In the circuit board industry, multi-layer circuit boards (high-precision PCB multi-layer boards) are generally defined as 4-20 or more layers called multi-layer circuit boards, which are more difficult to process than traditional multi-layer circuit boards, and their quality is reliable. High requirements, mainly used in communication equipment, industrial control, security, high-end servers, medical electronics, aviation, military and other fields. In recent years, the market demand for high-level PCB boards in the fields of application communications, base stations, aviation, and military is still strong. With the rapid development of China's telecommunications equipment market, the market prospects for high-level PCB boards are optimistic.
At present, the domestic high-level circuit board manufacturers that can produce circuit board proofing mainly come from foreign-funded enterprises or a few domestic-funded enterprises. The production of high-level circuit boards not only requires high investment in technology and equipment, 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, so the threshold for multi-layer circuit board proofing to enter the enterprise is relatively high. High, the realization of industrialized 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 enterprises. This paper 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-level circuit boards for your reference.
1. Difficulties in the production of main circuit boards
Compared with the characteristics of conventional circuit boards, high-level circuit boards have the characteristics of thicker PCB boards, more layers, denser lines and vias, larger unit size, and thinner dielectric layers. , impedance control and reliability requirements are more stringent.
a. Difficulty in alignment between layers
Due to the large number of high-level PCB layers, the customer design side has more and more stringent requirements on the alignment of each layer of the PCB. Usually, the alignment tolerance between layers is controlled to ±75μm. Considering the large size of the high-level board unit design and the ambient temperature and humidity of the graphics transfer workshop , and the dislocation stacking, interlayer positioning method and other factors caused by the inconsistency of expansion and contraction of different core layers, make it more difficult to control the interlayer alignment of high-rise boards.
b. Difficulties in making inner layers
The PCB high-level circuit board adopts special materials such as high TG, high speed, high frequency, thick copper, and thin dielectric layer, which puts forward high requirements for the production of inner layer circuits and control of graphic size, such as the integrity of impedance signal transmission, which increases the production of inner layer circuits. difficulty. The line width and line spacing are small, the open and short circuits increase, the micro-shorts increase, and the pass rate is low; there are many signal layers of fine lines, and the probability of missed inspection of the inner layer AOI increases; the thickness of the inner layer core board is thin, and it is easy to wrinkle, resulting in poor exposure and etching. It is easy to roll the board when passing the machine. Most of the multi-layer circuit boards are system boards. The unit size is large, and the cost of scrapping the finished product is relatively high.
c. Difficulties in pressing
Multiple PCB inner layer core boards and prepregs are superimposed, and defects such as sliding plates, 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, withstand voltage, amount of glue filling and dielectric thickness of the material, and set a reasonable PCB high-level circuit board pressing program. The number of layers is large, and the expansion and contraction control and size coefficient compensation cannot be consistent; the interlayer insulating layer is thin, which easily leads to the failure of the interlayer reliability test.
d. Difficulties in drilling production
The use of high-TG, high-speed, high-frequency, and thick copper special plates increases the difficulty of drilling roughness, drilling burrs and decontamination. The number of layers is large, the total copper thickness and plate thickness are accumulated, and the drilling tool is easy to break; there are many dense BGAs, and the CAF failure problem caused by the narrow hole wall spacing; the inclined drilling problem is easily caused by the plate thickness.
2. Control of key production processes
a, PCB material selection
With the development of high-performance and multi-functional electronic components, as well as high-frequency and high-speed signal transmission, electronic circuit materials are required to have low dielectric constant and dielectric loss, as well as low CTE and low water absorption. rate and better high-performance CCL materials to meet the processing and reliability requirements of high-rise boards. Commonly used plate suppliers mainly include A series, B series, C series, and D series. The main characteristics of these four inner layer substrates are compared in Table 1. For high-layer thick copper circuit boards, use prepregs with high resin content. The amount of glue flowing between the interlayer prepregs is enough to fill the inner layer pattern. If the insulating medium layer is too thick, the finished board will be too thick. On the contrary, if the insulating medium layer is too thin, it is easy to cause There are quality problems such as dielectric delamination and high-voltage test failure, so the selection of insulating dielectric materials is extremely important.
b. Design of laminated laminated structure
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 filling and the thickness of the dielectric layer, etc. The following main principles should be followed.
①. When the customer requires high TG sheet, the core board and the prepreg must use the corresponding high TG material.
②. Prepreg and core board manufacturers must be consistent. In order to ensure the reliability of PCB, all layers of prepreg should avoid using single 1080 or 106 prepreg (except customers have special requirements). If the customer has no dielectric thickness requirements, the thickness of the dielectric between each layer must be guaranteed according to IPC-A-600G ≥ 0.09mm.
③. The inner layer substrate is 3OZ or above, and the prepreg with high resin content is selected, such as 1080R/C65%, 1080HR/C68%, 106R/C73%, 106HR/C76%; but try to avoid the structural design of all 106 high-adhesive prepreg. In order to prevent the overlapping of multiple 106 prepregs, because the glass fiber yarn is too thin, the glass fiber yarn collapses in the large base material area, which affects the dimensional stability and the delamination of the explosion board.
