PCB Tech - Difficulty of making high-precision multilayer circuit boards 2

Difficulty of making high-precision multilayer circuit boards2

2. Control of key production processes

⑴ Circuit board material selection

With the development of high-performance and multi-functional electronic components, high-frequency, high-speed development of signal transmission is brought about, so the dielectric constant and dielectric loss of electronic circuit materials are required to be relatively low, as well as low CTE and low water absorption. Rate and better high-performance copper clad laminate materials to meet the processing and reliability requirements of high-level boards. Commonly used board suppliers mainly include A series, B series, C series, and D series. The main characteristics of these four inner substrates are compared, see Table 1. For high-rise 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 dielectric layer is too thick, the finished board may be too thick. On the contrary, if the insulating dielectric layer is too thin, it is easy to cause Quality problems such as dielectric delamination and high-voltage test failure, so the selection of insulating dielectric materials is extremely important.

(2) Circuit board laminated 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 filler, and the thickness of the dielectric layer. The following main principles should be followed.

1. When the customer requires high TG sheet, the core board and prepreg must use the corresponding high TG material.

2. If the customer has no special requirements, the thickness tolerance of the interlayer dielectric layer is generally controlled by +/-10%. For the impedance board, the dielectric thickness tolerance is controlled by IPC-4101C/M tolerance. If the impedance influence factor is related to the thickness of the substrate, then The tolerance of the sheet material must also be in accordance with IPC-4101C/M tolerance.

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

4. For the inner substrate 3OZ or above, use prepregs with high resin content, such as 1080R/C65%, 1080HR/C68%, 106R/C73%, 106HR/C76%; but try to avoid the structural design of 106 high-adhesive prepregs. Prevent the superposition of multiple 106 prepregs. Because the glass fiber yarn is too thin, the glass fiber yarn collapses in the large substrate area, which affects the dimensional stability and the delamination of the plate.

⑶ Alignment control between circuit boards

The accuracy of the inner core board size compensation and production size control requires a certain period of time to collect data and historical data experience in the production to accurately compensate the size of each layer of the high-layer board to ensure that the core board of each layer expands and shrinks. consistency. Choose high-precision, high-reliability interlayer positioning methods before pressing, such as four-slot positioning (PinLAM), hot-melt and rivet combination. Setting the appropriate pressing process and routine maintenance of the press is the key to ensuring the quality of the pressing, controlling the glue flow and cooling effect of the pressing, and reducing the problem of inter-layer misalignment. The layer-to-layer alignment control needs to comprehensively consider factors such as the inner layer compensation value, the pressing positioning method, the pressing process parameters, and the material characteristics.

⑷PCB inner layer circuit process

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 graphics resolution capability, and the resolution can reach about 20μm. The alignment accuracy of the traditional exposure machine is ±25μm, and the alignment accuracy between layers 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 the high-level board.

In order to improve the etching ability of the circuit, it is necessary to give proper compensation to the width of the circuit and the pad (or solder ring) in the engineering design, but also to make a more detailed design for the compensation amount of the special pattern, such as the return circuit and the independent circuit. consider. 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, and mass production can be done after the first piece is confirmed to be qualified. In order to reduce the etching side corrosion, 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 capability, and it is possible to carry out technical transformation of the equipment or introduce high-precision etching line equipment to improve etching uniformity and reduce etching burrs and unclean etching.

⑸PCB pressing process

At present, the positioning methods between layers before pressing mainly include: four-slot positioning (PinLAM), hot melt, rivet, hot melt and rivet combination, and different product structures adopt different positioning methods. For the high-level board, the four-slot positioning method (PinLAM) or the fusion + riveting method is used. The positioning hole is punched out by the OPE punching machine, and the punching accuracy is controlled within ±25μm. When fusing, adjust the machine to make the first board to use X-RAY to check the layer deviation, and the layer deviation can be produced in batches. During mass production, it is necessary to check whether each plate is fused into the unit to prevent subsequent delamination. The pressing equipment adopts high-performance supporting equipment. The press meets the alignment accuracy and reliability of the high-level board.

According to the laminated structure of the high-rise board and the materials used, study the appropriate pressing procedure, set the best heating rate and curve, in the conventional multilayer circuit board pressing procedure, appropriately reduce the heating rate of the laminated sheet and extend the high temperature The curing time allows the resin to flow and cure fully, while avoiding the problems of sliding plate and interlayer dislocation during the pressing process. Plates with different material 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 expansion and shrinkage coefficients given, the properties of different plates and prepregs are different, and the corresponding plates must be used The prepreg parameters are pressed together, and the special materials that have never been used need to verify the process parameters.

⑹ Circuit board drilling technology

Due to the superposition of each layer, the plate and copper layer are too thick, which will cause serious wear to the drill bit and easily break the drill bit. The number of holes, falling speed and rotation speed are appropriately reduced. Accurately measure the expansion and contraction of the board to provide accurate coefficients; the number of layers is ≥14, the hole diameter is ≤0.2mm, or the hole-to-line distance is ≤0.175mm, and the hole position accuracy is ≤0.025mm. The hole diameter is larger than φ4.0mm. Step drilling, with a thickness-to-diameter ratio of 12:1, adopts step-drilling and positive and negative drilling methods; control the drilling front and hole thickness, and the high-level board should be drilled with a new drill or a one-grind drill as far as possible, and the hole thickness should be controlled within 25um. In order to improve the drilling burr problem of high-rise thick copper plates, after batch verification, the use of high-density backing plates, the number of stacked plates is one, the drill bit grinding times are controlled within 3 times, which can effectively improve the drilling burrs

For high-level boards for high-frequency, high-speed, and massive data transmission, back-drilling technology is an effective way to improve signal integrity. The back drill mainly controls the length of the remaining stub, the consistency of the hole position of the two holes, and the copper wire in the hole. Not all drilling machine equipment has the back drilling function, the drilling machine equipment must be technically upgraded (with the back drilling function), or the drilling machine with the back drilling function must be purchased. The back-drilling technology used from industry-related literature and mature mass production applications mainly includes: traditional depth-controlled back-drilling method, inner layer is back-drilling with signal feedback layer, depth back-drilling is calculated according to the plate thickness ratio, which will not be repeated here.

Three, circuit board reliability test

The high-level board is generally a system board, which is thicker, heavier, and larger in unit size than the conventional PCB multilayer board. The corresponding heat capacity is also larger. When soldering, it needs more heat and the soldering high temperature time is longer. It takes 50 seconds to 90 seconds at 217°C (melting point of tin-silver-copper solder). At the same time, the cooling rate of the high-layer board is relatively slow, so the time for reflow soldering test is extended, and in accordance with IPC-6012C, IPC-TM-650 standards and industry requirements, The main reliability test for high-level circuit boards.