PCB Tech - Difficulties in Proofing Production of Multilayer Circuit Board 3

Difficulties in Proofing Production of Multilayer Circuit Board3

4. Inner 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 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.

5. 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 multi-layer circuit boards, 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 multilayer circuit board and the materials used, study the appropriate pressing procedure and set the best heating rate and curve. In the conventional multilayer circuit board pressing procedure, appropriately reduce the heating rate of the laminated sheet. Extend the high-temperature curing time to make the resin flow and cure fully, and at the same time avoid 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.

6. 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, and the drill bit grinding times are controlled within 3 times, which can effectively improve the drilling burrs.

For high-level circuit boards used for high-frequency, high-speed, and massive data transmission, back-drilling technology is an effective method 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, 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 soldering, more heat is required and the soldering high temperature time is longer. At 217°C (melting point of tin-silver-copper solder), it takes 50 to 90 seconds, and the cooling speed of multilayer circuit boards is relatively slow, so the time for reflow soldering test is extended, and it is combined with IPC-6012C, IPC-TM-650 standards and industry Requirements, the main reliability test of multilayer circuit boards.