Key production process control for High Level PCB guddiga

The high-rise circuit board is generally defined as a high-rise multi-layer circuit board with 10-20 floors or more, which is more difficult to process than the traditional multi-layer circuit board and has high quality and reliability requirements. It is mainly used in communication equipment, high-end server, medical electronics, aviation, industrial control, military and other fields. In recent years, the market demand for high-rise boards in the fields of application communication, base station, aviation and military is still strong. With the rapid development of China’s telecom equipment market, the market prospect of high-rise boards is promising.

Waqtigan xaadirka ah, PCB manufacturers that can mass produce high-rise PCB in China mainly come from foreign-funded enterprises or a few domestic enterprises. The production of high-rise PCB requires not only higher technology and equipment investment, but also the experience accumulation of technicians and production personnel. At the same time, the customer certification procedures for importing high-rise PCB are strict and cumbersome. Therefore, the threshold for high-rise PCB to enter the enterprise is high and the industrialization production cycle is long. The average number of PCB layers has become an important technical index to measure the technical level and product structure of PCB enterprises. This paper briefly describes the main processing difficulties encountered in the production of high-rise PCB, and introduces the key control points of key production processes of high-rise PCB for your reference.

1、 Main production difficulties

Compared with the characteristics of conventional circuit board products, high-rise circuit board has the characteristics of thicker boards, more layers, denser lines and vias, larger unit size, thinner dielectric layer, and stricter requirements for inner space, interlayer alignment, impedance control and reliability.

1.1 difficulties in interlayer alignment

Due to the large number of high-rise board layers, the customer’s design end has increasingly strict requirements on the alignment of PCB layers, and the alignment tolerance between layers is usually controlled to ± 75 μ m. Considering the large unit size design of high-rise board, the ambient temperature and humidity of graphics transfer workshop, the dislocation superposition and interlayer positioning mode caused by the inconsistent expansion and contraction of different core board layers, it is more difficult to control the interlayer alignment of high-rise board.

1.2 difficulties in making inner circuit

The high-rise 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 fabrication and graphic size control of the inner circuit, such as the integrity of impedance signal transmission, which increases the difficulty of the fabrication of the inner circuit. The line width and line spacing are small, the open and short circuits increase, the micro short increases, and the qualification rate is low; There are many signal layers of fine lines, and the probability of missing AOI detection in the inner layer increases; The inner core plate is thin, easy to fold, resulting in poor exposure, and it is easy to roll after etching; Most of the high-rise boards are system boards with large unit size, and the cost of scrapping the finished products is relatively high.

1.3 pressing manufacturing difficulties

When multiple inner core plates and semi cured sheets are superimposed, defects such as sliding plate, delamination, resin cavity and bubble residue are easy to occur in crimping production. When designing the laminated structure, it is necessary to fully consider the heat resistance, voltage resistance, glue filling amount and medium thickness of the material, and set a reasonable high-rise plate pressing program. There are many layers, and the control of expansion and contraction and the compensation of size coefficient cannot be consistent; The interlayer insulation layer is thin, which is easy to lead to the failure of interlayer reliability test. Fig. 1 is a diagram of the defect of bursting plate delamination after thermal stress test.

Fig.1

1.4 drilling difficulties

The use of high Tg, high speed, high frequency and thick copper special plates increases the difficulty of drilling roughness, drilling burr and drilling dirt removal. There are many layers, the total copper thickness and plate thickness are accumulated, and the drilling tool is easy to break; Caf failure caused by dense BGA and narrow hole wall spacing; Due to the plate thickness, it is easy to cause the problem of oblique drilling.

