PCB Tech - About PCB circuit board impedance test, some things have to be said

About PCB circuit board impedance test, some things have to be said

After the PCB circuit board has completed impedance control according to the customer's drawing requirements, it is also necessary to use the test COUPON to test to see if the characteristic impedance of the board meets the design requirements. Let professional PCB manufacturer introduce you to those things about PCB circuit board impedance testing:

1. Definition of characteristic impedance and impedance control

1.1. Characteristic impedance refers to the resistance of high-frequency signals or electromagnetic waves during the transmission of electronic equipment signal transmission lines, which is the characteristic impedance. Its symbol is Zo, referred to as impedance.

2. Impedance control means that customers have requirements for the control of the characteristic impedance of the board, and after calculating according to the impedance calculation formula, measures must be taken to control the impedance on the PCB board so that it is within a certain range (tolerance).

Impedance test range: 50-150Ω; single line: 50Ω/75Ω; paired line: 90Ω/100Ω/125Ω. The current impedance control tolerance is within 10% or even 5%, which is quite difficult.

PCB board

2. Introduction to impedance test COUPON

2.1. Benefits of using test COUPON:

(1) It is a test method that is easy to test and control impedance;

(2) It is not related to the actual circuit, but it can find out the incorrect finished product;

(3) A shorter signal transmission line (generally shorter than 150mm, usually ≥86.36mm) can be used to make accurate test results.

2.2. Attention should be paid when designing coupons:

(1) Whether the line width after etching is consistent with the line width in the forming area;

(2) Whether its placement position is likely to cause insufficient dielectric thickness due to the flow of glue on the edge of the board;

(3) Does the board line density affect the uniformity of the map; otherwise, the coupon loses its representativeness.

2.3. Matters needing attention in impedance test:

(1) Avoid contact of the staff's fingers and external substances with the test sample;

(2) The board should be placed flat on an anti-static rubber pad, and the tester should wear gloves and an anti-static ring;

(3) When testing, the probe needs to be aligned vertically with the test pattern, and lightly press down;

(4) The impedance line of the outer layer shall be tested at the level where it is located;

(5) Maintain constant indoor temperature and humidity.

3. impedance tolerance analysis and improvement

3.1. If there is impedance out-of-tolerance in batch spot check, follow the steps below to deal with and improve:

(1) The physics room conducts a full inspection of this batch of panels;

(2) Feedback related quality issues immediately, and the engineering department will deal with the abnormal board;

(3) Analyze whether the impedance design is correct;

(4) Check whether the impedance test pattern film is correct;

(5) Check whether the reference layer is made as required;

(6) Compare and analyze the actual measured value, the calculated value based on the slice result and the theoretical design value;

(7) Based on the analysis results, improvements are mainly made in aspects such as lamination and line width.

3.2. Ways to improve impedance control include:

(1) Use materials with low dielectric constant, low fiber content and uniform composition;

(2) Reduce the thickness of the dielectric layer;

(3) Reduce the thickness of copper foil;

(4) Control the uniformity of the wire;

(5) Use good impedance calculation software and excellent impedance testing equipment.

The above is the relevant knowledge about PCB circuit board impedance testing introduced by professional PCB manufacturers. I hope it will be helpful to you. ipcb is a high-precision, high-quality PCB manufacturer, such as: isola 370hr PCB,high-frequency PCB, high-speed PCB, ic substrate, ic test board, impedance PCB, HDI PCB, Rigid-Flex PCB, buried blind PCB, advanced PCB, microwave PCB, telfon PCB and other ipcb are good at PCB manufacturing.