PCB News - The problem of poor tin on PCB electroless nickel-gold plate

A PCB circuit board goes through dozens of processes on the production line, just like the Tang monk taking the scriptures, the monkeys have golden eyes, and the four masters and apprentices have gone through ninety-nine and eighty-one difficulties before the Phoenix Nirvana and the fire. Like rebirth, become a qualified high-quality PCB product.

The PCB soldering process is the last production process except for electrical testing, sampling inspection, and packaging. If a circuit board has quality problems in the last process of production, it will inevitably make people sigh.

Next, we will discuss the problem of poor tin on PCB electroless nickel-gold plate with the big coffees of the quality control department of Benqiang Circuit.

In order to take care of some small partners, we put aside the complicated process principles and chemical reaction processes, and try to use easy-to-understand language to discuss professional issues in the PCB manufacturing process.

In the PCB industry, in order to ensure the reliability and operability of downstream assembly, it is usually necessary to perform final surface treatment on the PCB.

Electroless Nickel Gold (ENIG), also known as the Immersion Nickel Gold process, provides an ideal plating layer for PCB boards that integrates solderability, conduction, and heat dissipation. With the advancement of technology, the process has developed rapidly in recent years and has been widely used in the PCB industry.

Immersion nickel gold is the electroless nickel plating and chemical immersion gold on the bare copper surface of the printed board pad. The plating layer has good contact conductivity and assembly welding performance. At the same time, it can also be used in conjunction with other surface treatment processes. Very important and widely used surface treatment process. Due to the versatility requirements of the immersion nickel-gold plate, and the extremely strict appearance requirements, coupled with the sensitivity of the immersion nickel-gold plate, quality problems are prone to occur, such as poor tin on the gold surface. In this paper, starting from the basic process of immersed nickel gold, using X-Ray, SEM, EDS and other analytical methods, the problem of poor tin on the immersion nickel gold plate is discussed.

For PCBs with poor soldering, we first check whether there are solder resist residues on the board. If there are, they must be cleaned, and then the thickness of nickel and gold is measured by the X-Ray tester to see if it meets the soldering requirements. condition.

Then cut the part and observe it with SEM and EDS. The results are shown in Figure 1 and Figure 2. It can be seen that the surface composition of the upper tin bad pad is mainly nickel (Ni), gold (Au), phosphorus (P), tin (Sn), and a small amount of carbon (C) and oxygen (O) are also present., The content of phosphorus element is 4.wt%, the presence of oxygen element indicates that the surface of the pad is oxidized, and the oxide layer can prevent the infiltration of liquid solder on the gold surface, which is one of the reasons for the poor soldering; in addition, it can also It was found that there were a small amount of corrosive spots and microcracks in the local areas of the poor solder pads.

Through X-Ray, SEM and EDS analysis, it is shown that the reason for the poor tin on the electroless nickel-gold plate is due to the oxidation of the pad, the slight corrosion of the nickel layer and the existence of the phosphorus-rich layer, which reduces the mechanical strength of the pad. When subjected to external force at high temperature, the soldering pad is not strong enough to cause poor soldering.

To correct the reason, like the case above, it may be that the chemical agent has been contaminated. Therefore, strengthening the management of various chemical agents in the production process is essential to prevent product quality accidents and improve the quality of PCB products.