Controlled Impedance

Introduction

Owing to significantly increasing transmission frequencies, i.e. shorter pulse rise times of electronic components, in high frequency technology, it has become necessary to treat the conductors concerned as a

component. Depending upon a number of parameters, HF signals are reflected on circuit boards. This means that the characteristic impedance differs from the output impedance of the transmitting component. The proper transmission of the signal is thus no longer guaranteed.

HemeixinPCB has created the option of checking the required impedance on the customer's circuit board, so as to modify the circuit board, or the tracks and the layer structure if necessary. The impedance is largely determined by the track geometry, the structure of the layers and the dielectric constant (Er) of the materials used.

After making the circuit board, the impedances are checked and recorded. The results of the measurements are available on request at any time.

Impedances on the circuit board

The microstrip is a very simple yet useful way to create a transmission line with a PCB. There are some advantages to using a microstrip transmission line over other alternatives. Modeling approximation can be used to design the microstrip trace. By understanding the microstrip transmission line, designers can properly build these structures to meet their needs.

The impedance specified by the customer will be checked during the PCB maker's preparatory work to see if it can be made. Depending on the definition of the layers in the stack and on the layout itself, from a set of 93 different models, the correct one must be selected to be able to calculate the impedance. As a result of this, a layer structure will be produced together with any necessary modifications to the geometry of the relevant tracks. The standard tolerance used for an impedance is ±10%. Depending upon various PCB characteristics, on request an improved tolerance (down to ±5%) can be offered.

Impedance PCB

Typical models for calculations

Impedance PCB

To be able to actually reach the calculated impedances in the printed circuit board, the manufacturer must understand their production processes very well and have good control of them. The angle of the track edges (resulting from W1, W2 and T1), the pressed thickness depending on the copper coating (H), the thickness of the solder resist varnish (C1, C2) and more, all influence the impedances on the circuit board. The tolerance range of all processes have to be considered here. These values are then added to the formulae for the calculation.

Depending on the complexity of the circuit board, some of the impedances of the circuit board can be checked during the manufacturing process. For this, new models for the corresponding situation (actual state of the circuit board) are separately defined and the results determined.

Dielectrics

To make controlled impedance circuit boards, differing materials are used. FR4 is standard here too. However, in applications with very high frequencies (>1 GHz), requirements such as low dielectric loss or a lower Er are of increasing importance.

Here, HemeixinPCB can offer a large portfolio of usable materials. Many of these are seen as standard and are regularly used, others are available at short notice. You will ἀnd further information in the "Materials".

Quality control

As impedances can no longer be measured once the components have been fitted, it is necessary to check and confirm the values prior to assembly. When the job is prepared, so-called "test coupons", which are similar to the relevant tracks, are added to the net area of the production blank.  The impedance of the test coupons represent those of the actual circuit board. HemeixinPCB has a CITS900s in their final inspection, the newest test equipment made by POLAR.