What are the difficulties in the production of multi-layer circuit board proofing

In the circuit board industry, multilayer circuit boards (high-precision PCB multilayer boards) are generally defined as multilayer circuit boards with 4 layers to 20 layers or more, which are more difficult to process than traditional multilayer circuit boards, and their quality and reliability High requirements, mainly used in communication equipment, industrial control, security, high-end server, medical electronics, aviation, military and other fields. In recent years, the market demand for PCB high-layer boards in the fields of communication, base station, aviation, military and other fields is still strong, and with the rapid development of China's telecom equipment market, the market prospect of PCB high-layer circuit boards is promising.

At present, domestic high-level circuit board manufacturers capable of circuit board proofing and production mainly come from foreign-funded enterprises or a few domestic-funded enterprises. The production of high-level circuit boards not only requires high technology and equipment investment, but also requires the accumulation of experience of technicians and production personnel. At the same time, the customer certification procedures for importing PCB multilayer circuit boards are strict and cumbersome. High, the realization of industrial production cycle is longer.

The average number of layers of PCB multilayer boards has become an important technical indicator to measure the technical level and product structure of PCB companies. This article briefly describes the main processing difficulties encountered in the production of multi-layer circuit board proofing, and introduces the control points of key production processes of high-layer circuit boards for your reference.

Compared with the characteristics of conventional circuit board products, high-level circuit boards have the characteristics of thicker PCB boards, more layers, denser lines and vias, larger unit sizes, and thinner dielectric layers. , impedance control and reliability requirements are more stringent.

1. Difficulties in alignment between layers

Due to the large number of layers of PCB high-layer boards, the customer design side has more and more strict requirements on the alignment of PCB layers. Usually, the alignment tolerance between layers is controlled at ±75um. Considering the large size design of high-layer board units and the ambient temperature and humidity of the graphics transfer workshop , and factors such as dislocation superposition and interlayer positioning methods caused by the inconsistency of expansion and contraction of different core board layers make it more difficult to control the interlayer alignment of high-layer boards.

2. Difficulties in making the inner circuit

PCB high-level circuit boards use special materials such as high TG, high speed, high frequency, thick copper, and thin dielectric layers, which put forward high requirements for inner layer circuit production and graphic size control, such as the integrity of impedance signal transmission, which increases the inner layer circuit production. difficulty. The line width and line spacing are small, the number of open and short circuits increases, the number of micro shorts increases, and the pass rate is low; there are many signal layers of fine and dense lines, and the probability of missing AO1 in the inner layer increases; It is easy to roll the board when passing the machine. Most of the multi-layer circuit boards are system boards, and the unit size is relatively large. The cost of scrapping the finished product is relatively high.

3. Difficulties in lamination

Multiple PCB inner-layer core boards and prepregs are superimposed, and defects such as slippery boards, delamination, resin voids, and bubble residues are prone to occur during lamination production. When designing the laminated structure, it is necessary to fully consider the heat resistance, voltage resistance, glue filling amount and dielectric thickness of the material, and set a reasonable PCB high-layer circuit board lamination program. If the number of layers is large, the expansion and shrinkage control and size factor compensation cannot be kept consistent; the interlayer insulation layer is thin, which may easily lead to the failure of the interlayer reliability test. Figure 1 is a defect diagram of cracked plate delamination after thermal stress test.

Multi-layer circuit board explosion board layered slice diagram

4. Difficulties in drilling

The use of high TG, high speed, high frequency, and thick copper special plates increases the difficulty of drilling roughness, drilling burrs and drilling dirt. The number of layers is large, the cumulative total copper thickness and plate thickness are easy to break when drilling; there are many dense BGAs, and the CAF failure problem is caused by narrow hole wall spacing; the problem of inclined drilling is easily caused by the thickness of the plate.

1. PCB material selection

With the development of high-performance and multi-functional electronic components, and high-frequency and high-speed signal transmission, it is required that the dielectric constant and dielectric loss of electronic circuit materials are relatively low, as well as low CTE and low water absorption. High efficiency and better high-performance CCL materials to meet the processing and reliability requirements of high-layer boards. Commonly used board suppliers mainly include A series, B series, C series, and D series. The comparison of the main characteristics of these four inner substrates is shown in Table 1. For high-layer thick copper circuit boards, prepregs with high resin content are used. The amount of glue flowing between layers of prepregs is enough to fill the inner layer graphics. If the insulating dielectric layer is too thick, the finished board will be too thick. On the contrary, if the insulating dielectric layer is too thin, it will easily cause Dielectric layering, high-voltage test failure and other quality problems, so the selection of insulating dielectric materials is extremely important.

2. Lamination structure design

The main factors considered in the design of the laminated structure are the heat resistance of the material, the withstand voltage, the amount of glue filled, and the thickness of the dielectric layer. The following main principles should be followed. ①. When customers require high TG boards, core boards and prepregs must use corresponding high TG materials.

. Prepreg and core board manufacturers must be consistent. In order to ensure the reliability of the PCB, avoid using a single 1080 or 106 prepreg for all layers (except for special requirements from customers). When the customer has no dielectric thickness requirements, the dielectric thickness between each layer must be guaranteed to be ≥ 0.09mm according to IPC-A-600G.

