At present, domestic PCB manufacturers that can mass produce high-level circuit boards 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 experience accumulation of technicians and production personnel. At the same time, the customer certification procedures for introducing high-level circuit boards are strict and cumbersome, so the threshold for high-level circuit boards to enter enterprises is high, and it takes a long time to realize industrial production.
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-level circuit boards, and introduces the control points of key production processes of high-level circuit boards, which can be used for reference by peers. First, the main production difficulties
Compared with the characteristics of conventional circuit boards, high-rise circuit boards have the characteristics of thicker boards, more layers, denser lines and vias, larger unit size, thinner dielectric layer, etc. The requirements of inner space, interlayer alignment, impedance control and reliability are stricter.
1.1 difficulty of interlayer alignment
Due to the large number of layers of high-rise boards, the customer’s design end has more and more strict requirements on the alignment degree of PCB layers, usually the interlayer alignment tolerance is controlled to ±75μm m. Considering the factors such as the large unit size design of high-rise boards, the environmental temperature and humidity of the graphic transfer workshop, the dislocation superposition caused by the inconsistency of expansion and contraction of different core boards, and the interlayer positioning mode, it is more difficult to control the interlayer alignment degree of high-rise boards.
1.2 difficulties in making inner circuit
The high-rise board is made of special materials such as high TG, high speed, high frequency, thick copper, thin dielectric layer, etc., which puts forward high requirements for inner circuit fabrication and graphic size control, such as the integrity of impedance signal transmission, which increases the difficulty of inner circuit fabrication. The line width is small, the short circuit is increased, the short circuit is increased, and the qualified rate is low; There are many fine signal layers in the line, and the probability of missing AOI in the inner layer increases; The thickness of the inner core plate is thin, so it is easy to fold, which leads to poor exposure, and it is easy to roll the plate when etching through the machine; Most of the high-level boards are system boards, and the unit size is large, so the cost of scrapping finished products is relatively high.
1.3 difficulties in pressing
Many inner core boards and prepregs are superimposed, so it is easy to produce defects such as sliding plate, delamination, resin cavity and bubble residue 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 lamination program for the high-rise board. There are too many layers, and the control of expansion and contraction and the compensation of size coefficient can’t keep the consistency; Thin interlayer insulation easily leads to failure of interlayer reliability test. Fig. 1 is the defect diagram of delamination after thermal stress test.
1.4 drilling difficulties
The use of high Tg, high speed, high frequency, thick copper special plate 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;Because of the thickness of the plate, it is easy to cause the problem of oblique drilling.
2、 Key production process control
2.1 material selection
With the development of high-performance and multi-functional electronic components, high-frequency and high-speed development of signal transmission, so it is required that the dielectric constant and dielectric loss of electronic circuit materials are relatively low, and low CTE, low water absorption and better high-performance copper clad laminate materials are required to meet the processing and reliability requirements of high-rise boards.Commonly used plate suppliers are a series, B series, C series, D series, the main characteristics of these four kinds of inner substrate comparison.
For high-rise thick copper circuit board, high resin content prepreg is selected. The flow of glue in the interlayer is enough to fill the inner layer. If the insulating medium layer is too thick, the finished board will become too thick. Otherwise, the dielectric layer is too thin, which will easily lead to quality problems such as dielectric delamination and high-voltage test failure. Therefore, it is very important to select insulating medium materials.
2.2 laminated structure design
The main factors to be considered in the design of laminated structure are the heat resistance, voltage resistance, glue filling amount and thickness of dielectric layer. The following main principles should be followed.
(1) The manufacturers of prepreg and core board must be consistent.In order to ensure the reliability of PCB, single 1080 or 106 prepreg shall not be used for all layers of prepreg (except for special requirements of customers). When the customer has no medium thickness requirement, the thickness of interlayer media must be guaranteed to be ≥ 0.09mm according to ipc-a-600g.
(2) When the customer requires high Tg board, the corresponding high Tg material should be used for the core board and the semi curing sheet.
(3) For inner substrate 3oz or above, semi cured tablets with high resin content are selected, such as 1080r / C65%, 1080hr / C 68%, 106R / C 73%, 106hr / C76%;However, the structural design of 106 high glue prepreg should be avoided as far as possible, so as to prevent the overlap of multiple 106 prepreg sheets. Because the glass fiber yarn is too thin and the glass fiber yarn collapses in the large base material area, the dimensional stability and plate delamination are affected.
(4) If the customer has no special requirements, the thickness tolerance of interlayer dielectric layer is generally controlled by +/-10%. For impedance plates, the thickness tolerance of dielectric layer is controlled by IPC-4101 C/M level tolerance. If the influencing factors of impedance are related to the thickness of base material, the plate tolerance must also be controlled by IPC-4101 C/M level tolerance.
2.3 Interlayer alignment control
The accuracy of inner core board size compensation and the control of production size need to accurately compensate the graphic size of each layer of high-rise board through the data collected in production for a certain period of time and historical data experience, so as to ensure the consistency of expansion and contraction of each layer of core board. Choose a high-precision and reliable interlaminar positioning method before pressing, such as Pin LAM, hot melt and rivet combination.