2、 Key production process control

2.1 material selection

With the development of electronic components in the direction of high-performance and multi-function, it also brings high-frequency and high-speed signal transmission. Therefore, it is required that the dielectric constant and dielectric loss of electronic circuit materials are relatively low, as well as low CTE, low water absorption and better high-performance copper clad laminate materials, so as to meet the processing and reliability requirements of high-rise boards. Common plate suppliers mainly include a series, B series, C series and D series. See Table 1 for the comparison of the main characteristics of these four inner substrates. For the high-rise thick copper circuit board, the semi cured sheet with high resin content is selected. The glue flow amount of the inter layer semi cured sheet is enough to fill the inner layer graphics. If the insulating medium layer is too thick, the finished board is easy to be too thick. On the contrary, if the insulating medium layer is too thin, it is easy to cause quality problems such as medium stratification and high-voltage test failure. Therefore, the selection of insulating medium materials is very important.

2.2 design of laminated structure

The main factors considered in the design of laminated structure are the heat resistance, voltage resistance, glue filling amount and dielectric layer thickness of the material, and the following main principles shall be followed.

(1) The manufacturer of semi cured sheet and core board must be consistent. In order to ensure the reliability of PCB, single 1080 or 106 semi cured sheet shall not be used for all layers of semi cured sheet (unless the customer has special requirements). When the customer has no medium thickness requirements, the medium thickness between layers must be guaranteed to be ≥ 0.09mm according to ipc-a-600g.

(2) When customers require high Tg board, core board and semi cured sheet shall use corresponding high Tg materials.

(3) For the inner substrate 3oz or above, select the semi cured sheet with high resin content, such as 1080r / C65%, 1080hr / C 68%, 106R / C 73%, 106hr / C76%; However, the structural design of all 106 high glue semi cured sheets shall be avoided as far as possible to prevent the superposition of multiple 106 semi cured sheets. Because the glass fiber yarn is too thin, the glass fiber yarn collapses in the large substrate area, which affects the dimensional stability and plate explosion delamination.

(4) If the customer has no special requirements, the thickness tolerance of interlayer dielectric layer is generally controlled by + / – 10%. For impedance plate, the dielectric thickness tolerance is controlled by ipc-4101 C / M tolerance. If the impedance influencing factor is related to the substrate thickness, the plate tolerance must also be controlled by ipc-4101 C / M tolerance.

2.3 interlayer alignment control

For the accuracy of inner core board size compensation and production size control, it is necessary to accurately compensate the graphic size of each layer of high-rise board through the data and historical data experience collected in production for a certain time to ensure the consistency of expansion and contraction of each layer of core board. Select high-precision and reliable interlayer positioning mode before pressing, such as pin Lam, hot-melt and rivet combination. Setting appropriate pressing process procedures and daily maintenance of the press are the key to ensure the pressing quality, control the pressing glue and cooling effect, and reduce the problem of interlayer dislocation. The control of interlayer alignment needs to be comprehensively considered from the factors such as inner layer compensation value, pressing positioning mode, pressing process parameters, material characteristics and so on.

2.4 inner line process

Because the analytical ability of the traditional exposure machine is less than 50 μ M. for the production of high-rise plates, laser direct imager (LDI) can be introduced to improve the graphics analysis ability, which can reach 20 μ M or so. The alignment accuracy of traditional exposure machine is ± 25 μ m. The inter layer alignment accuracy is greater than 50 μ m。 Using high-precision alignment exposure machine, the graphics alignment accuracy can be improved to 15 μ M, interlayer alignment accuracy control 30 μ M, which reduces the alignment deviation of traditional equipment and improves the interlayer alignment accuracy of high-rise slab.

In order to improve the etching capacity of the line, it is necessary to give appropriate compensation for the width of the line and the pad (or welding ring) in the engineering design, as well as more detailed design consideration for the compensation amount of special graphics, such as return line and independent line. Confirm whether the design compensation of inner line width, line distance, isolation ring size, independent line and hole to line distance is reasonable, otherwise change the engineering design. There are impedance and inductive reactance design requirements. Pay attention to whether the design compensation of independent line and impedance line is sufficient. Control the parameters during etching. Batch production can be carried out only after the first piece is confirmed to be qualified. In order to reduce etching side corrosion, it is necessary to control the chemical composition of each group of etching solution within the best range. The traditional etching line equipment has insufficient etching capacity. The equipment can be technically transformed or imported into high-precision etching line equipment to improve the etching uniformity and reduce the problems such as rough edge and unclean etching.