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 class tolerance. If the impedance influencing factor is related to the thickness of the substrate, Then the plate tolerance must also be in accordance with the IPC-4101C/M class tolerance.
c. Interlayer alignment control
The accuracy of the size compensation of the inner layer core board and the control of the production size need to be accurately compensated for 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 each layer of the core board Consistency of expansion and contraction. Choose high-precision, high-reliability interlayer positioning methods before lamination, such as four-groove positioning (PinLAM), hot melt and rivet combination. Setting appropriate pressing process procedures and daily maintenance of the press is the key to ensuring the quality of the pressing, controlling the glue and cooling effect of the pressing, and reducing the problem of interlayer dislocation. The interlayer alignment control needs to comprehensively consider factors such as the compensation value of the inner layer, the positioning method of the pressing, the parameters of the pressing process, and the material properties.
d. Inner layer circuit technology
Since the resolution capability of the traditional exposure machine is about 50μm, for the production of high-level boards, a laser direct imaging machine (LDI) can be introduced to improve the image resolution capability, and the resolution capability can reach about 20μm. The alignment accuracy of traditional exposure machines is ±25μm, and the interlayer alignment accuracy is greater than 50μm. Using a high-precision alignment exposure machine, the graphic alignment accuracy can be increased to about 15μm, and the interlayer alignment accuracy can be controlled within 30μm, which reduces the alignment deviation of traditional equipment and improves the interlayer alignment accuracy of high-rise boards.
In order to improve the line etching ability, it is necessary to give proper compensation to the width of the line and the pad (or solder ring) in the engineering design, and also need to make more detailed design for the compensation amount of special graphics, such as return lines, independent lines, etc. consider. Confirm whether the design compensation of inner layer line width, line spacing, isolation ring size, independent line, and hole-to-line distance is reasonable, otherwise, change the engineering design. There are impedance and inductance design requirements. Pay attention to whether the design compensation of independent lines and impedance lines is sufficient. Control the parameters during etching, and mass production can only be done after the first piece is confirmed to be qualified. In order to reduce the etching side erosion, it is necessary to control the composition of each group of the etching solution within the optimal range. The traditional etching line equipment has insufficient etching capacity, and the equipment can be technically transformed or imported into high-precision etching line equipment to improve the etching uniformity and reduce the etching burr, the etching is not clean and other problems.
e. pressing process
At present, the interlayer positioning methods before lamination mainly include: four-slot positioning (PinLAM), hot melt, rivet, hot melt and rivet combination, and different product structures adopt different positioning methods. For multi-layer circuit boards, the four-slot positioning method (PinLAM) is used, or the fusion + riveting method is used. The OPE punching machine punches out the positioning holes, and the punching accuracy is controlled at ±25μm. During fusion, X-RAY should be used to check the layer deviation of the first board. Only if the layer deviation is qualified, batches can be produced. In 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. Press to meet the accuracy and reliability of interlayer alignment of high-rise boards.
According to the laminated structure of the multi-layer circuit board and the materials used, the appropriate lamination procedure is studied, and the optimal heating rate and curve are set. Extend the high temperature curing time, make the resin flow and cure fully, and avoid problems such as sliding plates and interlayer dislocation during the lamination process. Plates with different TG values cannot be the same as grate plates; plates with common parameters cannot be mixed with plates with special parameters; to ensure the rationality of the given expansion and shrinkage coefficient, the performance of different plates and prepregs is different, and corresponding plates should be used Prepreg parameter lamination, special materials that have never been used need to verify process parameters.
f. Drilling process
Due to the superposition of each layer, the plate and copper layer are super thick, which will seriously wear the drill bit and easily break the drill blade. The number of holes, drop speed and rotation speed should be adjusted appropriately. Accurately measure the expansion and contraction of the board, and provide accurate coefficients; the number of layers is greater than or equal to 14 layers, the diameter of the hole is less than or equal to 0.2mm, or the distance from the hole to the line is less than or equal to 0.175mm. Step-by-step drilling, with a thickness-to-diameter ratio of 12:1, is produced by step-by-step drilling and forward and reverse drilling methods; control the drilling tip and hole thickness, and use a new drill or a grinding drill for high-rise boards as much as possible. Drill, and the hole thickness is controlled within 25um. In order to improve the drilling burr problem of high-rise thick copper plates, after batch verification, high-density backing plates are used, the number of stacked plates is one, and the drill grinding times are controlled within 3 times, which can effectively improve drilling burrs.
For high-frequency, high-speed, and massive data transmission high-level circuit boards, back-drilling technology is an effective way to improve signal integrity. Back drilling mainly controls the length of the residual stub, the consistency of the hole position of the two drillings, and the copper wire in the hole. Not all drilling machine equipment has the function of back drilling, and the drilling machine equipment must be technically upgraded (with the function of back drilling), or the drilling machine with the function of back drilling must be purchased. Back-drilling technologies from industry-related literature and mature mass production applications mainly include: traditional depth-controlled back-drilling method, back-drilling with signal feedback layer in the inner layer, and back-drilling depth calculated according to the ratio of plate thickness, which will not be repeated here.
3. Reliability test
PCB high-level boards are generally system boards, which are thicker, heavier, and larger in unit size than conventional multi-layer boards. The corresponding heat capacity is also larger. During welding, more heat is required, and the high temperature welding time experienced is longer. It takes 50 to 90 seconds at 217°C (melting point of tin-silver-copper solder), and the cooling rate of the multilayer circuit board is relatively slow, so the time for reflow soldering test is prolonged, and combined with IPC-6012C, IPC-TM-650 standards and industry Requirements, the main reliability test of multi-layer circuit boards.
English
Chinese