Setting proper pressing process and routine maintenance of press are the keys to ensure pressing quality, controlling the bonding and cooling effect, and reducing interlaminar dislocation. Interlayer alignment control needs comprehensive consideration from internal compensation value, lamination positioning mode, lamination process parameters, material characteristics and other factors.
2.4 inner circuit process
Because the resolving power of traditional exposure machine is about 50μm, for the production of high-rise board, LDI can be introduced to improve the graphic resolving power, which can reach about 20μm m. The alignment accuracy of traditional exposure machine is 25 μ m, and the alignment accuracy between layers is more than 50 μ m. By using high-precision alignment exposure machine, the alignment accuracy of graphics can be improved to about 15μm, and the alignment accuracy between layers can be controlled within 30μm, which reduces the alignment deviation of traditional equipment and improves the alignment accuracy between layers of high-rise boards.
In order to improve the etching ability of the circuit, it is necessary to give appropriate compensation to the width of the circuit and the pad (or solder ring) in the engineering design, and also to make more detailed design considerations for the compensation amount of special patterns, such as loop circuit and independent circuit. Confirm whether the design compensation of inner 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 inductive impedance design requirements. Pay attention to whether the design compensation of independent wire and impedance wire is enough. Control the parameters during etching, and batch production can be carried out only after the first piece is confirmed to be qualified. In order to reduce etching side erosion, it is necessary to control the components of etching solution in the best range. The traditional etching line equipment has insufficient etching ability, so the equipment can be technically reformed or imported into high-precision etching line equipment to improve etching uniformity, reduce etching burrs, unclean etching and other problems.
2.5 pressing process
At present, the interlaminar positioning methods before lamination mainly include: Pin LAM, hot melt, rivet, hot melt and rivet combination, and different positioning methods are adopted for different product structures. For the high-rise board, Pin LAM or fusion+riveting method is adopted. OPE punching machine punches the positioning hole, and the punching accuracy is controlled at ±25μm m.
X-RAY should be used to check the layer deviation of the first plate made by the fusing machine, and batch production can be made only if the layer deviation is qualified. In batch production, it is necessary to check whether each plate melts into the unit to prevent subsequent delamination. The pressing equipment adopts high-performance matching press to meet the interlayer alignment accuracy and reliability of high-rise plates.
According to the laminated structure of high-rise board and the materials used, the proper laminating procedure is studied, and the best heating rate and curve are set. On the conventional laminating procedure of multilayer circuit board, the heating rate of laminated board is appropriately reduced, and the high-temperature curing time is prolonged, so that the resin can fully flow and cure, and at the same time, the problems of sliding plate and interlayer dislocation in the laminating process are avoided.
Plates with different TG values cannot be the same as grate plates; The plate with ordinary parameters cannot be mixed with the plate with special parameters; To ensure the rationality of the given expansion and contraction coefficient, the properties of different plates and prepregs are different, so it is necessary to press the corresponding plate prepreg parameters, and the special materials that have never been used need to verify the process parameters.
2.6 drilling technology
Due to the superposition of various layers, the plate and copper layer are excessively thick, which seriously wears the drill bit and easily breaks the drill cutter. The number of holes, falling speed and rotating speed should be appropriately lowered. Measure the expansion and contraction of the plate accurately, and provide accurate coefficient; The number of layers is ≥14, the aperture is ≤0.2mm or the distance from hole to line is ≤0.175mm, and the drilling machine with hole position accuracy ≤0.025mm is used for production; The diameter over φ4.0mm is drilled step by step, and the thickness-diameter ratio of 12:1 is drilled step by step, and the method of positive and negative drilling is adopted. Control the drilling edge and hole diameter. The high-rise board shall be drilled with brand-new drill cutter or Grinding 1 drill cutter as far as possible, and the hole diameter shall be controlled within 25um. In order to improve the drilling burr problem of high-rise thick copper plate, through batch verification, the drilling burr can be effectively improved by using high-density backing plate, the number of stacked plates is one, and the grinding times of drill bit are controlled within three times.
For the high-level board with high frequency, high speed and mass data transmission, the back drilling technology is an effective method to improve the signal integrity. Back drilling mainly controls the length of the residual stub, the consistency of the hole position of the two drills and the copper wire in the hole, etc. Not all drilling machines have the function of back drilling, so it is necessary to upgrade the technology of drilling machines (with the function of back drilling) or purchase drilling machines with the function of back drilling. Back-drilling technology applied from relevant industry literature and mature mass production mainly includes: traditional depth-controlled back-drilling method, back-drilling with signal feedback layer as inner layer, and back-drilling with depth calculated according to the proportion of plate thickness, which will not be repeated here.
III. Reliability test
Generally, the high-rise board is a system board, which is thicker, heavier, larger in unit size and larger in corresponding heat capacity than the conventional multi-layer board. During welding, it needs more heat, and it takes a long time to experience high-temperature welding. 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 board is relatively slow, so the time of reflow soldering test is prolonged. Combined with IPC-6012C, IPC-TM-650 standards and industry requirements, the main reliability test of high-rise board is carried out.