.Inner substrate 30Z or above, choose high resin content prepreg, such as 108OR/C65%, 108OHR/C68%, 106R/C73%, 106HR/C76%; but try to avoid all the structural design of 106 high-resin prepreg, In order to prevent multiple 106 prepregs from stacking, because the glass fiber yarn is too thin, the glass fiber yarn will collapse in the large substrate area, which will affect the dimensional stability and burst delamination.

④. If the customer has no special requirements, the thickness tolerance of the interlayer dielectric layer is generally controlled by +/-10%. For the impedance plate, the dielectric thickness tolerance is controlled by the IPC-4101C/M level tolerance. If the impedance influencing factor is related to the thickness of the substrate, Then the plate tolerance must also be in accordance with IPC-4101C/M level tolerance.

3. Alignment control between layers

The accuracy of the size compensation of the inner core board and the control of the production size need to accurately compensate the graphic size of each layer of the multi-layer circuit board through the data and historical data collected in production for a certain period of time to ensure that the core board of each layer Consistency of expansion and contraction. Choose high-precision, high-reliability interlayer positioning methods before lamination, such as four-slot positioning (PinLAM), hot-melt and rivet combination. Setting up a suitable lamination process program and daily maintenance of the press is the key to ensuring the quality of lamination, controlling the glue flow and cooling effect of lamination, and reducing the problem of misalignment between layers. Inter-layer alignment control needs to be comprehensively considered from the inner layer compensation value, pressing positioning method, pressing process parameters, material properties and other factors.

4. Inner layer circuit technology

Since the resolution capability of the traditional exposure machine is around 50um, for the production of high-layer boards, a laser direct imaging machine (LDI) can be introduced to improve the graphics resolution capability, and the resolution capability can reach about 20um. The alignment accuracy of the traditional exposure machine is ±25um, and the interlayer alignment accuracy is greater than 50um. Using a high-precision alignment exposure machine, the pattern alignment accuracy can be increased to about 15um, and the interlayer alignment accuracy is controlled within 30um, which reduces the alignment deviation of traditional equipment and improves the interlayer alignment accuracy of high-layer boards.

In order to improve the line etching ability, it is necessary to give appropriate compensation to the width of the line and the pad (or welding ring) in the engineering design, and to make a more detailed design for the compensation amount of special graphics, such as return lines, independent lines, etc. consider. Confirm whether the inner layer line width, line spacing, spacer ring size, independent line, and hole-to-line distance design compensation are reasonable, otherwise change the engineering design. Impedance and inductive reactance design require attention to whether the independent line and impedance line design compensation are sufficient, the parameters are well controlled during etching, and mass production can only be carried out after the first part is confirmed to be qualified. In order to reduce etching side erosion, it is necessary to control the components of each group of etching solution within the optimal range. The etching capacity of traditional etching line equipment is insufficient. It is possible to carry out technical transformation on the equipment or introduce high-precision etching line equipment to improve etching uniformity and reduce etching burrs and unclean etching.

5. Lamination process

At present, the interlayer positioning methods before lamination mainly include: four-slot positioning (PinLAM), hot-melt, rivets, and combination of hot-melt and rivets. Different product structures adopt different positioning methods. For multi-layer circuit boards, use the four-slot positioning method (PinLAM), or use the fusion + riveting method to make, and the OPE punching machine punches out the positioning holes, and the punching accuracy is controlled at ±25um. When making the first board, the adjustment machine needs to use X-RAY to check the layer deviation, and the batch can only be produced if the layer deviation is qualified. During mass production, it is necessary to check whether each board is fused into the unit to prevent subsequent delamination. The pressing equipment adopts high-performance matching The press machine meets the alignment accuracy and reliability of the high-layer board.

According to the laminated structure of multilayer circuit boards and the materials used, study the appropriate lamination procedure, set the optimal heating rate and curve, and properly reduce the heating rate of the lamination sheet in the conventional lamination procedure of multilayer circuit boards. Prolong the curing time at high temperature, so that the resin can fully flow and cure, and at the same time avoid problems such as slippage and interlayer misalignment during the pressing process. Boards with different material TG values cannot be the same as fire grate boards; boards with common parameters cannot be mixed with boards with special parameters; the expansion and contraction coefficients are guaranteed to be reasonable, and the performance of different boards and prepregs is different, and corresponding boards should be used Prepreg parametric lamination, special materials that have never been used need to verify process parameters.

6. Drilling process

Due to the superimposition of each layer, the plate and copper layer are super thick, which will seriously wear the drill bit and easily break the drill bit. The number of holes, falling speed and rotation speed should be appropriately lowered. Accurately measure the expansion and contraction of the board and provide an accurate coefficient; the number of layers is ≥14 layers, the aperture is ≤0.2mm or the hole-to-line distance is ≤0.175mm, and the drilling machine with hole position accuracy ≤0.025mm is used for production; Step-by-step drilling, thickness-to-diameter ratio 12.:1 is produced by step-by-step drilling, front and back drilling methods; control the drilling edge and hole thickness, try to use a new drill or grind 1 drill for high-layer boards, and control the hole thickness within 25um . In order to improve the problem of drilling burrs on high-layer thick copper plates, it has been verified in batches that high-density backing plates are used, the number of stacked plates is one, and the number of drill grinding times is controlled within 3 times, which can effectively improve drilling burrs, as shown in Figure 2 and Figure 3 Show.