2.5 pressing process

At present, the interlayer positioning methods before pressing mainly include: pin Lam, hot melt, rivet, and the combination of hot melt and rivet. Different positioning methods are adopted for different product structures. For the high-rise slab, the four slot positioning method (pin Lam) or fusion + riveting method shall be used. The ope punching machine shall punch the positioning hole, and the punching accuracy shall be controlled within ± 25 μ m。 During fusion, X-ray shall be used to check the layer deviation of the first plate made by the adjusting machine, and the batch can be made only after the layer deviation is qualified. During batch production, it is necessary to check whether each plate is melted into the unit to prevent subsequent delamination. The pressing equipment adopts high-performance supporting press to meet the inter layer alignment accuracy and reliability of high-rise plates.

According to the laminated structure of high-rise board and the materials used, study the appropriate pressing procedure, set the best temperature rise rate and curve, appropriately reduce the temperature rise rate of pressed board in the conventional multi-layer circuit board pressing procedure, prolong the high-temperature curing time, make the resin fully flow and solidify, and avoid the problems such as sliding plate and interlayer dislocation in the pressing process. Plates with different TG values cannot be the same as grate plates; Plates with ordinary parameters cannot be mixed with plates with special parameters; To ensure the rationality of the given expansion and contraction coefficient, the properties of different plates and semi cured sheets are different, so the corresponding plate semi cured sheet parameters need to be pressed, and the process parameters need to be verified for special materials that have never been used.

2.6 drilling process

Due to the over thickness of plate and copper layer caused by the superposition of each layer, the drill bit is seriously worn and it is easy to break the drill bit. The number of holes, falling speed and rotating speed shall be appropriately reduced. Accurately measure the expansion and contraction of the plate to provide accurate coefficient; If the number of layers ≥ 14, the hole diameter ≤ 0.2mm or the distance from hole to line ≤ 0.175mm, the drilling rig with hole position accuracy ≤ 0.025mm shall be used for production; diameter φ The hole diameter above 4.0mm adopts step-by-step drilling, and the thickness diameter ratio is 12:1. It is produced by step-by-step drilling and positive and negative drilling; Control the burr and hole thickness of the drilling. The high-rise slab shall be drilled with a new drill knife or a grinding drill knife as far as possible, and the hole thickness shall be controlled within 25um. In order to improve the drilling burr problem of high-rise thick copper plate, through batch verification, the use of high-density backing plate, the number of laminated plates is one, and the grinding times of drill bit is controlled within 3 times, which can effectively improve the drilling burr

For the high-rise board used for soo noqnoqoshada sare, high-speed and massive data transmission, back drilling technology is an effective method to improve signal integrity. The back drilling mainly controls the residual stub length, the hole position consistency of the two boreholes and the copper wire in the hole. Not all drilling machine equipment has back drilling function, so it is necessary to upgrade the drilling machine equipment (with back drilling function) or purchase a drilling machine with back drilling function. The back drilling technology applied from industry related literature and mature mass production mainly includes: traditional depth control back drilling method, back drilling with signal feedback layer in the inner layer, and calculating the depth back drilling according to the proportion of plate thickness. It will not be repeated here.

3、 Reliability test

The high-rise board is generally a system plate, which is thicker and heavier than the conventional multi-layer plate, has larger unit size, and the corresponding heat capacity is also larger. During welding, more heat is required and the welding high temperature time is long. At 217 ℃ (melting point of tin silver copper solder), it takes 50 seconds to 90 seconds. At the same time, the cooling speed of high-rise plate is relatively slow, so the time of reflow test is prolonged. Combined with ipc-6012c, IPC-TM-650 standards and industrial requirements, the main reliability test of high-rise board is carried out.