or even ±5%, as long as the software with relatively high calculation accuracy is used for more accurate simulation prediction before production, combined with the results of the simulation prediction, the corresponding It is still achievable to make appropriate adjustments to some of the parameters in the production process, and to specially control the key processes in the production process.

references
(1) CITS500s tester from POLAR, UK

(2) CITS25 impedance calculation software from POLAR, UK

(3) Si8000K software, Polar Instruments.

(4) Lin Jindu, Characteristic Impedance and Electromagnetic Interference of PCB (Ⅱ), "Printed Circuit Information", 2000.10