Multilayer circuit board is generally defined as 10 ~ 20 layers or more of a high multilayer circuit board, which is more difficult to process than the traditional multilayer circuit board and requires high quality and reliability. It is mainly used in communication equipment, high-end servers, medical electronics, aviation, industrial control, military, and other fields. In recent years, the application of communication, base station, aviation, military, and other fields of multilayer market demand is still strong, and with the rapid development of China’s telecom equipment market, multilayer market prospects are good.
At present, the domestic PCB manufacturers that can mass-produce multilayer circuit boards mainly come from foreign enterprises or a few domestic enterprises. The production of multi-layer circuit board not only needs higher technology and equipment input but also needs the experience accumulation of technical personnel and production personnel. At the same time, the import of multi-layer board customer certification procedures are strict and cumbersome, so the entry threshold of multi-layer circuit board into enterprises is higher, and the industrialization production cycle is longer. The average number of PCB layers has become an important technical index to measure the technical level and product structure of PCB enterprises.
The main production difficulties
Compared with the characteristics of conventional circuit board products, the multi-layer circuit board has the characteristics of thicker parts, more layers, more dense lines and through holes, larger unit size, the thinner dielectric layer, etc., and the requirements of inner space, inter-layer alignment, impedance control, and reliability are more strict.
1.1 Difficulties in alignment between layers
Because laminated layer number is much, end customer design is more and more strict with PCB alignment of each layer, layer usually counterpoints between tolerance control + 75 microns, consider a multilayer unit size design, graphic transfer workshop environment temperature and humidity, and different core board layer increases and inconsistency of superposition of dislocation, interlayer factors such as positioning way, making a multilayer interlayer alignment control more difficult.
1.2 Difficulties in making inner line
Special materials such as high TG, high speed, high frequency, thick copper, and the thin dielectric layer are used in the multilayer plate, which put forward high requirements for inner layer circuit production and graphic size control. For example, the integrity of impedance signal transmission increases the difficulty of inner layer circuit production. The wire width is small, the short circuit is increased, the micro-short circuit is increased, the pass rate is low; As there are more signal layers in the fine circuit, the probability of AOI missing detection in the inner layer is increased. The inner core plate thickness is thin, easy to fold lead to poor exposure, etching the machine is easy to roll plate; Most of the multilayer boards are system boards, and the unit size is large, so the cost of scrap in finished products is relatively high.
1.3 Difficulties in pressing production
Multiple inner core plates and semi-cured sheets are superimposed, and defects such as sliding plate, lamination, resin cavity, and bubble residue are easily produced during pressing production. In the design of the laminated structure, it is necessary to fully consider the heat resistance of the material, voltage resistance, the amount of glue, and the thickness of the medium, and set a reasonable multi-layer pressing program. The number of layers, the expansion and shrinkage control, and size coefficient compensation can not keep the consistency; a Thin insulation layer can easily lead to failure of interlayer reliability test.
1.4 Difficulties in drilling production
Special plate with high TG, high speed, high frequency, and thick copper is used to increase the difficulty of drilling roughness, drilling burr, and dirt removal. The number of layers, the total copper thickness, and the thickness of the finished board drilling it easy to break the knife; CAF failure is caused by narrow hole wall spacing and dense BGA. The thickness of the plate can easily lead to the problem of oblique drilling.
Multilayer boards are generally system boards, which are thicker, heavier, and larger in unit size than conventional multilayer boards. The corresponding heat capacity is also larger. When welding, more heat is needed and the welding high-temperature time is longer. It takes 50 seconds to 90 seconds at 217℃(melting point of tin, silver, and copper solder). At the same time, the cooling rate of multi-layer PCB is relatively slow, so the time of the over-reflow welding test is extended. In combination with IPC-6012C, IPC-TM-650 standards, and industry requirements, the main reliability test of multi-layer PCB.
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