HDI board is the most sophisticated circuit board among PCB boards, and its board manufacturing process is also the most complex. The core steps include the formation of high-precision printed circuits, processing of micro via holes, and electroplating of surfaces and holes. Next, let’s take a look at these core steps in HDI PCB pattern making.
1. Ultra-fine line processing
With the development of science and technology, some high-tech equipment is becoming more and more miniaturized and sophisticated, which places higher and higher requirements on the HDI boards used.
The line width/line spacing of HDI circuit boards for some equipment has developed from the early 0.13 mm (5 mil) to 0.075 mm (3 mil), and has become a mainstream standard. As a leader in the HDI express circuit board industry, Shenzhen Benqiang Circuit Co., Ltd.'s related production technology has reached 38μm (1.5 mil), which is approaching the industry limit.
Increasingly higher line width/line spacing requirements have brought the most direct challenges to graphics imaging in the PCB manufacturing process. So how are the copper wires on these precision boards processed?
The current formation process of fine lines includes laser imaging (pattern transfer) and pattern etching.
Laser direct imaging (LDI) technology is to directly scan the surface of a copper-clad board with photoresist to obtain a refined circuit pattern. Laser imaging technology greatly simplifies the process and has become the mainstream in HDI PCB plate making. Process technology.
Nowadays, there are more and more applications of semi-additive method (SAP) and modified semi-additive method (mSAP), that is, pattern etching method. This technical process can also realize conductive lines with a line width of 5um.
2. Micro hole processing
The important feature of the HDI board is that it has micro via holes (pore diameter ≤0.10 mm), which are all buried blind hole structures.
Buried blind holes on HDI boards are currently mainly processed by laser processing, but CNC drilling is also used.
Compared with laser drilling, mechanical drilling also has its own advantages. When laser processing epoxy glass cloth dielectric layer through holes, the difference in ablation rate between the glass fiber and the surrounding resin will lead to slightly poor quality of the hole, and the residual glass fiber filaments on the hole wall will affect the reliability of the via hole. . Therefore, the superiority of mechanical drilling is reflected at this time. In order to improve the reliability and drilling efficiency of PCB boards, laser drilling and mechanical drilling technologies are steadily improving.
0.3 Electroplating and surface coating
How to improve plating uniformity and deep hole plating capabilities in PCB manufacturing and improve the reliability of the board. This depends on the continuous improvement of the electroplating process, starting from many aspects such as the proportion of electroplating liquid, equipment deployment, and operating procedures.
High-frequency sound waves can accelerate the etching ability; permanganic acid solution can enhance the decontamination ability of the workpiece. High-frequency sound waves will stir and add a certain proportion of potassium permanganate plating solution in the electroplating tank. This helps the plating solution flow evenly into the hole. Thereby improving the deposition ability of electroplated copper and the uniformity of electroplating.
At present, the copper plating filling of blind holes has also matured, and copper filling of through holes of different apertures can be carried out. The two-step method of copper plating hole filling can be suitable for through holes with different apertures and high aspect ratios. It has strong copper filling ability and can minimize the thickness of the surface copper layer.
There are many options for the final surface finish of PCB. Electroless nickel/gold plating (ENIG) and electroless nickel/palladium/gold plating (ENEPIG) are commonly used on high-end PCBs.
Both ENIG and ENEPIG have the same immersion gold process. Choosing the appropriate immersion gold process is very important for reliable installation welding or wire bonding. There are three types of immersion gold processes: standard displacement gold immersion, high-efficiency gold immersion with limited nickel dissolution, and reduction reaction gold immersion mixed with mild reducing agents. Among them, the effect of reduction reaction is better.
For the problem that the nickel layer contained in ENIG and ENEPIG coatings is not conducive to high-frequency signal transmission and the formation of fine lines, surface treatment can be used and electroless palladium/catalytic gold plating (EPAG) can be used instead of ENEPIG to remove nickel and reduce the metal thickness.