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The Test of Characteristic Impedance
Posted:04:04 PM August 17, 2018 writer: G

1. Measurement of TDR

It can be seen from the above that the characteristic impedance value in the overall transmission line not only needs to maintain uniformity, but also makes its value fall within the tolerance range required by the designer. The general measurement method is to use Time Domain Reflectometry (TDR). This TDR can generate a step wave (StepPulse or Step Wave) and send it into the transmission line to be tested to become an Incident Wave. Therefore, when the signal line has a wide and narrow change in the line width, the up and down oscillation of the Z0 ohm value also appears on the screen.

2. Low frequency does not need to measure Z0, high speed will use TDR

When the wavelength of the signal square wave (λ reading Lambda) far exceeds the length of the board surface, there is no need to consider the troublesome problem in the high speed field such as reflection and impedance control. For example, in the early 1989, the CPU was not fast, and its clock rate was only 10 MHz. Of course, the complicated problems of various signal transmissions did not occur. However, the current Pentium IV has an internal frequency of up to 1.7 GHz, which is naturally a problem. Compared with the huge difference of the year, it is only a muddy cloud! From the wave equation, the wavelength of the 10MHz square wave in the current year is:

But when the clock rate of the DRAM chipset has jumped to 800MHz, the wavelength of the square wave will also be shortened to 37.5cm; while the speed of the P-4 CPU is as high as 1.7GHz and its wavelength is shorter to 17.6cm, then its PCB motherboard The FSB transmitted between the two will also accelerate to the boundary of 400MHz and 75cm. It can be seen that the length of the line in the package carrier, even the line length on the motherboard, has been close to the wavelength of the signal. Of course, the transmission line effect must be emphasized, and the measurement of the TDR must be used. .

3. TDR has been around for a long time

Measuring the characteristic impedance (Z0) of a transmission line with a time domain reflectometer is not new. In the early years, it was used to monitor the safety of the Submarine Cable, and it was always noted whether it had a "disconnection" problem in the transmission quality. It has been gradually used in the field of high-speed computers and high-frequency communication.

4. TDR test of CPU carrier board

The packaging technology of active components has been continuously refurbished and accelerated in recent years. The C-DIP and P-DIP double-row jack-welding (PTH) of the 1970s are now extinct. The QFP (four-sided extension) or PLCC (four-sided hook) of the 80-year Lead Frame has also been rapidly reduced from HDI boards or hand-held models. Instead, there is an Area Array ball-type BGA or CSP, or a footless LGA. Even the interconnection of Chips to Substracts has progressed from Wire Bond to shorter and more direct Flip Chip (FC) technology. The fast pace of the electronics industry has almost changed rapidly!

Hioki's "1109 Hi Tester" was introduced at JPCA in June 2001. In order to accurately measure the Z0 of the 1.7GHz high-speed transmission FC/PGA carrier board, flying probes are no longer used. The touch test also gave up the SMA probe-type TDR manual-test (Press-type) approach. And the fixed high-frequency short-distance connection cable is fixed, and the fixed high-frequency stylus is accurately positioned, and at the automatic shifting distance and the contact point of the contact column to be tested, the high-precision automatic test with all-in-one interference is performed. .

In the XY displacement of the CCD camera monitor platform, and the drop in the Z direction of the Laser high and low sensor inspector, these double precise positioning and finding points, together with the rotatable contact stylus, can avoid The trouble of using traditional cables, connectors, switches, etc., greatly reduces the error of TDR measurement. This has made the "1109HiTESTER" measurement of Z0 on the package carrier much more accurate than other methods.

In fact, its probe combination uses a four-way probe set (one signal and two Gnds in each direction). The data will of course be more accurate when the CCD monitors one side and measures it. Any errors caused by temperature changes can also be minimized under the automatic correction of standard value ceramic plates.

5. Precisely fallen size

This latest 1109, not only can carry out Z0 measurement on the CPU of the highest-order package carrier, but also can complete the fine measurement of the remaining high-priced CSP, BGA, FC, etc. The size to be tested can be varied from 10mm × 10mm, to 500mm × 600mm, and it can be appreciated in the dramatic situation. In the future, the industry may have to measure Z0 on the actual signal line outside of Coupon. This difficult TDR technology is currently under development.

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