This high multilayer PCB is a 12-layer board, and the surface treatment process is Immersion Gold. Multilayer circuit boards with 10 to 20 layers or more are generally called high multilayer boards, which are more difficult to process than multilayer circuit boards with 8 layers and below, and their quality and reliability requirements are high. They are mainly used in communication equipment, high-end servers, Medical electronics, aviation, industrial control, military and other fields.
1. Stack up
2. Production process:
Cutting → inner layer → inner layer etching → inner layer AOI → brown oxidation → lamination → drilling → PTH and plating → outer layer → outer layer pattern plating → outer layer etching → outer layer AOI inspection → medium Test → impedance test 1 → Pretreatment of solder mask → Solder mask → impedance test 2 → immersion gold → silkscreen → routing→ E-test → FQC → Packing
3. Manufacturing difficulties
Compared with the characteristics of conventional circuit boards, high-level circuit boards have the characteristics of thicker boards, more layers, denser lines and vias, larger cell sizes, and thinner dielectric layers. The inner layer space, the degree of alignment between layers, Impedance control and reliability requirements are more stringent.
a. Difficulties in alignment between layers
Due to the large number of high-level boards, the customer design side has more and more stringent requirements for the alignment of each layer of the PCB. Usually, the alignment tolerance between layers is controlled by ±75μm. Considering the large-scale design of the high-level board unit and the ambient temperature and humidity of the graphics transfer workshop, As well as factors such as misalignment and superposition caused by inconsistency of expansion and contraction of different core layers, interlayer positioning methods, etc., it is more difficult to control the degree of alignment between layers of high-rise boards.
b. Difficulties in making inner circuit
The high-level board uses special materials such as high TG, high-speed, high-frequency, thick copper, thin dielectric layer, etc., which puts forward high requirements on the production of the inner circuit and the control of the pattern size, such as the integrity of the impedance signal transmission, which increases the difficulty of making the inner circuit. Small line width and line spacing, more open and short circuits, more short circuits, and low pass rate; more fine line signal layers, the probability of missing AOI detection in the inner layer is increased; the inner core plate is thinner, which is easy to wrinkle and cause poor exposure and etching It is easy to roll the board when it passes the machine; most of the high-level boards are system boards, the unit size is relatively large, and the cost of scrapping the finished product is relatively high.
c. Difficulties in pressing and making
When multiple inner core boards and prepregs are superimposed, defects such as sliding plates, delamination, resin voids, and air bubbles are likely to occur during lamination production. When designing the laminated structure, it is necessary to fully consider the heat resistance of the material, the withstand voltage, the amount of glue and the thickness of the medium, and set a reasonable high-level board pressing program. There are many layers, and the amount of expansion and contraction control and the compensation of the size factor cannot be kept consistent; the thin interlayer insulation layer can easily lead to the failure of the interlayer reliability test.
d. Difficulties in drilling drilling
The use of high-TG, high-speed, high-frequency, and thick copper special plates increases the difficulty of drilling roughness, drilling burrs and de-drilling. There are many layers, the cumulative total copper thickness and the plate thickness, the drilling is easy to break the knife; the dense BGA is many, the CAF failure problem caused by the narrow hole wall spacing; the plate thickness is easy to cause the inclined drilling problem.
4. Main points of production control:
a. Material selection:
As the number of layers increases, the probability of quality problems increases, so the selection of circuit boards with more than 10 layers is very important. For TG value, TG170 and above materials are preferred.
In addition, because many high-level boards have high-speed, high-frequency signal transmission requirements. Therefore, choosing a relatively low dielectric constant and dielectric loss, as well as low CTE, low water absorption and better high-performance copper clad laminate materials can meet the requirements.
b. Laminated 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 filling and the thickness of the dielectric layer, etc. The following main principles should be followed.
(1) 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.
(2) When customers require high TG sheets, the core board and prepreg must use corresponding high TG materials.
(3) For the inner substrate 3OZ or above, use prepregs with high resin content, such as 1080R/C65%, 1080HR/C 68%, 106R/C 73%, 106HR/C76%; but try to avoid using all 106 high-adhesive prepregs The structure is designed to 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.
(4) 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-4101 C/M tolerance. If the impedance affects the factor and the thickness of the substrate If relevant, the sheet tolerance must also be in accordance with IPC-4101 C/M tolerance.
c. Interlayer alignment control
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 a high-precision, high-reliability interlayer positioning methods before pressing, such as four-slot positioning (Pin LAM), hot melt and rivet combination. Setting the proper 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 interlayer 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.
d. 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. Design requirements for impedance and inductive reactance pay attention to whether the design compensation of independent lines and impedance lines is sufficient, control the parameters during etching, and mass production can be carried out 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 the technical transformation of the equipment or introduce high-precision etching line equipment to improve etching uniformity and reduce etching burrs and unclean etching.
e. Drilling process
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 edge and hole thickness, and use new drills or grinding drills for high-rise boards 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, the 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.
4. Application areas
This circuit board is used in communication base stations and is the base station mainboard. The number of circuit boards of communication base stations is generally relatively high, and since the base station requires continuous and uninterrupted work, the quality requirements of this type of circuit board are usually very high.