To make it easier for you to understand, here we will briefly introduce the main differences between sinking gold plating, hard gold plating and soft gold plating, and how to choose the appropriate gold plating method for your project.
PCB gold plating
It is also called electroplated nickel gold. When "gold" and "copper" are in direct contact, electron migration and diffusion will occur, so a layer of nickel should be plated on the surface before gold plating.
Electroplated gold can be divided into hard gold plating and soft gold plating. The difference between the two is whether they are "pure gold". The higher the purity of "gold", the softer it will be. To increase hardness, other metals (usually cobalt, nickel, or iron) are added to the chemical composition to form an alloy, that is, hard gold.
Hard gold plating and soft gold plating can also be distinguished in appearance. The surface of hard gold is smooth and looks brighter, while the surface of soft gold is a little dark.
The hard gold plated surface is hard and friction resistant, but its weld ability is poor. Therefore, it can be used in places that do not need welding but are subject to force and friction. It is often used for selective gold plating on specific parts of circuit boards, such as "gold finger". The reason why it is called "selective gold plating" is that there is no need for whole plate gold plating, which reduces the use of gold while taking into account the manufacturing cost and product performance.
Soft gold plating
Soft gold plating has good adhesion, and gold and aluminum can form a good alloy, so aluminum / gold bonding is generally used on cob (chip on board).
PCB gold deposit
Now PCB gold deposition generally refers to chemical gold deposition. The advantage of gold plating is that "nickel" and "gold" can be attached to copper without complex electroplating, and its surface is smoother than that of gold plating, which is conducive to finer spacing.
Due to the effect of surface gold layer produced by chemical substitution method, the maximum thickness of gold layer cannot reach the same thickness as that of electroplated gold in principle.
Because of the replacement principle, the gold deposit is "pure gold", so it is often classified as "soft gold", so some people use it for aluminum wire bonding. However, too thin gold layer will affect the adhesion of aluminum wire bonding, so it must be strictly required that the thickness of gold deposition coating should be at least 3~5 micro inches（ μ”）。 Generally speaking, it is difficult for the thickness of gold deposit to exceed 5 μ”， The thickness of gold plating can easily reach 15 μ” Above, the price will also increase with the thickness of the gold layer.
Flash gold
Flash gold just means fast gold plating. In fact, it is the pre process of hard gold plating. It uses high current and high gold content solution to form a dense but thin gold plating layer on the nickel layer, which is convenient for the subsequent electroplating of gold nickel or gold cobalt alloy. Some people see that "gold plated" PCBs can also be made, which is cheap and takes less time to make. Therefore, this kind of "flash gold" PCB is sold in the market.
Due to the lack of follow-up gold plating process, the cost of "flash gold" is much cheaper than that of real hard gold plating, but its gold layer is very thin and generally can not effectively cover all nickel layers under the gold layer. Therefore, the circuit board is prone to oxidation after long-term storage, which will affect the solderability.
Advantages and disadvantages of gold deposition and electroplating for PCB
From the perspective of cost, the cost of gold plating is relatively high compared with other surface treatment methods (such as gold precipitation and HASL). Unless there is a special purpose, it has no cost advantage over gold plating.
In terms of material properties, gold plating by gold deposition is easier to weld than hard gold plating, which will not cause poor welding. The crystal structure protects it from oxidation.
In terms of electrical performance, with the increasing density of PCB layout, gold plating is easy to cause short circuit of wires. The gold plated plate only has nickel gold on the pad, which can avoid short circuit to the greatest extent.
How to choose the appropriate gold plating method?
The following four aspects can be considered:
1. contact force absorption and wear resistance
Hard gold plating has better hardness and wear resistance. Hard gold plating shall be specified for applications requiring repeated sliding wear or on-off switching.
2. adhesion / weldability
The existence of "non precious metal" elements in hard gold plating makes welding more difficult than soft gold plating / gold plating. For very sensitive joints, such as wire bonding, constant temperature bonding or other sensitive connections, only soft gold plating / gold plating should be considered.
3. corrosion resistance / biocompatibility
Soft gold plating / gold plating has high purity and excellent corrosion resistance, while other components in hard gold plating reduce the overall corrosion resistance of the coating and are easy to oxidize at high temperature. Therefore, soft gold plating / gold plating is more suitable for medical applications.
4. contact resistance
Compared with hard gold plating, soft gold plating has lower contact resistance. Especially in high-temperature applications, due to the accelerated formation of oxides and other compounds, hard gold plating will increase the contact resistance. In this case, soft gold plating / gold plating is a better choice.
5. appearance
Soft gold / gold deposits are more likely to be scratched. In general, hard gold plating is recommended for applications that require beautiful gold contacts, such as visible interconnect applications.
summary
It can be said that in most cases, the selection priority of the three gold plating methods of hard gold plating, soft gold plating and gold plating should be gold plating > hard gold plating > soft gold plating.
However, the above is for reference only and other possibilities cannot be excluded. If you have any suggestions or unsolved problems, please contact BQC.