Microwave Tech - Three manufacturing difficulties of high-level circuit boards and the key technology of soft and hard board

Compared with the characteristics of conventional circuit boards, high-level circuit boards have the characteristics of thicker boards, more layers, denser lines and vias, larger cell sizes, and thinner dielectric layers. The inner layer space, the degree of alignment between layers, Impedance control and reliability requirements are more stringent.

1. Difficulties in alignment between layers

Due to the large number of high-level boards, the customer design side has more and more stringent requirements for the alignment of each layer of the PCB. Usually, the alignment tolerance between layers is controlled by ±75μm. Considering the large-scale design of the high-level board unit and the ambient temperature and humidity of the graphics transfer workshop, As well as factors such as misalignment and superposition caused by inconsistency of expansion and contraction of different core layers, interlayer positioning methods, etc., it is more difficult to control the degree of alignment between layers of high-rise boards.

2. Difficulties in the production of inner circuits

The high-level board adopts special materials such as high TG, high-speed, high-frequency, thick copper, thin dielectric layer, etc., which puts forward high requirements for the production of the inner circuit board and the control of the pattern size, such as the integrity of the impedance signal transmission, which increases the difficulty of the production of the inner circuit . Line width and line spacing are small, open and short circuits increase, short circuit increases, and pass rate is low; there are more fine circuit signal layers, and the probability of missing AOI detection in the inner layer increases; the inner core board is thinner, which is easy to wrinkle and cause poor exposure and etching It is easy to roll the board when it passes the machine; most of the high-level boards are system boards, and the unit size is relatively large, and the cost of scrapping the finished product is relatively high.

3. Difficulties in pressing production

When multiple inner core boards and prepregs are superimposed, defects such as sliding plates, delamination, resin voids, and air bubbles are likely to occur during lamination production. When designing the laminated structure, it is necessary to fully consider the heat resistance of the material, the withstand voltage, the amount of glue and the thickness of the medium, and set a reasonable high-level board pressing program. There are many layers, and the amount of expansion and contraction control and the compensation of the size coefficient cannot be kept consistent; the thin interlayer insulation layer can easily lead to the failure of the interlayer reliability test. Figure 1 is a defect diagram of the delamination of the plate after the thermal stress test.

The key technology of rigid-flex board

At present, from simple consumer electronic products to important aerospace devices, flexible circuit boards have become a key technology. Various key components, such as medical equipment, keyboards, drives, printers, mobile phones, etc., have applied this technology. The rigid-flex board (R-FPCB), which is a combination of the flexible part and the rigid part, is also called a rigid-flex board. It is a printed circuit board with both rigid PCB and flexible PCB characteristics.

The main key technologies of rigid-flex board include:

(1) Material matching technology: Rigid-flex board involves the matching of flexible substrates, rigid FR-4 substrates, non-flowing prepregs, cover films and other materials. Material selection is related to the workability and reliability of the product.

(2) Multi-material interlayer alignment and mixed pressure technology: Rigid-flex board has a variety of phases in a longitudinal section. To achieve good interlayer alignment accuracy and bonding strength, in addition to material selection, flexibility The prevention and control of expansion and shrinkage of the board and the rigid board substrate, the positioning method between the layers, the design of the laminated structure, and the pre-treatment and pressing process parameters are very important, and multiple sets of process tests are required to explore.

(3) Multi-layer interconnection technology: Rigid-flex boards may have designs such as multilayer interconnection, any layer of buried blind via interconnection, etc., involving key technologies such as drilling of different phases, hole metallization technology, etc., which need to pass the process Research and development to ensure a good combination between the plated hole wall and the inner hole ring to achieve good interlayer interconnection.

(4) Flexible board damage prevention technology: using existing conventional hard board production conditions to manufacture rigid-flex board, how to protect the flexible board area from various chemical attacks and mechanical external forces in the process to ensure the appearance quality of the flexible board area, Flexural resistance requirements, insulation reliability requirements, will be the key to the reliability of rigid-flex board.