High - level circuit board hub production process control

High level circuit board is generally defined as 10 layers ~ 20 layers or above high layer circuit board, more difficult than the traditional multi-layer circuit board processing, its high quality and reliability requirements, mainly used in communication equipment, high-end servers, medical electronics, aviation, industrial control, military and other fields. In recent years, the market demand for high-rise board in applied communication, base station, aviation, military and other fields is still strong, and with the rapid development of China's telecom equipment market, the prospect of high-rise board market is promising.

At present, the PCB manufacturers that can produce high level circuit boards in bulk are mainly from foreign enterprises or a few domestic enterprises. The production of high level circuit board not only needs higher technology and equipment investment, more need technical staff and production staff experience accumulation, at the same time into the high level board customer certification procedures are strict and cumbersome, so the high level circuit board into the enterprise threshold, to achieve a long industrial production cycle. PCB uniform layer number has become an important technical index to measure the technical level and product structure of PCB enterprises. This paper describes the main processing difficulties encountered in the production of high level circuit board, introduces the control points of high level circuit board hub production process, for peer reference and warning.

First, the main production difficulties

Compared with the characteristics of conventional circuit board products, the high level circuit board has thicker parts, more layers, more dense lines and holes, larger unit size, thinner dielectric layer and other characteristics, the inner space, the alignment between layers, impedance control and reliability requirements are more strict.

1.1 Difficulty of interlayer alignment

Because of the number of high-rise board layers, the customer design end of the PCB layer alignment requirements are more and more strict, usually the inter-layer alignment tolerance control ±75μm, considering the high-rise board unit size design, graphic transfer workshop temperature and humidity, as well as different core board layer growth and shrinkage inconsistency caused by dislocation superposition, inter-layer positioning mode and other factors, It makes it more difficult to control the alignment degree between the layers of the high floor.

1.2 Difficulties in making inner circuits

High TG, high speed, high frequency, thick copper, thin dielectric layer and other special materials are used in high-rise board, which puts forward high requirements for inner line fabrication and graphic size control. For example, the integrity of impedance signal transmission increases the difficulty of inner line fabrication. Line width line distance is small, open short circuit increase, micro short increase, low qualified rate; There are more signal layers in the fine line, and the probability of missing AOI in the inner layer increases. Inner core plate thickness is thin, easy to fold resulting in poor exposure, etching machine easily roll plate; Most of the high-rise boards are system boards, the unit size is large, and the cost of scrap in finished products is relatively high.

1.3 Difficulties in pressing production

The superposition of multiple inner core plates and semi-cured sheets can easily produce defects such as sliding plate, delamination, resin floating and bubble residue during pressing production. In the design of laminated structure, it is necessary to fully consider the heat resistance, voltage resistance, filling amount and medium thickness of the material, and set a reasonable high level plate pressing program. Because of the large number of layers, the volume control and size coefficient compensation cannot be consistent. The thin insulation layer between layers easily leads to the failure of inter-layer reliability test. FIG. 1 is the defect diagram of the layered burst plate after thermal stress test.

1.4 Difficulties in drilling

Special plates of high TG, high speed, high frequency and thick copper are adopted, which increases the difficulty of drilling roughness, drilling burr and drilling fouling removal. The number of layers, the cumulative total copper thickness and plate thickness, drilling easy to break the knife; CAF failure caused by dense BGA and narrow pore wall spacing; Because of the thickness of the plate easily lead to oblique drilling problems.

Two, hub production process control

2.1 Material Selection

With the development of electronic components in the direction of high function and multi-function, it also brings high frequency and high-speed development of signal transmission, so it requires low dielectric constant and dielectric loss of electronic circuit materials, as well as low CTE, low water absorption and better high function copper clad plate materials to meet the processing and reliability requirements of high-rise board. Commonly used plate suppliers mainly include series A, series B, series C and series D. The main characteristics of these four kinds of inner substrate are compared, as shown in Table 1. For the high thickness copper circuit board selection of high resin content of the semi-cured sheet, the flow glue between the layers of the semi-cured sheet is enough to fill the inner figure, the insulating medium layer is too thick easy to cause the finished plate super thick, on the other hand, the insulating medium layer is thin, easy to cause the dielectric layer, high pressure test failure and other quality problems, so the choice of insulating medium material is very important.