With the rapid development of Mini LED technology, the trend of chip miniaturization is becoming more and more obvious, and the size of LED chip pad is also correspondingly reduced, which poses a challenge to the packaging end of Mini LED. The major packaging plants continue to improve the process and equipment accuracy, and the technical route is also blooming.

There are many technical routes for the application of Mini LED, but many manufacturers have a common goal: to reduce costs and increase efficiency. In the process of Mini LED, the nickel gold pad substrate is widely used but the cost is high, so the concept of traditional SMT PCB pad plating design has entered the field of vision of Mini LED players.

In this context, tin spraying, Cu OSP, chemical tin and other pad application processes are swarming. This paper will introduce different pad coatings and their characteristics.

What is PCB surface treatment

The process difficulty of Mini LED PCB lies in solder resistance and surface treatment. The most basic purpose of PCB surface treatment is to ensure good solderability or electrical performance.

So, what is PCB surface treatment?

Because copper is very easy to be oxidized in the air, the oxidation will have a negative impact on the welding effect, so the pad surface will be coated (plated) with organic layer or metal layer to improve the wetting and bonding ability of solder. As shown in the figure below:

Introduction of plating types of different pads

The development of PCB technology has continued to this day, and many PCB surface treatment processes have been produced, such as hot-air leveling, organic coating OSP, electroless nickel plating/gold dipping and tin dipping.

Lead-free tin spraying (hot air leveling HASL)

Hot air leveling, also known as hot air solder leveling HASL (Hot Air Solder Leveled), is a process of coating molten tin-lead solder on the surface of PCB and leveling (blowing) with heated compressed air to form a coating layer that is not only resistant to copper oxidation, but also provides good weldability. Good solderability, hot air leveling solder and copper form copper-tin intermetallic compound at the joint.

Cu OSP (Organic Solderability Preservative) organic coating process OSP is different from other surface treatment processes. It acts as a barrier layer between copper and air to prevent the oxidation of the surface of copper pads; The organic coating process is simple and the cost is low, which makes it widely used in the industry.

Electroless nickel gold (ENIG) electroless nickel plating and immersion gold plating (ENIG) have good weldability. Ni is required to be ≥ 3um thick as the isolation layer and weldable coating; Au is the protective layer of Ni. If too much Au is dissolved into the solder joint (whether Sn-Pb or Sn-Ag-Cu), it will cause "gold brittleness". Therefore, the thickness of the Au layer must be limited. The thickness of the Au layer used for welding is ≤ 1? m （ENIG ：0．05～0．3?m）。

The tin whisker is easy to appear after the immersion tin process of PCB, and the tin whisker and tin migration will bring reliability problems during the welding process. After that, organic additives were added into the tin dipping solution, which made the tin layer in granular structure, overcome the previous problems, and also had good thermal stability and weldability.

Disadvantages: The biggest weakness of tin dipping is its short service life, especially when stored in high temperature and high humidity environment, the Cu/Sn intermetallic compound will continue to grow until it loses its weldability. Therefore, the tinplate can not be stored for too long.

How to select the surface treatment process?

Due to the limitation of HASL hot air leveling process, pad flatness is not recommended for applications with high requirements for pad flatness.

The order of weldability&wettability of pad surface is as follows:

Chemical nickel-gold>tin spraying (HASL)&tin dipping>Cu OSP In response to the requirements of Mini LED direct display applications, it is recommended to select nickel-gold pads with good solderability and wettability.

Many customers found the phenomenon of "black pad" in the welding process after selecting the chemical nickel-gold surface treatment process. As shown in the figure below:

Cause analysis is as follows:

(a) The structure of the gold coating and nickel coating on the bonding pad is not dense enough, there are cracks on the surface, the water in the air is easy to enter, and the acid solution in the gold dipping process is easy to remain in the nickel coating. During gold plating, the surface grains will present a rough, sparse and porous morphology to form many voids, and the plating solution will continue to react with the Ni atoms under the Au layer through these voids, making the Ni atoms continue to oxidize, while the undissolved Ni ions will be trapped under the Au layer, forming nickel oxide (NixOy). When the nickel layer is eroded by excessive oxidation, the so-called black pad is formed.

(b) The high or low phosphorus content of the nickel coating results in poor acid corrosion resistance of the coating, easy corrosion discoloration, "black disk" phenomenon, and poor weldability. (PH 3-4 is better)

(c) The gold-plated layer is too thin. When welding, the thin Au layer as a weldable protective coating quickly diffuses into the solder, exposing the surface of the over-oxidized and low-weldable Ni layer, which will inevitably make it difficult to form a uniform and continuous intermetallic compound (IMC) between the Ni and the solder, affecting the bonding strength of the solder joint interface, and may cause cracking along the solder joint/coating interface, If it is serious, it may lead to poor surface wetting, which may cause the components to fall off from the PCB or the nickel surface to become black, commonly known as "black nickel".

Cost&shelf life

Summary: With the application requirements of Mini LED products becoming higher and higher, the environmental protection requirements become more and more strict, and the surface treatment processes of solder pads are various. Regardless of the surface treatment process, the first thing is to understand the characteristics of the surface process and select the suitable product application scenario.