PCB Tech - What is the concept of gold in PCB circuit boards?

In the PCB manufacturing industry, regarding the difference between hard gold, soft gold, and flash gold on circuit boards, based on the conclusions obtained from dealing with relevant circuit board manufacturers and asking circuit board manufacturers for many years, the following are only personal opinions and experience, Please correct me if there are any errors.

There are many friends who work in the PCBA system assembly plant in the later stage. They have not been very clear and unclear about the "hard gold", "soft gold" and "flash gold" on the circuit board. Some people still think that electroplating gold is Must be hard gold? And chemical gold must be soft gold? In fact, this division method can only say that the answer is half right.

In fact, the difference between "hard gold" and "soft gold" in the PCB industry refers to "alloy" and "pure gold", because "pure gold" is actually softer, and "alloys" mixed with other metals are harder and more resistant. Friction, so the purer the gold, the softer it is.

Electroplated nickel gold

In fact, "electroplating gold" itself can be divided into hard gold and soft gold. Because electroplated hard gold is actually an electroplated alloy (that is, plated with Au and other metals), the hardness will be relatively hard, and it is suitable for use in places that require force and friction. In the electronics industry, it is generally used as the edge of the circuit board. Contact point (commonly known as "gold finger", as shown in the front picture). The soft gold is generally used for aluminum wire on COB (Chip On Board), or the contact surface of mobile phone keys. Recently, it has been widely used on the front and back of the BGA substrate.

To understand the origin of hard gold and soft gold, it is best to first understand the process of electroplating gold. Leaving aside the previous pickling process, the purpose of electroplating is basically to electroplate "gold" on the copper skin of the circuit board, but if "gold" and "copper" are in direct contact, there will be a physical reaction of electron migration and diffusion (potential It is necessary to electroplate a layer of "nickel" as a barrier layer first, and then electroplate gold on top of the nickel, so what we generally call electroplated gold, its actual name should be called "electroplated nickel gold".

The difference between hard gold and soft gold is the composition of the last layer of gold that is plated on. When plating gold, you can choose to electroplate pure gold or alloy. Because the hardness of pure gold is relatively soft, it is also called "soft gold". ". Because "gold" can form a good alloy with "aluminum", COB will particularly require the thickness of this layer of pure gold when making aluminum wires.

In addition, if you choose to electroplated gold-nickel alloy or gold-plated alloy, because the alloy will be harder than pure gold, it is also called "hard gold".

The electroplating procedure of soft gold and hard gold:

Soft gold: pickling - electroplating nickel - electroplating pure gold

Hard gold: pickling - nickel plating - pre-gold plating (flash gold) - plating gold nickel or gold-gu alloy

Chemical gold

The current "chemical gold" is mostly used to call this ENIG (Electroless Nickel Immersion Gold) surface treatment method. Its advantage is that "nickel" and "gold" can be attached to the copper skin without using the complicated copying process of electroplating, and its surface is flatter than electroplating gold, which is suitable for the shrinking electronic parts and the requirements for high flatness. The fine pitch is especially important.

Since ENIG uses a chemical replacement method to produce the effect of the surface gold layer, the maximum thickness of its gold layer cannot in principle reach the same thickness as electroplated gold, and the more the bottom layer is, the less gold content will be.

Because of the principle of replacement, ENIG's gold-plated layer belongs to "pure gold", so it is often classified as a kind of "soft gold", and some people use it as the surface treatment of COB aluminum wire, but it must It is strictly required that the thickness of the gold layer should be at least 3～5 micro-inches (μ"). Generally, it is difficult to achieve a gold thickness of more than 5μ". A too thin gold layer will affect the adhesion of the aluminum wire. ; The general electroplating gold can easily reach a thickness of 15 micro-inches (μ") or more, but the price will increase with the thickness of the gold layer.

Flash Gold

The term "flash gold" comes from the English Flash, which means fast gold plating. In fact, it is the "pre-gold plating" process of hard gold electroplating. The bath with thicker gold first forms a denser but thinner gold-plated layer on the performance of the nickel layer to facilitate subsequent electroplating of gold-nickel or gold-plated alloys. Some people see that PCBs with gold plating can also be made in this way, and the price is cheap and the time is shortened, so some people sell such "flash gold" PCBs.

Because "flash gold" lacks the subsequent gold electroplating process, its cost is much cheaper than real electroplating gold, but also because the "gold" layer is very thin, it generally cannot effectively cover all the nickel layers under the gold layer. Therefore, it is easier to cause the oxidation of the circuit board after being stored for too long, which will affect the solderability.

Judging from the many current PCB surface treatment methods, the cost of electroplating nickel gold is relatively high compared to other surface treatment methods (such as ENIG, OSP). With the current high price of gold, it has been seldom used. Unless there is a special purpose, such as the contact surface treatment of the connector, and the need for sliding contact elements (such as gold fingers...), etc.; but in terms of the current circuit board surface treatment technology, the electroplated nickel-gold coating has a good The anti-friction ability and excellent anti-oxidation ability are still unmatched.