Choose the right surface treatment process and you won’t have to worry about high-speed signal attenuation!

yvonneGan · Published on 2023-12-12 13:40

Choose the right surface treatment process and you won’t have to worry about high-speed signal attenuation! [Copy link]

Member of Mr. High Speed - Huang Gang

Normally, a PCB board will only have one surface treatment, but Mr. Gaosu doesn't follow the usual path and insists on applying multiple surface treatments to the same PCB board!

Is it just for fun? Of course, this is also one of Mr. Gaosuo’s ideas, that is, he wants to try whether the board factory can achieve it through some special processes!

But it is definitely more than just this idea. Mr. High Speed was created to study the high-frequency performance of the traces under different surface treatments. On the same board, the board and the trace length are guaranteed to be consistent. Under this premise, the difference in high-frequency loss of the traces obtained by different surface treatment processes can be well obtained.

Go check out the results quickly to see if it’s quite different from the loss ranking in everyone’s minds... Ha!

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Surface treatment classification

Why do we need surface treatment? The most basic purpose of PCB surface treatment is to ensure good solderability or electrical performance. Because copper is easily oxidized in the air and cannot maintain its own properties for a long time, other treatment methods are needed to avoid oxidation and maintain solderability.

The different colors we see on the PCB pads are actually the presentation of different surface treatment processes, which are mainly classified as follows.

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Silver plate

Immersion silver is a process of depositing silver onto the surface of a PCB pad. This method deposits a layer of silver plating on the pad surface by replacing copper (Cu) with silver (Ag).

Advantages and disadvantages coexist. The advantages are high solderability and flatness, while the disadvantages are high storage requirements and easy oxidation.

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Immersion Gold Plate

Immersion gold is a common surface treatment process that coats the copper surface with a thick layer of nickel-gold alloy with good electrical properties.

The benefits mainly include a smooth surface and a long shelf life.

The disadvantages are high cost and average welding strength.

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Green oil board

Green oil refers to the ink coated on the copper foil of PCB, also called liquid photoresist, which is an acrylic oligomer. It can be said to be the most common surface treatment process!

Advantages include high stability, low cost and wide application!

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Tin plate

Similar to the silver immersion process, it is a metal surface deposited by a chemical replacement reaction and applied directly to the base metal (copper) of the circuit board.

The advantage is high flatness and suitable for fine pitch devices, but the disadvantage is also short shelf life.

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OSP Board

OSP is to chemically grow an organic film on the clean bare copper surface. It uses a water-based organic compound that selectively combines with copper and provides an organic metal layer to protect the copper before welding.

The advantages are simple process and reworkability, while the disadvantages are that it is not suitable for PTH holes and has a short shelf life.

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Losses due to different surface treatments

After testing and de-embedding, we obtained the trace loss results for the same length with different surface treatments. Are you surprised? I didn’t expect the difference between them to be so big, right? In addition, have you guessed the ranking of their losses?

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Questions for this issue

Which surface treatment loss rankings surprised you? Could you please share your thoughts?